Impact of Integration of FO Membranes into a Granular Biomass AnMBR for Water Reuse

Abstract

The granular sludge based anaerobic membrane bioreactor (G-AnMBR) has gained emphasis in the last decade by combining AnMBR advantages (high quality permeate and biogas production towards energy positive treatment) and benefits of granular biomass (boosted biological activity and reduced membrane fouling). With the aim to further reduce energy costs, produce higher quality effluent for water reuse applications and improve system efficiency, a forward osmosis (FO) system was integrated into a 17 L G-AnMBR pilot. Plate and frame microfiltration modules were step by step replaced by submerged FO ones, synthetic wastewater was used as feed (chemical oxygen demand (COD) content 500 mg/L), with hydraulic retention time of 10 h and operated at 25 °C. The system was fed with granular biomass and after the acclimation period, operated neither with gas sparging nor relaxation at around 5 L.m^-2.h^-1 permeation flux during at least 10 days for each tested configuration. Process stability, impact of salinity on biomass, the produced water quality and organic matter removal efficiency were assessed and compared for the system working with 100% microfiltration (MF), 70% MF/30% FO, 50% MF/50% FO and 10% MF/90% FO, respectively. Increasing the FO share in the reactor led to salinity increase and to enhanced fouling propensity probably due to salinity shock on the active biomass, releasing extracellular polymeric substances (EPS) in the mixed liquor. However, above 90% COD degradation was observed for all configurations with a remaining COD content below 50 mg/L and below the detection limit for MF and FO permeates, respectively. FO membranes also proved to be less prone to fouling in comparison with MF ones. Complete salt mass balance demonstrated that major salinity increase in the reactor was due to reverse salt passage from the draw solution but also that salts from the feed solution could migrate to the draw solution. While FO membranes allow for full rejection and very high permeate purity, operation of G-AnMBR with FO membranes only is not recommended since MF presence acts as a purge and allows for reactor salinity stabilization.

Keywords: anaerobic membrane bioreactor; forward osmosis; granular biomass; membrane fouling.

Figure 1

Experimental setup of the FO-G-AnMBR pilot.

Figure 2

% COD removal (bars) and MF permeate COD concentration (black circles) for every MF/FO ratio.

Figure 3

Respective impact of RSD and feed salts concentration on conductivity increase in the G-AnOMBR reactor for each MF/FO ratio.

Figure 4

Representation of ion transfer during (a) 70% MF and (b) 40–60% MF steps. Rr (orange), Rt (yellow) and ΔDS (green) represent the measured concentration, the theoretical concentration in the reactor and the increase of concentration in the DS, respectively.

Figure 5

TMP increase during MF operation with various FO/MF extraction ratios and operation at constant flux (5 L.m⁻².h⁻¹).

Figure 6

Daily average of FO permeation flux with 70%, 40–60%, and 10–20% of MF.

Figure 7

(a) Days of operation before cleaning and (b) total solid (TS) fouling rate (in g TS/module/day) attached to the membrane surface for MF and FO modules for various MF/FO operating ratios.

Figure 8

(a) Protein (PN) and polysaccharide (PS) contents and (b) 3DEEM volume of fluorescence normalized for the fouling layer reported as function of the TS.