Environ. Eng. Res. 2015
Research Paper http://dx.doi.org/10.4491/eer.2015.005 pISSN 1226-1025 eISSN 2005-968X In Press, Uncorrected Proof
Forward Osmosis Membrane Filtration for Microalgae Harvesting Cultivated in Sewage Effluent Su-bin Kim1, Sachin Paudel2, Gyu Tae Seo2† 1 2
Department of Eco-friendly offshore plant FEED engineering. Changwon National University, Changwon 641-773, Korea Department of Environmental Engineering, Changwon National University, Changwon 641-773, Korea
Abstract The purpose of this study is to evaluate the performance of forward osmosis system for harvesting microalgae cultivated in secondary sewage effluent. Microalgae species used in this study were chlorella sp. ADE4. The drawing agents used for forward osmosis system were seawater and concentrate of sea water reverse osmosis (SWRO) system. Chlorella sp. ADE4 cultured in secondary sewage effluent illustrated moderate efficiency in removal of TN (68%) and superior performance in TP removal (99%). Comparison of seawater and SWRO concentrate as drawing agent were made in FO membrane separation of the microalgae. The result from this study depicts that SWRO concentrate is strong drawing agent in FO membrane system providing an average dewatering rate of 4.8 L/m2 h compared to seawater with average dewatering of 2.9 L/m2 h. Results obtained from this study indicated that FO system could be viable option for harvesting the microalgae for further biodiesel production. SWRO concentrate as a drawing agent could be very important finding in field of membrane technology for disposal of SWRO concentrate.
Keywords: Draw solution, Forward osmosis membrane, Microalgae, Seawater concentrate, Sewage effluent
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
Copyright © 2014 Korean Society of Environmental Engineers
Received January 11, 2015 Accepted February 10, 2015 †
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1. Introduction
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Due to rapid population growth, urbanization and industrialization world is facing problems
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associated with shortage of fresh water and energy to run people’s daily life. As population
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rises, the demand of freshwater gets elevated and so is the generation of wastewater and crisis
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of energy. Wastewater treatment plant requires more energy to treat this higher generation of
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wastewater. Treatments plants in existence are not 100% effective in removing nutrients at
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desirable limits for disposal. The ultimate fate of these pollutant nutrients in effluent is either
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in sea, land or any other water bodies which leads to increase in pollutant load and
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eutrophication in water bodies. Microalgae is being an acceptable field of interest from last
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five decades because of their possible applications in biofuels, cosmetics, pharmaceuticals,
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nutrition and food additives, aquaculture etc. Moreover, it is also potent medium for further
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removal of polluting nutrients from wastewater treatment plant effluent [1, 2, 3]. The high
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cost of microalgae cultivation is one of the major obstacles for commercial application of
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microalgae and production of biodiesel.
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Microalgae have potential of treating sewage by up taking nitrogen and phosphorus to get
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bloomed [4]. Difficulties in cultivating microalgae in commercial scale could be solved at
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some extent by using it for further removal of nutrient from sewage but there is still a problem
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for harvesting it for higher biomass concentration. Microalgae harvesting methods such as
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chemical coagulation, centrifuge, and other physical means are not economically viable in
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terms of energy and cost. Chemical coagulation contaminates the microalgae biomass and is
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not suitable for biodiesel production.
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microalgae harvesting seems economically difficult in terms of energy and membrane cost as
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well. One potential solution for harvesting issue could be the use of FO membrane (Forward
Membrane systems such as MF, UF and NF for
１
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Osmosis). FO membrane system which consumes less energy and does not require applied
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mechanical pressure has potential application including seawater and brackish water
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desalination and wastewater treatment biomass concentration, and food processing. [5, 6, 7, 8]
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FO is an osmotically driven membrane process that takes advantage of the osmotic pressure
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gradient to drive water across the semipermeable membrane from the feed solution (low
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osmotic pressure) side to the draw solution (high osmotic pressure) side [9]. The
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concentration difference across facilitates water transport by diffusion from low osmotic
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pressure impaired feed to the high osmotic pressure draw solution, while rejecting ions and
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molecules [10,7]. FO differs from traditional membrane separation technology because unlike
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MF, UF, NF, and RO a little or no hydraulic pressure is required and fouling propensity is also
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low [10].
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The availability of a suitable draw solution is crucial for advancing FO technology. An
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appropriate draw solution not only promotes the efficiency of the FO process, but also saves
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cost of the subsequent steps in recovering and replenishing the draw solute [11]. In addition to
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have minimal toxicity and low cost, an ideal draw solution needs to fulfill the following
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requirements. First, it should be able to generate a high osmotic pressure. As the osmotic
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pressure difference between the draw solution and feed solution across the membrane is the
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driving force for the FO process, the osmotic pressure of a draw solution must be higher than
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that of the feed solution to ensure a positive permeate flux [12]. Ideal drawing agent for FO
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system is problematic and economically ineffective. SWRO concentrate could list its name as
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a drawing agent for FO system due to its high concentration. The rationale behind selection of
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seawater as drawing agent is, it consists of various chemical ions at very high concentration
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compared to sewage effluent which ultimately creates higher osmotic gradient when used as ２
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drawing agent for FO system. Seawater has high solubility and is relatively easy to
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reconcentrate to high concentrations using conventional desalination process without risk of
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scaling [13]. SWRO concentrate is so highly concentrated with salt ions and elements of
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seawater which is being problematic for direct disposal to marine system. Engineers and
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researchers are working for better option for management of SWRO concentrate [14]. One
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solution for management of SWRO concentrate is to use it as a drawing agent in FO system
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as it has high concentration and possibly has higher osmotic efficiency.
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The objective of this study is to evaluate the potential of forward osmosis membrane process
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for harvesting microalgae for further biodiesel production. Microalgae performance in
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removal of nutrient from sewage was studied. Efficiency of seawater and SWRO concentrate
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as drawing agent was assessed.
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2. Materials and Methods
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2.1. Microalgae Cultivation
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Species of microalgae used in the experiment was provided by Pusan National University. It
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was cultivated in BG-11 medium as stock solution. The chlorella sp. ADE4 was inoculated to
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make the initial microalgae concentration of 0.32 g/L in 2L Erlenmeyer flask containing
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secondary sewage effluent. Erlenmeyer flasks were placed on a shaker for mixing at 120 rpm
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and with illumination at light intensity of 50 µmol/m2.s. The growth rate of microalgae was
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calculated using equation provided below.
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Growth rate(𝑔𝑔/𝑑𝑑) =
ln 𝑊𝑊2 − ln𝑊𝑊1 𝑡𝑡2 − 𝑡𝑡1
(1)
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Where, W1 and W2 represent dry weight or cell number at time t1 and t2, respectively.
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The secondary effluent was collected from local sewage treatment plant in Jinhae, South
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Korea. Chemical characteristics of the secondary effluent are shown in table 1. The average
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concentration of TN and TP was found to 19.9 and 0.15 mg/L, respectively.
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Table 1. Characteristic of secondary effluent Constituent
Average value
TN (mg/L)
19.9
TP (mg/L)
0.15
pH
7.1
Temperature (°C)
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2.2. FO Membrane
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Flat sheet membranes specifically developed for FO were used in this study. The membranes
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were manufactured by HTI (Hydration Technologies Inc., Albany, OR, USA). Salient features
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and properties of this membrane are repeatedly reported in previous studies [7, 15, 16].
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Membrane in this study was made up of Cellulose Triacetate (CTA) with embedded polyester
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screen mesh for mechanical support. HTI membranes are especially designed to minimize
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internal concentration polarization, so are thinner (
