Exploring Submerged Forward Osmosis for Water Recovery and Pre-Concentration of Wastewater before Anaerobic Digestion: A Pilot Scale Study

Abstract

Applying forward osmosis directly on raw municipal wastewater is of high interest for the simultaneous production of a high quality permeate for water reuse and pre-concentrating wastewater for anaerobic digestion. This pilot scale study investigates, for the first time, the feasibility of concentrating real raw municipal wastewater using a submerged plate and frame forward osmosis module (0.34 m2) to reach 70% water recovery. Membrane performance, fouling behavior, and effective concentration of wastewater compounds were examined. Two different draw solutions (NaCl and MgCl2), operating either with constant draw concentration or in batch with draw dilution over time, were evaluated. Impact of gas sparging on fouling and external concentration polarization was also assessed. Water fluxes up to 15 L m−2 h−1 were obtained with clean water and 35 g NaCl/L as feed and draw solution, respectively. When using real wastewater, submerged forward osmosis proved to be resilient to clogging, demonstrating its suitability for application on municipal or other complex wastewater; operating with 11.7 g NaCl/L constant draw solution, water and reverse salt fluxes up to 5.1 ± 1.0 L m−2 h−1 and 4.8 ± 2.6 g m−2 h−1 were observed, respectively. Positively, total and soluble chemical oxygen demand concentration factors of 2.47 ± 0.15 and 1.86 ± 0.08, respectively, were achieved, making wastewater more suitable for anaerobic treatment.

Recent Progresses of Forward Osmosis Membranes Formulation and Design for Wastewater Treatment

“…The resultant by-products may become more severe threats to both organisms and environment as possess higher stability and probably more poisonous. Currently, the A pilot scale submerged plate and frame FO module has been applied for the pre-concentration of real raw municipal wastewater prior to anaerobic digestion [119]. The pure water flux of 15 L m −2 h −1 was obtained with NaCl as feed and draw solution, respectively.…”

“…It was found that after four concentration cycles, the surface grafted PAMAM dendrimer was capable of sustaining the high NH4 + -N, the improved antifouling capacity allowed effective restoration the NH4 + -N retention rate to approximately its initial value, after a simple physical cleaning. A pilot scale submerged plate and frame FO module has been applied for the pre-concentration of real raw municipal wastewater prior to anaerobic digestion [119]. The pure water flux of 15 L m −2 h −1 was obtained with NaCl as feed and draw solution, respectively.…”

Forward Osmosis as Concentration Process: Review of Opportunities and Challenges

“…Ansari et al demonstrated the feasibility to concentrate, by 10-fold, WW, and to reach a 1000 mg·L −1 COD concentration, which is suitable for downstream anaerobic digestion [ 127 ]. Moving to a pilot scale, water concentration factors (WCF) of 5 and 2.5 were achieved with a spiral wound membrane and submerged plate and frame (P&F) TFC modules, respectively [ 128 , 129 ]. One of the limitations of raw WW concentration is the incomplete and heterogeneous recovery of nutrients (N, P, COD).…”

“…However, initial experiments using FO on primary treated (screened) WW demonstrated that the accumulated fouling layer was loose and easily reversible [ 111 ]. Additionally, the concentration of raw sewage using the submerged FO system proved to be promising with operation without clogging and no major fouling issue [ 128 ]. Another limitation of FO process concentration can be the toxicity of some compounds for downstream treatments—i.e., anaerobic digestion inhibitors, such as SO 4 2− , NH 3 , and salts.…”

“…Using the TFC membrane and NaCl as draw, Ferrari et al observed a very strong effect of cations/anions selectivity. Most cations present in the WW were poorly rejected (NH 4 + , K + , Na + , Ca 2+ ), while anions (SO 4 2− , PO 4 3− ) were effectively concentrated [ 128 ]. Additionally, bidirectional solute flux plays a role [ 226 , 228 , 229 ].…”