Comparison of membrane- and cloth-cathode assembly for scalable microbial fuel cells: Construction,performance and cost

The aim of this study is to compare the performance of different membrane cathode assembly (MCA) and cloth-cathode assembly (CCA) in air-chamber microbial fuel cells (MFCs) and provide an optimum cathode configuration for MFC scaling up. Two MCAs were prepared by hot-pressing carbon cloth containing cathodic catalyst to anion exchange membrane (AEM) and cation exchange membrane (CEM), respectively. A CCA was built by coating GORE-TEX^® cloth with a mixture of nickel-based conductive paint and cathodic catalyst. Under the fed-batch mode using brewery wastewater, the MFCs were compared with respect to power production, coulombic efficiency, COD removal, internal resistance and material cost. The experimental results show that CCA is a more favorable alternative than MCAs due to its easier preparation, higher maximum power density and COD removal, and lower internal resistance and cost. The optimum cathode assembly of CCA is cost-effective and mechanically robust enough to meet the important requirements for MFC scalability.     

Removal of carbonaceous and nitrogenous pollutants from a synthetic wastewater using a membrane-coupled bioreactor

Two modified Ludzack-Ettinger (MLE)-type membrane-coupled bioreactors (MBRs) were investigated in this study for the purpose of removing both nitrogenous and carbonaceous pollutants from a synthetic wastewater. During the first MBR experiment, removal efficiencies were high (>90%) for chemical oxygen demand (COD) and ammonia, but total nitrogenous pollutant removal efficiency was poor (~25%). Bacterial community analysis of ammonia oxidizing bacteria (AOB) by a nested PCR-DGGE approach detected two Nitrosomonas-like populations and one Nitrosospira-like population. During the initial portion of the second MBR experiment, COD and ammonia removal efficiencies were similar to the first MBR experiment until the COD of the influent wastewater was increased to provide additional electron donors to support denitrification. Total nitrogen removal efficiencies eventually exceeded 90%, with a hydraulic residence time (HRT) of 24 h and a recirculation ratio of 8. When the HRT of the MBR experiment was decreased to 12 h, however, ammonia removal efficiency was adversely affected. A subsequent increase in the HRT to 18 h helped improve removal efficiencies for both ammonia (>85%) and total nitrogenous compounds (~70%). Our research demonstrates that MBRs can be effectively designed to remove both carbonaceous and nitrogenous pollutants. The ability of the microbial community to switch between anoxic (denitrifying) and oxic (nitrifying) conditions, however, represents a critical process constraint for the application of MLE-type MBR systems, such that little benefit is gained compared to conventional designs.     

Treatment of leachate from a solid waste landfill site using a two-stage anaerobic filter

Raw leachate was treated using a two-stage upflow anaerobic filter process. Leachate from a solid waste landfill site, which received both municipal and industrial wastes, contained high organic matter (17-21 g/L COD, 13-14 g/L BOD, and 3.5-4.6 g/L volatile acids), and low metal (Zn and Fe) concentrations. Depending on sampling time, leachate composition and characteristics varied considerably. At an organic loading up to 4 g COD/day(2) media area, the BOD and COD removal percentages were 98 and 91%, respectively. The biofilters were also effective for metal removal. However, the filter effluent contained a high concentration of ammonia. System overloading was characterized by the accumulation of large quantities of volatile acids and by a now ratio of alkalinity/volatile acids, resulting in low COD removal and reduced gas production. Once the first filter was upset, the second stage could only partially respond to the volatile acids accumulated in the effluent of first filter.     

Airborne enteric micro-organisms and ammonia levels in diaper-changing rooms in kindergartens

Aims: We evaluated risks associated with diaper changing in Finnish kindergartens where children were using either modern disposable paper or reusable cloth diapers. Methods and Results: We determined enteric micro‐organisms and ammonia in diaper‐changing rooms in four kindergartens in autumn and winter in the ambient air. No coliphages were detected in the air. The numbers of faecal coliforms and enterococci in air were typically low regardless of whether the children used either paper or cloth diapers. Ammonia concentrations increased over the background level because of diaper changing. Conclusions: The numbers of bacteria or coliphages are not expected to pose any high air hygiene risks, and increased ammonia air concentrations are unlikely to impair the health of staff or children when diapers are changed in modern kindergartens. However, increased ammonia gas concentrations indicate that some other diaper‐related gas‐phase emissions should be studied to understand better diaper‐related health risks. Significance and Impact of the Study: Modern reusable cloth baby diapers and the modern paper baby diapers used in this study are equally safe with respect to risks from airborne virus, bacteria or ammonia.     

Control of COD/N ratio for nutrient removal in a modified membrane bioreactor (MBR) treating high strength wastewater

A modified membrane bioreactor (MBR) system has been developed to evaluate the efficiency of nutrient removal in treating synthetic high strength water. This study examined the effect of influent COD/N ratio on this system. Results showed that above 95.0% removal efficiencies of organic matter were achieved; indicating COD removal was irrespective of COD/N ratio. The average removal efficiencies of total nitrogen (TN) and phosphate (PO(4)(3-)-P) with a COD/N ratio of 9.3 were the highest at 90.6% and 90.5%, respectively. Furthermore, TN removal was primarily based on simultaneous nitrification and denitrification (SND) process occurred in the aerobic zone. Decreased COD/N ratios to 7.0 and 5.3, TN removal efficiencies in steady-states were 69.3% and 71.2%, respectively. Both aerobic SND and conventional biological nitrification/denitrification contributed to nitrogen removal and the latter played dominant effect. PO(4)(3-)-P-release and uptake process ceased in steady-states of COD/N 7.0 and 5.3, which decreased its removal efficiency significantly.     

Oxygen availability effect on the performance of air‐breathing cathode microbial fuel cell

The effect of the oxygen availability over the performance of an air‐breathing microbial fuel cell (MFC) was studied by limiting the oxygen supply to the cathode. It was found that anodic reaction was the limiting stage in the performance of the MFC while oxygen was fully available at cathode. As the cathode was depleted of oxygen, the current density becomes limited by oxygen transport to the electrode surface. The exerted current density was maintained when oxygen mole fraction was higher than 10% due to the very good performance of the cathodic catalysts. However, the current density drastically falls when working at lower concentrations because of mass transfer limitations. In this sense it must be highlighted that the maximum exerted power, when oxygen mole fraction was higher than 10%, was almost three times higher than that obtained when oxygen mole fraction was 5%. Regarding to the wastewater treatment, a significant decrease in the COD removal was obtained when the MFC performance was reduced due to the limited availability of oxygen, which indicates the significant role of the electrogenic microorganisms in the COD removal in MFC. In addition, the low availability of oxygen at the cathode leads to a lower presence of oxygen at the anode, resulting in an increase in the coulombic efficiency. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:900–907, 2015     

All‐Solid‐State,Foldable, and Rechargeable Zn‐Air Batteries Based on Manganese Oxide Grown on Graphene‐Coated Carbon Cloth Air Cathode

Pliable, safe, and inexpensive energy storage devices are in demand to power modern flexible electronics. In this work, a foldable battery based on a solid‐state and rechargeable Zn‐air battery is introduced. The air cathode is prepared by coating graphene flakes on pretreated carbon cloth to form a dense, interconnected, and conducting carbon network. Manganese oxide hierarchical nanostructures are subsequently grown on the large surface area carbon network, leading to high loading of active catalyst per unit volume while maintaining the mechanical and electrical integrity of the air cathode. Solid‐state and rechargeable Zn‐air battery with such air cathode exhibits similar polarization curve and resistance at its flat and folded states. The folded battery is able to deliver a power density as high as ≈32 mW cm^?2 and good cycling stability of up to 110 cycles. In addition, the flat battery shows similar discharge/charge curve and stable cycling performance after 100 times of repeated folding and unfolding, indicating its high mechanical robustness.     

人工湿地处理炼油废水的生态效益研究

来自茂名石化公司的炼油废水含有较高浓度的污染物 ,超过广东省规定的排放标准 ,未经处理不能直接排放。 4种草本植物 ,香根草、芦苇、宽叶香蒲和蒲草被用来测试建植人工湿地的效果以及所建植的人工湿地处理炼油废水的效果。在为期 2个月的处理过程中 ,人工湿地在前期对炼油废水的净化效率很高 ,它们对第一批高浓度废水中氨氮、COD、BOD和油的去除率分别是 97.7%、78.2 %、91 .4%和 95 .3% ,对第一批低浓度污水分别是 97.1 %、71 .5 %、73.7%和 89.8%。但是 ,随着时间的推移 ,湿地的净化效果会有一定程度的下降 ,然后逐渐趋于稳定。湿地对氨氮、COD、BOD和油类的去除效率始终表现为氨氮 >油类 >BOD>COD,但植物对它们的净去除量却是 COD>BOD>氨氮和油类。湿地建植之初 ,植物的净化功能很弱 ,但随植物的生长和生物量的增加而逐渐增强。然而 ,不同植物种对废水的净化率很接近 ,基本上无显著性差异。被测试的 4种植物在污水湿地中的生长表现都好过在清水湿地中的 ,但香根草、芦苇和宽叶香蒲在高浓度废水的分蘖数比在低浓度的废水少些 ,而蒲草相反 ,表明高浓度污水相对于低浓度污水而言已经对前面 3个种产生了伤害 ,却仍促进蒲草生长。在清水培养阶段 ,香根草产生分蘖的速度是 4个种中最低的 ;进入污水培养阶     

Effect of COD/N ratio and salinity on the performance of sequencing batch reactors

The effects of COD/N ratio (3-6) and salt concentration (0.5-2%) on organics and nitrogen removal efficiencies in three bench top sequencing batch reactors (SBRs) with synthetic wastewater and one SBR with fish market wastewater were investigated under different operating schedules. The solids retention time (SRT, 20-100 days) and aeration time (4-10h) was also varied to monitor the performance. For synthetic wastewater, chemical oxygen demand (COD) removal efficiencies were consistently greater than 95%, irrespective of changes in COD/N ratio, aeration time and salt concentrations. Increasing the salt concentrations decreased the nitrification efficiency, while high COD/N ratio's favored better nitrogen removal (>90%). The treatment of real saline wastewater ( approximately 3.2%) from a fish market showed high COD (>80%) and nitrogen (>40%) removal efficiencies despite high loading rate and COD/N fluctuations, which is due to the acclimatization of the biomass within the SBR.     

Biological treatment of a synthetic space mission wastewater using a membrane-aerated,membrane-coupled bioreactor (M2BR)

This paper describes the membrane-aerated, membrane-coupled bioreactor (M2BR), which was developed for wastewater treatment during long-term space missions because it achieves aeration and biomass separation using components that are compatible with microgravity conditions. In the experiments described herein, the M2BR was used to treat a synthetic wastewater formulated by NASA to simulate the wastewater typically collected during space missions. The M2BR was able to achieve more than 90% removal of both chemical oxygen demand (COD) and total nitrogen when it was fed a modified NASA wastewater that had a 4:1 COD to nitrogen ratio. When the full-strength synthetic wastewater was fed to the M2BR (COD:N=1), however, the nitrogenous pollutant removal efficiency was adversely affected because of either insufficient oxygen transfer to support nitrification (an air-fed M2BR) or insufficient electron donor to support denitrification (an oxygen-fed M2BR). In conclusion, the M2BR provides considerable promise for wastewater treatment during long-term space missions, although additional research is needed to identify the best approach to treat the space mission wastewater, which poses a unique challenge because of its low COD:N ratio.     

Analysis of ammonia loss mechanisms in microbial fuel cells treating animal wastewater

Ammonia losses during swine wastewater treatment were examined using single- and two-chambered microbial fuel cells (MFCs). Ammonia removal was 60% over 5 days for a single-chamber MFC with the cathode exposed to air (air-cathode), versus 69% over 13 days from the anode chamber in a two-chamber MFC with a ferricyanide catholyte. In both types of systems, ammonia losses were accelerated with electricity generation. For the air-cathode system, our results suggest that nitrogen losses during electricity generation were increased due to ammonia volatilization with conversion of ammonium ion to the more volatile ammonia species as a result of an elevated pH near the cathode (where protons are consumed). This loss mechanism was supported by abiotic tests (applied voltage of 1.1 V). In a two-chamber MFC, nitrogen losses were primarily due to ammonium ion diffusion through the membrane connecting the anode and cathode chambers. This loss was higher with electricity generation as the rate of ammonium transport was increased by charge transfer across the membrane. Ammonia was not found to be used as a substrate for electricity generation, as intermittent ammonia injections did not produce power. The ammonia-oxidizing bacterium Nitrosomonas europaea was found on the cathode electrode of the single-chamber system, supporting evidence of biological nitrification, but anaerobic ammonia-oxidizing bacteria were not detected by molecular analyses. It is concluded that ammonia losses from the anode chamber were driven primarily by physical-chemical factors that are increased with electricity generation, although some losses may occur through biological nitrification and denitrification.     

Nitrogen removal from polluted river by enhanced floating bed grown canna

Floating beds of canna can remove nitrogen from polluted river water, but the removal efficiency is not very high in a short time, so some enhanced methods must be used to improve nitrogen removal efficiency by the system. Immobilized denitrifying bacteria and aeration were added into the canna floating bed. The experimental results showed that these enhancements substantially improved the nitrogen removal efficiency of the floating beds. With the enhancements, total nitrogen removal was 72.1%, ammonia nitrogen oxidation was 100%, nitrate nitrogen removal was 75.8%, nitrite nitrogen removal was 95.9%, and COD removal was 94.6% in 5 days. Without the enhancements, the canna floating bed system removed only 50.4% of the total nitrogen, 22.4% of the nitrate nitrogen, 5.3% of the nitrite nitrogen and 39.9% of the COD, respectively, in 5 days, but did accomplish 100% oxidation of the ammonia nitrogen. The experimental results provide an improved method for the treatment of polluted river water.     

The effects on operation conditions of sludge retention time and carbon/nitrogen ratio in an intermittently aerated membrane bioreactor (IAMBR)

An intermittently aerated membrane bioreactor (IAMBR) system has been developed to improve the efficiency of nutrient removal, and for the stable treatment of organic matter which is contained as suspended solid (SS) in the influent. The important operating factors of an intermittently aerated bioreactor (IABR) are sludge retention times (SRTs) and carbon/nitrogen (C/N) ratios. Because research on IAMBR is young, this paper explores the effect of SRTs and C/N ratios on these systems. For SRTs of 20, 25, 30, and 40 days, there was little difference in the removal of COD, T–N, and T–P. In comparing C/N ratios of 4.5, 7, and 10, the COD concentration in permeate with a C/N ratio of 10 was most stable, although the concentration of organic matter in the influent was high. The removal efficiencies of T–N and T–P in permeate with a C/N ratio of 10 were the highest at 92.9% and 88.9%, respectively. This implies that a C/N ratio above 10 should be maintained for a nutrient removal efficiency of approximately 90%.     

生物膜法强化净化氨氮污染水体及其微生物群落解析

【目的】比较不同营养条件及挂膜方式下生物膜法对氨氮污染水体的净化效果及其功能微生物群落结构。【方法】设置空白(Blank)、自然成膜(Raw)、预附脱氮菌强化挂膜(PCC)3组生物膜反应器,利用末端限制性片段长度多态性(T-RFLP)技术和非度量多维标度(NMDS)分析方法对生物膜反应器转化氨氮过程中微生物群落结构及其演替过程进行动态解析。【结果】在C/N=1:1时,除PCC在起始阶段短暂具有较高的氨氮脱除效率外,Blank、Raw和PCC最终均表现出较低的氨氮转化效率(10%-20%)。改变C/N=2:1后,Raw和PCC对人工合成污水中NH4+-N的转化率均提高至95%以上,而且Raw与PCC的群落结构在C/N=2:1时具有较高的相似性,优势菌群主要为γ-变形菌纲(Gammaproteobacteria)、放线菌纲(Actinobacteria)和硝化螺菌纲(Nitrospira)。【结论】C/N是影响生物膜反应器氨氮去除效果及驱动生物膜反应器中细菌群落结构发生改变的重要因子。     

Reactivation characteristics of stored aerobic granular sludge using different operational strategies

Aerobic granules after 6 months storage were employed in identical sequencing batch reactors (SBRs) using synthetic wastewater to investigate the impacts of different operational strategies on granules' reactivation process. The SBRs were operated under three operational strategies for reactivation of (a) different organic loading rate (OLR); (b) different ammonia concentration; and (c) different shear force (a superficial upflow air velocity). The results indicated that granules after long-term storage could be successfully recovered after 7 days of operation, and the excellent granule reactivation performance was closely related to the operational strategies, since inappropriate operational strategies could cause the outgrowth of filamentous bacteria and granule disintegration. Based on comprehensive comparison of reactivation performance under different operational strategies, the optimal operation strategy for granule reactivation was suggested at OLR of 0.8 kg COD/m(3)/day, ammonia concentration of 15-20 mg/L, and a superficial upflow air velocity of 2.6 cm/s. After 7 days operation under the optimal strategy, the dark brown granules (12 months storage) restored their bioactivities to previous state, in terms of COD removal efficiency (97.44%) and specific oxygen uptake rate (40.63 mg O(2)/g SS h(-1)). The results shed light on the future practical application of stored aerobic granules as bioseed for reactor fast start-up.     

Air-cathode structure optimization in separator-coupled microbial fuel cells

Microbial fuel cells (MFC) with 30% wet-proofed air cathodes have previously been optimized to have 4 diffusion layers (DLs) in order to limit oxygen transfer into the anode chamber and optimize performance. Newer MFC designs that allow close electrode spacing have a separator that can also reduce oxygen transfer into the anode chamber, and there are many types of carbon wet-proofed materials available. Additional analysis of conditions that optimize performance is therefore needed for separator-coupled MFCs in terms of the number of DLs and the percent of wet proofing used for the cathode. The number of DLs on a 50% wet-proofed carbon cloth cathode significantly affected MFC performance, with the maximum power density decreasing from 1427 to 855 mW/m² for 1–4 DLs. A commonly used cathode (30% wet-proofed, 4 DLs) produced a maximum power density (988 mW/m²) that was 31% less than that produced by the 50% wet-proofed cathode (1 DL). It was shown that the cathode performance with different materials and numbers of DLs was directly related to conditions that increased oxygen transfer. The coulombic efficiency (CE) was more affected by the current density than the oxygen transfer coefficient for the cathode. MFCs with the 50% wet-proofed cathode (2 DLs) had a CE of >84% (6.8 A/m²), which was substantially larger than that previously obtained using carbon cloth air-cathodes lacking separators. These results demonstrate that MFCs constructed with separators should have the minimum number of DLs that prevent water leakage and maximize oxygen transfer to the cathode.     

Continuous removal of aromatic hydrocarbons by an AF reactor under denitrifying conditions

Removal of three typical aromatic hydrocarbons, benzene, biphenyl and naphthalene by an anaerobic filter (AF) reactor under continuous mode and denitrifying conditions was studied. Results showed that the AF reactor could degrade these aromatic hydrocarbons effectively under above-mentioned conditions. When influent wastewater contained 900 mg COD/l and about 60 mg (total aromatic hydrocarbons)/l, 90% and 84% removal efficiency could be achieved for them respectively. When COD/NO₃ ⁻-N ratio (C/N) was in the range 5–30, the removal of benzene was slightly influenced by C/N and it remained stable at about 90%. However, degradation of naphthalene, biphenyl and total COD was greatly influenced by C/N, and highest removal was achieved at C/N = 15, it was 90%, 85% and 82% for COD, naphthalene and biphenyl, respectively. Degradation of these three aromatic hydrocarbons followed the order: benzene > naphthalene > biphenyl.     

Degradation of predigested distillery effluent by isolated bacterial strains

Batch studies were conducted on degradation of anaerobically digested distillery wastewater by three bacterial strains, viz. Xanthomonas fragariae, Bacillus megaterium and Bacillus cereus in free and immobilized form, isolated from the activated sludge of a distillery wastewater treatment plant. The removal of COD and colour with all the three strains increased with time up to 48 hr and only marginal increase in COD and colour removal efficiency was observed beyond this period up to 72 hr. After this period removal efficiency remained fairly constant up to 120 hr. The maximum COD and colour removal efficiency varied from 66 to 81% and 65 to 75%, respectively for both free and immobilized cells of all the three strains. The strain Bacillus cereus showed the maximum efficiency of COD (81%) and colour (75%) removal out of the three strains. An interrelationship between the percent COD and colour removal was carried out by correlation and regression analysis and was justified by high values of coefficient of correlation (r = 0.99) for all the cases. The first order removal rate kinetics was also applied and rate constants were evaluated for COD and colour removal efficiencies.     

Influence of diligent disintegration on anaerobic biomass and performance of microbial fuel cell

Objectives

To enhance the performance of microbial fuel cells (MFC) by increasing the surface area of cathode and diligent mechanical disintegration of anaerobic biomass.

Results

Tannery effluent and anaerobic biomass were used. The increase in surface area of the cathode resulted in 78% COD removal, with the potential, current density, power density and coulombic efficiency of 675 mV, 147 mA m^?2, 33 mW m^?2 and 3.5%, respectively. The work coupled with increased surface area of the cathode with diligent mechanical disintegration of the biomass, led to a further increase in COD removal of 82% with the potential, current density, power density and coulombic efficiency of 748 mV, 229 mA m^?2, 78 mW m^?2 and 6% respectively.

Conclusions

Mechanical disintegration of the biomass along with increased surface area of cathode enhances power generation in vertical MFC reactors using tannery effluent as fuel.

     

Influence of COD/NH3–N ratio on organic removal and nitrification using a modified RBC

Synthetic wastewaters were prepared with different influent concentrations of ammonia nitrogen (NH₃–N) and COD and the treatment studies were conducted using a rotating biological contactor (RBC). If organic removal and nitrification can be simultaneously effected in one process, it will be an ideal solution to water pollution control. The RBC used in the present study was a four stage laboratory model and the discs were modified by attaching porous netlon sheets to enhance biofilm area. The COD loads (S ₀) used were about 1000 and 1500?mg/l whereas NH₃–N concentrations used were in the range of 20 to 185?mg/l. Hydraulic load (q) of 0.03?m³?.?m^-2?.?d^-1 and ammonia nitrogen loadings in the range of 0.66 to 5.5?g NH₃–N?.?m^-2?.?d^-1 were used. The RBC was operated at two different rotating speeds of 6 and 12?rpm. The results showed that the nitrification and percentage of COD removal were not affected up to the value of the COD/NH₃–N in the range from 47 to 23 at w=6?rpm and for an average influent COD of 1003?mg/l. Beyond that range only the nitrification rate decreased much whereas the percentage of COD removal was not affected. Similarly, at an influent COD load of 1557?mg/l, the nitrification and percentage COD removal were not affected for the value of the COD/NH₃–N in the range from 44 to 23 but beyond that range only the nitrification rate decreased while the percentage of COD removal was approximately constant and still high. A correlation plot between the NH₃–N removed and NH₃–N applied was presented at a rotating speed of 6?rpm and it was found that the nitrification rate of 3.93?g NH₃–N?.?m^-2?.?d^-1 was achieved at ammonia loading of 5.55?g NH₃–N?.?m^-2?.?d^-1. Also the results at w=12?rpm showed improvement of nitrification rate over those at 6?rpm.