Simultaneous desalination and nutrient recovery during municipal wastewater treatment using microbial electrolysis desalination cell
Graphical abstract
Introduction
Zero liquid discharge (ZLD) is a wastewater treatment strategy that eliminates contaminants and recycles most water for reuse (Tong and Elimelech, 2016). The wastewater contains nutrients, such as nitrogen and phosphate (Zuo et al., 2014), and salinity (Windey et al., 2005). The inadequate treatment of wastewater will cause eutrophication, affecting the reuse of wastewater. Furthermore, it is an incontrovertible fact that the phosphorus reserves are being depleted. Recovering phosphate from wastewater could alleviate the limited phosphate resource supply (Liu et al., 2017; Wang et al., 2015). However, existing biological treatments are efficient in removing nitrogen and phosphate, but inefficient in treating salinity during municipal wastewater treatment (Zuo et al., 2014). In view of these problems, some wastewater treatment processes capable of removing organics, desalination, and recovering nitrogen and phosphate resources are developed to satisfy the increasing need of environmental protection and sustainable development.
In order to achieve ZLD, advanced treatment technologies are needed. A bio-electrochemical system (BES) could realize the effective treatment of organic waste or biomass and capture electricity from wastewater, which was considered as a far-reaching sustainable development technology (Gao et al., 2018; Pan et al., 2017). As a type of BES, the microbial electrolytic cell (MEC) is mostly used to produce hydrogen, methane, and other chemicals (Huang et al., 2014; Logan et al., 2008; Zhen et al., 2015). Owing to metabolic activity of microorganisms on electrode, MECs have also been used to remove organics in wastewater, which offers great potential as an alternate to traditional wastewater treatments (Escapa et al., 2016). However, the quality of wastewater effluent of MEC was hardly satisfactory because it contained a certain amount of nutrient ions (phosphate and NH4+) and salts.
Electrodialysis (ED) technology could be used to separate different ions (such as NO3− and PO43−) from wastewater to realize nitrogen and phosphorus recovery (Thompson Brewster et al., 2016; Tice and Kim, 2014; Ward et al., 2018). As various ions concentrate into the product chamber of ED through ion exchange membranes (IEMs), nutrient recovery could be achieved (Alvarado and Chen, 2014; Strathmann, 2010). In previous reports, ED technology was widely used for desalination (Larsen et al., 2016; Li et al., 2007; Zhang et al., 2014). Moreover, ED could be utilized to enrich nitrogen and phosphate from urine or wastewater (Escher et al., 2006; Lee et al., 2003; Mondor et al., 2008). However, ED technology only separates ions from wastewater, and the residual organics will cause serious fouling of IEMs, making them require further treatment. Microbial desalination cell (MDC) has also been innovated and identified as a feasible approach of sustainable desalination by using the in situ generated electricity from waste organics in wastewater (Cao et al., 2009; Chen et al., 2017).
One of the disadvantages of MDC is that the processes of desalination and organic removal are performed separately in different chambers (Brastad and He, 2013; Wen et al., 2012), hindering its application in saline wastewater treatment, such as food processing, leather industry, and petroleum wastewater (Lefebvre and Moletta, 2006; Lefebvre et al., 2005). However, the salinity is one of the key factors limiting wastewater reuse and could potentially cause groundwater pollution and soil salinization (Rebhun, 2004; Tran et al., 2016). Therefore, the desalination of saline wastewater during wastewater treatment offers great significance, especially by properly utilizing the in-situ generated electricity from wastewater (Geng et al., 2018; Wang et al., 2017), which has potential to realize ZLD and improve the reusability of wastewater.
In previous study, the microbial electrolysis desalination cell (MEDC) reactor was generally used for chemical production, desalination (Li et al., 2017; Luo et al., 2011), and nitrogen and phosphate recovery (Tice and Kim, 2014), but was rarely used in wastewater treatment. In our MEDC, nitrogen and phosphate separation and recovery as well as desalination can be achieved in situ, partially utilizing the electricity generated from organic matters in wastewater, which meet the demand of ZLD of wastewater treatment. In the point, MEDC is a potential technology for wastewater treatment and energy and nutrients recovery.
In this study, by integrating ED into a BES, a microbial electrolysis desalination cell (MEDC) reactor was developed to simultaneously achieve organics removal, desalination, and nutrient recovery during wastewater treatment. The performances of MEDC in terms of nitrogen and phosphate separation and recovery, energy consumption, Coulombic efficiency, chemical oxygen demand (COD) removal efficiency, and desalination performance were investigated under various IEM pairs and voltages. Moreover, the working mechanisms of MEDC were further evaluated by characterizing the bioanode by cyclic voltammetry and analyzing the microbial community structure.
Section snippets
MEDC construction
The MEDC was constructed using Plexiglas blocks with an interior cubic chamber. The effective volumes of anode chamber and cathode chamber were 175 mL (5 × 5 × 7 cm) and 25 mL (5 × 5 × 1 cm), respectively. The larger anode chamber was employed to achieve effective removal of the organic matter in wastewater and higher energy output. A stack of IEMs was inserted between the anode and the cathode chambers (Fig. 1). An IEM stack consisted of one pair of cation-exchange membrane (CEM) and
Separation and concentration performance
The results in Fig. 2 exhibit the system performance of MEDC in terms of nutrient removal and resource recovery. The nutrient ions were diluted in the electrode chamber, while the NH4+ and HPO42− concentrated in the product chamber. It is obvious that NH4+ and HPO42− in the electrode chamber could be removed under each condition. The initial concentrations of NH4+ and HPO42− were 30 mg/L and 10 mg/L, respectively. The concentrations of NH4+ and HPO42− in MEDC effluent decreased as the applied
Conclusion
A MEDC system was developed for wastewater treatment. The performance of MEDC was improved by increasing the voltage and number of IEM pairs. During the treatment of wastewater, the MEDC with multi-IEM stacks exhibited better COD removal, desalination performance, and resource recovery. The energy consumption for netrients separation and recovery was below 0.14 kWh/m3 during the experiment. The recovery efficiencies of NH4+ and HPO42− could reach up to 66 ± 5.3% and 66.7 ± 4.7% in multi-IEM
CRediT authorship contribution statement
Jiahuan Li: Investigation, Validation, Writing - original draft. Rudong Liu: Investigation, Validation, Methodology, Formal analysis. Shan Zhao: Writing - review & editing. Shuguang Wang: Writing - review & editing. Yunkun Wang: Conceptualization, Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
We acknowledge the support received from the National Natural Science Foundation of China (NSFC 51878389 and 51709157), the State Key Laboratory of Microbial Technology Open Projects Fund (M2019-04), the China Postdoctoral Science Foundation (2017M620287) and Shandong Key Laboratory of Water Pollution Control and Resource Reuse (2019KF04).
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These authors contributed equally to this work.