Treatment and reclamation of hydrocarbon-bearing ...

Rev Chem Eng 2018; aop

Partha Kundua and Indra M. Mishra*

Treatment and reclamation of hydrocarbon-bearing oily wastewater as a hazardous pollutant by different processes and technologies: a state-of-the-art review https://doi.org/10.1515/revce-2017-0025 Received April 21, 2017; accepted October 9, 2017

1 Introduction

Abstract: Hydrocarbon-containing oily wastewater generated by various industries creates a major environmental problem all over the world since petroleum products are commonly used as energy sources and raw materials in various industries. In case of offshore/coastal oil recovery operations, produced water is discharged through either shore side outfalls or coastal rim releases. In many cases, current disposal practices leads to severe environmental pollution by contamination of petroleum hydrocarbon to the surface, ground, and coastal waterways. Therefore, it is necessary to evaluate the performance of various processes for the recovery of petroleum hydrocarbons from wastewater. In this paper, a detailed review on the different separation/treatment processes of oily wastewater is presented. Previous and recent research works are reviewed in the area of oil-water separation from wastewater and also highlight the new developments in these areas. Various separation processes and technologies such as gravity separation, flotation process, membrane process, adsorption process, biological treatment, freeze/thaw process, and photocatalytic oxidation process (PoPs)/advanced oxidation processes (AoPs) are discussed and reviewed. The adsorption properties of a wide variety of porous sorbent materials in oily wastewater treatment, particularly in the area of oil spill cleanup, are also reviewed. The advantages and disadvantages of each process are critically discussed and compared.

Potential oil sources are petroleum refineries, oil production wells, oil and gas fields, petrochemical industries, gasoline dispensing stations, transport vehicles carrying petroleum products, and/or leaking oil/gas pipelines, which contaminate/pollute water bodies and soil through spills or wastewater discharges. The petroleum, metallurgical, and transportation industries are concerned about the separation and recovery of oils from water and the treatment of oily wastewater. Petroleum refineries and oil/gas fields usually have a high concentration of oil in miscible and immiscible forms in the effluent. During the production of oil from oil wells/reservoirs, an increased amount of coproduction either oil-in-water (o/w) or waterin-oil (w/o) emulsions happens. Transportation of water along with oil increases additional energy expenditure and restricts the volumetric flow of oil. Several water desalination plants or reverse osmosis (RO) treatment units face serious problems due to the presence of oil in their influent water sources. The effluent wastewater must meet the discharge standards set by the environmental regulating agencies like Central and/or State Pollution Control Boards and other regulatory bodies. The Ministry of Environment and Government Forest (MOEF), India, has set the discharge limit of oil to wastewater as less than 5 mg/l (MOEF Notification 2008). The formation of emulsions (o/w or w/o) significantly alters the characteristics of effluent wastewater. Due to the formation of stable emulsions, the viscosity of discharge water significantly increased from a few Pa · s to about 1000 Pa · s and density could be as high as 1003 kg m−3 (Fingas et al. 1993, Kundu et al. 2015). A semisolid-like liquid mass was formed, which causes difficulty in commonly used wastewater treatment processes if it remained untreated. Primary oil separation methods include gravity separation, tilted plate interception, American Petroleum Institute (API) separator, floatation, and chemical coagulation, but these did not meet the discharge limit. A commercial method includes chemical deemulsification followed by air flotation or gravity separation (Hafiz et al. 2005, Bensadok et al. 2007) to overcome the problem. However, such a method requires costly chemicals and faces the serious disposal problem of large volume of sludge created during the process.

Keywords: coalescence; flotation; membrane; oily wastewater; pollution; wastewater treatment.

Present address: Petroleum Technology Research Centre (PTRC), Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada. *Corresponding author: Indra M. Mishra, Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttrakhand 247667, India; and Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, Dhanbad 826004, Jharkhand, India, e-mail: [email protected] Partha Kundu: Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttrakhand 247667, India a

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2 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater Physico-chemical treatment methods involve adsorption and membrane separation, which have advantages, like no use of additional chemicals, high removal efficiency, compactness of the treatment unit, and automated mode of operation (Benito et al. 1998). The operational complexity, high energy consumption, higher capital, and operating cost limit the applicability of membrane separation and adsorption compared with the coalescence method. Filtration of emulsion using different filter media such as sawdust (Cambiella et al. 2006), activated carbon, peat (Mathavan and Viraraghavan 1989), organoclay (Alther 1995), and bentonite (Viraraghavan and Moazed 2003) was also used for the removal of emulsified oil from oily wastewater. However, poor adsorption and selectivity of oil and poor regeneration of such media limit their usage. Adsorption is a reversible process due to the presence of ionic bonds, which make the possibility of breaking the hydrogen bond easier while maintaining the ionic bond intact (Rogues and Aurello 1991, Zhou et al. 2008). Various researchers used oil coalesers, which were found to be useful for the treatment of synthetic and industrial oily wastewater (Li and Gu et al. 2005, Zhou et al. 2009). The purpose of this review paper is to provide an overview of the various separation methods and treatment processes for oily wastewater including emulsions and to highlight the recent development in this area.

1.1 S ources of oil in wastewater

3. 4. 5.

6.

automobile industry and metal-forming/work units also discharge spent machining-cutting oils, coolants, and drawing compounds. Oily wastewater produced in coke plants derived from cooling, quenching coke, or scrubbing gases. Oily wastes are generated from leaks, spills, or cleaning operation of transportation industries. Oily matter is obtained from leaks in seals, condensers, or heat exchangers during the processing of the crude oil. Run-offs from industrial areas contaminated with hydrocarbon substrates during storms or natural catastrophes are also counted as a source of pollutant.

1.1.2 Municipal sources The major sources of oily wastes are from food preparation, garbage disposal, and cleaning. Cleaning includes laundry washing and household cleaning jobs. Grease and oily materials are removed at sewage treatment plants. Road oil and degraded asphalt are washed from roads into the storm sewers and streams. Typical domestic wastewater may contain 10–50 mg/l of oil and grease.

1.1.3 Natural sources Pine trees, coniferous trees, and shrubs also produce oily materials to run-off water.

1.1.1 Industrial sources 1. One of the principal industrial sources of oily wastewater is the petroleum process industry, like the chemical treatment of lubricating oils, waxes, burning oils, barometric condensers, desalting processes, etc. Oily wastes are generated from producing, refining, storing, or transporting operations. The process water includes the condensate and the wash water, which come into contact with the petroleum products at some stage of the processing. The wastewater thus contains free and emulsified oil, impurities of petroleum products, spent acid solution, etc. About 0.1%–2% of crude oil is discharged into the sewers as free oil. This oil may form emulsion when flowing in a sewer. 2. Another major oil source is the metal industry. Most of the oily waste comes from metal-working or metalforming operations. The o/w emulsions in large quantities were used as a coolant, which quenches fiction-heated surface and provides lubrication. The

1.2 Classification of oil and water mixture The oil in o/w mixtures can be classified as follows: 1. Free oil 2. Dispersed oil 3. Emulsified oil (o/w or w/o emulsion) 4. Soluble oil Free oils in water are normally present either as a floating mass or in the form of oil droplets of size larger than 150 μm. Free oil layer floating on water stream is removed by an overflow weir in the tank and a skimmer and the residual oil is removed by an adsorption tank (such as a carbon tank) which is filled with organ clay (Quevedo et al. 2009). Dispersed oil in the o/w mixture is present in droplet form in the size range of 20–150 μm. Dispersed oil mixes with water due to shear, which can result in, when the wastewater passes through a pump, wastewater splashes


P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater 3

Figure 1: Classification and size range of oil droplets.

Table 1: Effects of discharged oily effluent. Effect on environment

Effect on human health

Effects on marine life and wildlife

Effect on industries and water treatment process

(1) Free oil hinders the penetration of sunlight in river water distracting aquatic life and restricting natural cleansing of water in rivers or lakes

(1) Consumption of untreated chemically emulsified oil disposed in river causes several health problems (2) B athing in contaminated oily water causes skin cancer

(1) Very harmful to marine flora and fauna, e.g. marine birds, mammals, fish, mangroves, coral reefs, etc.

(1) In steam generation and cooling process, oil contaminated water causes foaming, priming, over heating of tubes, which lead to poor heat transfer from metal surface to water (2) Free and emulsified oil can clog and coat the filter and ion exchange beds, decreases effectiveness of filtration and interfere with backwashing

(2) Undesirable odour emanates from oily waste

(3) Oily waste may coat the gills of fish and stop the oxygen transfer which is fatal for them

(3) F ish affected by toxic oils, if consumed can cause nausea and vomiting

(4) Untreated oily waste forms a layer on the banks of river, causes spoiling of vegetation present on banks and the aesthetics environment gets adversely affected

(2) Oil destroys the insulating ability of fur-bearing mammals, such as sea otters, and the water-repelling abilities of a bird’s feathers, thus exposing these creatures to the harsh climate/ environmental conditions (3) Habitat destruction

into a tank and anything that will break up and disperse larger oil droplets. An o/w emulsion contains oil droplets of size smaller than 20 μm. Figure 1 shows the classification and size range of oil droplets that were found in wastewaters. When the oil droplets size is less than 5 μm, then oil is said to be soluble. Soluble oils can comprise materials such as phenolic-type aromatic compounds or cutting oils used in

(3) In biological treatment of wastewater a layer of oil adheres to the microorganism creating additional resistances to oxygen and nutrient transfer to biomass and thus reduces the treatment efficiency

metal working which are selectively extracted to a varying degree by solvents.

1.3 Effects of oil spill on the environment Oils discharged into the water bodies/on soils cause adverse impact in ecology. It also affects the water


4 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater

Primary Corrugated plate interceptor

Oily wastewater

Parallel plate interceptor

Secondary

Tertiary

Chemical coagulation Dissolved air flotation Adsorption Membrance Ozone

Coalescence API separator

Treated wastewater

High rate filtration

Figure 2: Conventional oily wastewater treatment scheme.

treatment processes. The effects of oil-containing effluents are shown in Table 1. It is therefore necessary that oil-field effluent must be treated before disposal, which in turn improves the water quality, water reuse, and protection of downstream facilities. These are also in compliance with the environmental regulations and conditions imposed by regulatory agencies. This section presented a review of the open literature available on the treatment of oily wastewaters. Various methods of oily wastewater including o/w emulsion treatment were also reported in this section. Oil field water also contains suspended solids (SSs), which include clay, sand, and scale corrosion products like iron sulfide and iron oxide. Thus, prior the treatment of this wastewater, it cannot be disposed off, which causes ecological imbalance and water pollution hazards. There are three types of basic treatment methods for emulsified wastewater, as shown in Figure 2. The treatment of oily wastewater follows three stages: (a) droplets migrate to the interface between the oil and water bulk phases; (b) at the interface, droplets coalesce and are taken up into the bulk oil phases; and (c) the separated oil is removed from the water surface (Patterson 1985).

2 T echniques and methods of oil separation from oily/emulsified wastewater Various techniques have been used for the separation of oil from o/w emulsion. These methods are described hereafter. Oil water separator (OWS) is specially designed equipment used to separate oil and water mixtures into

individual phases. Different types of oil-water separator are selected on the basis of oil separation capabilities. Oil separation efficiency and life cycle cost are the key parameters for the design and selection of these specific OWSs.

2.1 Gravity separator The oil in water can be classified as free oil, dispersed oil, emulsified oil, and soluble oil. Free oils in water normally present either as a floating mass or in the form of oil droplets of size larger than 150 μm. Free oil can be removed by an overflow weir in the tank, gravity separator, and skimmer. According to Stoke’s law, the rise rate of the oil droplet can be varied by changing the oil density, water viscosity, water density, or the oil droplet size. The principal governing gravity separation of oil is expressed by Stoke’s Law:

V=

g (dw − do ) D2 , 18 µ

(1)

where V = oil droplet rise rate, g = acceleration due to gravity, dw = water density, do = oil density, D = oil droplet diameter, µ = water viscosity. The first three variables are controlled by temperature, while the addition of chemical coagulants alters the last variable and oil droplet size. The gravity separator consists of an empty vessel that provides long retention time to liquids. This will assist in settling out and forming two distinct layers under gravity (Hafiz et al. 2005). As shown in Figure 3, proper hydraulic design and longer retention time increase the separation efficiency.



Stoke’s law implies that larger droplets will rise faster and separate easily as compared to smaller droplets. The oleophilic nature of the corrugated plates attracts the oil droplets and enhances the coalescence, which helps to form larger droplets to rise faster. Its compact modular design makes the system advantageous as compared to the inclined plate modules. Plugging of the plate packs by deposited solids and solvent corrosion are the main drawbacks of this system. 2.1.3 API separator

Figure 3: Schematic diagram of gravity separator.

2.1.1 Skimmer A skimmer is the mechanical separator that skims oil collected on the free water surface. Four different types of skimmers are available, such as belt, collector tube, wheel, and aerator skimmers. Belt and wheel skimmers are generally used for oil skimming.

2.1.2 Flat corrugated (horizontal sinusoidal) plate separator Flat corrugated plate separators are made of oleophilic polypropylene plates that are stacked vertically and fastened into packs with rods or wires. In this process, a collective action of laminar flow, coalescence, and oleophilic attraction is used for the separation of oil (Rubio et al. 2002).

API separators are widely used in oil refineries and chemical processing industries handling relatively large amounts of oil in effluent. The conventional API separator consists of channels through which the oily effluent passes horizontally at a low velocity, which allows the oil droplets to rise on the upper surface of water and skimmed off (see Figure 4). The standard API separator was designed to achieve the separation of oil droplets of 150 μm diameter and larger. Simple design, low cost, resistance to plugging with solids, and low maintenance are main advantages of API separator. The drawbacks are the size and surface area of an API separator, which allow disturbance of the separation process by wind turbulence and short-cutting of the contaminant. 2.1.4 Inclined plate separator These systems are usually made in large modules constructed of fiberglass plates packaged in steel or stainless steel (SS) frames. The oil droplets enter the system and rise until they reach the plate above then migrate along the plate until they reach the surface.

Figure 4: Schematic of API separator.


6 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater The advantages of this system over API-type separators include improved efficiency in removing both solids and oil and also resistance to plugging with solids. This improvement is mainly due to the substantial decrease in the effective rise height that must be traversed by a rising oil droplet. On the other hand, these systems are only able to achieve the separation of oil droplets below 60 μm.

2.2 Flotation processes The flotation process has a great potential due to the high throughput and efficiency of modern equipment available (Zable 1992, Parekh and Miller 1999, Rubio et al. 2002). The flotation technique follows the gravity separation concept for the removal of oil and grease from wastewater as oil is lighter than water. The process of flotation consists of four basic steps: (i) air bubble generation, (ii) effective contact between gas bubble and oil droplets, (iii) attachment of gas bubbles to oil droplets, and (iv) rising up of air-oil combination. The most important part of a successful flotation process is the effective attachment of oil droplets with gas bubbles until the bubbles reach the surface of the flotation cell. Flotation can be performed either in rectangular/ cylindrical mechanically agitated vessel or in flotation columns. The mechanical vessel is equipped with a mixer and air diffuser at the bottom of the mixing tank to introduce air and assist in mixing action. In flotation columns, air spargers are used at the bottom of a tall column to introduce air while introducing the slurry from the top of the column.

2.2.1 Electroflotation In the electroflotation (EF) technique, microbubbles are generated under the influence of electric field between two electrodes. The basis of the microbubble generation is the electrolysis of diluted aqueous solutions and or dispersions that produce gas bubbles at both electrodes. The EF technique has three principal advantages. First, extremely fine and uniform (average bubble diameter, 20 μm) dispersed gas bubbles are formed by electrolysis. Second is the variation of current density, which gives the possibility of varying gas bubble concentrations in the flotation medium and increasing the bubble-oil drop collision probabilities. Third is the selection of appropriate electrode surface and solution conditions, which permits obtaining optimum process conditions (Hosny 1992). The disadvantages are the low throughput, the emission of H2 bubbles, the cost of the electrodes, their maintenance, and the handling of voluminous sludge produced. Andre et al. (2000) reported an electrolytic coagulation/flotation system that works using reversible polarity aluminum electrodes. They observed that aluminum ions were releasing from the anodes which initiating coagulation and hydrogen bubbles were generating from the aluminum cathode which enables flotation of the flocks.

2.2.2 Dispersed (induced) air flotation In dispersed air flotation, air bubbles are mechanically generated by a combination of a high-speed mechanical agitator and an air injection system (Van Ham et al. 1983, Bennett 1988, Zheng and Zhao 1993). Gas is introduced at the top, and the liquid becomes fully intermingled and

Figure 5: Diagrammatic sketch of DAF unit.



forms bubbles in the size range of 700–1500 μm diameter after passing through a disperser. Besides the petrochemical industry, this technology was well adopted in mineral processing.

2.2.3 Dissolved-air flotation The dissolved-air flotation (DAF) technique is uncourageously used for treatment of oily wastewater. Oil droplets with size greater than 40 mm can be effectively removed by applying DAF units (Barker et al. 1971). A schematic diagram of a DAF unit is shown in Figure 5. The DAF process has three basic flowsheets: (a) total pressurization of influent wastewater, (b) partial pressurization, and (c) recycle pressurization. In DAF, air is dissolved in wastewater under high pressure, as the solubility of gas in liquid increases with an increase in pressure. The solubility of air decreases when the pressure of the wastewater is reduced and the dissolved air comes out of the wastewater in the form of bubbles. The supersaturated water is forced through needle valves or special orifices, which produces clouds of bubbles of 30–100 μm down-stream of the constriction. The DAF process is a widely used flotation method for the treatment of industrial effluents, clarification of refinery wastewater, and wastewater reclamation.

2.2.4 Nozzle flotation

of an NF unit is shown in Figure 6. The advantages of NF systems are as follows: 1. Lower initial costs and energy use due to the usage of a single pump. 2. Lower maintenance and longer equipment life because of the absence of moving parts. The NF technique has been used extensively in the petrochemical industry for the separation of oil/water emulsions and treatment of oily metal-laden wastewater (Gopalratnam et al. 1988). 2.2.5 Column flotation A schematic diagram of a column flotation (CF) unit is shown in Figure 7. In case of mineral processing and wastewater treatment, feed enters about one-third of the way down from the top of the column used and descends against a rising swarm of bubbles generated by a sparger. Now, the recent upgrading in column design includes external gas spargers operating with or without the addition of surfactant or frothers, columns with internal baffles, and coalescer. In the presence of surface-active reagents, microbubbles are generated, as in the microcell column (Yoon et al. 1992). For conventional flotation column, the most important aspect is the efficient contact between gas bubble and particles, which lies in the development of a proper column design. Dobby and Finch (1991) proposed a CF

The nozzle flotation (NF) process involves a gas aspiration nozzle similar to an educator or an exhauster to draw air into recycled water, which is discharged into a flotation vessel. This will generate a two-phase mixture of air and water. Bubbles are in the size range of 400–800 μm in diameter (Gopalratnam et al. 1988). A schematic diagram

Figure 6: Conventional NF unit.

Figure 7: Conventional column flotation unit.


8 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater circuit equipped with CF cleaning circuits that can be configured in either the conventional countercurrent cleaning configuration or in the scavenger arrangement. The main objective of bubble generation in a CF unit is to produce relatively small gas bubbles (bubble diameter db = 0.5–1.5 mm) at a moderate gas rate (typically, superficial gas velocity Jq = 0.5–2.0 cm/s). In CF, various bubble generation techniques are adopted, such as mechanical shear contacting, static shear contacting, and sparging through a porous media, with and without high external shear and jetting.

2.2.6 Centrifugal flotation The centrifugal flotation process is a combination of a separator and a contactor. A centrifugal field is developed by hydrocyclone and aeration done either by injecting or suctioning of air, through flow static mixers or nozzles. Jordan and Susko (1992) mentioned that medium bubbles with a size of 100–1000 μm diameter were generated. The air-sparged hydrocyclone (ASH) is an example of a centrifugal flotation unit. It basically consists of an aeration system whereby air is sparged through a jacketed porous tube wall and is sheared into numerous small bubbles by the high-velocity swirl flow of the aqueous phase. An advanced ASH type of flotation was reported in applications to remove oil, grease, chemical oxygen demand (COD)/biological oxygen demand (BOD), etc. A bubble accelerated flotation (BAF) system is another type of centrifugal flotation unit that was designed on the basis of the contactor-separation concept, with very low detention times in the contactor. A schematic diagram of a BAF unit is shown in Figure 8. Depending on the bubble generation system, units are categorized into induced-air BAF, vacuum BAF, and electro-flotation BAF.

2.2.7 Jet flotation A jet flotation unit consists of three compartments such as aeration/contact zone-downcomer, aggregate disengagement zone-tank proper pulp area, and a cleaning or froth forming zone-tank proper zone. A jet (Jameson) flotation cell was designed on the basis of high bubble surface area flux to achieve fast flotation (Santander et al. 2011). The bubbles (medium size) formed in this cell may range from 100 to 600 μm in diameter (Clayton et al. 1991, Harbort et al. 1994). The Jameson Cell is a vertical pipe where the effluent and air are introduced at the top and travel downward

Figure 8: Schematic of BAF (bubble accelerated centrifugal flotation) device.

through the downcomer. A liquid jet is formed when pulp enters through a feed line. Air is entrained into the liquid by a vacuum effect and produces many bubbles. Therefore, a susceptible condition was created in the downcomer for particle collection by bubbles (air hold-up >40%) (Cowburn et al. 2005, Tasdemir et al. 2007). The advantages of a conventional jet flotation cell are its compact design, low capital cost, low maintenance cost due to no moving parts, low power consumption, low residence times (90%, at a high hydraulic loadings rate of more than 130 mh−1. Process efficiency was depended on minimum head loss. Mimimun head loss in the flocculator was 0.5–1.0 kgf/cm2. Bubble size of 100 μm (microbubbles) was generated in FF system. Advantages of this process are adequate turbulence, low area required, absence of mobile parts, simple design, and low mechanical and electrical energy required

Rosa and Rubio (2005)

Froth flotation

Oil (diesel) removal was done under colloidal gas aphron (CGA) conditions. The froth flotation column was operated at an air flow rate of 0.30 l/min with the feed solution prepared under non-equilibrium and the CGA conditions at 0.1 wt.% C14–15(PO)5SO4Na (extended surfactant), 3 wt.% NaCl, with a stirring speed of 5000 rpm and a stirring time of 5 min, gave the highest oil removal of 97%

Watcharasing et al. (2008)

Jet flotation

Efficiency of conventional jet flotation cell (CJC) was about 80% but in modified jet flotation cell (MJC) oil removal increased up to 85%

Santander et al. (2011)



1. DC power supply 2. Electrocoagulation tank

5

3. Electrolytic cell 4. Magnetic stirrer

+

–

1

5. Calibrated gas holder 6. Rotameter 7. Peristaltic pump 8. Feed tank 3

2

6

8 7 4

Figure 11: Schematic diagram of the EC system.

Walkowiak (1992) explained that a CAF unit utilizes a specially designed aerator that draws ambient air and injects microbubbles directly into the untreated wastewater. However, there was no information available regarding fundamental work with this flotation technique. Work done by various researchers using different flotation techniques are mentioned in Table 2.

The EC process depends on the response of water contaminants to stronger electric fields and electrically induced oxidation and reduction reaction. The EC process occurs in three consecutive steps (Babu et al. 2007): 1. Coagulant formation by electrolytic oxidation of the “sacrificial electrode.” 2. Destabilization of the contaminants, particulate suspension, and emulsion breaking. 3. Aggregation of the destabilized phases to form flocks.

2.3 Electrocoagulation

An external electric field is applied between electrodes, the anode material undergoes oxidation, the cathode material undergoes reduction, and elemental

Electrocoagulation (EC) techniques are widely used for the treatment of oily wastewater. This electrochemical process does not require any chemical pretesting or chemical adjustment of the wastewater. It produces much less sludge and dissolved solids than chemical treatment does (Öğütveren and Koparal 1997). EC is an electrolytic treatment process. A schematic diagram of an EC system is shown in Figure 11. The principle of EC is based on the stability of colloids, suspensions, and emulsions by the influence of electric charges. In most cases, aluminum and iron are used as anodes and cathodes (Sbnchez et al. 2003, Zaroual et al. 2005, Babu et al. 2007). Cations were generated by the dissolution of sacrificial anodes, inducing flocculation of the dispersed pollutants (see Figure 11). Hydrogen bubbles are produced at the cathode. Thus, the emulsion may be destabilized both by oxidative destruction of the chemical emulsifier and by neutralization of the emulsion-droplet charge by ferric/or aluminum ions in EC (Ryan 1986).

Table 3: Reactions occurred between electrodes and wastewater at EC system. Anode

Cathode

4OH−–4e− = 2H2O + O2 (g)

2 H2O–4e− = O2 (g) + 4H+

Al-anode Al(s)–3e− = Al3+(aq)

2 H3O+ + 2e− = H2 (g) + 2H2O (in acidic solution) 2H2O + 2e− = H2 (g) + 2OH− (in alkaline solutions)

Al3+(aq) + 3H2O = Al(OH)3 + 3H+

Fe-anode Fe(s)–2e− = Fe2+(aq) Fe2+(aq) + 2H2O = Fe(OH)2 + 2H2O Fe2+–e− = Fe3+ Fe3+ + 3H2O = Fe(OH)3 + 3H+

Al(s) + 4OH− =[Al(OH)4]− + 3e− (at very high pH)

Fe(OH)3 + OH− =[Fe(OH)4]− [Fe(OH)4]− + 2OH− =[Fe(OH)6]3+ (at very high pH)


12 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater metals are deposited on the cathodic surface. Ferrous (Fe3+) or aluminum (Al3+) ions form at the anode, and hydroxyl ions are generated at the cathode. Reactions occurring between two electrodes in the EC system are shown in Table 3. The generated Fe3+ and Al3+ ions subsequently undergo spontaneous reaction to produce corresponding hydroxides and polyhydroxides. This process is followed by in situ oxidation to the ferric state and subsequent precipitation as ferric/or aluminum hydroxides. Hydrolysis products of Al3+ and Fe3+ were responsible for polymerization during particle aggregation. Metal hydroxides (Table 4) that are formed in the process of the EC possess have very high affinity for dispersed particles as well as counter ions for absorption. The formation of colloidal particles is a result of this step. From Figure 12, it can be observed that the nucleus of a colloidal particle was formed by metal hydroxides. Nucleuses are surrounded by the adsorption layer of cations and anions, which form positively charged granules of the colloidal particles. These coagulated particles attract and absorb different ions and microcolloidal particles from the wastewater. The flocks formed in the water are transported to the surface by the gas (H2O2, etc.) bubbles, which were produced in the electrolysis process. As a consequence, the demulsified oil separates from the emulsified wastewater by sorption on the highly dispersed ferric-/or aluminumhydroxide microflocks (Black 1960, Inan et al. 2004). Table 4: Hydroxides and polyhydroxides of Al3+ and Fe3+ generated by EC process. Hydroxides of Al3+

Al(H2O)63+, Al(H2O)5OH2+, Al(H2O)4(OH)2+, Al(OH)2+, Al2(OH)24+, Al(OH)4+, Al6(OH)153+, Al7(OH)174+, Al18(OH)204+, Al13O4(OH)247+, Al13(OH)175+.

Hydroxides of Fe3+ Fe(OH)3, Fe(H2O)63+, Fe(H2O)5(OH)2+, Fe(H2O)4(OH)2+, Fe2(H2O)8(OH)24+, Fe2(H2O)6(OH)44+.

Detailed highlights of the work done by various researches are given in Table 5. The electrochemical reactions occurring between two electrodes are given below.

2.4 Membrane process The membrane process is one of the sophisticated separation processes for treating oily wastewater and o/w emulsions. This process is more effective than other conventional processes in terms of economics and quality of discharged water. However, the basis for selecting membranes and operating conditions depends empirically on achieving adequate rejection of oil and emulsified material (Xu et al. 2010).

2.4.1 Conventional separation mechanism of oil/water in membrane process The separation mechanism of oil using the membrane process according to Leiknes and Semmens (1999) is described below. Transport mechanism of emulsion through membrane was achieved by three consecutive steps (Figure 13): 1. Convective transport of the oil droplet from the bulk solution to the membrane surface. Hydrodynamic force is the driving force. 2. Attachment and adsorption of the oil droplets on the membrane surface. This step depends on the wettability and contact between the oil droplets and the surface. 3. Transport through the membrane pores. Deformation of oil droplet transports through a membrane pore takes place when membrane pore is smaller than oil droplet. Then membrane pressure is required to be great enough to overcome the surface tension forces.

Granule {[Fe (OH)3]m /n FeO+ + (n-x) Anion–}+ Nucleus

Adsorption layer

+ x Anion– Diffusion layer

Granule {[AI (OH)3]m /n AI3+ + (3n-x) Anion–}+ Nucleus

Adsorption layer

+ x Anion– Diffusion layer

Figure 12: Ionic-charge on a colloidal particle (micelle).

2.4.2 A pplication of membrane in oily/emulsified wastewater treatment In recent times, membranes are increasingly being used for treating oily wastewater. Membranes are most useful with stable emulsions, particularly water-soluble oily wastes (Cheryan and Rajagopalan 1998). Several studies have reported on the applicability of commercially available membranes for the treatment of macroemulsions. Brought to you by | Göteborg University - University of Gothenburg Authenticated Download Date | 1/24/18 6:43 PM


Table 5: Various studies reported on electro-coagulation technique for oily wastewater treatment. Sample no.

Highlights of work

Electrodes used

1.

Cutting mineral oil B22 (used for drilling and machining operation) was used for the preparation of o/w emulsion. Optimal pH was found equal to 6–7, maximum possible COD and turbidity removal was found to be 92% and 99% respectively

Al alloy AU4G (2017-Al) was used as cell electrodes. The Al alloy contains Cu at 4%, Fe, Mg and Mn each at 0.7%, Si at 0.5% and lower percentages of Zn and Cr. Dimension of Al alloy electrodes was 100 mm × 50 mm × 12 mm, which were embedded in each of the two valves: only one face was exposed to the solution. The effective electrode area was 5.0 × 10−3 m2 and the electrode gap was maintained constant at 20 mm

Bensadok et al. (2007)

2.

Electro-coagulation of concentrated oil dispersions was investigated. Removal of suspended matter was analysed by determination of the chemical oxygen demand (COD) and total organic carbon (TOC) of the clear fractions of the samples collected. Removal yields of COD and TOC higher than 90% was obtained upon dissolution of 0.5 g/l. The maximum 93% oil removal was obtained by dissolution of 0.6 g/l. The sludge produced by the EC process had a low Al3+ content in comparison with that of organic matter

Al-based alloy, A-U4G (2017-Al) was used as cathode and anode. Alloy contains Cu (4%), with the presence of Fe (0.7%), Mg (0.7%), Mn (0.7%), Si (0.5%), Zn (0.25%) and Cr (0.1%)

Khemis et al. (2005)

3.

Tannery effluent was treated by electrocoagulation technique. Operating parameters such as flow rate, current density and electrolysis time were investigated. Reduction of COD, BOD, TDS were studied at different current density (15, 20, 25 mA/cm2) and different flow rate (2, 4, 6, 8 lpm). According to experimental results, it was concluded that the reduction of COD, BOOD, TDS were favored at higher charge density and electrolysis time. Experimental results showed that the optimal current density and flow rate were 20 mA/cm2 and 6 lpm

Iron and aluminium plates were used as anode and cathode. Specification of electrodes was 100 × 100 × 2 mm and effective area was 100 cm2

Babu et al. (2007)

4.

A pilot plant with iron electrodes was designed to treat emulsified oil having concentration between 300 and 7000 mg l−1 at a flow rate of 4 l/min. Process reduces the effluent oil concentration to 10 mg/l or less when the dissolution rate of iron was increased in this process. They explained that the ferric ion breaks the stable emulsion by neutralising the charge on the droplets and precipitated as ferric hydroxide, while the oil was adsorbed onto the flocculent

Iron electrodes were used for the study

Weintraub et al. (1983)

5.

Synthetic oily wastewater was treated. Effects of applied potential, initial oil concentration, and supporting electrolyte concentration on the rate of efficient removal of oil were investigated. These experiments were performed with initial oil concentrations of 50, 200, and 500 mg/l. The removal efficiency was found to be as high as 100%. The experimental results showed that the aluminium electrodes were more efficient than iron electrodes. Due to the high adsorption capacity of hydrous aluminium oxides. Thus, energy consumption for aluminium reactor was much lower than iron reactor

Iron and aluminium were used as electrodes in the batch mode of experiments

References

Öğütveren and Koparal (1997)


14 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater Table 5 (continued) Sample no.

Highlights of work

6.

7.

Electrodes used

References

Efficiencies of the chemical and the electrochemical break-up of o/w emulsions with aluminium salts were studied. The break-up of the emulsions only took place in the range of pHs between 5 and 9, and the amount of aluminium necessary to produce the destabilization of the emulsion was proportional to the oil concentration. They observed that the treatment of emulsions containing chloride ions as electrolyte had better efficiencies than sulfate medium

Aluminium electrodes (type HE 18) were used placed in a single compartment electrochemical flow cell. Both electrodes (anode and cathode) were square in shape (10 cm side) and the electrode gap was 9 mm

Canizares et al. (2008)

An industrial electro-coagulation unit was used for treatment of different soluble oily wastes. The maximum oil removal was expressed in terms of COD and suspended solid matter (SS) by conducting preliminary tests in a small batch cell. Experimental result showed a reduction of 70%–90% of the initial pollution was done. For soluble oil maximum removal COD-91.3% and SS-46.4% were achieved from initial condition COD 55,000 mg/l and SS of 12,000 mg/l. They concluded that, COD removal was a decreasing function of the electrode gap and SS removal which was weakly affected by the electrode gap. Conversely, COD and SS removal efficiencies increased with the current density and the concentration of coagulant

Aluminum was electrodes, and aluminum hydroxide was the coagulant. Specification of plate type Al electrodes was 10 × 5 × 1.2 cm. These electrodes were embedded in two methyl polymethacrylate cell. The electrode gap was varied from 5 to 15 mm, with 5 mm being standard

Sbnchez Calvo et al. (2003)

1) Transport of bulk O/W emulsion to membrane surface 2) Attachment and adsorption of oil droplets Membrane surface Membrane support 3) Transpor to membrane pores

Figure 13: Transport mechanism of o/w emulsions over membrane.

Bansal (1975) investigated the ultrafiltration (UF) membrane process for treatment of o/w emulsion especially for lubricant oil, rolling oil, and cutting oil. The oil removal efficiency varied in between 99.64% and 99.99%. Santos and Wiesner (1997) investigated the application of UF membrane on the treatment of oily effluent. They reported that the oil and grease concentrations in the UF permeate

were lower than 14 mg/l. The total organic carbon (TOC) rejections were also high, ranging from 94.8% to 98.7%. Bhattacharyya et al. (1979) commented that excellent oil rejections were obtained with UF treatment with oil concentration of less than 10 mg/l. Hlavacek (1995) explored that the membrane can also act as a coalescer. Demulsification basically happens during passage of emulsions through the membrane under certain operating pressure. Daiminger et al. (1995) also found similar phenomena. It was found that spontaneous droplet coalescence takes place during the flow of oil dispersion through thin microporous hydrophobic membranes with a pore size similar to the oil droplet size. It should be mentioned that coalescence was not observed with the hydrophobic membrane. Lipp et al. (1988) investigated o/w separation using UF membranes. They observed that the UF unit had oil rejections greater than 99.9% and more than 96% rejection of TOC. Bodzek and Konieczny (1992) investigated the applicability of polyacylonitrile (PAN) and poly(vinyl chloride) UF membranes for emulsified wastewater. They reported that the oil rejection of the UF unit was 95%–99% and COD



Oily wastewater stream

Recycle stream

Free floating oil Prefilter

Settling tank

Permeate stream (oil free)

Process vessel Membrane

Residual solids

Highly concentrated oil

Figure 14: Schematic of typical membrane system for treatment of oily wastewater.

was 91%–98%. Bensadok et al. (2007) obtained new results for treatment of metal-working wastewater using a commercial PAN UF membrane. Ezzati et al. (2005) worked with a hydrophobic polytetrafluoroethylene membrane with 0.45 mm pore size and optimized various parameters such as water and emulsifier content in feed, operating pressure, operating temperature, and feed residence time. Wu et al. (2016) used microfiltration (MF) carbon membranes for treatment of oily wastewater. Membrane was prepared using phenolic resin. A schematic diagram of a UF unit for treatment of oily wastes is shown in Figure 14. The membrane unit is operated in a semibatch recycle mode. The feed wastewater is added to the process tank at the same rate as clean permeate was withdrawn, maintaining a constant level in the tank. The retentate contains the residual oil and grease, which is recycled to the process tank. The feed is stopped when the oils, grease, and other suspended matter reach a limiting concentration in the tank. Peng et al. (2017) prepared a novel superhydrophilic/ underwater superoleophobic PAN UF membrane using hydroxylamine-induced phase inversion process. The PAN membrane displayed a high flux, ranging from 2200 to 3806 l m−2 h−1 bar−1 and desirable separation efficiency for various o/w emulsions. The PAN membrane showed superior antifouling property and recyclability due to its ultralow oil-adhesion property. The authors explained that during the hydroxylamine-induced phase inversion process, the amidoximation of PAN introduces an amine group and a hydroxyl group into the PAN chains, which upgrades the hydrophilicity of PAN membrane and endowing the PAN membrane with superhydrophilicity, underwater superoleophobicity, and ultralow oil-adhesion property.

2.4.3 Membrane surface modification Anderson and Saw (1987) discussed that the surface characteristic of UF membranes could be altered by adsorption

with an emulsifier. Significant improvement in oil rejection and membrane flux of hydrophilic membrane was achieved by an anionic emulsifier, which makes the process suitable for oily wastewater treatment. Chen et al. (1992) also found similar results. They also reported that the presence of an anionic emulsifier in the feed helps to reduce membrane fouling due to the electrostatic interaction between the membrane surface and the charge of the emulsifier. Membrane technology has been acknowledged as an advanced separation process of surfactant-stabilized emulsions with allowable discharge quality and a relatively simple process. Despite these unique advantages, the real application of using membranes in industrial fields for treating oily wastewater remains limited. The major problem was severe membrane fouling caused by surfactant adsorption and/or pore plugging by oil droplets as well as degradation over long-term application due to its polymeric-based structure. In this regard, carbon-based membrane technology was considered as an attractive technique to fill the gap between membrane technology and existing oily wastewater treatment. Carbon membrane is a new type of porous inorganic membrane prepared by carbonization of various carbonaceous materials. Carbon membranes are a good alternative for treatment of oily wastewater due to their stability in aggressive (vapor or solvents and nonoxidizing acids or bases) and adverse (Ismail and David 2001) conditions. Recently, Al-anzi and Siang (2017) reviewed the recent advances in nanomaterials and carbon-based nanocomposite membrane for effectively treating emulsified oil/ water mixtures. They covered the current developments of carbon-based nanomaterials, nanofibers, and membranes based on their fabrication, characterization, and separation performance. Challenges facing the development of carbon-based membranes for treating industrial oily wastewater were also highlighted. In the last 2 years, advanced materials have emerged in different forms, like aerogels (Nardecchia et al. 2013,


16 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater Lee et al. 2015), foam membranes (FMs; Chaudhary et al. 2014), polysaccharide agents (Srinivasan 2013), surface modified fabrics, and inorganic meshes for successful separation of oil/water mixtures (Zhang et al. 2015, Wang et al. 2015a,b). A unique three-dimensional network of hydrophilic aerogels preferably selects water over oil and transforms surface to hydrophobic leads to preferable selection of oil over water. These new classes of materials have shown excellent separation properties because of their large surface area and high porosity and can be easily custom-made to fit the final application (Sai et al. 2015). However, owing to their distinctive features like sustainability and biodegradability, in addition to superhydrophilicity and high surface area, bio-based aerogels are a better choice for o/w emulsion separation. Chaudhary et al. (2014) developed a novel FM, which was fabricated from agarose (Agr) and gelatin (Gel) in combination with the natural cross-linker genipin (G). FMs attained good capillary microstructures of 10–45 μm between the MF and UF range due to the controlled lyophilization process, which allows selective permeation of water. Microporous foam was generated as high as >500 l m−2 h−1 continuous flux with ~98% pure product water. Chaudhary et al. (2015) investigated direct recovery of water from stable emulsion (o/w) waste using macroporous aerogel membrane. They transformed chitosan-based gel into highly porous aerogel membrane using bio-origin genipin as cross-linking agent. The highly cross-linked chitosan acted as a support network along with inducing hydrophilicity to aerogel. Agarose was used as a poreforming agent, as well as surface coating, on a highly

cross-linked chitosan network. The attractive properties of aerogel membranes include their natural abundance, less to no toxicity, biodegradability, and ease of processing and disposing. The macroporous membrane produces ~99% pure water with a reflux rate of 600 l m−2 h−1 bar−1. Table 6 shows the different processed used for the treatment of oily wastewater based on dispersed oil droplets. In recent years, interfacial materials with special wettability have attracted extensive interests, especially pressure-driven filtration membranes (Xue et al. 2014, Chu et al. 2015, Wang et al. 2015a,b, 2016), in the field of oil water separation. Combined with surface modification technologies, a variety of nanomaterials and nanotechnologies have been widely exploited to construct superwetting surfaces, generally superhydrophobicsuperoleophilic or superhydrophilic-underwater superoleophobic materials. However, due to their large pore sizes, these superwetting filtration membranes cannot be applied to various emulsions. Recently, superhydrophobic and underwater superoleophobic filtration membranes with small pore sizes were designed to separate w/o and o/w emulsions, respectively (Zhang et al. 2013, Cao et al. 2014, Gao et al. 2014, Yang et al. 2014, Si et al. 2015, Song and Xu 2016, Ma et al. 2017). A dual pH- and ammoniavapor-responsive polyimide (PI)-based nanofibrous membrane with high permeate flux and stability was developed for separation of oil-water (Ma et al. 2017). The PI nanofibrous membrane (SNP/DA-TiO2/PI) was designed and fabricated by electrospinning and solution dip-coating techniques. The novel SNP/DA-TiO2/PI membrane exhibits superhydrophobicity in air and superoleophilicity

Table 6: Oil and grease removal technologies based on size of oil/grease droplets. Process

Smallest oil droplet removed (average microns, μm)

API separator Centrifuges Tilted plate Separator Flat corrugated Plate

150 100 60 45–50

Dispersed air Flotation Dissolved air Flotation Granular media Filters Coalescers

50

35

25–30

15

Membranes

99%) and was reusable. The membrane was highly stable under extreme conditions. Recently, interfacial materials (especially filtration membranes) with special wettability were broadly developed to solve the environmental problems by virtue of their advantages in energy saving, high flux, and good selectivity. However, the given wetting property (superhydrophilicity or superhydrophobicity) and pore size and poor stability of filtration membranes limit their widespread applications, which is far from meeting a wide variety of oily wastewater.

2.5 O il removal by adsorption Adsorption is a surface phenomenon that involves the contact of a free aqueous phase (solvent) with a rigid solid phase (adsorbent), which has the property of removing or storing one or more solutes (adsorbate) selectively. Selection of adsorbent for oily wastewater treatment is important for the development of an efficient adsorption-based treatment process. Activated carbon, one of the popular inorganic adsorbents, is extensively used in advanced treatment process due to its highly porous structure and large specific surface area. Maretto et al. (2014) reviewed different adsorption technologies and two different adsorbent materials (microporous materials); i.e. one was a natural zeolite, which is called clinoptilolite, and other was polymeric chelating resin (Purolite Resin S910), for the removal of dissolved heavy metals and treatment of petrochemical wastewater. Recently, Pintor et al. (2016) reviewed sorption method using different types of sorbents for oil sorption and recovery.

2.5.1 Mechanism of coalescence Coalescence mainly depends on the stability of the thin liquid film of the continuous phase separating them. The coalescence phenomena of oil droplet in a fibrous filter bed was done on the basis of three major steps: interception, diffusion, and impaction, i.e. approach of dispersed oil droplets to a fiber surface, attachment of the droplet on the fiber surface, and release of the enlarged droplets from the

downstream side of the filter (Hazlett 1969). The interception mechanism is the most significant mechanism in the coalescence process. Once an oil droplet reaches a fiber, oil droplets are attached and spread over the fiber. When the continuous phase approaches the solid surface, it is trapped in a dimple as a liquid drop. The drop is deformed by the excess pressure developed by the film drainage process. The interfacial tension plays a significant role that would affect the deformability of a drop (Princen 1963). It was noted that easily deformed drops have a longer film drainage time. Droplet release depends on three factors: flow velocity, surfactant content, and fiber size. Based on single fiber studies, Dickinson (1992) showed that the detachment occurred more readily from a smaller-diameter fiber than a larger-diameter fiber. A higher velocity removed smaller drops. A high concentration of surfactants encourages easy detachment as droplets slide on the fibrous material. Droplet detachment takes place due to a force imbalance between the oil droplets and the fiber. Basically, the hydrodynamic force acting on the droplets must overcome the adhesive force between the droplet and the fiber before detachment can occur. Coalescence of liquid-liquid dispersions (o/w or w/o emulsions) followed by flow-through porous media has been proved as an effective process in a variety of industrial applications and wastewater treatment. Coalescence of emulsions can be achieved by flow through a porous medium consisting of fibrous or granular packing, such as glass filaments or pebbles. According to various researchers, the fibrous coalescer acts by capturing suspended droplets that grow on the fibers by further capture and coalescence until they eventually become so large that hydrodynamic drag causes breakaway. After the break away, these large droplets travel through the pores and are released at the downstream face (Sherony and Kintner 1971, Rosenfeld and Wasan 1974). Fibrous media appeared to be more attractive than coarse granular material due to the higher porosities and higher specific surface area. When two-phase flow occurs through porous media, some complicated capillary phenomena occur simultaneously in the separation of liquid-liquid dispersions. During coalescence in porous media, three distinct regimes of coalescing phase are considered: oil microdrops suspended in capillary-conducted water; capillary-conducted oil, which forms connected channels; and held-up oil as discrete coalescing globules (Spielman and Su 1977). This third regime was taken to be in local capillary-equilibrium with the solid and water phases. Spielman and Goren (1972) and Spielman and Su (1977) proposed a detailed modeling approach of coalescence of o/w suspensions in porous media.


18 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater Table 7: Classification of oil sorbent. Sample no.

Category

Sorbents material

1

2

Inorganic mineral material Synthetic organic material

Zeolites, silica, perlite, graphite, vermiculites, sorbent clay and diatomite Polymericmaterials, polypropylene, polyurethane foams

3

Organic vegetable material

Straw, corn corb, wood fiber, cotton fiber, cellulosic kapok fiber, kenaf, milkweed floss and peat moss

2.5.2 Oil removal using different sorbent materials A wide range of materials are used for oil removal, such as dispersants, absorbents, solidifiers, booms, and skimmers (Delaune et al. 1999, Pelletier and Siron 1999, Teas et al. 2001, Lim and Huang 2007, Fingas 2008). Dispersant materials actually disperse the oil molecule and separate it from the water by absorption. Absorbents collect oil molecules into its matrix and are separated from the waste water by absorption. Solidifiers are basically hydrophobic polymers and are dry granular in nature, which react with the oil and transform it to a cohesive solidified mass that floats on the water surface. Booms are specifically used to confine the spilled oil to a specific area (containment) or stop the oil from entering a given area (diversion). Classification oil sorbent is shown in Table 7. Absorbent materials are more attractive because of availability and complete removal of the oil from the spill site. The addition of absorbents to oil spill areas facilitates a transformation from liquid to semisolid phase, and once this change is achieved, the removal of the oil from these absorbent structures then becomes much easier. Moreover, these materials can be recycled several times. Important characteristics of good absorbent materials include hydrophobicity and oleophilicity, high uptake capacity, high rate of uptake, retention time, and the reusability and biodegradability of the absorbents (Delaune et al. 1999, Pelletier and Siron 1999, Teas et al. 2001). Different sorbent materials used for oil spill cleanup are shown in Table 8. Different sorbent materials can also be used for removal of oil from oily wastewater. Rajakovic et al. (2007) investigated two types of sorbents: organic sorbents based on wool [loose natural wool fibers (NWFs) and recycled wool-based nonwoven material (RWNM)] and inorganic sorbent, like sepiolite. The sorption process was done in a continuous tubular contractor (initial oil concentration of 1511 mg/dm3) and in a sorption tank (initial oil

Remarks Porous in nature and ability to absorb oil in the presence of water. Commonly used commercial sorbents in oil spill cleanup. High oleophilic and hydrophobic characteristics But they degrade very slowly Poor buoyancy characteristics, relatively low oil sorption capacity and low hydrophobicity

concentration of 5066 mg/dm3). They concluded that the wool-based sorbents could be preferentially used for treatment of oily wastewater. They observed that NWF showed higher sorption capacity (5.56 g/g for NWF and 5.48 g/g for RWNM) compared to sepiolite (0.19 g/g). Gammoun et al. (2007) investigated the ability of chrome shavings to remove motor oils, oily wastes, and hydrocarbons from water. Chrome shavings used as an adsorbent, which were light-density porous tanned waste granules, float on the surface of water and remove hydrocarbons and oil films. The main advantages of using these tanned solid wastes as sorbent were the low density and high buoyancy of fibers, porosity and nontoxicity of the wastes, significant and quasi-instantaneous sorption of oils and hydrocarbons, and easy and efficient removal of saturated sorbents. They found that these tanned solid wastes were capable of absorbing many times their weight in oil or hydrocarbons (6.5–7.6 g of oil and 6.3 g of hydrocarbons per gram of chrome shavings). Brandao et al. (2010) investigated the adsorption ability of sugarcane bagasse to remove hydrocarbon byproducts from contaminated wastewater. Results showed that it was able to absorb up to 99% of gasoline and 90% of n-heptane in solutions containing about 5% of these contaminants. The use of sugarcane bagasse as adsorbent of nonpolar substances such as gasoline and n-heptane was very favorable. This showed the potential of sugarcane bagasse as a good alternative adsorbent. Sokker et al. (2011) investigated the adsorption of crude oil (initial concentration 0.5–30 g/l) from an aqueous solution using hydrogel of chitosan-based polyacrylamide. They prepared chitosan-g-P(AAm) hydrogel by a grafting method using gamma irradiation. Experiments were carried out as a function of different initial concentrations of oil residue, acrylamide (AM) concentration, contact time, and pH to determine the optimum condition for the adsorption of residue oil from aqueous solution and sea water. Adsorption was carried out through a series of


P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater 19 Table 8: Different sorbent materials used for oil-spill cleanup. Sorbent material

Granular solidifier, Nochar A 650 (Nochar 10333 N. Meridian, Suite 215, Indianapolis, IN 46290) Kapok fiber (agricultural product)

Nonwoven polypropylene sorbent

Modified bentonite clay

Cotton grass fibre

Milkweed (Asclepias) fiber and cotton fiber (natural sorbent)

Hydrophobic aerogels

Zeolites

Characteristics

Dry granular, non-toxic, non-hazardous solidifying agent. The solidifier was premeasured into 0.45 kg bags and applied by hand to the oiled plots. More than 70% of oil was recovered Hollow structure with large lumen. Its packing densities 0.02 g cm3 and oil sorption capacities were 36, 43 and 45 g/g for diesel, hydraulic oil (ASW46) and engine oil (HD 40), respectively. Oil recovery was obtained more than 83% with centrifugation, from the densely packed assemblies Consolidated fibrous materials having low density, low water uptake and excellent physical and chemical resistance Clay minerals are comprised of small crystalline particles that are formed from silica tetrahedral sheets and the aluminum or magnesium octahedral sheet The efficiencies of the clays in removing BTEX compounds were 75%, 87%, 89%, and 89%, respectively Cotton grass fibers, a by-product of peat excavation. In removing diesel oil from the surface of water, the removal efficiency was over 99% up to an absorbing factor of 20 times its own weight It was a hollow cellulosic material. Absorb higher amount of cruid oil than polypropylene fiber. Sorption capacity was approximately 40 g of crude oil/g of fiber at room temperature. More than 90% of the sorbed oil was removed from the sorbents by a simple mechanical action CF3-functionalized silica aerogel was used for oil-spill cleanup. Aerogels were solid metal-oxides with open foamtype structures allowing for penetration of various sizes of compounds into the solid having higher oil absorbing capacity. 100% of the oil was absorbed by the CF3-functionalized aerogel and was recovered through soxhlet extraction of the dried aerogel solid. It absorbs oil 40–140 times better than the nonfunctionalized silica aerogel High quality hydrophobic zeolite having high water and oil absorption capacity and cataion exchange ability used extensively for oil spill cleanup

batch adsorptions. The prepared hydrogel at 40% AM concentration and 5 kGy radiation doses showed maximum adsorption of crude oil (2.3 g/g at pH 3) compared to other hydrogels prepared at different AM concentrations (20%, 30%, and 50%) and different radiation doses (10, 15, and 20 kGy). The adsorption capacity of the prepared hydrogel was 1.8 g/g for crude oil from sea water. Vlaev et al. (2011) investigated the adsorption capacities of crude oil and diesel fuel at different temperatures onto black rice husk ash (BRHA) and white rice husks ash (WRHA) using an adsorption technique. BRHA and WRHA were prepared via pyrolysis of raw rice husks in a pilot plant fluidized-bed reactor at different conditions. The authors observed that BRHA had higher adsorption capacity than WRHA did. The equilibrium sorption capacity of diesel fuel onto BRHA was estimated to be about

References Delaune et al. (1999)

Lim and Huang (2007)

Wei et al. (2005) Gitipour et al. (1997)

Suni et al. (2004)

Choi and Cloud (1992)

Reynolds et al. (2001)

Adebajo et al. (2003)

5 g/g, while onto WRHA, it was only 2.8 g/g. The BRHA surface was nonpolar and that of WRHA was polar, which facilitated the adsorption of diesel fuel onto BRHA, as the hydrocarbons are nonpolar. Thus, due to their high adsorption capacity and low cost, BRHA and WRHA can be successfully used as effective adsorbents for the cleanup of bilge water and spills of oil and oil products in water basins. Li et al. (2013) used yellow horn shell residues treated by ionic liquid (IL) for oil removal from water. They evaluated the comprehensive sorption capacities of the shell residues for five types of oils in pure oil and oily water medium. They found that the maximum sorption capacities of IL-treated shell residues (0.39–0.61 g/g) were about 1.5-fold those of untreated shell residues (0.32–0.42 g/g). However, the sorption capacity of modified shell residues


20 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater was not outstanding. But reusability, biodegradability, simple preparation, and low cost made it an ideal biosorbent for oil removal from water. Thus IL treatment could be an emerging method for modification of biomaterial in wastewater treatment. Likon et al. (2013) used poplar seed hair fibers obtained from Populus nigra Italica for oil absorption. They reported that the exceptional chemical, physical, and microstructural properties of poplar seed hair fibers enable superabsorbent behavior with high absorption capacity for heavy motor oil and diesel fuel. The absorption values were 182–211 g heavy oil/g fiber and 55–60 g heavy oil/g fiber for packing densities of 0.005 g/cm3 and 0.02 g/cm3, respectively. They recommended that the use of poplar seed fibers for the production of oil absorbents was sustainable, had a low carbon footprint, had low energy demand, and was clean. Pintor et al. (2015) studied treatment of oil and grease using granular cork materials. Batch and continuous studies were performed. They observed that pH has a significant effect on destabilization of emulsion, which increases the efficiency of sorption process over 80% at pH 2. Recently, Pachathu and Ponnusamy (2016) studied the treatment of synthetic emulsified wastewater using different chemically/morphologically modified adsorbents. They used raw and reformed/modified cornhusk as an adsorbent using rotary shaker (RS), microwave (MW), and ultrasound (US)-assisted techniques for removal of oil from o/w emulsions. The chemical modifications were done using a cetyl pyridinium chloride solution (CPC) on pretreated sodium hydroxide adsorbent. They performed batch studies by varying process parameters such as contact time, pH, dosage, speed, and temperature to confirm the enhanced condition for maximum adsorption capacity. Further, Pachathu et al. (2016a) used modified bagasse and rice straw as absorbents for the removal of emulsified oil by a MW-assisted technique. The raw adsorbents were chemically modified with sodium hydroxide and surfactant solution for better adsorption of oil. They found a maximum oil removal of 98.07% and 98.72% for MW-assisted bagasse and rice straw at 313 K. Several researches have reported that some natural fibers (NFs) (cotton, milkweed, kenaf, and wool) have higher oil sorption capacities as compared to the commercially available synthetic fibers (Johnson and Manjrekar 1973, Barrer 1989, Choi and Moreau 1993, Radetic et al. 2003). Excellent oil sorption properties and high biodegradability of NFs were also the reason for alternative to synthetic fibers (Choi and Moreau 1993, Radetic et al. 2003).

As we know, oil spills impose serious pollution on the environment. Various sorbent materials were developed to remove oil from contaminated or spilled areas. The spill cleanup process involves the transfer of oil from the contaminated area to some transportable form of temporary storage with the help of oil sorbents (Choi et al. 1993). The treated sorbents end up in landfills or in incineration. These options lead to producing another source of pollution and also an increase in the oil recovery cost. As an alternative, biosurfactants are used to clean the treated sorbents (Wei et al. 2005). Biosurfactants are produced biologically. They are biodegradable and enhance the biodegradation of oil by supplying the bioavailability of hydrophobic compounds. The use of biosurfactant is new. Wei et al. (2005) utilized rhamnolipid biosurfactant JBR215, which is very effective in removing oil from sorbent materials. It is a biodegradable and extracellular natural substance produced during fermentation processes utilizing certain bacterial strains. The washing time is about 120 min when more than 99% of soaked oil was removed from the oil sorbent (Wei et al. 2005). Asha and Vira raghavan (2010) performed a batch study to evaluate the efficiencies of four types of biomaterials to remove oil from water. Two fungal biomasses of Mucor rouxii and Absidia coerulea, along with chitosan and walnut shell media, were used as biomaterials. The study demonstrated that the removal efficiencies by M. rouxii for these oils were in the range of 77%–93% at a pH of 5.0.

2.5.2.1 T reatment of emulsified wastewater by sawdust/ bagasse fly ash bed filter Sawdust is an easily available by-product in timber and pulp mills. They exhibit good sorbent characteristics for the treatment of wastewater containing various pollutants such as heavy metals, dyes, and phenolic compounds. It has also been used as the filter media for the treatment of o/w emulsions (Cambiella et al. 2006). Metal working fluid/cutting oil (3 vol% oil) is used for the preparation of o/w emulsion. Sawdust is chemically modified by inorganic salt (calcium sulfate), which acts as a coagulant for the breakdown of emulsions. Experimental results showed that more than 99% of oil removal was achieved under optimum process conditions. The easy availability of sawdust and calcium sulfate makes them an attractive choice for waste cutting oil treatment and oil removal. The characteristics of eucalyptus sawdust are given elsewhere (Cambiella et al. 2006). The most common separation method for the separation of secondary emulsions, with drop sizes less than 100 μm, is the steady-state fiber-bed coalescence (Secerov



Sokolovic et al. 2009). Coalescer was also used for the separation of both stable and unstable emulsions (Speth et al. 2002). One of the main advantages of a coalescence bed is the possibility of bed oil self-cleaning. They are compact and easy to install and maintain. Besides that, the need to replace the bed from time to time depending on the influent concentration is a limitation of this process (Li and Gu 2005). Sareen et al. (1966) and Hazlett (1969) investigated the effect of coalescing bed length on the separation efficiency of w/o emulsions. Sareen et al. (1966) concluded that there was an optimal bed length that enables maximal value of critical velocity. Hazlett (1969) commented that a bed length above a minimum has no influence on coalescence. Secerov Sokolovic et al. (1997, 2007) studied the effect of bed length on steady-state bed coalescence. The important properties of fiber bed with respect to bed coalescence were morphological and geometrical characteristics, including fiber size, shape, arrangement, and heterogeneity (Sareen et al. 1966, Hazlett 1969, Secerov Sokolovic et al. 2007). Based on the chemical nature of fiber surface, bed materials were classified in two categories: low-surface energy and high-surface energy materials. The wettability of bed materials also plays a crucial role in coalescence phenomena. Secerov Sokolovic and Sokolovic (2004) investigated coalescence with lowenergy materials and found a marked difference in bed coalescence efficiency.

Unlike saw dust, bagasse fly ash (BFA) is also an abundant and low-cost agricultural waste. Kundu and Mishra (2016) investigated the oil removal efficiency of BFA from oily wastewater. They used a modular continuous coalescing bed reactor packed with BFA, particles and optimal removal was achieved about 84.8% under optimal operating conditions. Pachathu et al. (2016b) studied the potential removal efficiency of chemically modified bagasse and corn husk using a packed-bed column. The adsorbent surfaces were modified using sodium hydroxide and CPC under normal RS, MW, and US medium. The removal efficiency of modified bagasse and corn husk was found to be 91%.

2.5.2.2 Adsorption by resin The basic phenomenon of oil removal by adsorbents is coalescence. Coalescence depends on the stability of the thin liquid film of continuous phase separating them from the solid surface because of three major steps: interception, diffusion, and impaction, i.e. approach-attachmentrelease of droplets (Hazlett 1969). Resin coalescer was effectively used for the treatment of industrial effluents (reducing high levels of oil and grease) and separation of o/w emulsions in appropriate droplet size range (Secerov Sokolovic et al. 2003, 2006, Li and Gu 2005, Zhou et al. 2009). Zhou et al. (2008) developed a modified resin coalescer for the treatment of emulsified oily wastewater and

Collector Resin bed

Reactor Figure 15: Schematic diagram of resin coaleser for treatment oily wastewater.


22 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater compared the removal efficiency with other commercially available materials [i.e. polypropylene granular, granular activated carbon (GAC), and ceramic filter (Figure 15). They used a polystyrene (PS) resin that was modified chemically with cetyltrimethyl-ammonium bromide (R-CTAB). The fidelity of a hybrid system consisting of R-CTAB+GAC and GAC+R-CTAB was evaluated. They found that the combination of R-CTAB+GAC gave the best results. This combination removed more than 90% oil from the oily wastewater. In this process, modified resin served as a pretreatment of the activated carbon adsorption process, which did not require high frequency of backwashing. Zhou et al. (2008) explained that due to hydrogen bond formation between hydrocarbon molecules and the free hydrophilic part of the particular surfactant, the R-CTAB had a satisfactory adsorption capacity and oil removal ability. It could change the zeta potential of effluents and coalescing oil droplets. Maiti et al. (2011) and Kundu and Mishra (2013) treated oily wastewater more precisely than emulsified wastewater using granular resin particles, which are a copolymer of styrene and di-vinyl benzene. A modular packed-bed reactor was used for o/w emulsion treatment. The effects of several process variables such as oil concentration, bed height, and flow velocity on the oil separation efficiency were investigated. Kundu and Mishra (2013) optimized the coalescing bed process and proposed a statistical model for predicting the oil removal efficiency of resin bed system. They found that the resin was very efficient in oil droplet adsorption and coalescence. The resin bed acted as an effective oil coalescer in removing oil droplets from o/w emulsion. Carmona et al. (2017) treated the oily wastewater using two resins, such as AMBERLITE™ ROC110 and DOWEX™ OPTIPORE™ L493. These two resins showed a high efficiency in removing organics from water. From the two resins tested, DOWEX™ OPTIPORE™ L493 achieved a complete removal of hydrocarbons while AMBERLITE™ ROC110 did not. They recommended that if complete removal is required, only DOWEX™ OPTIPORE™ L493 can be considered. But it required high cost due to the need of regeneration.

2.5.2.3 T reatment of oily wastewater by aerogels/ inverse fluidization of aerogels GAC is widely used as sorbent for wastewater treatment due to its highly porous structure. However, silica aerogels (CF3), which is hydrophobic in nature having larger absorption capacity than GAC, can be used for removing oil and other organic contaminants (Reynolds et al. 2001). Hydrophobic aerogel (Nanogel) granules are available in different sizes. Aerogel of sizes between 500 and 850 μm, 1.7 and 2.3 mm, and 0.5 and 2.3 mm are generally

used for treatment of oily wastewater. Particles are fluidized by downward flow of oil emulsified wastewater in an inverse fluidization mode (Quevedo et al. 2009). Aerogel particles are nanostructured, extremely light, highly porous in nature, and sufficiently robust to be fluidized. Quevedo et al. (2009) studied the hydrodynamic characteristics of inverse aerogel fluidized beds and the oil removal capacity of the aerogel granules. Minimum fluidization velocity for small granules is about 0.007 m/s and for large granules (1.7–2.3 mm in diameter) is about 0.02 m/s. Intermediate particles with size distribution of 0.5–2.3 mm had a minimum fluidization velocity of 0.013 m/s. Quevedo et al. (2009) concluded that taller fluidized bed (more granules) with smaller granules and low fluid velocities gave satisfactory results. Low-pressure drop, which translated in low energy consumption, makes the inverse fluidized bed a better choice over a packed-bed filter. The used saturated aerogels could be recovered by various ways, such as pressing, washing with hydrocarbons or other solvents, or use of superheated steam, which make the process more economical. Wang et al. (2010) also emphasized on inverse fluidization by aerogels (nanogel) because this high oil adsorption capacity makes an ideal material for removing hydrocarbon compounds from water. Table 9 shows the different adsorbents used by various researchers for oil removal. Gao et al. (2017) developed amphiphilic aerogels based on the SiO2-chitosan composite system with strong purifying capacity for treatment of emulsified wastewaters. The hybrid aerogels were prepared by integrating the hydrophilic chitosan into an organosilane modified SiO2 nano-framework via sol-gel and ambient drying processes. The hybrid aerogel powders exhibited good compatibility with and high absorption of both water and oily liquids (up to 6.3 g g−1 for propylene carbonate), together with good regeneration efficiencies (>90% after 10 cycles). Rapid (5 min) and effective treatment of emulsified wastewater was performed, which exhibited high absorption capacity and high purification efficiencies much better than activated carbon.

2.6 Biological treatment of oily wastewater Zaho et al. (2006) studied the biological treatment process and commented that the immobilization of microbial cells for treatment of wastewater has received considerable attention in recent years, especially for the removal of toxic organic and refractory substances from industrial effluents. Biological process was also used for the treatment of oil field emulsified wastewater treatment (Wang



Table 9: Oil removal by different adsorbent materials. Adsorbent

Remarks

References

Natural wool fibers (NWF) Recycled wool based nonwoven material (RWNM) Inorganic fiber (sepiolite) Sawdust Polypropylene granular (pp) Polystyrene resin chemically modified with cetyltrimethyl-ammonium bromide (R-CTAB) Polyurethane (PU) foam bed

Sorption capacity–5.56 g/g Sorption capacity–5.48

Rajakovic et al. (2007) Rajakovic et al. (2007)

Sorption capacity–0.19 g/g Removal efficiency ~99% Removal efficiency ~70% Removal efficiency ~81.2%

Rajakovic et al. (2007) Cambiella et al. (2006) Zhou et al. (2009) Zhou et al. (2009)

Mean separation efficiency 82.0%–99.8%

Granular-activated carbon (GAC) R-CTAB + GAC were about (oil concentration-1.0 g/l) for GAC + R-CTAB. (oil concentration-1.0 g/l) Ceramic filter Surfactant modified barley straw (SMBS) Polyacrylonitrile fiber (PANF)

Removal efficiency ~92.6% Removal efficiency ~93.6%

Secerov Sokolovic et al. (1997) Zhou et al. (2008, 2009) Zhou et al. (2008)

Removal efficiency ~78.2%

Zhou et al. (2008)

Removal efficiency ~36% Adsorption capacity was 576.0 ± 0.3 mg/g at 25°C

Zhou et al. (2009) Ibrahim et al. (2010)

Oil removal efficiencies of the PANF bed at the optimal condition for three different velocities (10, 24, and 36 dm3/h) were 95%, 82%, and 71%, respectively The oil removal efficiencies of the MPANF bed at the optimal condition for three different velocities (10, 24, and 36 dm3/h) were 97.3%, 93.5%, and 84.9%, respectively The removal efficiency was >99%. Typical fluid velocities during operation were in the range of 1–2 cm/s for granules less than 1 mm in size Nano-silica Aerosil R812 having large surface area (260 m2/g). Oil removal efficiency was 97%–97%, at pH 7

Ji et al. (2009)

Ji et al. (2009)

Quevedo et al. (2009)

Syed et al. (2011)

Modified polyacrylonitrile fiber (MPANF)

Nanogel

Hydrophobic nano-silica (aersosil R 812)

et al. 2002, Erhan et al. 2004). The biological aerated filter (BAF) is operated on the basis of immobilized whole cell reactor. The main advantages of a BAF reactor are that it can maintain high hydraulic loading rates and retain a high biomass concentration to reduce environmental shock, which causes less sludge formation, and promote microbial growth (Zf and Jr 2002). For aerobic upflow, submerged BAFs were also used to degrade oil at low COD load and high salinity condition (Yang et al. 2000). Zaho et al. (2006) investigated the performance of BAF reactors using two immobilized microorganisms on functional polyurethane foams carriers. One was B350 and the other was B350M. It was found that B350 and B350M groups of microorganisms were effective for the treatment of oil field emulsified wastewater. The performance of the reactor containing immobilized B350M microorganisms was better than that of the reactor containing B350 microorganisms over a long period of time, from 26 to 142 days. The reactor containing B350M

microorganisms achieved a mean TOC degradation efficiency of 78% and means oil degradation efficiency of 94%, at an hydraulic retention time of 4 h and a volumetric load of 1.07 kg COD (m3 d−1). Lee et al. (2005) investigated the effect of the synthesized mycolic acid on the biodegradation of diesel oil by Gordonia nitida strain LE31. Water containing diesel oil with concentrations of up to 15,000 ppm was efficiently degraded by this strain. They found that at a concentration of 20,000 ppm, the degradation by this strain was not effective. The synthetic mycolic acid behaved like a surfactant, which enhanced the degradation to a greater extent. Biological treatment using the aerobic active sludge system can degrade the petroleum hydrocarbons effectively and has been regarded as an efficient method to deal with hydrocarbon-containing oily wastewater. Zhang et al. (2009a,b) used rhamnolipids in an aerated active sludge system for biotreatment of oily wastewater. They applied rhamnolipid-containing cell-free culture broth,


24 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater which in turn enhances the biodegradation of crude oil and lubricating oil in a conventional aerobically activated sludge system. They observed that lubricating-oilbearing wastewater was more biodegradable than crude oil. Rhamnolipids greatly facilitated the biodegradation of lubricating oil at 20 and 25°C. Rhamnolipid treatment (22.5 mg/l) for 24 h at 20°C significantly increased the removal rate of oil by 92% in comparison to the removal rate of 24% in the absence of rhamnolipids. Yeber et al. (2012) evaluated the remediation efficiency of oily wastewater by an integrated photocatalytic and biological treatment. A batch reactor was used for that purpose and TiO2 was used as the photocatalyst. For biological treatment, they used the bacteria Pseudomonas aeruginosa in the form of biofilms. This integrated chemical and b iological treatment achieved 99.0% of oil and 78.6% TOC (mg l−1) removal. Cristóvão et al. (2016) evaluated the potential of aerobic biological process via activated sludge. The process showed high organic matter degradation rates (average value of 4900 mgO2/gCOD · d). Li et al. (2017) investigated the microbial communities in the functional areas of biofilm reactors with large height-diameter (L/D) ratio using the anaerobic-aerobic (A/O) reflux process to treat heavy oil refinery wastewater without pretreatment. In this process, COD and total nitrogen removal reached 93.2% and 82.8%, respectively. The no-pretreatment A/O process is useful to resolve the problems of a time-consuming treatment process and secondary pollutant produced by flocculant can be further used in the chemical oil removal unit.

2.7 F reeze/thaw method for separation of oil The freeze/thaw method is commonly found in the oily emulsion sludge treatment in cold regions. It is found to be very effective as a large amount of oily sludge was generated in petroleum refinery, which required processing to reduce its oil content for safe disposal. The oily sludge was characterized as a w/o emulsion (water content varying from 38 to 77 wt.%). Freeze/thaw treatment is actually a physical sludge conditioning method that includes: (i) improving certain sludge dewatering characteristics significantly, (ii) changing the floc structure into a more compact form, and (iii) reducing the sludge bound water content (Vesilind et al. 1991, Lee and Hsu 1994). In freeze/thaw treatment, the most critical parameter is freezing speed, and instant freezing is not necessary (Logsdon and Edgerley 1971). According to various researchers, the sludge dewatering efficiency decreases with increasing freezing speed. However, a long freezing

time will make the process uneconomical when compared with other conditioning/dewatering processes (Randall et al. 1975, Ezekwo et al. 1980). The freeze/thaw process of oily sludge was done by separating solid particles from the ice lattices followed by the compacting of the bio-solids during freezing. This will change the physical nature of the solids so that they cannot hold initially existing water after thawing. The initial water content, freezing temperature, freezing time, thawing rate, and thawing temperature are the important parameters that influence the freeze/thaw process. Freezing and thawing temperature depends on the types of oily sludge. It was found that optimal freezing temperature ranged around −15°C to −40°C, and thawing was done at room temperature for the oily sludge (Lee and Hsu 1994, Jean et al. 1999, Chen and He 2003). The most important step was demulsification, which involved removing water from a w/o emulsion. Chen and He (2003) explained the detailed freeze/thaw demulsification mechanism. The mechanism is briefly described below. In o/w emulsion, the film at the oil-water interface and the surfactant distributed on it play a big role in the stability of the emulsion. Step 1: Freezing – when temperature decreases, water droplets freeze. During freezing, surfactant molecules are expelled out from the ice lattices at the interface, shown in Figure 16A–B. After a sufficiently long time, the expelled surfactant molecule diffuses into the oil phase. Step 2: Thawing – during an increase in thawing temperature, more surfactant molecules were diffused away from the ice because the interface film would melt first. Due to the absence of surfactant on the interfacial film, the water droplets were coalesced together to form larger water droplets, and some surfactant molecules would gather together to form micelles inside the water droplets with a trace amount of oil, which is shown in Figure 16C–D. A relative comparison among different process is discussed in Table 10.

2.8 Photocatalytic oxidation process/ advanced oxidation process Photocatalytic oxidation processes (PoPs)/advanced oxidation processes (AoPs) are attractive wastewater treatment processes. In this process, organic and inorganic components are mineralized by photocatalytic degradation. It is also an efficient and cost-effective technique that is suitable for petroleum refinery effluent (PRE) treatment



A

B

Water

Water

Surfactant

Surfactant

Oil Oil

C

D

Water

Water

Micelles

Surfactants

Oil Oil

Figure 16: Mechanistic view of demulsification phenomenon of freeze/thaw process. (A, B) Surfactant molecules are expelled out from the ice lattices at the interface; (C, D) the water droplets were coalesced together due to lack of surfactant at the bubble interface.

at the advanced stage. Photocatalysis has a large capability for the refinery wastewater treatment. The drawback of this method is that of it being slow compared with traditional methods but at the same time it has the advantage of not leaving toxic byproducts or sludge to be disposed. Ziolli and Jardim (2001) studied the photocatalytic destruction of the water-soluble crude oil fraction (WSF) using TiO2/ultraviolet (UV)-Vis. The process was shown to be quantitatively efficient in the destruction of WSF. The process reached around 90% of degradation for carbon concentrations, which ranged between 9 and 45 ppm. The total destruction of water-soluble compounds originating from oil residues indicated that the photocatalysis is an extremely attractive potential process that could be employed for water treatment (petrochemical waste water), despite the fact that mineralization reaches values as high as 90%. During photocatalysis, the generation of

transient toxic species was observed, which should be taken with special care. Diyauddeen et al. (2011) presented a review focused on photocatalytic degradation of PRE by discussing the process principles and its applicability for treating typical organic pollutants found in refinery effluents. Emam and Aboul-Gheit (2014) investigated the removal of o/w emulsions via PoP. They used synthetic wastewater samples or artificial seawater to investigate the effects of (i) initial oil concentration, (ii) catalyst concentration, (iii) irradiation time, and (iv) pH. The photocatalytic activities of three commercial TiO2 types such as Degussa P25, Hombikat UV-100, and Millennium PC50 were compared with those of other two TiO2 samples prepared using an artificial UV source. They found that the Degussa P25 photocatalyst and the TiO2-SG, both in distilled water and in artificial seawater, were the most active


26 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater Table 10: Summary of various oil removal processes. Process

Gravity separator – API separator

– Skimmer – Inclined plate separator

– Corrugated plate separator Flotation – Dispersed (induced) air flotation (IAF)

– Dissolved air flotation (DAF)

– Nozzle flotation (NF)

– Electro-flotation (EF)

– Column flotation

– Jet flotation

Advantages Simple design, easy to operate, cheap and low maintenance Popular for oil skimming resistant to plugging with solid, removing both solid and oil Compact design, small in size, low retention time Remove relatively free oil & SS. Compact size, lower capital cost

Disadvantages

Large retention time, large area required. Separation efficiency limited upto 150 μm droplet diameter

Efficiency limited upto 60 μm droplet diameter

Plugging of plate, corrosion by solvent like BTEX

No moving parts, so less maintenance cost. High throughput, high efficiency and moderate equipment cost Lower initial costs and energy use because a single pump provides the mixing and air supply Removes soluble oils, Clarity of the treated wastewater. High efficiency, low cost, no secondary sludge disposal problem Low maintenance cost due to no moving parts in the cell. Lower water usage than educators No moving parts. High throughput, high efficiency and moderate equipment cost Compact size and lower capital cost

Requires chemicals, high power requirement, less flocculation flexibility. Performance is depending on hydraulic loading Explosion risk in oil industry, problem in Sludge disposal when coagulant is used Large amount of water is required

– Centrifugal flotation

System uses the contactor–separation concept with very low detention times in the contactor

Low throughput, the emission of H2 bubbles, high cost of electrodes and maintenance and problem of generation of large volume of sludge Large residence time and additional of pH regulators requires High power requirement and less flocculation flexibility Large residence time and maintenances cost of the moving parts High power requirement and high capital cost

Filtration

Handles high solids

Requires back washing

Electrocoagulation

Removes soluble oils, BOD and COD, high oil removal efficiency, low cost

Periodic replacement of aluminium or iron electrode required

Biological treatment

Remove soluble oils. It maintains high hydraulic loading rates and retains a high biomass concentration to reduce environmental shock, resulting in less sludge formation, and promoting microorganisms grow

High tolerance for oil and grease. Pretreatment requires. Applicability limited to degrade oil with low N and P compounds under low COD load and high salinity condition

Membrane process (ultrafiltration)

Soluble oil removal, stability in aggressive (vapor or solvents, and non-oxidizing acids or bases) and adverse (high temperature and pressure operation) environments

Low flux rates, suffer from membranes fouling and reduced membrane life

– Cavitation air flotation (CAF)

Hybrid ultrafiltration (UF) and membrane distillation (MD)

Very high reduction of the total organic carbon (99.5%) and total dissolved solids (99.9%) was found

Susceptible to fouling and costly

Inverse Fluidization of Aerogels

Extremely low energy consumption (low pressure drop) and large absorption capacity

Pressure drop increases proportionally to the flow rate, and the bed voidage reduces as the filter saturates with the contaminants which lead to a lower removal capacity

Higher oil removal efficiency, cost effective, no prechemical treatment of effluent required The coalescence performance is insensitive to the length of the granular bed

The higher the inlet oil concentration lowers the overall coalescence efficiency

Coalescing using fibrous and granular beds

Adsorption – Adsorption by various adsorbent

– Absorption by resin

Easily available adsorbent and more than 99% of oil content in the influent stream was removed More than 80% of emulsified oil was removed. Process, high frequency of backwashing is no longer required and regenerated after treatment

A chemical pre-treatment and surfactant modification of adsorbent is needed to activate the sorption sites Removal efficiency enhanced by either chemical modification or grafting. Not easily available and costly



among other photocatalysts used under study. Removal of 43% of oil was achieved by Degussa P25 in distilled water and in artificial seawater, respectively, after 3 h of irradiation. They observed that the photocatalytic degradation of o/w emulsion was strongly hindered in artificial seawater. They concluded that the TiO2 can efficiently catalyze the photodegradation of o/w emulsion in the presence of radiation and air. Moslehyani et al. (2015) investigated the potential of a novel flat sheet nanocomposite membrane as a photocatalytic separator in the photocatalytic membrane reactor (PMR) for the removal of hydrocarbons from bilge water. Bilge water is basically composed of a corrosive mixture of different hydrocarbons from engine oil and the fuel tank of ships. The nanocomposite membrane was made of titanium dioxide (TiO2), halloysite nanotubes (HNTs), and polyvinylidene fluoride (PVDF). They developed a PMR that consists of photocatalytic nanocomposite membrane. They found that 99.9% of hydrocarbons were removed by the PMR within 8 h, which was due to uniform distribution and the high effectiveness of the TiO2-HNTs photocatalyst.

2.9 C ombined/mixed treatment process There were several methods for the purification of oily wastewater, including conventional physical and chemical methods. Adsorption (activated carbon, organoclay, copolymers, zeolites, and resins), filter, and coalesers are categorized under physical treatment, and oxidation, electrochemical process, photocatalytic treatment, ILs, and demulsifier are categorized under chemical treatment methods. These conventional methods have their own drawbacks, such as high cost, use of toxic compounds, need for a large space for installation, and generation of secondary pollutants. Keeping these drawbacks in view, membrane separation processes serve as an emerging technology. However, the major problem attacking the membrane separation processes is membrane fouling. Membrane fouling still remains one of the most technical challenges in the separation industries. Recently, in oily wastewater treatment, the combined method has shown to be a more promising technique. In combined systems, different physical, chemical, and biological methods and PMR were used for the pretreatment of membrane units. The fact is that the combined methods are efficient in purification but are a more costly process. For reuse of oilfield-produced water, in order to maintain drinking water quality standards, hybrid processes using different physical and chemical pretreatments are required before the RO treatment.

Optimized pretreatment and unique separation (OPUS) is a process in which both physical and chemical treatments are used for producing boiler feed water. The degasification, chemical softening, media filtration, ion-exchange softening, and cartridge filtration are some of the physical and chemical treatments, followed by RO separation, considered in OPUS technology (Padaki et al. 2015). Doran et al. (1997) proposed different arrangements of combined systems for oily wastewater treatment. Dissolved gas floatation, walnut shell filtration, warm softening, and membrane bioreactor followed by RO systems were proposed by Tsang and Martin (2004). Moslehyani et al. (2015) used a PMR for oil-water separation. They combined the photocatalytic reactor with the UF permeation cell to separate the remained oil and suspended TiO2 (photocatalyst) after degradation.

2.10 A dvanced smart materials for oily wastewater treatment Over the past decade, nanomaterials and nanotechnology have rapidly transformed from an academic pursuit to commercial reality. To efficiently separate the increasing amount of oily wastewater, four kinds of surface wetting properties must be considered for selecting the proper nanomaterials (Ma et al. 2016), i.e. hydrophobicoleophilic, hydrophilic-oleophobic, superhydrophilicsuperoleophobic, and responsive/switchable wetting properties. Recently, Al-anzi and Siang (2017) presented a brief summary of various nanomaterials used for the separation of emulsified oil/water mixture. In recent time, superwetting materials have attracted considerable attention. Among them, porous materials with special wettability are more popular since this kind of material is easy to fabricate and is both cost and time saving. Moreover, by combining the design of special wettability with the proper pore size, the porous materials could achieve the separation of oil/water mixtures. Recently, Zhang et al. (2017) presented a detailed review about the application of superwetting porous materials for separation of immiscible oil/water mixtures. They mainly summarized the use of two types of superwetting materials; one was water blocking porous materials with superhydrophobic/superoleophilic wettability and other was oil blocking porous materials with superhydrophilic/underwater superoleophobic wettability. Through combining the special wettability with proper porous substrates, the superwetting materials are able to achieve both the immiscible oil/water separation and emulsion separation. Porous materials with relatively large pore size are able to separate immiscible oil/


28 P. Kundu and I.M. Mishra: Treatment and reclamation of oily wastewater water mixtures. But when the pore size of the porous substrates becomes smaller, the materials could separate w/o or o/w emulsions. As for water blocking porous materials, the antifouling property is of great significance since it will determine the service life of materials directly. When the porous materials are oil blocking, the anticorrosive property should be taken into consideration. Although superwetting porous materials have shown great potential in the separation of both immiscible oil/water mixtures and emulsions, they have some limitations when used in real industrial fields. Much effort is still needed to fabricate more stable superwetting porous materials in the future. Dang et al. (2016) designed a novel pH responsive non-fluorine-containing copolymer. The copolymer, together with silica, can be dip-coated on different materials, including cotton fabric, filter paper, and polyurethane foam. The coated materials exhibited switchable superhydrophilicity and superhydrophobicity and can be applied in continuous separation of oil/water/oil three phase mixtures, different surfactant stabilized emulsion (o/w, w/o, and oil-in-acidic water), as well as oil uptake and release via in situ and ex situ pH change. Herein, carbon-based nanomaterials have drawn tremendous attention among membrane scientists due to their low cost, superior chemical and mechanical stability, and highly integrated operation. Apart from various carbon-based nanomaterials, carbon nanotubes (CNTs) and graphene have caused a lot of interest in oil removal studies due to their exceptional one-dimensional structure, large specific surface area, and oleophilic and hydrophobic nature (Gupta and Tai 2016, Thines et al. 2017). Recently, the fabrication of nanofibrous materials in terms of sorbents (Zhu et al. 2011), membranes (Shang et al. 2012), and aerogels (Si et al. 2015) has shown a rapid expansion of research due to its simple and effective oil/water separation. Song et al. (2017) used a superoleophobic BiVO4 coated mesh with sunlight-driven self-cleaning property for oil/water separation. The coating had micro/nanostructures and showed underwater superoleophobicity for various oils and organic solvents. They reported that the coating cannot be easily contaminated by common pollutants, like organic solvents, strong acid, strong alkali, high-concentration salt solution, and antibiotic solutions. It also exhibited excellent durability against abrasion, cavitation erosion, and ultralow temperature. However, when the contaminant was completely decomposed by the BiVO4, the coating recovered its wetting behavior and could be used for oil/water separation again. All the features endowed the BiVO4 coated mesh with high oil/water separation efficiency and possibility to apply in a continuous oil/water separation system.

Raturi et al. (2017) developed a smart surface mesh with reversible wetting properties for on-demand oilwater separation. They synthesized ZnO-nanowire (NW)-coated mesh, which showed superhydrophilic/ underwater superoleophobic behavior. ZnO NWs were obtained from the chemical vapor deposition method and coated on an SS mesh (SSM). The wetting property of ZnO-NW-coated mesh switched easily from superhydrophilic to superhydrophobic state and vice versa by simply annealing it at 300°C alternatively under hydrogen and oxygen environment. Thus, the reversible wettability of ZnO NWs provides a smart surface mesh, which can be switched between “oil-removing” and “water-removing” modes. The separation was done by gravity. They found that for more than 10 cycles of mesh reutilization in both modes alternatively, the separation efficiency of 99.9% stayed relatively invariant, which indicates a prolonged antifouling property and excellent recyclability. Liu et al. (2017) prepared polypyrrole (PPy)-coated SSMs with underwater superoleophobicity and under oil superhydrophobicity as well as controllable pore size by adopting cyclic voltammetry. Under gravity only, oily wastewater can be economically and effectively separated by using the PPy-coated meshes with appropriate pore sizes regardless of oil density (immiscible oil-water mixtures) or the type of disperse phase (emulsions). In recent decades, polylactic acid (PLA) has been regarded as the most promising eco-friendly materials due to its outstanding properties and is also expected to be developed into novel biodegradable separation materials. Gu et al. (2017) reported the fictionalization of PLA nonwoven fabric as superoleophilic and superhydrophobic material for efficient treatment of oily wastewater. Moreover, it was used first to modify PLA nonwoven fabric. They developed a hierarchical micro/nanoparticle assistant strategy for the fabrication of biodegradable superhydrophobic PLA nonwoven fabric for gravity-driven oil-water separation. The hierarchical micro/nanoparticles consisting of hydrophobic PS microspheres and silica oxide (SiO2) nanoparticles were deposited densely on the polydopamine-modified PLA fabric to enhance its surface roughness. The resultant SiO2/PS/PLA hybrid nonwoven fabric with hierarchical porous structures exhibited high oil-absorption capacity and selectivity. In recent years, the magnetic oil-water multiphase separation technique has attracted considerable attention, in which surface-modified magnetic nanoparticles (MNPs) are used as the demulsifier. Lu et al. (2016) developed thermosensitive and reusable MNPs for removing emulsified oil droplets from aqueous media. They had used a convenient “grafting through” reaction for



anchoring poly (N-isopropylacrylamide) (PNIPAM) molecular chains onto the surfaces of MNPs to facilely develop advanced thermoresponsive MNPs. Magnetic iron oxide nanoparticles were prepared by coprecipitation method, followed by coating with a silica layer and further modification with γ-methacryloxypropyl triisopropoxidesilane. The efficiency of synthetic MNPs for treating emulsified oily wastewater was evaluated as a function of dosage, pH, temperature, and reusability. In diesel-in-water emulsion, the synthetic MNPs could rapidly assemble to the oil-water interface, resulting in efficient removal of emulsified oil in the presence of magnetic field. They observed that the demulsification performance was thermoresponsive, not pH responsive. At higher temperatures (~33°C), the PNIPAM-grafted MNPs tended to desorb from the emulsified oil droplets, and therefore the MNPs could be facilely regenerated by using hot water and be reused up to seven cycles without showing a significant decrease in its demulsification efficiency. Thermosensitive and reusable magnetic demulsifier could potentially be a promising material for the treatment of emulsified oily wastewater. Duan et al. (2017) prepared core-shell magnetic and thermosensitive composite nanoparticles, named M-DMEA, by grafting polyoxyallcylated N,N-dimethy lethanolamine onto magnetite (Fe3O4) nanoparticles. They found that the M-DMEA could separate emulsified oil droplets from oily wastewater produced from flooding (OWPF) rapidly under an external magnetic field with high oil removal rate (92.3% at the dosage of 2.5 g/l) at 65°C. However, due to its excellent magnetic response, M-DMEA was collected and recycled for another three cycles. They also investigated the flocculation performance of M-DMEA in OWPF treatment. The obtained M-DMEA was applied to treat real OWPF, and the results showed that the oil removal increased with its dosage. When the dosage of M-DMEA was 2.5 g/l, the oil removal reached 92.3% at 65°C. Furthermore, M-DMEA was recycled and reused. However, there was a significant decrease in the oil removal observed after three cycles because of the adsorption of polymer residue in OWPF. In fact, it is expected that M-DMEA might find promising application in the emulsion treatment field in the future. A new type of porous network material named metal organic frameworks (MOFs) has attracted great attention among the scientific community. MOFs have recently been proposed as adsorbents to remove contaminants from water owing to their high surface area and versatile tunability. These materials are synthesized by combining metal ions and organic ligands to form a porous crystalline network. Lin et al. (2014) prepared a copper-based MOF, i.e. HKUST-1 (CuBTC), for separation oil droplets from

water. They found that HKUST-1 exhibits a high removal capacity, about six times higher than a commercial activated carbon. HKUST-1 can be well dispersed in the o/w emulsion and capture oil droplets after a certain time of mixing. Once oil droplets are adsorbed on HKUST-1, the oil-rich HKUST-1 tends to coagulate and settles down in the aqueous phase to complete the phase change. They estimated that the maximal removal capacity of HKUST-1 was 4000 mg g−1 by the Langmuir isotherm model. Furthermore, HKUST-1 was successfully regenerated using an ethanol washing process. The sample exhibited 95% of its original removal capacity during the five testing cycles.

3 F uture perspective and challenges The rapid industrial growth (i.e. oil and gas, petrochemical, pharmaceutical, metallurgical, and food industries) has led to the large production of oily wastewater. The need to treat oily wastewater is an inevitable challenge. One of the solutions for addressing this issue is the reuse of water, which requires the adoption of advanced technologies, such as membrane technologies, PMR, etc. Few UF and MF processes showed prominent results in the removal of oily wastewater treatment. However, inorganic membranes have been shown more interest due to their efficiency. Some future aspects of treatment process regarding oily wastewater are highlighted in the following: 1. For the existing problems of technology and processes, research and development of a new combined process are very much needed to maximize the advantages of various methods to avoid its limitations. 2. Mixed purification systems were reported by many researchers up to pilot scale. More emphasis on the economically acceptable mixed methods is required to establish a bench scale system. 3. An appropriate method for the surface modification is necessary to control the surface modification. Hydrophilic surface modification increases the efficiency of the membrane. Meanwhile, an appropriate modifier is also one of the dominant requirements in the future. 4. The development of low-cost novel ceramic membranes is more effective and very essential for oily wastewater treatment. Upgrading of processes for backwashing of the ceramic membranes is needed. Therefore, it is very essential to come out with a novel ceramic material that causes less fouling and is easy to backwash. 5. Membrane separation technology is one of the promising separation technologies in the treatment of oily wastewater. The membrane separation process is



6.

7.

8.

9.

10.

economically effective as compared to other commercial methods. Several approaches were adopted to further improve membrane performance. Surface modification by particular monomers enhances membrane performance and showed better antifouling properties. Some of the surface modified membrane has been used in pilot plant scale and had promising results. Like polymer membranes, ceramic membranes also showed more prominence in the separation process. Continuous efforts in the development of efficient membranes are required, which will provide an effective and economically feasible process for oily wastewater treatment. Heterogeneous photocatalysis using TiO2 was found as a promising process to minimize the impact of hydrocarbon oil compounds on contaminated waters. An in-depth study of oily wastewater degradation mechanism using advanced photo-oxidation is required to improve oily wastewater treatment efficiency and reduce processing costs, which will provide a solid theoretical foundation. The performance of the PMR system is found to be excellent for treatment of hydrocarbon-bearing wastewater. PMR is a very novel method in oily wastewater treatment, which showed very promising performance on the degradation of toxic hydrocarbon compounds. The modified PLA nonwoven fabric provides a new pathway to fabricate oil/water separation materials, taking thorough consideration of both high separation efficiency and formidable posttreatment of the used separation materials, which shows attractive potential applications in water purification. This process is simple, fast, and cost-effective. The separation mesh is found to be highly efficient with a high purity of filtrate. These smart surface meshes provide a simple, efficient, and scalable approach for on-demand oil-water separation and opens up new perspectives in the field of oily wastewater treatment. Adopting a green technology approach in oily wastewater treatment research is very much needed in the future, such as supercritical water oxidation (SCWO), which is a promising green technology. SCWO processes completely convert hazardous wastewaters into innocuous products and also allow energy recovery.

4 Conclusions A detailed review on various separation techniques of hydrocarbon bearing oily wastewater treatment was presented. Among various flotation techniques, the induced

air flotation (IAF) technique was found to be most promising for oily wastewater (o/w or w/o emulsions) treatment. IAF allows longer flotation time and column height, which improved oil separation efficiency. The application of insoluble electrodes in electroflotation technique is a new concept that can be implemented in oil separation processes. But the potential application of electroflotation techniques using insoluble anodes was limited due to electrical energy consumption. The presence of NaCl reduces the energy consumption of electroflotation technique significantly, which makes the electroflotation technique a useful method in treating emulsified wastewater. The most effective method of oily wastewater treatment including o/w or w/o emulsions was the fixed-bed coalescence. A coalescence process involving various coalescing media was presented. The efficiency of bed coalescence depends on the bed characteristics, emulsion properties, and working conditions. Mechanical coalescence was done either by fibrous bed or by granular materials. Fibrous media was preferred to granular solids due to higher porosities and higher specific surface than coarse granular media. But periodic cleaning was required to regenerate active pore site due to the deposition of emulsified substance. Studies on fibrous material were done less extensively because fibrous media were not as common as granular materials. Mechanisms of coalescence involving fibrous media were less understood concerning the correlation of resistance and the structure of the medium. Therefore, more study involving fibrous materials are required. Membrane process was found to be an advanced and effective method in the field of oil separation from oily wastewater/emulsions. According to various research works, UF and MF membranes were suitable for oily wastewater treatment. Fouling was the main drawback of the MF process. Moreover, mixed or hybrid methods were not cost-effective. Moreover, membrane fouling caused by the adsorption of other organic molecules (i.e., surfactants) is a problem which remains overlooked. Nevertheless, the impacts of key operating parameters such as cross flow velocity, operating pressure, oil concentration, and pH also contributed to the deterioration of membrane performance. More research is still required to resolve the aforementioned issues as well as to enhance the performance efficiency, reliability, and stability of carbon-based membrane for real industrial implementation. It is recommended that advanced pretreatment stage such as photocatalytic oxidation, flotation, gravity separation, and flocculation prior to membrane filtration might be advantageous. Although it might take years to resolve the



remaining challenges in this field, carbon-based membranes have the potential to deal with a large variety of industrial oily wastewater in the future. The extraordinary properties of the carbon based nanomaterials are able to provide leap forward opportunities to revolutionize traditional oily wastewater treatment. The astonishing properties of graphene oxide, CNTs, and carbon fiber have proven their potential benefits to the real practical application. Further modification on these carbon-based nanomaterials is required to improve their properties and cost-effectiveness. This might show a bright future for these nanomaterials toward oily wastewater. More research focused on responsive/switchable wettable materials will be required as their wettability can be switched upon applying an external stimulus such as light, pH, and temperature or treatment with specific chemicals. Materials with responsive wettability can switch from wetting (superwetting) to antiwetting (super-antiwetting), or vice versa. Therefore, the responsive wettable material offers a significant advantage for controllable oil/water separation in harsh environmental conditions. PNIPAMgrafted MNPs could potentially be a new promising and environmentally friendly approach for effectively treating emulsified wastewater. Acknowledgments: We thankfully acknowledge the financial support provided by the Ministry of Human Resources Development (MHRD), Government of India, for carrying out this study. The authors are grateful to the Department of Chemical Engineering, IIT Roorkee, for providing the support to study and do research on the present work. One of the author (P.K) also extended thanks to Mr. Soumitra Maiti for meaningful discussions and input.

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Bionotes Partha Kundu Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttrakhand 247667, India

Partha Kundu received his PhD (2016) and MTech degree (2011) from Department of Chemical Engineering, IIT Roorkee. He obtained his BTech (2009) in Chemical Engineering from Calcutta University, India. At present, he is a post-doctoral fellow at University of Regina, Canada. His research interest includes colloid and interfacial science, separation processes, petroleum engineering, environmental engineering, wastewater treatment and hydrocarbon processing.

Indra M. Mishra Department of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttrakhand 247667, India; and Department of Chemical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, Dhanbad 826004, Jharkhand, India [email protected] Indra M. Mishra has served as a Professor of Chemical Engineering, and Dean at IIT Roorkee. At present, he is serving as Professor & Head of Department of Chemical Engineering at IIT (ISM), Dhanbad. He obtained his PhD (1977) degree in Chemical Engineering from IIT (B.H.U) Varanasi and carried out post-doctoral research at the University of Hannover, Germany (1980–1982). He specializes in the areas of chemical, hydrocarbon engineering, environmental engineering, energy engineering, and transport phenomena.


Rev Chem Eng

2018 | Volume x | Issue x

Graphical abstract Partha Kundu and Indra M. Mishra Treatment and reclamation of hydrocarbon-bearing oily wastewater as a hazardous pollutant by different processes and technologies: a state-of-the-art review https://doi.org/10.1515/revce-2017-0025 Rev Chem Eng 2018; x(x): xxx–xxx

Review: Various processes and technologies for treatment of oily wastewater are discussed, the current state of knowledge regarding emulsified wastewater management is described and challenges in existing separation techniques are presented which will help to choose the appropriate process for oily wastewater treatment. Keywords: coalescence; flotation; membrane; oily wastewater; pollution; wastewater treatment.


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