Performance of submerged anaerobic membrane bioreactor at different SRTs for domestic wastewater treatment.

Three submerged anaerobic membrane bioreactors (SAnMBR) with solids retention times (SRTs) of 30, 60 and 90d were set up for domestic wastewater treatment. Total COD removal rates higher than 83% were achieved. Maximum biogas production was 0.5L CH4d(-1) (0.010L CH4 (gMLVSS)(-1)d(-1); MLVSS: mixed liquor volatile suspended solids) at a SRT of 90d. A longer SRT benefited biomass accumulation and biogas production due to enhanced dominancy of methanogens. With the SRT increasing from 30 to 90d, enhancement of hydrolysis from 35 to 56% was identified as the major reason for the better efficient acidification and methanogenesis observed. Best treatment performance and membrane fouling control were observed for the SAnMBR operated at a SRT of 60d. Soluble microbial products or specific soluble microbial products that accumulated with SRT decrease from 60 to 30d led to a faster membrane fouling. With an increase of SRT from 60 to 90d, higher mixed liquor suspended solids concentration caused more particle deposition on the membrane surface, while more soluble microbial products or specific soluble microbial products was attributed to more metabolism products generation.

Optimization of a microbial fuel cell for wastewater treatment using recycled scrap metals as a cost-effective cathode material.

Microbial fuel cell (MFC) for wastewater treatment is still hindered by the prohibitive cost of cathode material, especially when platinum is used to catalyze oxygen reduction. In this study, recycled scrap metals could be used efficiently as cathode material in a specially-designed MFC. In terms of raw power, the scrap metals ranked as follows: W/Co > Cu/Ni > Inconel 718 > carpenter alloy; however, in terms of cost and long term stability, Inconel 718 was the preferred choice. Treatment performance--assessed on real and synthetic wastewater--was considerably improved either by filling the anode compartment with carbon granules or by operating the MFC in full-loop mode. The latter option allowed reaching 99.7% acetate removal while generating a maximum power of 36 W m(-3) at an acetate concentration of 2535 mg L(-1). Under these conditions, the energy produced by the system averaged 0.1 kWh m(-3) of wastewater treated.

Effect of increasing anodic NaCl concentration on microbial fuel cell performance.

High salinity effluents represent an estimated 5% of the wastewater generated worldwide. In microbial fuel cells, high salinity is usually considered beneficial to power production because increased conductivity facilitates proton transfer and therefore decreases the internal resistance of the system. However, high salt concentrations are known to adversely affect the physiology of anaerobic microbial consortia. In this study, the effect of increasing NaCl concentration in the anode chamber of a microbial fuel cell fed with sodium acetate was tested. Adding up to 20 g L(-1) of NaCl enhanced the overall performance of the system, reducing the internal resistance by 33% and increasing the maximum power production by 30%. Higher NaCl concentration proved detrimental to the system. However, the Coulombic efficiency started to be affected at a much lower NaCl concentration of 10 g L(-1), showing that the anodophilic bacteria are sensitive to NaCl at relatively low concentrations.

Microbial fuel-cell-based toxicity sensor for fast monitoring of acidic toxicity.

Wastewater may contain various potential toxicants. A microbial fuel cell (MFC) is a device in which bacteria convert the chemical energy into electricity. If a toxic event occurs, microbial activity is inhibited and thus the power output of the MFC decreases. Therefore, an MFC could serve as an early toxicity warning device. A real-time biomonitoring system was developed using MFCs to detect the inflow of toxic substances into wastewater treatment systems. After the MFCs reached steady state, a toxic incident was created by adding HCl into the wastewater to alter its pH. Consequently, a rapid decrease in voltage was observed immediately, followed by a subsequent recovery. The optimal MFC design was a single-chamber air cathode MFC, where the anode and cathode were separated by a Selemion proton exchange membrane. Under an external resistance of 5 Ω, the maximum power averaged 0.23 ± 0.023 mW with domestic wastewater. The optimized MFC showed high sensitivity and fast recovery when exposed to the acidic toxic event. When the hydraulic retention time was decreased from 22 to 3.5 min, sensitivity of the MFC increased substantially. Finally, the extent of inhibition observed was found to be related to the toxicity level, suggesting that a dosage-response relationship exists.

Electrical performance of low cost cathodes prepared by plasma sputtering deposition in microbial fuel cells.

Microbial fuel cells (MFCs) could potentially be utilized for a variety of applications in the future from biosensors to wastewater treatment. However, the amount of costly platinum (Pt) used as a catalyst should be minimized via innovative deposition methods such as sputtering. In addition, alternative and low-cost catalysts, such as cobalt (Co), should be sought. In this study, ultra low Pt or Co cathodes (0.1 mg cm(-2)) were manufactured by plasma sputtering deposition and scanning electron micrographs revealed nano-clusters of metal catalyst in a porous structure favorable to the three-phase heterogeneous catalytic reaction. When operated in single-chamber air-cathode MFCs, sputtered-Co cathodes generated on average the same power as sputtered-Pt cathodes (0.27 mW cell(-1)) and only 27% less than conventional Pt-ink cathodes with a catalyst load 5 times higher (0.5 mg cm(-2)). Finally, microscopy and molecular analyses showed evidence of biocatalysis activity on metal-free cathodes.

A comparison of membranes and enrichment strategies for microbial fuel cells.

The external resistance is perhaps the easiest way to influence the operation of a microbial fuel cell (MFC). In this paper, three enrichment strategies, whereby the external resistance was fixed at: (1) a high value in order to maximize the cell voltage (U strategy); (2) a low value in order to maximize the current (I strategy); and (3) a value equal to the internal resistance of the MFC to maximize the power output (P strategy), were investigated. The I strategy resulted in increased maximum power generation and the likely reason is that electron transfer was facilitated under low external resistance, which in turn, favored the development of an electrochemically active biofilm. This experiment was conducted in a single-chamber MFC system equipped with a membrane electrode assembly, and a comparison of the performance achieved by five different membranes is also provided. Selemion was found to be a suitable alternative to Nafion.

Submerged anaerobic membrane bioreactor for low-strength wastewater treatment: effect of HRT and SRT on treatment performance and membrane fouling.

Three 6-L submerged anaerobic membrane bioreactors (SAnMBRs) with solids retention times (SRTs) of 30, 60 and infinite days were setup for treating synthetic low-strength wastewater at hydraulic retention times (HRTs) of 12, 10 and 8 h. Total COD removal efficiencies higher than 97% were achieved at all operating conditions. Maximum biogas production rate was 0.056 L CH(4)/g MLVSS d at an infinite SRT. A shorter HRT or longer SRT increased biogas production due to increased organic loading rate or enhanced dominancy of methanogenics. A decrease in HRT enhanced growth of biomass and accumulation of soluble microbial products (SMP), which accelerated membrane fouling. A drop in carbohydrate to protein ratio also inversely affected fouling. At 12-h HRT, the effect of SRT on biomass concentration in SAnMBRs was negligible and membrane fouling was controlled by variant surface modification due to different SMP compositions, i.e., higher carbohydrate and protein concentrations in SMP at longer SRT resulted in higher membrane fouling rate. At 8 and 10-h HRTs, infinite SRT in SAnMBR caused highest MLSS and SMP concentrations, which sped up particle deposition and biocake/biofilm development. At longer SRT, lower extracellular polymeric substances reduced flocculation of particulates and particle sizes, further aggravated membrane fouling.

Optimization of a Pt-free cathode suitable for practical applications of microbial fuel cells.

Microbial fuel cells (MFCs) are considered as a promising way for the direct extraction of biochemical energy from biomass into electricity. However, scaling up the process for practical applications and mainly for wastewater treatment is an issue because there is a necessity to get rid of unsustainable platinum (Pt) catalyst. In this study, we developed a low-cost cathode for a MFC making use of sputter-deposited cobalt (Co) as the catalyst and different types of cathode architecture were tested in a single-chambered air-cathode MFC. By sputtering the catalyst on the air-side of the cathode, increased contact with ambient oxygen significantly resulted in higher electricity generation. This outcome was different from previous studies using conventionally-coated Pt cathodes, which was due to the different technology used.

Floating-type microbial fuel cell (FT-MFC) for treating organic-contaminated water.

This study examines a floating-type microbial fuel cell (FT-MFC) that can be applied to treat organic-contaminated water without mechanical aid. The bottom of the anode compartment was left open to the aquatic environment and the cathode was exposed above the water surface. When four FT-MFCs were inoculated with anaerobic digest fluid (ADF) obtained from a brewery wastewater treatment system (Gwangju, Korea), the open circuit voltages were around 0.4-0.5 V. The initial chemical oxygen demand (COD) of ADF was 1700 ppm and decreased to 380 ppm over a 2-day period under the open circuit mode. Two of the four FT-MFCs were then switched to closed circuit mode to view the current production under batch operation. The current developed was around 0.25 mA and the COD value decreased to 230 ppm after 5 days. The acetate concentration used was varied from 5 to 50 mM to observe whether or notthe substrate was limited bythe growth of bacteria involved in current generation. It was found that the current was maximized at 0.67 mA when 5 mM of acetate was fed at a feeding rate of 0.08 mL/min. Maximum current density and maximum power density were 138 mA/m2 and 8 mW/m2, respectively. These results indicate that FT-MFCs could be applied in situ to treat organic-contaminated water in natural aquatic environments without mechanical aid. Further cathode operation optimization is expected to enhance the treatment of organic materials and corresponding current production.

Performance of forward (direct) osmosis process: membrane structure and transport phenomenon.

The performance of a forward (direct) osmosis (FO) process was investigated using a laboratory-scale unit to elucidate the effect of membrane structure and orientation on waterflux. Two types of RO membrane and a FO membrane were tested using ammonium bicarbonate, glucose, and fructose as the draw solution to extract water from a saline feed solution. The FO membrane was able to achieve higher water flux than the RO membranes under the same experimental conditions while maintaining high salt rejection of greater than 97%. Increasing operating temperature increased the water flux in FO process. To investigate the effect of membrane orientation on water flux, the FO membrane was tested normally (dense selective layer facing draw solution) and reversely (dense selective layer facing feed solution). Explanations on transport phenomenon in FO process were proposed which explain the observation that the FO membrane, when used in the normal orientation, performed better due to lesser internal concentration polarization. This study suggests that an ideal FO membrane should consist of a thin dense selective layer without any loose fabric support layer.

Membrane fouling of submerged membrane bioreactors: impact of mean cell residence time and the contributing factors.

In this study, four bench-scale pre-anoxic submerged membrane bioreactors (MBR) were operated simultaneously at different mean cell residence times (MCRTs) (3, 5, 10, and 20 days) to systematically elucidate the contributing factors of membrane fouling. Severe membrane fouling was first observed in the 3-day followed by the 5-day MCRT MBRs. Minimal membrane fouling was detected in the 10 and 20-day MCRT MBRs. The fouling of microfiltration membrane was not controlled by mixed liquor suspended solids concentration or zeta potential of biomass. Instead, membrane fouling rate increased with increasing soluble microbial products and extracellular polymeric substances concentrations, which both increased with decreasing MCRT. Total organic carbon, protein, carbohydrate, and UV254 absorbance in the mixed liquor supernatant increased with decreasing MCRT and were consistently higher than those of the effluent. Accumulation of carbohydrates rather than proteins in the mixed liquor supernatant was found to decrease with increasing MCRT. Normalized capilliary suction time value rather than the capilliary suction time value would indicate membrane fouling potential of a mixed liquor. Image analysis of the fouled membrane using scanning electron microscope and confocal laser scanning microscope showed that biofilm formation was the cause of membrane fouling.

Specific resistance to filtration of biomass from membrane bioreactor reactor and activated sludge: effects of exocellular polymeric substances and dispersed microorganisms.

This study investigates the effect of dispersed microorganisms and exocellular polymeric substances on biomass dewaterability. Specific resistance to filtration (SRF) was measured for biomass from a membrane bioreactor and a completely mixed activated sludge system. Both laboratory-scale reactors were fed with synthetic wastewater and operated at a high food-to-microorganism ratio (F/M) (1 to 11 kilograms chemical oxygen demand per kilogram mixed liquor volatile suspended solids per day [kgCOD/(kg MLVSS.d)]) and short solids retention times (0.25 to 5 d). The SRF values were affected by strong interactions of three parameters: (1) the mixed liquor suspended solids concentration, (2) the amount of dispersed microorganisms, and (3) the exocellular polymeric substances (EPS) concentration. At F/M smaller than 2 kg COD/(kg MLSS.d) and mixed liquor suspended solids (MLSS) concentration higher than 2000 mg/L, increasing amount of dispersed microorganisms in the biomass yielded higher SRF values. However, at high F/M (> 5 kg COD/kg MLSS.d) and low MLSS concentrations (< 600 mg/L), lower EPS concentrations resulted in slightly smaller SRF values, even though the amount of dispersed microorganisms in the biomass was much higher. Thus, at low MLSS concentrations, EPS concentrations rather than the amount of dispersed microorganisms tend to control SRF.

Membrane bioreactor operation at short solids retention times: performance and biomass characteristics.

This study investigated the performance and biomass characteristics of a membrane bioreactor (MBR) and a completely mixed activated sludge (CMAS) system operated at short solids retention times (SRT) ranging from 0.25 to 5 d and hydraulic retention times of 3 and 6 h. The lab-scale reactors were fed with synthetic wastewater to ensure consistency in feed composition. Results show the MBR was capable of achieving excellent quality effluent regardless of the extremely short SRT. The MBR removal efficiencies ranged from approximately 97.3-98.4% (TCOD) in the MBR, compared to 77.5-93.8% (TCOD) and 94.1-97.0% (SCOD) in the CMAS. Nitrification completely ceased when SRT was < 2.5 d. The MBR biomass was composed of small, weak and uniform-sized flocs with large mass of short filamentous organisms and mainly dispersed microorganisms at SRT of 5 and 0.25 d, respectively. In contrast, the CMAS sludge was composed of large flocs with filamentous organisms as a backbone at SRT > 2.5 d. The CMAS flocs were smaller and weaker at shorter SRT. The MBR sludge contained a much higher fraction of non-flocculating microorganisms. This fraction increased significantly with decreasing SRT. It was found that the concentrations of protein and carbohydrates in the exocellular polymeric substances for both the MBR and the CMAS decreased with increasing F/M ratio or decreasing SRT. The combination of increasing amounts of non-flocculating microorganisms and a reduction of EPS at shorter SRT in both reactors contributed to deteriorating sludge settling properties. A significant presence of dispersed biomass and small flocs in MBR contributed to better reactor performance probably due to less mass transfer resistance.