Enhanced hydrogen production from waste activated sludge by cascade utilization of organic matter in microbial electrolysis cells

Fermentative hydrogen production from waste activated sludge (WAS) has low H₂ yield because WAS contains limited amounts of carbohydrate suitable for use by hydrogen-producing bacteria. Here, augmentation of hydrogen production from WAS by microbial electrolysis cells (MECs) was implemented. H₂ yields of 3.89 ± 0.39 mg-H₂/g-DS (5.67 ± 0.61 mg-H₂/g-VSS) from raw WAS and 6.78 ± 0.94 mg-H₂/g-DS (15.08 ± 1.41 mg-H₂/g-VSS) from alkaline-pretreated WAS were obtained in the two-chamber MECs (TMECs). This was several times higher than yields obtained previously by fermentation. Single-chamber MECs (SMECs) with low internal resistance showed a H₂ production rate that 13 times that of TMECs with similar H₂ yield when alkaline-pretreated WAS was used. However, methanogenesis was detected after several batch cycles. A yield balance calculation revealed that carbohydrates were not the only substrates for electrohydrogenesis. Protein and its acidification products, such as volatile fatty acids are also responsible for a portion of H₂ generation in MEC. Characterization of WAS in TMECs by three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy with parallel factor analysis indicated that electrohydrogenesis reacted on the extracellular polymeric substances and intracellular substances of WAS. Cascade utilization of organic matter in MECs increased hydrogen production from WAS. MECs showed high hydrogen yield from WAS, fewer H₂ sinks, and insensitivity to temperature. Optimizing MEC configurations and operation conditions and improving the pretreatment processes of WAS are necessary before practical application can take place on a large scale.     

Hydrogen production using single-chamber membrane-free microbial electrolysis cells

Microbial electrohydrogenesis provides a new approach for hydrogen generation from renewable biomass. Membranes were used in all the reported microbial electrolysis cells (MECs) to separate the anode and cathode chambers. To reduce the potential losses associated with membrane and increase the energy recovery of this process, single-chamber membrane-free MECs were designed and used to investigate hydrogen production by one mixed culture and one pure culture: Shewanella oneidensis MR-1. At an applied voltage of 0.6V, this system with a mixed culture achieved a hydrogen production rate of 0.53m(3)/day/m(3) (0.11m(3)/day/m(2)) with a current density of 9.3A/m(2) at pH 7 and 0.69m(3)/day/m(3) (0.15m(3)/day/m(2)) with a current density of 14A/m(2) at pH 5.8. Stable hydrogen production from lactic acid by S. oneidensis was also observed. Methane was detected during the hydrogen production process with the mixed culture and negatively affected hydrogen production rate. However, by employing suitable approaches, such as exposure of cathodes to air, the hydrogenotrophic methanogens can be suppressed. The current density and volumetric hydrogen production rate of this system have potential to increase significantly by further reducing the electrode spacing and increasing the ratio of electrode surface area/cell volume.     

Electricity generation from swine wastewater using microbial fuel cells

Microbial fuel cells (MFCs) represent a new method for treating animal wastewaters and simultaneously producing electricity. Preliminary tests using a two-chambered MFC with an aqueous cathode indicated that electricity could be generated from swine wastewater containing 8320 +/- 190 mg/L of soluble chemical oxygen demand (SCOD) (maximum power density of 45 mW/m2). More extensive tests with a single-chambered air cathode MFC produced a maximum power density with the animal wastewater of 261 mW/m2 (200 omega resistor), which was 79% larger than that previously obtained with the same system using domestic wastewater (146 +/- 8 mW/m2) due to the higher concentration of organic matter in the swine wastewater. Power generation as a function of substrate concentration was modeled according to saturation kinetics, with a maximum power density of P(max) = 225 mW/m2 (fixed 1000 omega resistor) and half-saturation concentration of K(s) = 1512 mg/L (total COD). Ammonia was removed from 198 +/- 1 to 34 +/- 1 mg/L (83% removal). In order to try to increase power output and overall treatment efficiency, diluted (1:10) wastewater was sonicated and autoclaved. This pretreated wastewater generated 16% more power after treatment (110 +/- 4 mW/m2) than before treatment (96 +/- 4 mW/m2). SCOD removal was increased from 88% to 92% by stirring diluted wastewater, although power output slightly decreased. These results demonstrate that animal wastewaters such as this swine wastewater can be used for power generation in MFCs while at the same time achieving wastewater treatment.     

Pyrosequencing reveals highly diverse microbial communities in microbial electrolysis cells involved in enhanced H2 production from waste activated sludge

Renewable H₂ production from a plentiful biomass, waste activated sludge (WAS), can be achieved by fermentation, but the yields are low. The use of a microbial electrolysis cell (MEC) can increase the H₂ production yields to several times that of fermentation. We have proved that the enhancement of H₂ production was due to the ability of MECs to use a wider range of organic matter in WAS than in fermentation. To support this result strongly, we here investigated the microbial community structures of WAS and anode biofilms in WAS-fed MECs. A pyrosequencing analysis based on the bacterial 16S rRNA gene showed that dominant populations in MECs were more diverse than those in WAS (inoculum and substrate) after enrichment, and there was a clear distinction between MECs and WAS in microbial community structure. Diverse acid-producing bacteria and exoelectrogens (predominance of Geobacter) were detected in MECs but they were only rarely found in WAS. It has been reported that these acid-producing bacteria can ferment various sugars and amines with acetate, propionate, and butyrate as their major by-products. This was consistent with our chemical analyses. Detected exoelectrogens are known to use these organic acids (mainly acetate) and certain sugars to directly produce current for H₂ generation at the cathodes in the MECs. Using quantitative real-time PCR, we demonstrated that a consistent feed of alkaline-pretreated WAS containing large amounts of acetate led to a predominance of acetoclastic methanogens, while hydrogenotrophic methanogens were abundant in MECs fed both raw and alkaline-pretreated WAS. Syntrophic interactions between phylogenetically diverse microbial populations in anodophilic biofilms were found to drive the efficient cascade utilization of organic matter in WAS.     

Hydrogen and methane production from household solid waste in the two-stage fermentation process

A two-stage process combined hydrogen and methane production from household solid waste was demonstrated working successfully. The yield of 43 mL H(2)/g volatile solid (VS) added was generated in the first hydrogen production stage and the methane production in the second stage was 500 mL CH(4)/g VS added. This figure was 21% higher than the methane yield from the one-stage process, which was run as control. Sparging of the hydrogen reactor with methane gas resulted in doubling of the hydrogen production. pH was observed as a key factor affecting fermentation pathway in hydrogen production stage. The optimum pH range for hydrogen production in this system was in the range from 5 to 5.5. The short hydraulic retention time (2 days) applied in the first stage was enough to separate acidogenesis from methanogenesis. No additional control for preventing methanogenesis in the first stage was necessary. Furthermore, this study also provided direct evidence in the dynamic fermentation process that, hydrogen production increase was reflected by acetate to butyrate ratio increase in liquid phase.     

Innovative self-powered submersible microbial electrolysis cell (SMEC) for biohydrogen production from anaerobic reactors

A self-powered submersible microbial electrolysis cell (SMEC), in which a specially designed anode chamber and external electricity supply were not needed, was developed for in situ biohydrogen production from anaerobic reactors. In batch experiments, the hydrogen production rate reached 17.8 mL/L/d at the initial acetate concentration of 410 mg/L (5 mM), while the cathodic hydrogen recovery (RH₂) and overall systemic coulombic efficiency (CE_os) were 93% and 28%, respectively, and the systemic hydrogen yield (YH₂) peaked at 1.27 mol-H₂/mol-acetate. The hydrogen production increased along with acetate and buffer concentration. The highest hydrogen production rate of 32.2 mL/L/d and YH₂ of 1.43 mol-H₂/mol-acetate were achieved at 1640 mg/L (20 mM) acetate and 100 mM phosphate buffer. Further evaluation of the reactor under single electricity-generating or hydrogen-producing mode indicated that further improvement of voltage output and reduction of electron losses were essential for efficient hydrogen generation. In addition, alternate exchanging the electricity-assisting and hydrogen-producing function between the two cell units of the SMEC was found to be an effective approach to inhibit methanogens. Furthermore, 16S rRNA genes analysis showed that this special operation strategy resulted same microbial community structures in the anodic biofilms of the two cell units. The simple, compact and in situ applicable SMEC offers new opportunities for reactor design for a microbial electricity-assisted biohydrogen production system.     

Hydrogen and electricity production from a food processing wastewater using fermentation and microbial fuel cell technologies

Hydrogen can be produced from fermentation of sugars in wastewaters, but much of the organic matter remains in solution. We demonstrate here that hydrogen production from a food processing wastewater high in sugar can be linked to electricity generation using a microbial fuel cell (MFC) to achieve more effective wastewater treatment. Grab samples were taken from: plant effluent at two different times during the day (Effluents 1 and 2; 735+/-15 and 3250+/-90 mg-COD/L), an equalization tank (Lagoon; 1670+/-50mg-COD/L), and waste stream containing a high concentration of organic matter (Cereal; 8920+/-150 mg-COD/L). Hydrogen production from the Lagoon and effluent samples was low, with 64+/-16 mL of hydrogen per liter of wastewater (mL/L) for Effluent 1, 21+/-18 mL/L for Effluent 2, and 16+/-2 mL/L for the Lagoon sample. There was substantially greater hydrogen production using the Cereal wastewater (210+/-56 mL/L). Assuming a theoretical maximum yield of 4 mol of hydrogen per mol of glucose, hydrogen yields were 0.61-0.79 mol/mol for the Cereal wastewater, and ranged from 1 to 2.52 mol/mol for the other samples. This suggests a strategy for hydrogen recovery from wastewater based on targeting high-COD and high-sugar wastewaters, recognizing that sugar content alone is an insufficient predictor of hydrogen yields. Preliminary tests with the Cereal wastewater (diluted to 595 mg-COD/L) in a two-chambered MFC demonstrated a maximum of 81+/-7 mW/m(2) (normalized to the anode surface area), or 25+/-2 mA per liter of wastewater, and a final COD of <30 mg/L (95% removal). Using a one-chambered MFC and pre-fermented wastewater, the maximum power density was 371+/-10 mW/m(2) (53.5+/-1.4 mA per liter of wastewater). These results suggest that it is feasible to link biological hydrogen production and electricity producing using MFCs in order to achieve both wastewater treatment and bioenergy production.     

Disinfection of swine wastewater using chlorine, ultraviolet light and ozone

Veterinary antibiotics are widely used at concentrated animal feeding operations (CAFOs) to prevent disease and promote growth of livestock. However, the majority of antibiotics are excreted from animals in urine, feces, and manure. Consequently, the lagoons used to store these wastes can act as reservoirs of antibiotics and antibiotic-resistant bacteria. There is currently no regulation or control of these systems to prevent the spread of these bacteria and their genes for antibiotic resistance into other environments. This study was conducted to determine the disinfection potential of chlorine, ultraviolet light and ozone against swine lagoon bacteria. Results indicate that a chlorine dose of 30 mg/L could achieve a 2.2-3.4 log bacteria reduction in lagoon samples. However, increasing the dose of chlorine did not significantly enhance the disinfection activity due to the presence of chlorine-resistant bacteria. The chlorine resistant bacteria were identified to be closely related to Bacillus subtilis and Bacillus licheniformis. A significant percentage of lagoon bacteria were not susceptible to the four selected antibiotics: chlortetracycline, lincomycin, sulfamethazine and tetracycline (TET). However, the presence of both chlorine and TET could inactivate all bacteria in one lagoon sample. The disinfection potential of UV irradiation and ozone was also examined. Ultraviolet light was an effective bacterial disinfectant, but was unlikely to be economically viable due to its high energy requirements. At an ozone dose of 100 mg/L, the bacteria inactivation efficiency could reach 3.3-3.9 log.     

Altering protein conformation to improve fermentative hydrogen production from protein wastewater

One important reason for low hydrogen production from protein wastewater is due to the native folded conformation of protein. In this study the enhancement of bio-hydrogen production from protein wastewater by altering protein conformation via pretreatment was reported. Firstly, the effect of different pretreatment methods (acid, alkali, heat, and ultraviolet) on hydrogen production from synthetic protein wastewater was compared. The hydrogen production from the ultraviolet pretreated wastewater was 111.3 mL/g-protein, which was 3.79-, 3.73-, 3.54-, and 1.36-fold of that from the unpretreated (blank), acid, alkali, and heat pretreated wastewater, respectively. Then, the reasons for ultraviolet pretreatment showing significantly higher hydrogen production than other pretreatments were investigated. It was found that all pretreatments did not cause the cleavage of peptide bond, but the ultraviolet one caused much greater damage of hydrogen bonding networks and unfolding of protein. Thus, during anaerobic fermentation much higher protease activity and protein utilization were observed, which resulted in the bio-hydrogen production being remarkably improved. Further studies indicated that the photo-oxidization of aromatic residues in protein was not the reason for ultraviolet pretreatment remarkably improving bio-hydrogen production. Finally, the application of ultraviolet pretreatment to enhance hydrogen production from real protein wastewater was testified.     

矿化垃圾与碱性添加剂对餐厨垃圾发酵产氢的影响

研究了不同配比的矿化垃圾与碱性添加剂对餐厨垃圾发酵产氢的影响。试验结果表明,矿化垃圾主要起种子菌作用,此外其高孔隙率和高阳离子交换能力可提高传质效果;而碱性添加剂则能够抑制耗氢菌活性,缓冲体系的pH。在矿化垃圾与碱性添加剂的协同作用下,餐厨垃圾与污泥进行联合发酵产氢,获得最大的氢气浓度为41％,氢气产率为95．8mL／gVS;产氢体系进行的主要是丁酸型发酵,且氢气的产生量与VFA的产生量直接相关;水解反应生成的有机酸进一步发酵转化为气相的过程成为发酵产氢的限制性步骤,只有在矿化垃圾和碱性添加剂的协同作用下,微生物才可将产生的大量有机酸迅速向气相转化。     

Ammonium recovery and energy production from urine by a microbial fuel cell

Nitrogen recovery through NH₃ stripping is energy intensive and requires large amounts of chemicals. Therefore, a microbial fuel cell was developed to simultaneously produce energy and recover ammonium. The applied microbial fuel cell used a gas diffusion cathode. The ammonium transport to the cathode occurred due to migration of ammonium and diffusion of ammonia. In the cathode chamber ionic ammonium was converted to volatile ammonia due to the high pH. Ammonia was recovered from the liquid-gas boundary via volatilization and subsequent absorption into an acid solution. An ammonium recovery rate of 3.29 g_N d⁻¹ m⁻² (vs. membrane surface area) was achieved at a current density of 0.50 A m⁻² (vs. membrane surface area). The energy balance showed a surplus of energy 3.46 kJ g_N⁻¹, which means more energy was produced than needed for the ammonium recovery. Hence, ammonium recovery and simultaneous energy production from urine was proven possible by this novel approach.