Development of a novel membrane bioreactor for cost-effective wastewater treatment and microalgae harvesting

The rapid depletion of fossil fuels has raised increasing attention worldwide, and initiated intensive research for sustainable alternatives for energy production. Among these, biodiesel from microalgae has appeared as one of the most promising candidate due to their ability to accumulate large amount of lipids. Indeed, microalgae can achieve a productivity up to 25 higher than other crop sources without need of cultivatable soil, therefore without competing with food production. In the meantime, microalgae have also shown promising results for the treatment of various kind of wastewaters. However, the cultivation of microalgae for energy production suffers from the large costs of harvesting and dewatering of biomass, prior to lipid extraction and biofuel production, which accounts for up to 50% of operating costs. Therefore, the search for cost-effective methods of harvesting and dewatering of microalgae biomass has become necessary to optimize their usage. This study investigates forward osmosis (FO) for the dewatering of microalgae biomass and its implementation within a photobioreactor used for wastewater treatment. FO is a cost-effective filtration process based on the differences of osmotic pressure across a semi-permeable membrane. The use of FO for microalgae dewatering is of high interest, given the high fouling ability of microalgae biomass and the low fouling promises of FO. First, the feasibility of using FO for microalgae dewatering was assessed, focusing on better understanding the fouling mechanisms involved. The filtration performances have been investigated under various operating parameters. It has been found that when Ca2+-containing draw solutions were used, microalgae responded to the back diffusion of calcium ions by an extensive excretion of carbohydrates, accelerating the formation of algal flocs, thus enhancing the rate and extent of flux decline and reducing the algae dewatering efficiency. However, most of the fouling was reversible by simple hydraulic flushing. In addition, substantial adsorption of algal biomass was observed on the feed spacer. Also, Scenedesmus obliquus and Chlamydomonas reinhardtii, with fructose and abundant glucose and mannose in its cell wall, showed strong response to the back diffusion of calcium ions which encouraged S. obliquus to produce more extracellular carbohydrates and formed a stable gel network between algal biomass and extracellular carbohydrates, leading to algae aggregation and severe loss in both water flux and algae biomass during FO dewatering with Ca2+-containing draw solution. Among the species investigated, Chlorella vulgaris without fructose was the most suitable microalgae species to be dewatered by FO with a high algae recovery and negligible flux decline regardless of which draw solution was applied. These findings improve mechanical understanding of FO membrane fouling by microalgae; have significant implications for the algae species selection; and are critical for the development and optimization of FO dewatering processes. Finally, the implementation of FO dewatering with continuous microalgae biomass production and synthetic wastewater treatment was investigated. Two systems (External FO ; Immersed FO) have been studied and compared in order to provide insights on the advantages and disadvantages of each system. Constant parameters have been set identical for both systems: operation time; photobioreactor; hydraulic retention time; biomass production; FO permeate volume. The results reveals that the wastewater treatment efficiency (nutrients removal), as well as the production of biomass were greater with the immersed system due to a greater microalgae growth. However, these may not be sustainable in a long term operation of the immersed system. The external FO system was found better in terms of salinity build-up and FO dewatering performances. Overall, an external FO dewatering is recommended due to its better flexibility and sustainability

Microalgae (Scenedesmus obliquus) dewatering using forward osmosis membrane: Influence of draw solution chemistry

Forward osmosis (FO) is gaining increasing interests for its potential applications in biofuel generation. In this study, bench-scale experiments were conducted to investigate the FO performance for microalgae dewatering which is one of the technical challenges in algal biofuel production. The filtration performance was assessed by analyzing permeate water flux and algal biomass concentration in the feed solution. Compared to the active layer facing draw solution (AL-DS) orientation (> 45% flux reduction), active layer facing feed solution (AL-FS) was more efficient (< 15% flux reduction) due to the lower membrane fouling and higher cleaning efficiency (> 90% water flux recovery after deionized water flushing). In the AL-FS orientation, FO performance strongly depended on the draw solution chemistry with NaCl exhibiting the best results. When Ca2 +-containing solution was used as draw solution, microalgae responded to the back diffusion of calcium ions by an extensive excretion of carbohydrates, accelerating the formation of algal flocs, thus enhancing the rate and extent of flux decline and reducing the algae dewatering efficiency. However, most of the flux decline was reversible by simple hydraulic flushing without any chemical cleaning reagents and air scouring. In addition, substantial adsorption of algal biomass was observed on feed spacer. This study has the implication for Scenedesmus obliquus dewatering using FO technology. Selection of AL-FS orientation, Ca2 +-free draw solutions and prevention of microalgae adhesion onto feed spacer may significantly improve the efficiency and productivity of the dewatering process

The Influence of Forward Osmosis Module Configuration on Nutrients Removal and Microalgae Harvesting in Osmotic Photobioreactor

Microalgae have attracted great interest recently due to their potential for nutrients removal from wastewater, renewable biodiesel production and bioactive compounds extraction. However, one major challenge in microalgal bioremediation and the algal biofuel process is the high energy cost of separating microalgae from water. Our previous studies demonstrated that forward osmosis (FO) is a promising technology for microalgae harvesting and dewatering due to its low energy consumption and easy fouling control. In the present study, two FO module configurations (side-stream and submerged) were integrated with microalgae (C. vulgaris) photobioreactor (PBR) in order to evaluate the system performance, including nutrients removal, algae harvesting efficiency and membrane fouling. After 7 days of operation, both systems showed effective nutrients removal. A total of 92.9%, 100% and 98.7% of PO4-P, NH3-N and TN were removed in the PBR integrated with the submerged FO module, and 82%, 96% and 94.8% of PO4-P, NH3-N and TN were removed in the PBR integrated with the side-stream FO module. The better nutrients removal efficiency is attributed to the greater algae biomass in the submerged FO-PBR where in situ biomass dewatering was conducted. The side-stream FO module showed more severe permeate flux loss and biomass loss (less dewatering efficiency) due to algae deposition onto the membrane. This is likely caused by the higher initial water flux associated with the side-stream FO configuration, resulting in more foulants being transported to the membrane surface. However, the side-stream FO module showed better fouling mitigation by simple hydraulic flushing than the submerged FO module, which is not convenient for conducting cleaning without interrupting the PBR operation. Taken together, our results suggest that side-stream FO configuration may provide a viable way to integrate with PBR for a microalgae-based treatment. The present work provides novel insights into the efficient operation of a FO-PBR for more sustainable wastewater treatment and effective microalgae harvesting

Microalgal biomass dewatering using forward osmosis membrane: influence of microalgae species and carbohydrates composition

The potential application of forward osmosis (FO) in microalgae dewatering requires an improved understanding of the factors that control membrane fouling which can reduce dewatering performance in terms of water flux through membrane and algae recovery. The aim of this study was to elucidate the influence of algae cell wall carbohydrate composition on the FO dewatering performance using three types of draw solutions (sea salts, MgCl2 and CaCl2). Experimental results suggest that the interaction between microalgae and back diffused draw solutes plays a key role. Scenedesmus obliquus with fructose and abundant glucose and mannose in its cell wall showed strong response to the back diffusion of calcium ions which encouraged S. obliquus to produce more extracellular carbohydrates and formed a stable gel network between algal biomass and extracellular carbohydrates, leading to algae aggregation and severe loss in both water flux and algae biomass during FO dewatering with Ca2 +-containing draw solution. Chlamydomonas reinhardtii without fructose but great galactose showed a similar response to the calcium back diffusion but to a lower extent. Both S. obliquus and C. reinhardtii did not cause obvious membrane fouling but dramatic algae biomass loss at the end of FO filtration with MgCl2 draw solution due to their interaction with back diffused Mg2 + ions which led to the deposition of algae flocs onto membrane surface and/or feed spacer. Chlorella vulgaris without fructose was the most suitable microalgae species to be dewatered by FO with algae recovery over 81% and negligible flux decline regardless of which draw solution was applied. The findings improve mechanical understanding of FO membrane fouling by microalgae; have significant implications for the algae species selection; and are critical for the development and optimization of FO dewatering processes

Imparting antimicrobial and anti-adhesive properties to polysulfone membranes through modification with silver nanoparticles and polyelectrolyte multilayers

The antimicrobial and bacterial anti-adhesive properties of polysulfone (PSU) membranes modified with silver nanoparticles (AgNPs) and polyelectrolyte multilayers (PEMs) composed of poly(allylamine hydrochloride) and poly(acrylic acid) were investigated. The membranes’ antimicrobial properties were evaluated using a colony forming unit (CFU) enumeration method, while the anti-adhesive properties of the membranes were examined using a direct microscopy observation membrane filtration system. The AgNP mass loading required for the inhibition of bacterial growth on the AgNP/PEM-modified membranes was significantly lower than the AgNP loadings reported in other studies for membranes with the nanoparticles dispersed within the membrane matrix. The immobilization of AgNPs on the membrane surface maximized the opportunities for bacteria-nanoparticle contact, which allowed for effective bacteria inactivation. Furthermore, in comparison to unmodified PSU membranes, the bacterial deposition kinetics on all the modified membranes were reduced by ca. 50% and the bacterial removal efficiencies were significantly increased from close to 0% to as high as over 90%. Three-cycle filtration and rinsing experiments were also performed to evaluate the effectiveness of the surface modification over an extended time period of use

Human urine as a forward osmosis draw solution for the application of microalgae dewatering

Human urine is a unique solution that has the right composition to constitute both a severe environmental threat and a rich source of nitrogen and phosphorous. In fact, between 4-9% of urine mass consists of ions, such as K+, Cl-, Na+ or NH4+. Because of its high ionic strength, urine osmotic pressure can reach values of up to 2000 kPa. With this in mind, this work aimed to study the effectiveness of real urine as a novel draw solution for forward osmosis. Water flux, reverse nitrogen flux and membrane fouling were investigated using fresh or hydrolysed urine. Water flux as high as 16.7 +/- 1.1 L m(-2) h(-1) was recorded using real hydrolysed urine. Additionally, no support layer membrane fouling was noticed in over 20 h of experimentation. Urine was also employed to dewater a Chlorella vulgaris culture. A fourfold increase in algal concentration was achieved while having an average flux of 14.1 L m(-2) h(-1). During the algae dewatering, a flux decrease of about 19% was noticed; this was mainly due to a thin layer of algal deposition on the active side of the membrane. Overall, human urine was found to be an effective draw solution for forward osmosis