versus coagulation/dissolved air flotation

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Nov 25, 2009 - coagulation/dissolved air flotation (C/DAF) for pre-treatment of personal care ... in the 1st scheme and dissolved air flotation in the 2nd one.
Desalination 252 (2010) 106–112

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Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l

Comparative study between chemical coagulation/precipitation (C/P) versus coagulation/dissolved air flotation (C/DAF) for pre-treatment of personal care products (PCPs) wastewater F. El-Gohary a, A. Tawfik a,b,⁎, U. Mahmoud c a b c

National Research Center, Water Pollution Research Department, El-Behouth St., Dokki, P. Box 12622, Cairo, Egypt Centre of Excellence for Advanced Sciences, Renewable Energy Group, National Research Center, El-Behouth St., Dokki, P. Box 12311, Cairo, Egypt Sanitary Engineering Department, Faculty of Engineering, El-Azhar University, Egypt

a r t i c l e

i n f o

Article history: Received 17 August 2009 Received in revised form 18 October 2009 Accepted 22 October 2009 Available online 25 November 2009 Keywords: Industrial wastewater Coagulation DAF COD Oil and grease Sludge

a b s t r a c t For pretreatment of wastewater discharged from personnel care products (PCPs) factory, two treatment schemes were investigated. The 1st step in both schemes was chemical coagulation followed by precipitation in the 1st scheme and dissolved air flotation in the 2nd one. Ferric chloride (FeCl3·6H2O), alum (Al2 (SO4)3·18H2O) and ferrous sulfate (FeSO4·6H2O) were used as coagulants. Lime (CaO) was used as coagulant aid and for pH adjustment. For C/P, the three coagulants investigated were found to be more or less similar in their performance. Maximum CODtotal removal obtained by ferric chloride, ferrous sulfate and alum was 75.8 ± 9.7, 77.5 ± 9.6 and 76.7 ± 9.9%, respectively. Corresponding BOD5 total removal values were 78 ± 15.8, 78.7 ± 15.6 and 74.1 ± 19.3%, respectively. However, the optimum dose of ferric chloride and alum was 600 and 700 mg/l while that of ferrous sulfate was 850 mg/l. Alum produced a voluminous sludge, but with the least solids content. Ferric chloride produced compact sludge with a good settleability as reflected by the low sludge volume index (SVI) of 76.3 ± 28.8 ml/gTSS. In the coagulation–dissolved air flotation (C/DAF) experiments, the results showed that alum produced higher COD removal (77.5 ± 3.2%) as compared to ferric chloride (71.6 ± 2.9%) and ferrous sulfate (67.7 ± 3.7%). A cost evaluation of the initial investment and the running costs using the different coagulants at their optimum operating conditions were calculated. The investment and running cost for C/P process is higher by 27.3 and 23.7% than C/DAF. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Wastewater generated from personal care products (PCPs) industry is characterized by high content of organic compounds which are not easily biodegradable [1]. The presence of detergents (anionic surfactant), oil and grease (O&G), the main constituents of PCPs in wastewater treated via activated sludge systems, increases the formation of filamentous organisms (Actinomycete Nocardia Amarae), which creates scum and foam layers in the aeration tanks [2]. In addition, O&G can be adsorbed on the surface of the sludge, which may limit the transfer of soluble substrates and oxygen to the biomass. This leads to reduction in the rate of substrate conversion. Furthermore, the composition of wastewater from PCPs industry varies greatly from day to day and hour to hour, depending on the industrial process (soap; shampoo; toothpaste; creams and liquid soap). Reif et al. [3] investigated the fate of PCPs during the operation of

⁎ Corresponding author. National Research Center, Water Pollution Research Department, El-Behouth St., Dokki, P. Box 12622, Cairo, Egypt. Tel./fax: +20 2 33351573. E-mail address: tawfi[email protected] (A. Tawfik). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.10.016

membrane bioreactor (MBR) treating domestic wastewater. The MBR was operated at sludge retention time (SRT) of 44–72 days. PCPs were partially sorbed onto the sludge and their elimination from the effluent was very limited, below 9%. Likewise, Matamoras et al. [4] investigated the occurrence and removal efficiencies of PCPs as well as BOD5, and TSS in thirteen onsite household secondary wastewater treatment systems, including two compact bio-filters, two biological sand filters, and five horizontal subsurface flow constructed wetlands. As expected, all systems removed TSS and BOD5 efficiently (>95% removal). The PCPs removal efficiencies were not exceeding 50%. For removal of PCPs from industrial wastewater electro-coagulation and electro-Fenton process was investigated by Aloui et al. [5]. The best results were obtained using electroFenton process, exceeding 98 and 80% of anionic surfactants and COD removal, respectively. Likewise Wang et al. [6] investigated the treatment of PCPs wastewater by Fenton oxidation process at the following operating conditions: pH=8, Fe2+ dosage=600 mg/l and H2O2 dosage=120 mg/l. The COD and PCPs were removed by a value of 85 and 95%, respectively. Although PCPs in wastewater industry can be effectively removed by chemical oxidation processes, the costs of these techniques are still high.

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Coagulation/precipitation (C/P) processes have been found to be cost effective, easy to operate, and energy saving treatment alternatives [1]. C/P processes have been mainly used for wastewater treatment to separate suspended and/or fatty particles [7]. The most widely used coagulants are alum; iron salts and lime [8]. The main function of coagulants is to flocculate colloidal particulates into larger particles that can be removed by sedimentation or flotation [9]. The mode of action is generally explained in terms of two distinct mechanisms: 1) neutralization of negatively charged colloids by cationic hydrolysis products and, 2) incorporation of impurities in an amorphous hydroxide precipitate, so-called sweep flocculation [10]. The relative importance of these mechanisms depends on factors such as pH and coagulant dosage. C/P has been used for the treatment of wastewater discharged from pulp and paper mills [11], dyes [12], hospitals [13], and beverage processing [14]. Coagulation–dissolved air flotation (C/DAF) is a preferable technique for treatment of oily wastewater [15], olive oil mills [16], slaughterhouses [17], chemical mechanical polishing processes [18], and refineries [19]. The process variables, which control the DAF system, namely the saturation pressure, air/solids (A/S) ratio and hydraulic surface loading rate, depend on wastewater characteristics and the effluent quality requirements [20]. It is worth mentioning that, an adequate aggregation of the particulate matter represents an essential requirement for efficient flotation technique [21]. The objective of the experiments is to investigate chemical coagulation followed by either precipitation (C/P) or dissolved air flotation (C/DAF) using alum, ferric chloride and ferrous sulfate for pretreatment of personnel care products (PCPs) wastewater. The optimum pH values and coagulant dose were determined for each coagulant. Emphasis will be afforded to the removal efficiency of the COD, BOD5, TSS, oil and grease. Moreover, characteristics of the floated and settled sludge were assessed. Initial investments as well as the operational cost of the two treatment processes (C/P and C/DAF) were estimated. 2. Material and methods 2.1. Wastewater and analytical methods The industrial wastewater used in this study was provided from Unilever Mashreq Company for manufacturing of personnel care products (PCPs) (4th industrial zone–6th October city, Egypt). The main surfactant used in this factory, is the anionic surfactant (sodium lauryl ether sulphate). Some other chemical compounds are also used in the formulation of different PCPs products, such as: calcium carbonate, silica, sorbitol, stearic acid niacinamide, butyl methoxy cinnamat, carbopol 980 (acrylic polymer–carbomer), dimethyl ammonium (Dehardened Tallow) and sodium hydroxide. Flow measurement of the wastewater indicated that the wastewater discharged is around 90 m3/d. Physico-chemical analyses of 24 h composite samples were carried out according to APHA [22]. The analysis covered total chemical oxygen demand (CODtotal), biochemical oxygen demand (BOD5), total suspended solids (TSS), volatile suspended solids (VSS), total phosphorous (total-P), oil and grease (O&G). 0.45 μm membrane filter paper (Schleicher & Schuell ME 25) was used for determination of soluble chemical oxygen demand (CODsoluble) and soluble biochemical oxygen demand (BOD5 soluble). The CODparticulate and BOD5 particulate were calculated by the difference between CODtotal and CODfiltered, BOD5 total and BOD5 filtered, respectively. Due to a lack of facilities, anionic surfactant was not measured. Sludge characteristics were determined according to APHA [22]. This covers sludge volume (SV), solids content and sludge volume index (SVI). 2.2. Treatment processes To develop design parameters for treatment of end of pipe effluent of the wastewater under investigation, two treatment schemes were

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studied: a) coagulation–precipitation (C/P) and, b) coagulation– dissolved air flotation (C/DAF). The coagulants used were ferric chloride, alum, and ferrous sulfate. The optimum pH values and coagulant dose were determined for each coagulant. All coagulants were aided with lime. Lime has many positive effects. It adjusts the pH to the optimum value, acts as a coagulant aid, improves sludge settleability and stability. However, it may increase the dry solids content of the sludge by 20–30% [23]. C/P experiments were conducted using a Phipps & Bird standard jar test unit Model 7790-400. Each sample to be coagulated was placed under a state of rapid stirring (267 rpm) (G value= 1000 s), while the coagulant was added slowly to the solution under stirring for 60 s. The speed of mixing was reduced at a regular stepwise manner; covering a range of 50 rpm every 60 s. until the flocculation stage was reached. Stirring was then maintained at 25 rpm (G value= 50 s) for further 20 min for optimum floc formation. Characteristics of the chemically treated effluent were determined after 1.0 h settling. Samples for analysis were taken by means of a suction device allowing the withdrawal of accurate amounts from all jars at the same time. These experiments were repeated seven times to cover variations in wastewater characteristics. C/DAF experiments were performed using a flotation unit, the schematic diagram of which is presented in Fig. 1. The main components of the flotation unit are: air compressor, a pressure retention tank, and a floatation cell. The pressure tank is designed to withstand a pressure up to 10 bar. The pressure within the tank was regulated via a pressure gauge mounted on the exit line. The flotation unit is made of a calibrated plexi-glass column, 66 cm in length and 8.0 cm diameter. The pressurized air/water mixture was released from the retention tank to the flotation cell unit through a valve located at the bottom of the column. Water in the retention tank was saturated with air at a pressure of 4.0 bar. The required amount of air pressurized water was released gradually to the flotation cell. Coagulants were injected using graduated syringes. The treated wastewater samples were then collected for analysis from the tap beneath the floating sludge layer at the top of the flotation cell. The performance of a flotation system can be explained by air to solid (A/S) ratio as in the following equation, which affects particle–bubble collision, particle separation and removal [17]: A = S = 1:3Sa ðfP−1ÞX Where Sa is the air solubility, 18.7 ml/l at 20 °C, f is the air saturation ratio at pressure P (0.8), P is the pressure (gauge pressure +1) and X is the solid content, mg/l. 3. Results and discussion 3.1. Wastewater characteristics Wastewater is produced from production lines and equipments and floor cleaning operations. Main characteristics of wastewater are presented in Table 1. The BOD5/COD ratio averaged 0.46. A significant amount of COD (on average 40%) is in the form of insoluble material. 3.2. Coagulation–precipitation (C/P) process 3.2.1. Use of ferric chloride 3.2.1.1. Effect of pH. For determination of the optimum pH value of ferric chloride, different pH values covering a range from 2.97 to 9.8 were tested. The ferric chloride dose was kept constant at 800 mg/l. Data presented in Fig. 2a shows that there are two optimum pH values, one in the acidic side (4.6) and the other in the alkaline range (8.23). At pH 4.6, the main Fen+ species present in solution is Fe

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Fig. 1. Schematic diagram of dissolved air flotation (DAF) unit.

+ (OH)+ 2 , while at pH around 8.23, it is Fe(OH)3 [14]. Fe(OH)2 can neutralize negatively charged materials like organic substances and suspended particles [23]. On the other hand, Fe(OH)3, a hydrophobic compound, can adsorb contaminants in particulate form by surface interactions, which in some cases can lead to polymeric entities [24].

600 mg/l (Fig. 2b). This will allow coagulant saving and consequently lower sludge production. The effectiveness of coagulation–precipitation (C/P) process using ferric chloride aided with polyelectrolyte (non-ionic polyacrylamide) for the treatment of beverage industrial wastewater was investigated by Amud and Amoo [14]. 73% of COD was removed by the addition of 300 mg/l ferric chloride at pH value of 9 whereas, 91% removal of COD was eliminated with the addition of 25 mg/l polyelectrolyte to 100 mg/l ferric chloride.

3.2.1.2. Effect of ferric chloride dose. The use of different doses of ferric chloride ranging from 200 to 1000 mg/l was examined at the predetermined optimum pH value (8.23). The removal of COD increased with increasing dose of ferric chloride as shown in Fig. 2b. The COD percentage removal increased from 20.3 to 75.8% when the dose of the ferric chloride was increased from 200 to 600 mg/l, respectively. Increasing the dose of the ferric chloride would increase the super saturation of the Fe (OH) 3 which increased the nucleation rate and hence the floc growth rate. As a result, suspension of greater number of flocs was enhanced, and subsequently, removal of larger amounts of COD was achieved, due to the availability of larger surface area on which adsorption of the organic matter took place. On the contrary, low doses of ferric chloride led to the formation of larger but fewer flocs as a result of faster growth rate relative to nucleation rate, which resulted in a smaller surface area on which adsorption of organic matter occurred [14]. The optimum dose of a coagulant is defined as the value above which there is no significant difference in the increase of removal efficiency with a further addition of coagulant or flocculant [23]. Fig. 2b shows that increasing the ferric chloride dose up to 1000 mg/l exerted slight improvement in the coagulation efficiency. Thus the optimum dose of ferric chloride that enhanced COD removal will not exceed

3.2.2. Use of alum 3.2.2.1. Effect of pH. The pH of the wastewater was changed from 3.2 to 9.32 at a fixed alum dose of 1000 mg/l. From the available results presented in Fig. 3a, it can be seen that the optimum pH value is around 5.2. However, the COD removal achieved at pH value of 6.9 was also acceptable (Fig. 3a). Therefore, pH 6.9 was selected as an optimum pH value for alum to avoid pH correction of the wastewater prior to disposal into sewer network. 3.2.2.2. Effect of alum dose. Changing the alum dose from 500 to 1000 mg/l, at a pH value of 6.9 gave the results presented in Fig. 3b. Available data indicates that the COD removal efficiency increased from 37.8 to 72% by increasing the alum dose from 500 to 700 mg/l respectively. The predominant removal mechanism at low doses of alum is adsorption and charge neutralization. However, at high doses of coagulant is sweepfloc coagulation by enmeshment in the aluminum hydroxide precipitate [25]. Further increase of the alum dose from 700 to 1000 mg/l exerted

Table 1 Comparison between the efficiency of different coagulants at optimum operating conditions during C/P process. Parameters

Unit

Raw wastewater

Lime dose pH CODtotal CODsoluble CODparticulate BOD5 total TSS (105 °C) VSS (550 °C) Total-P Oil and grease BOD5/COD ratio

– mgO2/l mgO2/l mgO2/l mgO2/l mg/l mg/l mg/l mg/l

7.46 2276 ± 311 1371 ± 179 905 ± 132 1056 ± 182 406 ± 170 152 ± 14 7.9 ± 2.3 169.7 ± 17 0.46

Sludge analysis Sludge volume (SV) Sludge weight (105 °C) Sludge volume index (SVI)

ml/l g/l ml/gTSS

Ferric chloride (500–700 mg/l) (300–500 mg/l) 8.23 550.3 ± 173 509 ± 160 41.3 ± 13 231.3 ± 171 17.3 ± 13 12 ± 7.8 3.1 ± 4 41 ± 1.7 0.39

122 ± 25.5 1.6 ± 0.87 76.3 ± 28.8

%R

– 75.8 ± 9.7 62.9 ± 14.8 95.4 ± 1.8 78.0 ± 15.8 96 ± 3 92.0 ± 4.7 60.2 ± 40 75.8 ± 3.3 –

Alum (600–800 mg/l ) (120–200 mg/l) 6.9 530 ± 167 434 ± 113 96 ± 57 274 ± 198 26 ± 17 19.6 ± 9 0.9 ± 0.2 40.3 ± 1.5 0.52

133 ± 65 0.9 ± 0.46 147.8 ± 51.8

%R

– 76.7 ± 9.9 68.0 ± 10.6 89.4 ± 8.9 74.1 ± 19.3 93.6 ± 4.7 87.0 ± 5.6 88.1 ± 3.2 76.2 ± 1.9 –

Ferrous sulfate (700–1000 mg/l) (200–300 mg/l) 9.1 511.3 ± 173 472 ± 164 39.3 ± 23 225 ± 169 23 ± 16.5 16.1 ± 13.3 1.4 ± 0.9 40 ± 6.1 0.44

127 ± 31 1.33 ± 0.78 95.5 ± 28.8

%R

– 77.5 ± 9.6 65.6 ± 15.3 95.7 ± 2.6 78.7 ± 15.6 94.3 ± 3.3 89.4 ± 8.1 82.2 ± 7.4 76.4 ± 3.1

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changed from 6.12 to 9.7. It can be seen from Fig. 4a that percentage removal of COD was increased from 19.5 to 77.5% by increasing the pH of the reaction from 6.12 to 9.1 respectively. Further increase of the pH above 9 lead to a reduction in COD removal. Therefore, pH 9.1 is recommended as the optimum pH for this experimental run. Hove et al. [26] found that the average oxidation rate at pH 9 was 9.8 mg (Fe2+)/l/min, while it was only 2.1 mg (Fe2+)/l/min at pH 6.0. 3.2.3.2. Effect of ferrous sulfate dose. To find out the optimum ferrous sulfate dose, different doses ranging from 200 to 1000 mg/l were tested. The pH was adjusted using 260 mg/l lime at the predetermined pH value of 9.1. The effect of coagulant dose on COD reduction is presented in Fig. 4b. Available data indicates that, maximum COD reduction of 71.8% was obtained at 800 mg/l ferrous sulfate. The treatment of textile wastewater with ferrous sulfate, regulating pH in the range of 8.5–9.5 by lime, was proved to be very effective in removing COD (50–60%) from textile wastewater [27]. 3.3. Comparison between the efficiency of the different coagulants at their optimum operating conditions

Fig. 2. a. Determination of optimum pH value at fixed dose of ferric chloride (800 mg/l). b. Determination of optimum dose of ferric chloride at constant pH value of 8.23.

slight improvement in COD removal i.e. by a value of 2.6% (Fig. 3b). Therefore, the optimum dose of alum that enhanced maximum removal of COD was taken as 700 mg/l. A higher dose of alum (3.0 g/l) aided with 1 g/ l lime was used for coagulation–precipitation of cosmetic wastewater industry [5]. Anionic surfactant, COD and BOD5 removal was 53.3, 37.3, and 51.2% respectively. 3.2.3. Use of ferrous sulfate 3.2.3.1. Effect of pH. To determine the optimum pH-value for ferrous sulfate, a fixed dose equivalent to 1000 mg/l was used. The pH was

Fig. 3. a. Determination of optimum pH value at fixed dose of alum (1000 mg/l). b. Determination of optimum dose of alum at constant pH value of 6.9.

The results presented in Table 1 reveal that the three coagulants investigated were found to be more or less similar in their performance. Maximum CODtotal removal obtained by ferric chloride and ferrous sulfate were 75.8±9.7 and 77.5±9.6%, respectively. Corresponding BOD5 total removal values were 78±15.8 and 78.7±15.6%, respectively. However, the optimum dose of ferric chloride was 600 mg/l while that of ferrous sulfate was 850 mg/l to achieve the same removal efficiency. In case of alum, maximum removal efficiency of CODtotal was 76.7±9.9%, whereas, BOD5 total removal was 74±19.3%. Ferric chloride, alum and ferrous sulphate did not show much variation with regard to TSS, oil and grease removal (Table 1). Treatment of an emulsified polymeric wastewater was investigated by Al-Malack et al. [28] using coagulation–precipitation process. The results showed that ferric chloride reduced the COD values from 700 to 280 mg/l at an optimum pH value of 8.5, resulting in 96% removal efficiency. Alum and ferrous achieved a removal efficiency of 85 and 74% of the COD at optimum pH values of 8.5 and 9 respectively. In another study, alum and ferric chloride was used for COD removal from dyeing wastewater at dosage of 4 g/l and 1.5 g/l and pH values of 7 and 10

Fig. 4. a. Determination of optimum pH value at fixed ferrous sulfate dose of (1000 mg/l). b. Determination of optimum dose of ferrous sulfate at constant pH value of 9.1.

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tion of the sludge indicated the formation of small flocs with a compact structure. This is due to the increased of oxidation rate which resulted in high primary nucleation rates and depressed growth of the particles. It is because of the high oxidation rates that the number of particles formed also increased significantly at pH 9.1 and 8.23 compared to pH 6.9 [26]. The other reason for the bigger particles at pH 6.9 is that this is closer to the point of zero charge of alum hydroxide which is around 7.0 [33].

respectively [29]. 90% of COD removal was achieved for both coagulants. Haydar and Aziz [30] investigated the effectiveness of chemically enhanced primary treatment in removing pollutants from tannery wastewater. Alum, ferric chloride and ferrous sulfate were tested. Alum was found to be the suitable coagulant for tannery wastewater at a dose of 200–240 mg/l. COD and TSS removal efficiency was 95 and 98% respectively. The results presented in Table 1 reveal that the coagulation– precipitation process using alum performs better with respect to the total-P removal (88.1±3.2%) as compared to ferric chloride (60.2±40%) and ferrous sulfate (82.2.2±7.4%). The removal mechanism of phosphates using alum could be due to 1) the interaction of phosphates with the soluble aluminum forms (chemical complexation), or with the nonsoluble (charge neutralization) complexes, producing compounds with generic formulae Al(OH)3 −x(PO4) x which can either adsorb onto the positively charged Al (III) hydrolysis species, or may act as cores for the precipitation of Al (III) hydrolysis products. 2) The direct adsorption of phosphates onto aluminum hydrolysis products, mainly referred to precipitation with insoluble Al(OH)3 (sweep flocculation) [31]. According to Yeber et al. [32], the biodegradability of the organic matter present in wastewater can be estimated by the BOD5/COD ratio, and values below 0.5 are considered unsatisfactory. Table 1 show that the BOD5/COD ratio increased with the use of alum as the coagulant, while it was below the limit when ferric chloride and ferrous sulfate was used. Therefore, the treated effluent by alum aided with lime at their optimum operating conditions was found to be more favorable for biological treatment, as compared to the other two coagulants since the BOD5/COD ratio of the treated effluent was 0.52. On the contrary, Aloui et al. [5] found that the chemically treated effluent of cosmetic wastewater using alum aided with lime was not favorable for biological treatment as the BOD5/COD ratio of effluent was within the value of 0.21. Wang et al. [6] found that the pre-treated surfactant wastewater by means of coagulation–flocculation with 600 mg Fe2+/l, the value of BOD5/ COD increased from 0.3 to 0.6. A comparison of the sludge production at the optimum operating conditions of the three coagulants investigated is presented in Table 1. Available data shows that the use of alum at pH 6.9 in the coagulation process provided voluminous sludge but with less solids content (0.9± 0.46 g/l). Ferric chloride at pH 8.23 and ferrous sulfate at pH 9.1 on the other hand, provided less sludge volume but with high solids content as shown in Table 1. Corresponding sludge volume index (SVI) was 76.3 ± 28.8 and 95.5 ± 28.8 ml/gTSS respectively. Furthermore, visual inspec-

3.4. Coagulation–dissolved air flotation (C/DAF) The results of the 2nd run of experiments using coagulation– dissolved air flotation (C/DAF) process are presented in Table 2. The use of alum at its optimum operating conditions achieved removal efficiency of 77.5 ±3.2% for CODtotal; 78.6 ± 0.8% for O&G and 88.6 ± 2.3% for TSS. For ferrous sulfate (700–1000 mg/1), the following percentage removal was obtained: 67.7 ± 3.7% for CODtotal, 72 ± 4.2% for O&G and 84 ± 14% for TSS. When ferric chloride was used, COD, O&G and TSS removal efficiency was found to be 71.6 ± 2.9, 73 ± 5 and 85 ± 7.8%, respectively. The results obtained indicated no significant difference between the three coagulants in terms of BOD5 and VSS removal. However, the use of alum resulted in better total phosphorous removal (88.6 ± 2.3). Similar removal efficiencies of TSS (74%) and COD (77%) was achieved via C/DAF process treating poultry slaughterhouse wastewater using 24 mg/l poly-aluminum chloride (PAC) aided with 1.5 mg/l anionic polymer at pH value of 7.25 [17]. Azbar and Yonar [34] investigated C/DAF for treatment of vegetable oil refining industry wastewater using ferric chloride and alum. At a dose of 250 mg/l, alum achieved percentage removal of 88% for COD, 72% for O&G and 80% for TSS. At the same dose, ferric chloride achieved removal efficiency of COD (84%), O&G (67%) and TSS (80%). One of the interesting features of the application of DAF is its ability to produce a relatively concentrated sludge. This is the result of the fact that the float layer, which is formed during the flotation process, is partially pushed above the water surface by the continuous flow of air bubbles. The float layer is drained resulting in an increase of the dry solids concentration. Consequently, less amount of sludge is produced with high solids content. The characteristics of the sludge produced during the C/DAF process are highly dependent on the specific coagulant used and on the operating conditions [35]. The results of the present study are shown in Table 2. The combined use of alum and lime produced the lowest sludge volume (85 ± 54 ml/l) with the lowest solids content (0.59 ± 0.16 g/l). However, SVI was higher (144 ± 80 ml/gTSS) than that of the sludge produced by the

Table 2 Comparison between the efficiency of different coagulants at optimum operating conditions during C/DAF process. Parameters Lime dose A/S ratio pH CODtotal BOD5 total TSS (105 °C) VSS (550 °C) Total-P Oil and grease BOD5/COD ratio Sludge analysis Sludge volume (SV) Sludge weight (105 °C) Sludge volume index (SVI)

Unit

Raw wastewater

ml air/mgTSS mgO2/l mgO2/l mg/l mg/l mg/l mg/l

7.46 2276 ± 311 1056 ± 182 406 ± 170 152 ± 14 7.9 ± 2.3 169.7 ± 17 0.46

Ferric chloride (500–700 mg/l) (300–500 mg/l) (0.06–0.14) 8.41 ± 0.15 645.7 ± 53 244 ± 160 61.3 ± 57 37 ± 36 1.6 ± 0.7 45.6 ± 10 0.38

%R

– 71.6 ± 2.9 76.9 ± 14.7 85 ± 7.8 75.7 ± 21.3 77 ± 1.2 73 ± 5 –

Alum (600–800 mg/l ) (120–200 mg/l) 0.06244–0.1494 6.93 ± 0.2 512 ± 154 251 ± 154 46 ± 35 40.3 ± 29 0.8 ± 0.3 36.3 ± 4 0.49

%R

– 77.5 ± 3.2 76.2 ± 14.8 88.7 ± 8.7 73.5 ± 18.4 88.6 ± 2.3 78.59 ± 0.8 –

Ferrous sulfate (700–1000 mg/l) (200–300 mg/l) 0.06244–0.1494 8.9 ± 0.2 735 ± 64 241 ± 225 65 ± 58 53 ± 49 1.7 ± 0.8 47.5 ± 0.7 0.32

ml/l

122 ± 25.5

85 ± 54

122 ± 30

g/l

1.6 ± 0.87

0.59 ± 0.16

1 ± 0.14

ml/gTSS

76.3 ± 28.8

144 ± 80

122 ± 13.4

%R

– 67.7 ± 3.7 77.1 ± 25 84 ± 14 65.1 ± 31 75.7 ± 12 72 ± 4.2 –

F. El-Gohary et al. / Desalination 252 (2010) 106–112 Table 3 Annual cost estimation (L.E./year) for treatment of 90 m3/d wastewater. Item

Coagulation–precipitation (C/P) scheme (I)

Construction cost (L.E)⁎ Civil work Electromechanical Chemicals cost (L.E)⁎ Ferric chloride Alum Ferrous sulfate Power cost (L.E)⁎

Coagulation–dissolved air flotation (C/DAF) scheme (II)

7000 4000

3000 5000

11,858.4 15,115.8 46,994.4 153.72

11,858.4 15,115.8 46,994.4 219.6

Sludge treatment and disposal cost (L.E)⁎ Ferric chloride 106,140 Alum 58,560 Ferrous sulfate 87,840 Total annual cost 73,829.52 (L.E)⁎

74,298 40,992 61,488 56,327.4

Total annual cost (L.E)⁎: calculated based on alum; (L.E)⁎: Egyptian pound.

other two coagulants. In general, SVI value of 100 ml/gTSS or less is an indication of good quality sludge [36]. 3.5. Cost evaluation Chemicals may form the largest individual share of operating costs in chemical treatment processes [37]. The proper determination of type and dosage of chemicals will not only improve the process, but will also influence the running cost. A cost evaluation of the initial investment and the running costs using the different coagulants at their optimum operating conditions are presented in Table 3. From the results of the comparison, it is obvious that the initial investment for C/P is higher by 27.3% than C/DAF. Also, the running cost is higher by 23.7%. Therefore, chemical coagulation followed by dissolved air flotation (C/DAF) is more economical for the treatment of the wastewater under investigation. It is also worth mentioning that the land area required for C/DAF is less by 30% compared to C/P. 4. Conclusions • Chemical coagulation precipitation (C/P) process using ferric chloride (500–700 mg/l), ferrous sulfate (700–1000 mg/l), and alum (600– 800 mg/l) achieved CODtotal removal efficiencies of 75.8± 9.7, 77.5 ± 9.6 and 76.7 ± 9.9%, respectively. Corresponding BOD5 total removal values were 78± 15.8, 78.7 ± 15.6 and 74.1 ± 19.3%, respectively. However, the treated effluent by alum was found to be more favorable for biological treatment, as compared to the other two coagulants since the BOD5/COD ratio of effluent was 0.52. • Alum produced a voluminous sludge, but with the least solids content. Ferric chloride produced compact sludge with a good settleability as reflected by the low sludge volume index (SVI) of 76.3 ± 28.8 ml/gTSS. • Coagulation-dissolved air flotation (C/DAF) process showed that alum produced higher COD removal (77.5 ± 3.2%) as compared to ferric chloride (71.6 ± 2.9%) and ferrous sulfate (67.7 ± 3.7%). Also, the use of alum produced less sludge volume (85 ± 54 ml/l) with almost half solids content (0.59 ± 0.16 g/l) of that produced when ferric chloride or ferrous sulfate were used. • The calculated investment for C/P process is higher by 27.3% than C/ DAF. Also, the running cost is higher by 23.7%. Therefore, chemical coagulation followed by dissolved air flotation (C/DAF) is more economical for the treatment of the wastewater under investigation. It is also worth mentioning that the land area required for (C/ DAF) is less by 30% compared to C/P process.

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