Sustainable water desalination and electricity generation in a separator coupled stacked microbial desalination cell with buffer free electrolyte circulation

Renewable Energy-Driven Desalination: New Trends and Future Prospects of Small Capacity Systems

New trends and future prospects for small capacity systems of Renewable Energy-driven DESalination (REDES) are reviewed and assessed in this paper over a nominal desalination capacity range of 3–1000 m3/d. A thorough literature review is reported in order to evaluate current research and developing activities. Outstanding commercial prospects in the near future are identified for two off-grid REDES technologies under development. First, wave energy converters with direct coupling to seawater desalination. Second, solar micro gas turbines with biofuel backup coupled to reverse osmosis (RO) desalination and/or zero liquid discharge water treatment. These systems, as well as mature REDES plants (namely PV/RO and wind turbines/RO), will benefit from forthcoming advances in energy efficiency in the RO process itself. The Closed Circuit RO desalination (CCROTM) concept may be a key configuration for enhancing RE-driven RO desalination. Additionally, opportunities for innovation in seawater RO desalination with variable power consumption are highlighted. On the other hand, our conclusions highlight opportunities for developing novel portable REDES systems based on solar membrane distillation with a portable linear Fresnel concentrator manufactured by SOLATOM. Additionally, the concept of portable systems could foster the commercial development of microbial desalination cells combined with solar PV energy and RO powered by tidal currents.

A state-of-the-art review on microbial desalination cells

The rapid growth in population has increased the demand for potable water. Available technologies for its generation are the desalination of sea water through reverse osmosis, electrodialysis etc., which are energy and cost intensive. In this context, microbial desalination cell (MDC) presents a low-cost and sustainable option which can simultaneously treat wastewater, desalinate saline water, produce electrical energy and recover nutrients from wastewater. This review paper is focussed on presenting a detailed analysis of MDCs starting from the principle of operation, microbial community analysis, basic architecture, evolution in design, operational challenges, effect of process parameters, scale-up studies, application in multiple arenas and future prospects. After thorough review, it can be inferred that MDCs can be used as a stand-alone option or pre-treatment step for conventional desalination techniques without the application of external energy. MDCs have been used in multiple applications ranging from desalination, remediation of contaminated water, recovery of energy and nutrients from wastewater, softening of hardwater, biohydrogen production to degradation of waste engine oil. Although, MDCs have been used for multiple applications, still a number of operational challenges have been reported viz., interference of co-existing ions during desalination, membrane fouling, pH imbalance and limited potential of exoelectrogens. However, the re-circulation of anolytes with electrodialysis chamber has led to the maintenance of optimal pH for favourable microbial growth leading to improvement in the overall performance of MDCs. In future, genetic engineering may be used for improving the electrogenic activity of microbial community, next generation materials may be used as anode and cathode, varied sources of wastewater may be explored as anolytes, life cycle analysis and exergy analysis may be carried out to study the impact on environment and detailed pilot scale studies have to be carried out for assessing the feasibility of operation at large scale.

Energy production from treatment of industrial wastewater and boron removal in aqueous solutions using microbial desalination cell

As a result of a much needed paradigm shift worldwide, treated saline water is being considered as a viable option for replacing freshwater resources in agricultural irrigation. Vastly produced geothermal brine in Turkey may pose a significant environmental risk due to its high ionic strength, specifically due to boron. Boron species, which are generally found uncharged in natural waters, are costly to remove using high-throughput membrane technologies such as reverse osmosis. Recent advances in bioelectrochemical systems (BES) has facilitated development of energetically self-sufficient wastewater treatment and desalination. In this study, removal of boron from synthetic solutions and real geothermal waters, along with simultaneous energy production, using the microbial desalination cell (MDC) were investigated. Optimization studies were conducted by varying boron concentrations (5, 10, and 20 mg L^-1), air flow rates (0, 1, and 2 L min^-1), electrode areas (18, 24, 36, and 72 cm²), catholyte solutions, and operating modes. Even though the highest concentration decrease was observed for 20 mg-B L^-1, 5 mg-B L^-1 concentration experiment gave the closest result to the 2.4 mg-B L^-1 limit value asserted by WHO. Effect of electrode surface area was proven to be significant on boron removal efficiency. Employing the optimum conditions acquired with synthetic solutions, boron and COD removal efficiencies from real geothermal brine were 44.3% and 90.6%, respectively. MDC, being in its early levels of technology readiness, produced promising desalination and energy production results in removal of boron from geothermal brine.

Pushing microbial desalination cells towards field application: Prevailing challenges, potential mitigation strategies, and future prospects

Microbial desalination cells (MDCs) have been experimentally proven as a versatile bioelectrochemical system (BES). They have the potential to alleviate environmental pollution, reduce water scarcity and save energy and operational costs. However, MDCs alone are inadequate to realise a complete wastewater and desalination treatment at a high-efficiency performance. The assembly of identical MDC units that hydraulically and electrically connected can improve the performance better than standalone MDCs. In the same manner, the coupling of MDCs with other BES or conventional water reclamation technology has also exhibits a promising performance. However, the scaling-up effort has been slowly progressing, leading to a lack of knowledge for guiding MDC technology into practicality. Many challenges remain unsolved and should be mitigated before MDCs can be fully implemented in real applications. Here, we aim to provide a comprehensive chronological-based review that covers technological limitations and mitigation strategies, which have been developed for standalone MDCs. We extend our discussion on how assembled, coupled and scaled-up MDCs have improved in comparison with standalone and lab-scale MDC systems. This review also outlines the prevailing challenges and potential mitigation strategies for scaling-up based on large-scale specifications and evaluates the prospects of selected MDC systems to be integrated with conventional anaerobic digestion (AD) and reverse osmosis (RO). This review offers several recommendations to promote up-scaling studies guided by the pilot scale BES and existing water reclamation technologies.

Resource recovery from wastewater can be an application niche of microbial desalination cells

Microbial desalination cells (MDCs) have been studied as an emerging technology to accomplish simultaneous wastewater treatment and saline water desalination. A good amount of effort has been invested to understand fundamental problems and develop functional systems of the MDC technology. However, a revisit of MDCs' desalination function reveals that the unique requirements like co-location of wastewater and saline water will greatly limit the application of this technology. In addition, the relatively low desalination rate of MDCs will result in a large reactor size and thus higher capital cost. Because of the need for wastewater (as a substrate for electricity generation), the MDC technology may have a promising niche of application for resource recovery from wastewater. A proper design of MDCs will allow the current-driven separation of ammonia, phosphorus, and volatile fatty acids (VFAs) from wastewater for further recovery. Based on the literature data, we conduct a case study analysis of mass flow for MDC-based resource recovery and demonstrate the potential of this function. Resource recovery can be a new function of interest to MDCs and worth further exploration of its technical and economic feasibility.

Construction and application of a 1-liter upflow-stacked microbial desalination cell

As increasing demand of global reuse water, microbial desalination cell (MDC) is developed as a potential desalination approach to drive ion migration and separation through biodegradation without any additional energy. A novel, efficient, stable reactor coupled stacked MDC with upflow MDC was constructed, which was named as upflow-stacked MDC (USMDC). Compared with the traditional stacked MDC and upflow MDC, the desalination and generation performance of the USMDC was evaluated. Results showed that, after 24 h, the desalination ratio of USMDC can reach 91.9% when the external resistance was 1.5 Ω, which was 1.18 and 1.48 times higher than SMDC and UMDC, respectively. The long-term performance of the desalination efficiency was tested, which was maintained at 87.2-96.0% and stable for consecutive 120 days. Then, it was also the investigated that the relationship between desalination rate and external resistance during every period. The USMDC produced a maximum power density of 32.91 W m^-3. In addition, the difference of current density between USMDC and SMDC indicates the turbulence generated by cylindrical structure could effectively decrease the internal resistance. It was also corroborated that salt concentration gradient and bipolar electrodialysis would decline the charge transfer efficiency. Accordingly, USMDC was verified having the superior desalination performance thus providing the possibility for application in wastewater reuse.

Enhancing biodegradation and energy generation via roughened surface graphite electrode in microbial desalination cell

The microbial desalination cell (MDC) is known as a newly developed technology for water and wastewater treatment. In this study, desalination rate, organic matter removal and energy production in the reactors with and without desalination function were compared. Herein, a new design of plain graphite called roughened surface graphite (RSG) was used as the anode electrode in both microbial fuel cell (MFC) and MDC reactors for the first time. Among the three type of anode electrodes investigated in this study, RSG electrode produced the highest power density and salt removal rate of 10.81 W/m³ and 77.6%, respectively. Such a power density was 2.33 times higher than the MFC reactor due to the junction potential effect. In addition, adding the desalination function to the MFC reactor enhanced columbic efficiency from 21.8 to 31.4%. These results provided a proof-of-concept that the use of MDC instead of MFC would improve wastewater treatment efficiency and power generation, with an added benefit of water desalination. Furthermore, RSG can successfully be employed in an MDC or MFC, enhancing the bio-electricity generation and salt removal.

Bioelectrochemical systems-driven directional ion transport enables low-energy water desalination, pollutant removal, and resource recovery

Bioelectrochemical systems (BESs) are integrated water treatment technologies that generate electricity using organic matter in wastewater. In situ use of bioelectricity can direct the migration of ionic substances in a BES, thereby enabling water desalination, resource recovery, and valuable substance production. Recently, much attention has been placed on the microbial desalination cells in BESs to drive water desalination, and various configurations have optimized electricity generation and desalination performance and also coupled hydrogen production, heavy metal reduction, and other reactions. In addition, directional transport of other types of charged ions can remediate polluted groundwater, recover nutrient, and produce valuable substances. To better promote the practical application, the use of BESs as directional drivers of ionic substances requires further optimization to improve energy use efficiency and treatment efficacy. This article reviews existing researches on BES-driven directional ion transport to treat wastewater and identifies a few key factors involved in efficiency optimization.

Self-Driven Desalination and Advanced Treatment of Wastewater in a Modularized Filtration Air Cathode Microbial Desalination Cell

Microbial desalination cells (MDCs) extract organic energy from wastewater for in situ desalination of saline water. However, to desalinate salt water, traditional MDCs often require an anolyte (wastewater) and a catholyte (other synthetic water) to produce electricity. Correspondingly, the traditional MDCs also produced anode effluent and cathode effluent, and may produce a concentrate solution, resulting in a low production of diluate. In this study, nitrogen-doped carbon nanotube membranes and Pt carbon cloths were utilized as filtration material and cathode to fabricate a modularized filtration air cathode MDC (F-MDC). With real wastewater flowing from anode to cathode, and finally to the middle membrane stack, the diluate volume production reached 82.4%, with the removal efficiency of salinity and chemical oxygen demand (COD) reached 93.6% and 97.3% respectively. The final diluate conductivity was 68 ± 12 μS/cm, and the turbidity was 0.41 NTU, which were sufficient for boiler supplementary or industrial cooling. The concentrate production was only 17.6%, and almost all the phosphorus and salt, and most of the nitrogen were recovered, potentially allowing the recovery of nutrients and other chemicals. These results show the potential utility of the modularized F-MDC in the application of municipal wastewater advanced treatment and self-driven desalination.

Analysis of long-term performance and microbial community structure in bio-cathode microbial desalination cells

A microbial desalination cell (MDC) could desalinate salt water without energy consumption and simultaneously generate bioenergy. Compared with an abiotic cathode MDC, an aerobic bio-cathode MDC is more sustainable and is less expensive to operate. In this study, the long-term operation (5500 h) performance of a bio-cathode MDC was investigated in which the power density, Coulombic efficiency, and salt removal rate were decreased by 71, 44, and 27 %, respectively. The primary reason for the system performance decrease was biofouling on the membranes, which increased internal resistance and reduced the ionic transfer and energy conversion efficiency. Changing membranes was an effective method to recover the MDC performance. The microbial community diversity in the MDC anode was low compared with that of the reported microbial fuel cell (MFC), while the abundance of Proteobacteria was 30 % higher. The content of Planctomycetes in the cathode biofilm sample was much higher than that in biofouling on the cation exchange membrane (CEM), indicating that Planctomycetes were relevant to cathode oxygen reduction.

Bioelectricity inhibits back diffusion from the anolyte into the desalinated stream in microbial desalination cells

Microbial desalination cells (MDCs) taking advantage of energy in wastewater to drive desalination represent a promising approach for energy-efficient desalination, but concerns arise whether contaminants in wastewater could enter the desalinated stream across ion exchange membranes. Such back diffusion of contaminants from the anolyte into the desalinated stream could be controlled by two mechanisms, Donnan effect and molecule transport. This study attempted to understand those mechanisms for inorganic and organic compounds in MDCs through two independently conducted experiments. Donnan effect was found to be the dominant mechanism under the condition without current generation. Under open circuit condition, the MDC fed with 5 g L(-1) salt solution exhibited 1.9 ± 0.7%, 10.3 ± 1.3%, and 1.8 ± 1.2% back diffusion of acetic, phosphate, and sulfate ions, respectively. Current generation effectively suppressed Donnan effect from 68.2% to 7.2%, and then molecule transport became more responsible for back diffusion. A higher initial salt concentration (35 g L(-1)) and a shorter HRT (1.0 d) led to the highest concentration gradient, resulting in the most back diffusion of 7.1 ± 1.2% and 6.8 ± 3.1% of phosphate and sulfate ions, respectively. Three representative organic compounds were selected for test, and it was found that organic back diffusion was intensified with a higher salt concentration gradient and molecular weight played an important role in compound movement. Principal component analysis confirmed the negative correlation between Donnan effect and current, and the positive correlation between molecule transport and concentration gradient related conditions.

Novel Self-driven Microbial Nutrient Recovery Cell with Simultaneous Wastewater Purification

Conventional wastewater purification technologies consume large amounts of energy, while the abundant chemical energy and nutrient resources contained in sewage are wasted in such treatment processes. A microbial nutrient recovery cell (MNRC) has been developed to take advantage of the energy contained in wastewater, in order to simultaneously purify wastewater and recover nutrient ions. When wastewater was circulated between the anode and cathode chambers of the MNRC, the organics (COD) were removed by bacteria while ammonium and phosphate (NH₄⁺-N and PO₄³⁻-P) were recovered by the electrical field that was produced using in situ energy in the wastewater without additional energy input. The removal efficiencies from wastewater were >82% for COD, >96% for NH₄⁺-N, and >64% for PO₄³⁻-P in all the operational cycles. Simultaneously, the concentrations of NH₄⁺ and PO₄³⁻ in the recovery chamber increased to more than 1.5 and 2.2 times, respectively, compared with the initial concentrations in wastewater. The MNRC provides proof-of-concept as a sustainable, self-driven approach to efficient wastewater purification and nutrient recovery in a comprehensive bioelectrochemical system.

Integrated experimental investigation and mathematical modeling of brackish water desalination and wastewater treatment in microbial desalination cells

Desalination of brackish water can provide freshwater for potable use or non potable applications such as agricultural irrigation. Brackish water desalination is especially attractive to microbial desalination cells (MDCs) because of its low salinity, but this has not been well studied before. Herein, three brackish waters prepared according to the compositions of actual brackish water in three locations in Israel were examined with domestic wastewater as an electron source in a bench-scale MDC. All three brackish waters could be effectively desalinated with simultaneous wastewater treatment. The MDC achieved the highest salt removal rate of 1.2 g L(-1) d(-1) with an initial salinity of 5.9 g L(-1) and a hydraulic retention time (HRT) of 0.8 d. The desalinated brackish water could meet the irrigation standard of both salinity (450 mg L(-1) TDS) and the concentrations of major ionic species, given a sufficient HRT. The MDC also accomplished nearly 70% removal of organic compounds in wastewater with Coulombic efficiency varied between 5 and 10%. A previously developed MDC model was improved for brackish water desalination, and could well predict salinity variation and the concentrations of individual ions. The model also simulated a staged operation mode with improved desalination performance. This integrated experiment and mathematical modeling approach provides an effective method to understand the key factors in brackish water desalination by MDCs towards further system development.

Synthesis of iron oxide/partly graphitized carbon composites as a high-efficiency and low-cost cathode catalyst for microbial fuel cells

Waste cornstalks and pomelo skins are used as carbon resources for preparing nanocomposites of iron oxide and partly graphitized carbon (Fe3O4/PGC-CS and Fe3O4/PGC-PS). The results showed that Fe3O4 with a face-centered cubic structure is uniformly dispersed on the skeleton of Fe3O4/GC, and the highest SBET values of Fe3O4/PGC-CS (476.5 m(2) g(-1)) and Fe3O4/PGC-PS (547.7 m(2) g(-1)) are obtained at 1000 °C. The electrical conductivity and density of catalytic active sites are correspondingly improved by the introduction of Fe species. Microbial fuel cells (MFCs) with a mixed composite (Fe3O4/PGC-CS:Fe3O4/PGC-PS = 1:1) cathode (three-dimensional structures) generate the highest power density of 1502 ± 30 mW m(-2), which is 26.01% higher than that of Pt/C (1192 ± 33 mW m(-2)) and only declines by 7.12% after 18 cycles. The Fe3O4/PGC-CS cathode has the highest Coulombic efficiency (24.3 ± 0.7%). The Fe3O4/PGC composites exhibit high oxygen reduction reactivity, low charge transfer resistances, and long-term stability and can be used as a low-cost and high-efficiency catalyst for MFCs.

A ten liter stacked microbial desalination cell packed with mixed ion-exchange resins for secondary effluent desalination

The architecture and performance of microbial desalination cell (MDC) have been significantly improved in the past few years. However, the application of MDC is still limited in a scope of small-scale (milliliter) reactors and high-salinity-water desalination. In this study, a large-scale (>10 L) stacked MDC packed with mixed ion-exchange resins was fabricated and operated in the batch mode with a salt concentration of 0.5 g/L NaCl, a typical level of domestic wastewater. With circulation flow rate of 80 mL/min, the stacked resin-packed MDC (SR-MDC) achieved a desalination efficiency of 95.8% and a final effluent concentration of 0.02 g/L in 12 h, which is comparable with the effluent quality of reverse osmosis in terms of salinity. Moreover, the SR-MDC kept a stable desalination performance (>93%) when concentrate volume decreased from 2.4 to 0.1 L (diluate/concentrate volume ratio increased from 1:1 to 1:0.04), where only 0.875 L of nonfresh water was consumed to desalinate 1 L of saline water. In addition, the SR-MDC achieved a considerable desalination rate (95.4 mg/h), suggesting a promising application for secondary effluent desalination through deriving biochemical electricity from wastewater.

Enhancing charge harvest from microbial fuel cells by controlling the charging and discharging frequency of capacitors

Capacitor is a storage device to harvest charge produced from microbial fuel cells (MFCs). In intermittent charging mode, the capacitor is charged by an MFC first, and then discharged through an external resistance. The charge harvested by capacitor is affected by the charging and discharging frequency. In the present study, the effect of the charging and discharging frequency on charge harvest was investigated. At the switching time (ts) of 100 s, the average current over each time segment reached its maximum value (1.59 mA) the earliest, higher than the other tested conditions, and the highest COD removal (63%) was also obtained, while the coulombic efficiency reached the highest of 67% at the ts of 400 s. Results suggested that lower ts led to higher current output and COD removal, but appropriate ts should be selected in consideration of charge recovery efficiency.

Competitive migration behaviors of multiple ions and their impacts on ion-exchange resin packed microbial desalination cell

Mixed ion-exchange resins packed microbial desalination cell (R-MDC) could stabilize the internal resistance, however, the impacts of multiple ions on R-MDC performance was unclear. This study investigated the desalination performance, multiple ions migration behaviors and their impacts on R-MDCs fed with salt solution containing multiple anions and cations. Results showed that R-MDC removed multiple anions better than multiple cations with desalination efficiency of 99% (effluent conductivity <0.05 ms/cm) at hydraulic retention time of 50 h. Competitive migration order was SO4(2-)>NO3(-)>Cl(-) for anions and Ca(2+)≈Mg(2+)>NH4(+)>Na(+) for cations, jointly affected by both their molar conductivity and exchange selectivity on resins. After long-term operation, the existence of higher concentration Ca(2+) and Mg(2+) caused the electric conductivity of mixed resins decrease and scaling on the surface of cation-exchange membrane adjoined with cathode chamber, suggesting that R-MDC would be more suitable for desalination of water with lower hardness.

Improving the flexibility of microbial desalination cells through spatially decoupling anode and cathode

To improve the flexibility of microbial desalination cell (MDC) construction and operation, a new configuration with decoupled anode and cathode was developed and examined in this study. A higher salt concentration resulted in higher current generation, as well as a higher salt removal rate. The effect of the distance between the anode and the cathode on the MDC performance was not obvious, likely due to a sufficient conductivity in the salt solution. Because the cathode was identified as a limiting factor, adding one more cathode unit increased the current generation from 72.3 to 116.0 A/m(3), while installing additional anode units did not obviously alter the MDC current production. Changing the position of the anode/cathode units exhibited a weak influence on the MDC performance. Parallel connection of electrical circuits generally produced more current than the individual connections, and a strong competition was observed between multiple units sharing the same opposite unit.