Bioelectrochemically assisted osmotic membrane bioreactor with reusable polyelectrolyte draw solutes

Highlights

• PAA-Na can serve as an effective DS to minimize RSF in BEA-OMBR.

• BES can drive accumulated cations from the feed/anode into the cathode.

• PAA-Na DS can enhance water recovery and improve treatment performance.

• A dynamic balance is observed between PAA biodegradation and PAA residue.

• The recovered PAA-Na DS can achieve water extraction in multiple cycles.

Abstract

The aim of this work was to study reverse solute flux (RSF) from osmotic membrane bioreactor (OMBR) and consequent solute buildup in the feed side. A polyelectrolyte (PAA-Na) served as a draw solute (DS) to minimize RSF in OMBRs. In addition, a bioelectrochemical system (BES) was employed to drive accumulated cations from the feed/anode side into the cathode compartment, subsequently achieving PAA-Na DS recovery with the aid of high catholyte pH. Compared to the 1 M NH₄HCO₃ DS, the 0.48 g mL⁻¹ PAA-Na DS produced consistently stable water flux, enhanced water recovery and increased ammonium removal efficiency. Due to a dynamic balance between PAA removal and continuing RSF, the residual PAA concentration was 72 mg L⁻¹ on the feed side (27.0% of TOC). These results demonstrate the advantages of integrating a PAA-Na DS with a BES to mitigate RSF and to support further development of OMBR technology.

Introduction

Osmotic membrane bioreactors (OMBR) have been studied due to their promise as a technology that can produce high quality effluent from wastewater by combining biological treatment and forward osmosis (FO) membrane separation (Wang et al., 2016, Yan et al., 2018). A highly selective FO membrane can effect a net water flow by utilizing the osmotic pressure difference between a low-salinity feed solution and a high-salinity draw solution (Achilli et al., 2009, Cornelissen et al., 2008). Compared to traditional membrane bioreactors (MBRs) that employ a pressure-driven membrane process, the use of FO membranes in OMBRs can decrease energy consumption, alleviate membrane fouling, and achieve high contaminant removal due to a high rejection capacity, retaining persistent trace organic contaminants and facilitating their biodegradation (Alturki et al., 2012, Holloway et al., 2015a, Yap et al., 2012). Therefore, OMBRs have been extensively investigated for application for indirectly and directly reusing potable water, recovering nutrients, and removing heavy metals or trace organic chemicals (Aftab et al., 2017, Blandin et al., 2018, Holloway et al., 2015b, Morrow et al., 2018, Qiu and Ting, 2014).

A key challenge for the operation of an OMBR is solute buildup (i.e., salt accumulation) on the feed side due to rejection of solutes by the FO membrane and the reverse solute flux (RSF) from the draw side (Chekli et al., 2016, Wang et al., 2016). Solute buildup has adverse impacts on both membrane separation and biological treatment, such as decreased water flux, aggravated membrane fouling, deteriorated product water quantity, effects on bacterial community structure and activity, and increased operating costs due to the need for post disposal (Lay et al., 2011, Qiu and Ting, 2013, Song et al., 2018). It has been reported that when the conductivity of the mixed liquor increased from 2.0 to 13.6 mS cm⁻¹, the COD removal efficiency decreased from 90% to approximately 75%, and the specific oxygen utilization rate decreased from 3.42 to 2.34 mgO₂/g MLVSS/min, suggesting a reduction in biological activity (Qiu and Ting, 2013). The most common method for controlling salt accumulation is periodic supernatant discharge (Nawaz et al., 2013). An alternative strategy to mitigate salt accumulation is the integration of a side-stream microfiltration (MF) or ultrafiltration (UF) unit into an OMBR, resulting in a hybrid MF/UF-OMBR process (Holloway et al., 2015b, Wang et al., 2014). However, these membrane separation techniques face the problem of additional energy input, and the disposal of the extracted supernatant with a high salt concentration is a challenge. Electrodialysis (ED) has been employed to remove solute buildup, but this process still requires a significant amount of energy input (Lu and He, 2015).

A new strategy for controlling salt accumulation is the incorporation of a bioelectrochemical system (BES) into the feed side to reduce the solute buildup and potentially recover the solute for reuse (Yang et al., 2018). BES uses electrochemically active bacteria on anodes to oxidize organic matter and transfer the generated electrons to a terminal electron acceptor on cathodes (Logan et al., 2006). It has been demonstrated that BESs can be synergistically linked to OMBRs to achieve mutual benefits, such as reducing sludge production, alleviating membrane fouling, and simultaneously recovering energy, water, and nutrients from wastewater (Hou et al., 2017, Liu et al., 2017, Zhang et al., 2017). Increased solute conductivity and buffer capacity greatly enhance current generation in a BES, further promoting salt movement (Hou et al., 2016). Although a proposed bioelectrochemically assisted osmotic membrane bioreactor (BEA-OMBR) system could reduce anolyte conductivity from 24.1 to 9.0 mS cm⁻¹, the residual elevated salinity (i.e., NH₄⁺) from the NH₄HCO₃ draw solute (DS) can still affect bacterial enzyme activity and respiration rate (Qiu and Ting, 2013, Yang et al., 2018).

The use of a biodegradable DS may help to reduce DS residue. This approach has been investigated using a novel DS, namely, poly (acrylic acid, PAA), that is pH-responsive and can not only produce high osmotic pressures in the form of alkali metal but also reduce RSF due to its high molecular weight (Yang et al., 2017a). In this study, PAA-Na was incorporated into a BEA-OMBR to alleviate the RSF and maintain a healthy environment for the microorganism community. Moreover, the special configuration of the BEA-OMBR exhibited the unique benefit of effectively recovering PAA-Na by means of a pH adjustment recovery process with lower operation costs. Continuous operation of the system will result in the alkalization of the catholyte due to the consumption of protons and accumulation of hydroxyl. Such a high catholyte pH can be used to switch protonated PAA to polyelectrolyte and subsequently reduce resource consumption. The specific objectives of this study were to (1) evaluate the performance of a BEA-OMBR with the PAA DS compared to that of a BEA-OMBR with the NH₄HCO₃ DS; (2) examine the effects of cathodic or anodic buffer and PAA DS concentrations on BEA-OMBR performance; (3) investigate the biodegradation of PAA on the feed side of BEA-OMBR; and (4) investigate the reuse of the PAA DS in the BEA-OMBR.