How Yong Ng

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.

Carbon nanotube supported MnO2 catalysts for oxygen reduction reaction and their applications in microbial fuel cells.

Three types of manganese dioxide, α-MnO(2), β-MnO(2), γ-MnO(2) were tested as alternative cathode catalysts for oxygen reduction reaction (ORR) in air-cathode microbial fuel cells (MFCs). Prepared by solution-based methods, the MnO(2) nanomaterials were comprehensively characterized, and their electrocatalytic activities in neutral electrolyte were investigated with the supporting material of carbon nanotubes (CNTs) by cyclic voltammetry (CV). The CV results showed that all MnO(2) species could catalyze ORR in neutral NaCl solution with different catalytic activities. β-MnO(2) had the highest catalytic activity due to its intrinsic structure and better interaction with CNTs. Three MnO(2) species were further used as cathode catalysts under optimized conditions in air-cathode cubic MFCs, in which mixed culture was inoculated as biocatalysts and domestic wastewater was used as the substrate in the anode chamber. It was also found that β-MnO(2) based MFC yielded the best performance with a power density of 97.8 mWm(-2) which was 64.1% that of the Pt-based MFC, and a lower internal resistance of 165 Ω. Furthermore, the COD removal efficiency of β-MnO(2) based MFC was estimated as 84.8%, higher than that of the Pt-based MFC. This study demonstrated that using β-MnO(2) on CNT support instead of Pt could potentially improve the feasibility of scaling up air-cathode MFCs for practical applications by lowering the material cost.

Ozone-biological activated carbon as a pretreatment process for reverse osmosis brine treatment and recovery

Ozonation was used in this study to improve biodegradability of RO brine from water reclamation facilities. An ozone dosage ranging from 3 to 10 mg O(3)/L and contact times of 10 and 20 min in batch studies were found to increase the biodegradability (BOD(5)/TOC ratio) of the RO brine by 1.8-3.5 times. At the same time, total organic carbon (TOC) removal was in the range of 5.3-24.5%. The lab-scale ozone-biological activated carbon (BAC) at an ozone dosage of 6.0mg O(3)/L with 20-min contact time was able to achieve 3 times higher TOC removal compared to using BAC alone. Further processing with Capacitive Deionization (CDI) process was able to generate a product water with better water quality than the RO feed water, i.e., with more than 80% ions removal and a lower TOC concentration. The ozone-BAC pretreatment has the potential of reducing fouling in the CDI process.

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.

Characterisation of initial fouling in aerobic submerged membrane bioreactors in relation to physico-chemical characteristics under different flux conditions

The initial fouling characteristics of aerobic submerged membrane bioreactors (MBRs) were analysed under different flux conditions. Physico-chemical analyses of the mixed liquor hinted that carbohydrates were more important to membrane fouling than proteins. However, this contrasted with the characterisation of foulants on the membrane surfaces. Micro-structural analyses of the foulants on the membrane surfaces showed that the dominant foulants were different under different flux conditions. Membrane fouling occurred through a biofilm-dominated process under lower flux conditions, but the mechanism shifted towards a non-biofilm, organic fouling process as the flux was increased. In spite of the differences in fouling mechanisms, it was found that the protein fraction on the membrane surfaces, in the initial stages of MBR operations, had the greatest impact in the rise of transmembrane pressure.

Effect of membrane type and material on performance of a submerged membrane bioreactor.

This study evaluated the impact of membrane type and material on filtration performance in a submerged membrane bioreactor (MBR) for municipal wastewater treatment. Three types of microfiltration membranes with similar pore size of 0.1 microm but different materials and types, phase-inversed polytetrafluoroethylene (PTFE), track-etched polycarbonate (PCTE) and track-etched polyester (PETE), were used. Changes in permeability with time for the PCTE and PTFE membranes appeared similarly, whereas the PETE membrane exhibited the most rapid flux decline. Lower TOC in the permeate compared to the supernatant was probably due to a combination of biodegradation by the biofilm (cake layer) developed on the membrane surface and further filtration by cake layer and narrowed pores. The faster permeability decline and higher TOC removal rate of the PETE membrane were attributed to an initial permeate flux higher than an average design flux, which led to a faster rate of fouling and thicker cake layer. Therefore, an MBR should not be operated at a flux higher than the average design flux for a specific type of membrane. A gradual increment of biomass concentration did not significantly affect membrane permeability of each membrane investigated. Dissolved organic carbon fractionation results showed that the composition of each fraction between the supernatant and permeates did not change significantly with time, suggesting that membrane hydrophobicity was not a dominant factor affecting MBR fouling in this study. The organic foulants desorbed from the PCTE membrane contained approximately 60% of hydrophobic fraction, which was probably attributable to the extracellular polymeric substances proteins released from the biomass attached to the membrane. While the total filtration resistance of the PTFE membrane was influenced by a higher surface roughness, those of the PETE and PCTE membranes, which had a similar and lower roughness, were affected by the initial operating flux.

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.

Determination of charge transfer resistance and capacitance of microbial fuel cell through a transient response analysis of cell voltage

An alternative method for determining the charge transfer resistance and double-layer capacitance of microbial fuel cells (MFCs), easily implemented without a potentiostat, was developed. A dynamic model with two parameters, the charge transfer resistance and double-layer capacitance of electrodes, was derived from a linear differential equation to depict the current generation with respect to activation overvoltage. This model was then used to fit the transient cell voltage response to the current step change during the continuous operation of a flat-plate type MFC fed with acetate. Variations of the charge transfer resistance and the capacitance value with respect to the MFC design conditions (biocatalyst existence and electrode area) and operating parameters (acetate concentration and buffer strength in the catholyte) were then determined to elucidate the validity of the proposed method. This model was able to describe the dynamic behavior of the MFC during current change in the activation loss region; having an R(2) value of over 0.99 in most tests. Variations of the charge transfer resistance value (thousands of Omega) according to the change of the design factors and operational factors were well-correlated with the corresponding MFC performances. However, though the capacitance values (approximately 0.02 F) reflected the expected trend according to the electrode area change and catalyst property, they did not show significant variation with changes in either the acetate concentration or buffer strength.

Comparison in performance of sediment microbial fuel cells according to depth of embedded anode.

Five rigid graphite plates were embedded in evenly divided sections of sediment, ranging from 2 cm (A1) to 10 cm (A5) below the top sediment layer. The maximum power and current of the MFCs increased in depth order; however, despite the increase in the internal resistance, the power and current density of the A5 MFC were 2.2 and 3.5 times higher, respectively, than those of the A1 MFC. In addition, the anode open circuit potentials (OCPs) of the sediment microbial fuel cells (SMFCs) became more negative with sediment depth. Based on these results, it could be then concluded that as the anode-embedding depth increases, that the anode environment is thermodynamically and kinetically favorable to anodophiles or electrophiles. Therefore, the anode-embedding depth should be considered an important parameter that determines the performance of SMFCs, and we posit that the anode potential could be one indicator for selecting the anode-embedding depth.

Sequential anaerobic-aerobic treatment of pharmaceutical wastewater with high salinity.

In this study, pharmaceutical wastewater with high total dissolved solids (TDSs) and chemical oxygen demand (COD) content was treated through a sequential anaerobic-aerobic treatment process. For the anaerobic process, an up-flow anaerobic sludge blanket (UASB) was applied, and a COD removal efficiency of 41.3±2.2% was achieved with an organic loading rate of 8.11±0.31gCOD/L/d and a hydraulic retention time of 48h. To evaluate the salinity effect on the anaerobic process, salts in the wastewater were removed by ion exchange resin, and adverse effect of salinity was observed with a TDS concentration above 14.92g/L. To improve the anaerobic effluent quality, the UASB effluent was further treated by a membrane bioreactor (MBR) and a sequencing batch reactor (SBR). Both the UASB+MBR and UASB+SBR systems achieved excellent organic removal efficiency, with respective COD removal of 94.7% and 91.8%. The UASB+MBR system showed better performance in both organic removal and nitrification.