Progress in osmotic membrane bioreactors research: Contaminant removal, microbial community and bioenergy production in wastewater

Highlights

• Microbial communities required to produce energy in OMBR were reviewed.

• Capabilities of OMBR units were discussed in relation to energy production.

• Capabilities of OMBRs in improving wastewater treatment were reviewed.

• OMBRs removed >80% of TOC, PO₄^3-, NH₄⁺ and emerging contaminants from wastewater.

• OMBR has great potential to produce energy and treat wastewater simultaneously.

Abstract

Renewable energy, water conservation, and environmental protection are the most important challenges today. Osmotic membrane bioreactor (OMBR) is an innovative process showing superior performance in bioenergy production, eliminating contaminants, and low fouling tendency. However, salinity build-up is the main drawback of this process. Identifying the microbial community can improve the process in bioenergy production and contaminant treatment. This review aims to study the recent progress and challenges of OMBRs in contaminant removal, microbial communities and bioenergy production. OMBRs are widely reported to remove over 80% of total organic carbon, PO₄^3-, NH₄⁺ and emerging contaminants from wastewater. The most important microbial phyla for both hydrogen and methane production in OMBR are Firmicutes, Proteobacteria and Bacteroidetes. Firmicutes' dominance in anaerobic processes is considerably increased from usually 20% at the beginning to 80% under stable condition. Overall, OMBR process has great potential to be applied for simultaneous bioenergy production and wastewater treatment.

Introduction

The rapid industrialization, urbanization, and global population growth have led to considerable problems in the environmental and energy fields. Today, fossil fuel is the most widely used energy source in industry, agriculture, transport and household throughout the world. It has been reported that the global energy demand will be increasing considerably in the next few decades, as energy is the most fundamental driver of the global economy. More fossil fuel consumption has resulted in increasing emission of greenhouse gases and contaminants into the atmosphere and, consequently, global warming and deteriorating air quality (Huang et al., 2019, Hosseinzadeh et al., 2020a). Therefore, striking a harmony between the anthropogenic activities and sustainability of the environment is of great importance (Ali et al., 2016) resulting in more attention in renewable energy production (Sun et al., 2019b). It is expected that renewable energy sources will contribute to more than 50% of the total electricity generation by 2040, which more than doubles the value of 22% in 2016 (Alassi et al., 2019). In addition, water shortage is currently a serious problem worldwide, which is exacerbated by climate change. The annual water requirement is growing rapidly owing to world population increase and industrialization (Hosseinzadeh et al., 2020a, Hosseinzadeh et al., 2020b). There are different technologies to tackle each of these challenges individually, e.g. by applying advanced oxidation (Bao et al., 2020), adsorption (Alidadi et al., 2018) or membrane processes (Cheng et al., 2018, Kheirieh et al., 2018, Luo et al., 2018) for water treatment and reclamation, and by producing energy from renewable resources, e.g. geothermal, ocean, solar, hydro, wind and wave in lieu of fossil fuels (Tran and Smith, 2017, Wu et al., 2021). More interestingly, the development of processes which can simultaneously address all three challenges of renewable energy production, water resources conservation and environmental protection are extremely important for the society today, as part of our efforts to meet the UN sustainable development goals (Hosseinzadeh et al., 2020a).

Membrane bioreactor (MBR) technology, which integrates conventional activated sludge with physical processes of membrane separation like ultrafiltration (UF) and microfiltration (MF), has been extensively developed to treat and reclaim wastewater (Cheng et al., 2018). This technology is promising and reliable by easier maintenance and operation, smaller footprint, lower generation of sludge, and better effluent quality (Liu et al., 2014, Yurtsever et al., 2015). In addition, anaerobic MBR (AnMBR) is considered as a remarkable process for wastewater treatment and energy production (Liu et al., 2021), due to the high degradation capacity of anaerobic microorganisms, longer sludge retention time, and better effluent qualities (Cheng et al., 2018). Therefore, AnMBR has a great potential to produce energy, treat wastewater and consequently protect the environment in one process (Liu et al., 2014).

Conventional activated sludge process has recently been combined with forward osmosis (FO) to create a new process called osmotic membrane bioreactor (OMBR) (Achilli et al., 2010, Hosseinzadeh et al., 2020b). In the OMBR process, the difference of osmotic pressures between two sides of the membrane is the driving force for purified water from a low salinity feed solution into the draw solution (DS) through a FO semipermeable membrane. Subsequently, some other desalination processes such as distillation and reverse osmosis (RO) may be applied to regenerate clean water from DS for different usages like irrigation as fertilizers and potable water (Alturki et al., 2012, Cai, 2016, Hosseinzadeh et al., 2020b). Concerning the orientation of the membranes, two different modes, FO and pressure retarded osmosis (PRO) are proposed for this process. In a way that when DS runs against the selective thin and support layers, the process will be called FO and PRO, respectively (Ge et al., 2013). OMBR has some advantages over the conventional MBRs such as the low energy consumption, low fouling propensity and superior performance in the removal of contaminants particularly emerging contaminants e.g. endocrine disrupting chemicals, steroid hormones, pesticides and pharmaceutically active compounds, which are of the greatest concern currently (Luo et al., 2018). The results demonstrated that the emerging contaminants with a high molecular weight (>266 Da) were removed by more than 80%, while the removal of low molecular weight compounds was sporadic, due to the fact that FO membrane can more effectively retain high molecular weight contaminants resulting in their longer retention time and more biological degradation (Alturki et al., 2012, Blandin et al., 2018). Luo et al. (2018) reported that by using a novel biomimetic aquaporin FO membrane, 30 trace organic contaminants (TrOC) were removed by over 85% regardless of their physicochemical properties. Despite these advantages, the salinity build-up is one of the most important disadvantages of this process, which occurs in the bioreactor by virtue of the DS reverse diffusion and salt rejection (Hosseinzadeh et al., 2020b). Several MBRs review articles have focused on various characteristics such as high strength wastewater treatment with MBRs (Mutamim et al., 2012), OMBR (Viet et al., 2019), OMBR salinity build-up (Song et al., 2018), and extracellular polymeric substances in MBRs (Lin et al., 2014). Furthermore, the capability of the OMBRs in energy-nutrient-water solute recovery was reviewed and concluded that the energy balance of either electrodialysis or bioelectrochemical based OMBR processes was negative. The anaerobic OMBRs were regarded as energy efficient systems; however, the salinity build-up of OMBRs is regarded as a considerable drawback hindering such capability (Yang et al., 2021). In a biological-driven process such as OMBR, the microorganisms play a crucial role in its overall energy and environmental performance. Yet, there is a lack of study concerning the microbial community in OMBRs to assess the capability of these systems for different applications especially energy recovery. Therefore, this study aims to address the recent advances in OMBR process, particularly the microbial community controlling the process efficiency. In addition, the potential of energy production by OMBR with a focus on microbial community and other components of OMBR will be discussed. Finally, the main challenges and potential solutions are addressed in future outlook.