Development of anaerobic osmotic membrane bioreactor for low-strength wastewater treatment at mesophilic condition
Graphical abstract
Introduction
Membrane bioreactor (MBR) combines activated sludge process and membrane filtration for biomass retention into one single integrated system [1], [2]. In a conventional MBR, a porous microfiltration (MF) or ultrafiltration (UF) membrane is used to retain suspended solids in bioreactor [3]. Where waste water reuse is targeted, further treatment such as nanofiltration (NF), reverse osmosis (RO), or advanced oxidation may be necessary after the MBR to further improve the effluent water quality [4], [5]. Compact space usage and consistent effluent quality make MBR technology superior to other wastewater treatment options [1], [6].
Anaerobic membrane bioreactor (AnMBR) integrates anaerobic biological treatment process with membrane filtration [3]. Merits of AnMBR include lower sludge yield and energy requirement achieved by eliminating the need for aeration and producing energy in the form of methane [3], [7], [8]. AnMBR has generally been used to treat high-strength wastewaters especially industrial wastewaters. There are some recent advances demonstrating the technical feasibility of AnMBR systems for treating low-strength wastewaters [7], [9]; however the process involves more difficulties. The efficiency of AnMBR for treating low strength domestic wastewater is typically limited due to its dilute organic loading and the slow growth rate of methanogens [10], [11], [12]. Moreover, due to the limitation of membrane rejection, soluble organics and trace organic pollutants will end up in the treated effluent inevitably.
In recent years, there is growing research interest in applying high-retention membranes into bioreactors [13], [14], [15]. Smaller size contaminants such as hydrolyzed organic matter could be effectively retained by membrane, prolonging their residence time in reactor and potentially leading to an improved biodegradation efficiency and effluent quality. One highlight among those high-retention MBRs is osmotic membrane bioreactor (OMBR) combining forward osmosis (FO) process with MBR [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. FO is a membrane separation process in which water flows from a low-osmotic-pressure feed solution (FS) to a high-osmotic-pressure draw solution (DS) across a semi-permeable membrane [29], [30]. Where seawater is used as DS in an open loop to avoid the need for DS regeneration, FO requires no additional energy input to drive the filtration process compared to conventional pressure-driven separation processes such as reverse osmosis (RO) and nanofiltration (NF). Previous studies on OMBR have generally focused on wastewater treatment under aerobic conditions [31], and very limited attention has been paid to the use of OMBRs for anaerobic treatment [32], [33]. Utilizing FO membrane in the anaerobic bioreactor to retain and concentrate soluble organics, a novel anaerobic osmotic membrane bioreactor (AnOMBR) is expected to achieve high treatment efficiency of dilutive domestic wastewater, high effluent quality, low energy demand and high methane yield in one integrated system.
In the current study, feasibility of the AnOMBR for treating low-strength wastewater at mesophilic condition was evaluated. Specific objectives of the study include: (1) to study the FO membrane performances including flux level, draw solute back transport and nutrient removal rate; (2) to evaluate the efficiency of bioprocess including sludge bioactivity and biogas production rate of the system.
Section snippets
Membranes
A cellulose triacetate (CTA) FO membrane produced by Hydration Technology Inc (HTI, Albany, OR) was used in the current study. The properties of membrane and the detailed characterization methods had been reported in our previous studies [34], [35], [36]. In brief, the CTA membrane has a water permeability of 2.8×10−12 m/s Pa, salt permeability of 17.5×10−8 m/s and rejection to sodium chloride (NaCl) of 89.5%. The membrane surface is negative charged with zeta potential of −2.1±0.3 mV at pH 7.
Synthetic wastewater
The
Flux performance, salt accumulation and fouling
Fig. 2(a) shows the mixed liquor conductivity in the reactor and the flux performance of the CTA membrane in reactor versus time is plotted in Fig. 2(b). Both the flux and conductivity show good repetition comparing among different cycles. In each cycle, the flux decreased from a nearly identical initial flux of around 10 L/m2/h to a final flux of around 3 L/m2/h. Correspondingly, the conductivity increased from 1.1 mS/cm to around 20 mS/cm over time. The increase of conductivity is due to
Implications and perspectives
The AnOMBR system enjoys many advantages. A dense FO membrane can be used to retain organic waste (including sCOD) in dilute wastewater and to prolong their retention time in the reactor. This can potentially enable efficient anaerobic treatment of dilute wastewater (e.g., municipal wastewater). Although it is not investigated in the current study, the dense FO membrane is presumably able to retain some trace organic pollutants [62], [63], [64], [65], which allows greater exposure time for
Conclusions
A novel anaerobic osmotic membrane bioreactor (AnOMBR) was developed and its feasibility for treating dilute wastewater was evaluated. The bioreactor was operated for a period of 120 days at SRT of 90 days at mesophilic conditions:
Flux through membrane gradually declined due to both feed conductivity build-up and membrane fouling, although the former played a more dominant role.
The AnOMBR showed good and stable sCOD removal and nearly complete total phosphorous removal. The FO membrane had
Acknowledgement
The authors thank the Environment and Water Industry Program Office under the National Research Foundation of Singapore for the scholarship provided to Y. Gu. The work is financially supported by the Singapore Ministry of Education (grant no. MOE2011-T2-2-035, ARC 3/12). The authors also thank all the staff in Environmental Lab of Nanyang Technological University for continuous technical supports; Hydration Technologies Innovations for the provision of the FO membranes; Dr. Zhang Jinsong for
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