Effects of draw solutes on an integrated forward osmosis-Microbial fuel cell system treating a synthetic wastewater.

Microbial fuel cells (MFCs) and forward osmosis (FO) are both attractive and versatile wastewater treatment technologies that possess disadvantageous qualities that prevent their optimal performance. This study aimed to investigate how draw solute selection for FO treatment would affect MFC performance in a coupled FO-MFC system. Two types of draw solutes, NH4 HCO3 and NaCl, were studied, and it was found that 1.0 M NH4 HCO3 (FO-MFC-A) and 0.68 M NaCl (FO-MFC-B) had similar water fluxes of 6.04 to 3.39 LMH and 6.25 to 3.54 LMH, respectively. The reverse salt flux from the draw decreased the feed solution resistance for both draw solutes, but the FO-MFC-A system (0.32 W m-2 ) had a higher maximum power density than the FO-MFC-B system (0.26 W m-2 ). The current density for the FO-MFC-B system increased due to continuous solution resistance decrease, whereas it remained constant for the FO-MFC-A. The difference in Coulombic efficiencies (32.8% vs. 25.6%) but similar Coulombic recoveries (10.2% vs. 11.4%) between the FO-MFC-A and FO-MFC-B systems suggested that the FO-MFC-A might have the inhibited microbial activity by high ammonium/ammonia. The FO-MFC-A system had the lower energy consumption for nutrient removal (2.01 kWh kg-1 NH4 + -N) and recovery (8.87 kWh kg-1 NH4 + -N). These results have shown that NH4 HCO3 as a draw solute can have advantages of higher power density, higher Coulombic efficiency, and recoverability for draw regeneration, but its potential inhibition on microbial activity must also be considered. PRACTITIONER POINTS: Forward osmosis can be connected to microbial fuel cells for wastewater treatment. Water recovery by forward osmosis can greatly reduce the wastewater volume to microbial fuel cells. Ammonium draw solutes can result in lower volumetric energy consumption. Ammonia inhabitation of anode microbes will decrease organic removal.

Highly selective adsorption of vanadium (V) by nano-hydrous zirconium oxide-modified anion exchange resin.

Adsorption is widely used in removal of toxic vanadium (V) [V(V)] from water streams, and a fit-for-purpose adsorbent plays a vital role in this process. Herein HZrO@D201, an adsorbent with decoration of nanosized hydrous zirconium oxide (HZrO) on anion exchange resin D201, is fabricated for efficient V(V) removal. Compared to pristine D201, HZrO@D201 excelled in V(V) removal with a maximum adsorption capacity of 118.1 mg/g, due to potential formation of inner sphere complexation between V(V) and HZrO. HZrO@D201 could also functioned well in a wide pH range (3.00 to 9.00) and exhibited outstanding selective V(V) adsorption under the presence of competing anions (chloride, nitrate, sulfate, and phosphate). The adsorption thermodynamics was in accordance with the Langmuir model, while adsorption kinetics followed the Pseudo-Second-Order model. When treating actual vanadium contaminated groundwater from Panzhihua region (China), HZrO@D201 indicated a satisfactory lifespan in the column experiment for V(V) removal (2.41 times longer than D201), and the treated groundwater could meet the vanadium standard of drinking water source in China (less than 50 µg/L). Regeneration of HZrO@D201 was easily achievable with negligible capacity loss. Results from this work suggests a promising application potential of HZrO@D201 in vanadium pollution control.

Mitigation of bidirectional solute flux in forward osmosis via membrane surface coating of zwitterion functionalized carbon nanotubes.

Forward osmosis (FO) has emerged as a promising membrane technology to yield high-quality reusable water from various water sources. A key challenge to be solved is the bidirectional solute flux (BSF), including reverse solute flux (RSF) and forward solute flux (FSF). Herein, zwitterion functionalized carbon nanotubes (Z-CNTs) have been coated onto a commercial thin film composite (TFC) membrane, resulting in BSF mitigation via both electrostatic repulsion forces induced by zwitterionic functional groups and steric interactions with CNTs. At a coating density of 0.97 g m-2, a significantly reduced specific RSF was observed for multiple draw solutes, including NaCl (55.5% reduction), NH4H2PO4 (83.8%), (NH4)2HPO4 (74.5%), NH4Cl (70.8%), and NH4HCO3 (61.9%). When a synthetic wastewater was applied as the feed to investigate membrane rejection, FSF was notably reduced by using the coated membrane with fewer pollutants leaked to the draw solution, including NH4+-N (46.3% reduction), NO2--N (37.0%), NO3--N (30.3%), K+ (56.1%), PO43--P (100%), and Mg2+ (100%). When fed with real wastewater, a consistent water flux was achieved during semi-continuous operation with enhanced fouling resistance. This study is among the earliest efforts to address BSF control via membrane modification, and the results will encourage further exploration of effective strategies to reduce BSF.

Tackle reverse solute flux in forward osmosis towards sustainable water recovery: reduction and perspectives.

Forward osmosis (FO) has emerged as a potentially energy-efficient membrane treatment technology to yield high-quality reusable water from various wastewater/saline water sources. A key challenge remained to be solved for FO is reverse solute flux (RSF), which can cause issues like reduced concentration gradient and loss of draw solutes. Yet no universal parameters have been developed to compare RSF control performance among various studies, making it difficult to position us in this "battle" against RSF. In this paper, we have conducted a concise review of existing RSF reduction approaches, including operational strategies (e.g., pressure-, electrolysis-, and ultrasound-assisted osmosis) and advanced membrane development (e.g., new membrane fabrication and existing membrane modification). We have also analyzed the literature data to reveal the current status of RSF reduction. A new parameter, mitigation ratio (MR), was proposed and used together with specific RSF (SRSF) to evaluate RSF reduction performance. Potential research directions have been discussed to help with future RSF control. This review intends to shed more light on how to effectively tackle solute leakage towards a more cost-effective and environmental-friendly FO treatment process.

Evaluation of energy consumption of treating nitrate-contaminated groundwater by bioelectrochemical systems.

Nitrate contamination of groundwater is a mounting concern for drinking water production due to its healthy and ecological effects. Bioelectrochemical systems (BES) are a promising method for energy efficient nitrate removal, but its energy consumption has not been well understood. Herein, we conducted a preliminary analysis of energy consumption based on both literature information and multiple assumptions. Four scenarios were created for the purpose of analysis based on two treatment approaches, microbial fuel cells (MFCs) and controlled biocathodic denitrification (CBD), under either in situ or ex situ deployment. The results show a specific energy consumption based on the mass of NO3--N removed (SECN) of 0.341 and 1.602 kWh kg NO3--N-1 obtained from in situ and ex situ treatments with MFCs, respectively; the main contributor was the extraction of the anolyte (100%) in the former and pumping the groundwater (74.8%) for the latter. In the case of CBD treatment, the energy consumption by power supply outcompeted all the other energy items (over 85% in all cases), and a total SECN of 19.028 and 10.003 kWh kg NO3--N-1 were obtained for in situ and ex situ treatments, respectively. The increase in the water table depth (from 10 to 30 m) and the decrease of the nitrate concentration (from 25 to 15 mg NO3--N) would lead to a rise in energy consumption in the ex situ treatment. Although some data might be premature due to the lack of sufficient information in available literature, the results could provide an initial picture of energy consumption by BES-based groundwater treatment and encourage further thinking and analysis of energy consumption (and production).

Modeling assisted evaluation of direct electricity generation from waste heat of wastewater via a thermoelectric generator.

The thermal energy represents a significant portion of energy potential in municipal wastewater and may be recovered as electricity by a thermoelectric generator (TEG). Converting heat to all-purpose electricity by TEG has been demonstrated with large heat gradients, but its application in waste heat recovery from wastewater has not been well evaluated. Herein, a bench-scale Bi2Te3-based waste heat recovery system was employed to generate electricity from a low temperature gradient through a combination of experiments and mathematical modeling. With an external resistance of 7.8â€¯Ω and a water (hot side) flow rate of 75 mL min-1, a maximum normalized energy recovery of 4.5 × 10-4 kWh m-3 was achieved under a 2.8 °C temperature gradient (ΔT). Model simulation indicated a boost in both power output and energy conversion efficiency from 0.76 mW and 0.13% at ΔT = 2.8 °C to 61.83 mW and 1.15% at ΔT = 25 °C. Based on the data of two-year water/air temperature obtained from the Christiansburg Wastewater Treatment Plant, an estimated energy generation of 1094 to 70,986 kWh could be expected annually with a saving of $163 to $6076. Those results have revealed a potential for TEG-centered direct electricity generation from low-grade heat towards enhanced resource recovery from wastewater and encouraged further exploration of this approach.

Enhancing recovery of magnesium as struvite from landfill leachate by pretreatment of calcium with simultaneous reduction of liquid volume via forward osmosis.

Landfill leachate contains substances that can be potentially recovered as valuable resources. In this study, magnesium in a landfill leachate was recovered as struvite with calcium pretreatment; meanwhile, the leachate volume was reduced by using a submerged forward osmosis (FO) process, thereby enabling significant reduction of further treatment footprint and cost. Without pretreatment, calcium exhibited strong competition for phosphate with magnesium. The pretreatment with a Ca2+: CO32- molar ratio of 1:1.4 achieved a relatively low loss rate of Mg2+ (24.1±2.0%) and high Ca2+ removal efficiency (89.5±1.7%). During struvite recovery, 98.6±0.1% of magnesium could be recovered with a significantly lower residual PO43--P concentration (<25mgL-1) under the condition of (Mg+Caresidual): P molar ratio of 1:1.5 and pH9.5. The obtained struvite had a similar crystal structure and composition (19.3% Mg and 29.8% P) to that of standard struvite. The FO process successfully recovered water from the leachate and reduced its volume by 37%. The configuration of calcium pretreatment - FO - struvite recovery was found to be the optimal arrangement in terms of FO performance. These results have demonstrated the feasibility of magnesium recovery from landfill leachate and the importance of the calcium pretreatment, and will encourage further efforts to assess the value and purity of struvite for commercial use and to develop new methods for resource recovery from leachate.

Efficiently "pumping out" value-added resources from wastewater by bioelectrochemical systems: A review from energy perspectives.

Bioelectrochemical systems (BES) can accomplish simultaneous wastewater treatment and resource recovery via interactions between microbes and electrodes. Often deemed as "energy efficient" technologies, BES have not been well evaluated for their energy performance, such as energy production and consumption. In this work, we have conducted a review and analysis of energy balance in BES with parameters like normalized energy recovery, specific energy consumption, and net energy production. Several BES representatives based on their functions were selected for analysis, including direct electricity generation in microbial fuel cells, hydrogen production in microbial electrolysis cells, nitrogen recovery in BES, chemical production in microbial electrosynthesis cells, and desalination in microbial desalination cells. Energy performance was normalized to water volume (kWh m-3), organic removal (kWh kg COD-1), nitrogen recovery (kWh kg N-1), chemical production (kWh kg-1), or removed salt during desalination (kWh kg-1). The key operating factors such as pumping system (recirculation/feeding pumps) and external power supply were discussed for their effects on energy performance. This is an in-depth analysis of energy performance of various BES and expected to encourage more thinking, analysis, and presentation of energy data towards appropriate research and development of BES technology for resource recovery from wastewater.

Energy consumption by forward osmosis treatment of landfill leachate for water recovery.

Forward osmosis (FO) is an alternative approach for treating landfill leachate with potential advantages of reducing leachate volume and recovering high quality water for direct discharge or reuse. However, energy consumption by FO treatment of leachate has not been examined before. Herein, the operational factors such as recirculation rates and draw concentrations were studied for their effects on the quantified energy consumption by an FO system treating actual leachate collected from two different landfills. It was found that the energy consumption increased with a higher recirculation rate and decreased with a higher draw concentration, and higher water recovery tended to reduce energy consumption. The highest energy consumption was 0.276±0.033kWhm-3 with the recirculation rate of 110mLmin-1 and 1-M draw concentration, while the lowest of 0.005±0.000kWhm-3 was obtained with 30mLmin-1 recirculation and 3-M draw concentration. The leachate with lower concentrations of the contaminants had a much lower requirement for energy, benefited from its higher water recovery. Osmotic backwashing appeared to be more effective for removing foulants, but precise understanding of membrane fouling and its controlling methods will need a long-term study. The results of this work have implied that FO treatment of leachate could be energy efficient, especially with the use of a suitable draw solute that can be regenerated in an energy efficient way and/or through combination with other treatment technologies that can reduce contaminant concentrations before FO treatment, which warrants further investigation.

Electrolysis-assisted mitigation of reverse solute flux in a three-chamber forward osmosis system.

Forward osmosis (FO) has been widely studied for desalination or water recovery from wastewater, and one of its key challenges for practical applications is reverse solute flux (RSF). RSF can cause loss of draw solutes, salinity build-up and undesired contamination at the feed side. In this study, in-situ electrolysis was employed to mitigate RSF in a three-chamber FO system ("e-FO") with Na2SO4 as a draw solute and deionized (DI) water as a feed. Operation parameters including applied voltage, membrane orientation and initial draw concentrations were systematically investigated to optimize the e-FO performance and reduce RSF. Applying a voltage of 1.5 V achieved a RSF of 6.78 ± 0.55 mmol m-2 h-1 and a specific RSF of 0.138 ± 0.011 g L-1 in the FO mode and with 1 M Na2SO4 as the draw, rendering ∼57% reduction of solute leakage compared to the control without the applied voltage. The reduced RSF should be attributed to constrained ion migration induced by the coactions of electric dragging force (≥1.5 V) and high solute rejection of the FO membrane. Reducing the intensity of the solution recirculation from 60 to 10 mL min-1 significantly reduced specific energy consumption of the e-FO system from 0.693 ± 0.127 to 0.022 ± 0.004 kWh m-3 extracted water or from 1.103 ± 0.059 to 0.044 ± 0.002 kWh kg-1 reduced reversed solute. These results have demonstrated that the electrolysis-assisted RSF mitigation could be an energy-efficient method for controlling RSF towards sustainable FO applications.

Enhancing wastewater reuse by forward osmosis with self-diluted commercial fertilizers as draw solutes.

Using fertilizers as draw solutes in forward osmosis (FO) can accomplish wastewater reuse with elimination of recycling draw solute. In this study, three commercial fast-release all-purpose solid fertilizers (F1, F2 and F3) were examined as draw solutes in a submerged FO system for water extraction from either deionized (DI) water or the treated wastewater. Systematic optimizations were conducted to enhance water extraction performance, including operation modes, initial draw concentrations and in-situ chemical fouling control. In the mode of the active layer facing the feed (AL-F or FO), a maximum of 324 mL water was harvested using 1-M F1, which provided 41% of the water need for fertilizer dilution for irrigation. Among the three fertilizers, F1 containing a lower urea content was the most favored because of a higher water extraction and a lower reverse solute flux (RSF) of major nutrients. Using the treated wastewater as a feed solution resulted in a comparable water extraction performance (317 mL) to that of DI water in 72 h and a maximum water flux of 4.2 LMH. Phosphorus accumulation on the feed side was mainly due to the FO membrane solute rejection while total nitrogen and potassium accumulation was mainly due to RSF from the draw solute. Reducing recirculation intensity from 100 to 10 mL min(-1) did not obviously decrease water flux but significantly reduced the energy consumption from 1.86 to 0.02 kWh m(-3). These results have demonstrated the feasibility of using commercial solid fertilizers as draw solutes for extracting reusable water from wastewater, and challenges such as reverse solute flux will need to be further addressed.

High-efficient nitrogen removal by coupling enriched autotrophic-nitrification and aerobic-denitrification consortiums at cold temperature.

This study paid particular attention to total nitrogen removal at low temperature (10°C) by excellent coupling of enriched autotrophic nitrifying and heterotrophic denitrifying consortiums at sole aerobic condition. The maximum specific nitrifying rate of the nitrifying consortium reached 8.85mgN/(gSSh). Further test in four identical lab-scale sequencing batch reactors demonstrated its excellent performance for bioaugmentation in potential applications. On the other hand, the aerobic denitrifying consortium could achieve a specific denitrifying rate of 32.93mgN/(gSSh) under dissolved oxygen of 1.0-1.5mg/L at 10°C. Coupling both kinds of consortiums was proved very successful for a perfect total nitrogen (TN) removal at COD/N of 4 and dissolved oxygen of 1.5-4.5mg/L, which was hardly reached by any single consortium reported previously. The encouraging results from coupling aerobic consortiums implied a huge potential in practical treatment of low-strength domestic wastewater (200-300mg/L COD) during wintertime.