Taira Hidaka

Comparative performance and microbial diversity of hyperthermophilic and thermophilic co-digestion of kitchen garbage and excess sludge.

The objective of this study was to evaluate the performance characteristics of a hyperthermophilic digester system that consists of an acidogenic reactor operated at hyperthermophilic (70 degrees C) conditions in series with a methane reactor operated at mesophilic (35 degrees C), thermophilic (55 degrees C), and hyperthermophilic (65 degrees C) conditions. Lab-scale reactors were operated continuously, and were fed with co-substrates composed of artificial kitchen garbage (TS 9.8%) and excess sludge (TS 0.5%) at the volumetric ratio of 20:80. In the acidification step, COD solubilization was in the range of 22-46% at 70 degrees C, while it was 21-29% at 55 degrees C. The average protein solubilization was 44% at 70 degrees C. The double bond fatty acid removal ratio at 70 degrees C was much higher than at 55 degrees C. These results suggested that the optimal operation conditions for the acidogenic fermenter were about 3.1 days of HRT and 4 days of SRT at 70 degrees C. Methane conversion efficiency and the VS removal percentage in the methanogenic step following acidification was around 65% and 64% on average at 55 degrees C, respectively. The optimal operational conditions for this system are acidogenesis performed at 70 degrees C and methanogenesis at 55 degrees C. The key microbes determined in the hyperthermophilic acidification step were Anaerobic thermophile IC-BH at 6.4 days of HRT and Thermoanaerobacter thermohydrosulfuricus DSM 567 at 2.4 days of HRT. These results indicated that the hyperthermophilic system provides considerable advantages in treating co-substrates containing high concentrations of proteins, lipids, and nonbiodegradable solid matter.

Comparison of thermophilic anaerobic digestion characteristics between single-phase and two-phase systems for kitchen garbage treatment

Lab-scale single-phase and two-phase thermophilic methane fermentation systems (SPS and TPS, respectively) were operated and fed with artificial kitchen waste. In both SPS and TPS, the highest methane recovery ratio of 90%, in terms of chemical oxygen demand by dichromate (CODcr), was observed at an organic loading rate (OLR) of 15 gCODcr/(l.d). The ratio of particle CODcr remaining to total CODcr in the influent was 0.1 and the ratio of NH(4)-N concentration to the input total nitrogen concentration was 0.5 in both SPS and TPS. However, the propionate concentration in the SPS reactor fluctuated largely and was 2 gCODcr/l higher than that in TPS, indicating less stable digestion. Regardless, efficient kitchen waste degradation can be accomplished in both SPS and TPS at an OLR of <20 gCODcr/(l.d), even though TPS may be more stable and easier to maintain. Bacillus coagulans predominated with an occupied ratio of approximately 90% in the acid fermentation reactor of TPS, and then a richer microbial community with a higher Shannon index value was maintained in the methane fermentation reactor of TPS than in the SPS reactor.

Comparative performance of mesophilic and thermophilic anaerobic digestion for high-solid sewage sludge

In local cities, many small sewage and waste treatment facilities are operated independently. To encourage processing by anaerobic digestion at a centralized sewage treatment plant (STP), high-solid sewage sludge is helpful because it reduces the energy and cost required for transporting the sludge from other STPs. Mesophilic and thermophilic anaerobic digestion of sewage sludge at total solids concentrations (TS) of 7.5% and 10% were evaluated using laboratory-scale continuous reactors. Under the mesophilic condition, sewage sludge of 10% TS was successfully treated. Under the thermophilic condition, sewage sludge of 7.5% TS was not successfully treated when the total ammonia concentration was over 2000 mg N/L. Batch experiments showed that it takes a few weeks for the methane fermentation activity to recover after being inhibited. The effectiveness of adding easily biodegradable organic matter was confirmed. These results show that high-solid sewage sludge is suitable for small facilities by controlling the operating conditions.

Anaerobic co-digestion of sewage sludge with shredded grass from public green spaces

Adding greenery from public spaces to the co-digestion process with sewage sludge was evaluated by shredding experiments and laboratory-scale batch and continuous mesophilic anaerobic fermentation experiments. The ratio of the shredded grass with 20mm or less in length by a commercially available shredder was 93%. The methane production was around 0.2NL/gVS-grass in the batch experiment. The continuous experiment fed with sewage sludge and shredded grass was stably operated for 81days. The average methane production was 0.09NL/gVS-grass when the TS ratio of the sewage sludge and the grass was 10:1. This value was smaller than those of other reports using grass silage, but the grass species in this study were not managed, and the collected grass was just shredded and not ensiled before feeding to the reactor for simple operation. The addition of grass to a digester can improve the carbon/nitrogen ratio, methane production and dewaterability.

Two-phased hyperthermophilic anaerobic co-digestion of waste activated sludge with kitchen garbage

For co-digestion of waste activated sludge with kitchen garbage, hyperthermophilic digester systems that consisted of an acidogenic reactor operated at hyperthermophilic (70 degrees C) and a methanogenic reactor operated at mesophilic (35 degrees C), thermophilic (55 degrees C) or hyperthermophilic (65 degrees C) conditions in series were studied by comparing with a thermophilic digester system that consisted of thermophilic (55 degrees C) acidogenic and methanogenic reactors. Laboratory scale reactors were operated continuously fed with a substrate blend composed of concentrated waste activated sludge and artificial kitchen garbage. At the acidogenic reactor, solubilization efficiencies of chemical oxygen demand (COD), carbohydrate and protein at 70 degrees C were about 39%, 42% and 54%, respectively, and they were higher than those at 55 degrees C by around 10%. The system of acidogenesis at 70 degrees C and methanogenesis at 55 degrees C was stable and well-functioned in terms of treatment performances and low ammonium nitrogen concentrations. Microbial community analysis was conducted using a molecular biological method. The key microbe determined at the hyperthermophilic acidogenesis step was Coprothermobacter sp., which was possibly concerned with the degradation of protein in waste activated sludge. The present study proved that the hyperthermophilic system was advantageous for treating substrate blends containing high concentrations of waste activated sludge.

A simple biofilm model of bacterial competition for attached surface.

A simple biofilm model of competition in bacterial growth for an attached surface is developed. Competition for the attached surface is expressed with the crowded and detachment effects. The developed model is verified by comparing simulated results with data obtained in the experiments of batch culture of nitrifier and continuous treatment of actual sewage with biofilm reactor. This model can favorably simulate the growth competition between autotrophic and heterotrophic bacteria for the attached surface. Then some parameters for nitrification process are discussed with this model. It is clarified that the effective removal of organic matter before nitrification tank is required for effective nitrification in the biofilm reactor.

Effect of temperature on performance and microbial diversity in hyperthermophilic digester system fed with kitchen garbage.

The objective of this study was to evaluate the performances and microbial diversities for development of the effective hyperthermophilic digester system that consists of hyperthermophilic reactor and hyperthermophilic or thermophilic reactor in series. Lab-scale reactors were operated continuously fed with artificial kitchen garbage. The effect of temperature on the acidification step was firstly investigated. Results indicated that 43.1% of COD solubilization was achieved at 70 degrees C, while it was about 21% at 80 degrees C. The average protein solubilization reached to 31% at 80 degrees C. Methane conversion efficiency following the acidification was around 85% in average at 55 degrees C, but decreased with increasing temperature and methane gas was not produced over 73 degrees C. The upper temperature limits for growth of microbes were secondly observed and shown to be 73 degrees C for acetate oxidizers, 65 degrees C for propionate oxidizers, 70 degrees C for iso-butyrate oxidizers, 80 degrees C for lactate oxidizers and 65 degrees C for protein degrading bacteria in the methane fermenter. As well as, microbes affiliated with methanogens dominated the population below the 65 degrees C, while those affiliated with acidogens were predominant over the 73 degrees C. These results indicated that the hyperthermophilic processes have considerable benefits to treat wastewater or waste containing high concentration of protein.

Advanced treatment of sewage by pre-coagulation and biological filtration process.

A pre-coagulation and bio-filtration process for advanced treatment of sewage was developed and experimentally discussed with a pilot plant. The bio-filtration unit consists of a denitrification filter, a nitrification filter with side stream to the denitrification filter, and a polishing filter with anoxic and aerobic parts. Concentrations of SS, T-COD(Cr), T-carbonaceous BOD, T-N and T-P in the effluent were stably kept at less than 3, 20, 5mg/L, 2mg N/L and 0.2mg P/L, respectively, and transparency at higher than 100 cm, under total hydraulic retention time of 3.2h in the bio-filtration parts (filter-bed). ORP in an anoxic tank before a nitrification tank should be at a low level of less than -120 mV to keep remaining NO(-)(x) - N less than 1mg N/L, but must be maintained at a level higher than -150 mV. The maximum nitrogen-loading rate under a water temperature of 18 degrees C should be less than 0.25 kg N/(m(3)-filter-bed.d). Concentrations of microorganisms kept in the reactors were as high as 4000-5000 mg COD/L-filter-bed. Denitrification activity of 0.4 or 0.7 kg N/(m(3)-filter-bed.d), and nitrification activity of 0.3 kg N/(m(3)-filter-bed.d) were obtained, respectively, under a water temperature of about 18 degrees C. Backwashing in each tank as well as methanol addition and aeration in the polishing filter were operated successfully by the automatic control systems. These results proved that this process is applicable to advanced treatment of sewage with easy maintenance.

Enhancement of anaerobic digestion of shredded grass by co-digestion with sewage sludge and hyperthermophilic pretreatment

Anaerobic co-digestion of shredded grass with sewage sludge was investigated under various temperature conditions. The conversion of grass to methane was difficult to achieve under mesophilic conditions, while its methane yield was 0.19 NL/g VS-grass under thermophilic conditions. The mixture ratio of grass to sludge affected the methane yield, and the highest synergistic effect was obtained at a C/N ratio of around 10. In a continuous experiment, hyperthermophilic (80 °C) pretreatment promoted a methane yield of 0.34 NL/g VS-mixture, higher than that under mesophilic and thermophilic conditions (0.20 and 0.30 NL/g VS-mixture, respectively). A batch experiment with hyperthermophilic pretreatment showed that 3 days of treatment was sufficient for subsequent methane production, in which the highest dissolution of particulate COD, carbohydrate and protein was 25.6%, 33.6% and 25.0%, respectively.

Applicability of random cloning method to analyze microbial community in full-scale anaerobic digesters.

Microbial communities were analyzed in six full-scale anaerobic digesters and a lab-scale digester using the random cloning method. The purposes were to confirm the applicability, reproducibility, and error range of this method; to discuss the difference in the dominant microbes determined by this method in different operational conditions of temperature (mesophilic and thermophilic) and substrate (garbage, sewage sludge, and livestock waste); and to determine key microbes in each digester. Each sample was analyzed in triplicate. In one of the samples, 373 clones were analyzed to study the composition of microbial community in the digester. Time course analysis was conducted from the start-up period for approximately one year in one of the digesters. Similar microbial diversity was obtained corresponding to the type of substrate change (sewage sludge to garbage). Operational taxonomic units (OTUs) closely related to Coprothermobacter sp. and unidentified bacterium clones TUG14 and TUG22 disappeared during the first 40 d, while OTUs closely related to Bacillus sp. and Clostridium sp. increased later. Microbial diversity in digesters is strongly affected by the operational conditions, and similar microbial diversity can be obtained in triplicate analysis and under similar operational conditions. The present study verified the applicability of this method to discuss the overall difference in microbial communities.