Haruhiko Sumino

Closed DHS system to prevent dissolved methane emissions as greenhouse gas in anaerobic wastewater treatment by its recovery and biological oxidation.

Anaerobic wastewater treatment has been focused on its eco-friendly nature in terms of the improved energy conservation and reduction in carbon dioxide emissions. However, the anaerobic process discharges unrecovered methane as dissolved methane. In this study, to prevent the emission of dissolved methane from up-flow anaerobic sludge blanket (UASB) reactors used to treat sewage and to recover it as useful gas, we employed a two-stage down-flow hanging sponge (DHS) reactor as a post-treatment of the UASB reactor. The closed DHS reactor in the first stage was intended for the recovery of dissolved methane from the UASB reactor effluent; the reactor could successfully recover an average of 76.8% of the influent dissolved methane as useful gas (containing methane over 30%) with hydraulic retention time of 2 h. During the experimental period, it was possible to maintain the recovered methane concentrations greater than 30% by adjusting the air supply rate. The remaining dissolved methane after the first stage was treated by the next step. The second closed DHS reactor was operated for oxidation of the residual methane and polishing of the remaining organic carbons. The reactor had a high performance and the influent dissolved methane was mostly eliminated to approximately 0.01 mgCOD L(-1). The dissolved methane from the UASB reactor was completely eliminated--by more than 99%--by the post-treatment after the two-stage closed DHS system.

Recovery and biological oxidation of dissolved methane in effluent from UASB treatment of municipal sewage using a two-stage closed downflow hanging sponge system

A two-stage closed downflow hanging sponge (DHS) reactor was used as a post-treatment to prevent methane being emitted from upflow anaerobic sludge blanket (UASB) effluents containing unrecovered dissolved methane. The performance of the closed DHS reactor was evaluated using real municipal sewage at ambient temperatures (10-28 °C) for one year. The first stage of the closed DHS reactor was intended to recover dissolved methane from the UASB effluent and produce a burnable gas with a methane concentration greater than 30%, and its recovery efficiency was 57-88%, although the amount of dissolved methane in the UASB effluent fluctuated in the range of 46-68 % of methane production greatly depending on the temperature. The residual methane was oxidized and the remaining organic carbon was removed in the second closed DHS reactor, and this reactor performed very well, removing more than 99% of the dissolved methane during the experimental period. The rate at which air was supplied to the DHS reactor was found to be one of the most important operating parameters. Microbial community analysis revealed that seasonal changes in the methane-oxidizing bacteria were key to preventing methane emissions.

Development of a treatment system for molasses wastewater: The effects of cation inhibition on the anaerobic degradation process

This study evaluated the process performance of a novel treatment system consisting of an acidification reactor, an upflow staged sludge bed (USSB) reactor, an upflow anaerobic sludge blanket reactor, and an aerobic trickling filter for the treatment of a high-strength molasses wastewater with a chemical oxygen demand (COD) of up to 120,000mg/L. The USSB operating at 35°C was capable of achieving an organic loading rate of 11kgCOD/m(3) day with a methane recovery of 62.4% at an influent COD of 120,000mg/L. The final effluent COD was 4520mg/L. The system was effective with regard to nitrification and sulfur removal. Fifty percent inhibition of the bacterial activity of the retained sludge by the cations was determined at 8gK/L for sucrose degradation, 16gK/L for sulfate reduction, and 12gK/L or 9gNa/L for acetoclastic methane production. Cation inhibition of anaerobic degradation reduced the process performance of the USSB.

Performance of a pilot-scale sewage treatment: An up-flow anaerobic sludge blanket (UASB) and a down-flow hanging sponge (DHS) reactors combined system by sulfur-redox reaction process under low-temperature conditions

Performance of a wastewater treatment system utilizing a sulfur-redox reaction of microbes was investigated using a pilot-scale reactor that was fed with actual sewage. The system consisted of an up-flow anaerobic sludge blanket (UASB) reactor and a down-flow hanging sponge (DHS) reactor with a recirculation line. Consequently, the total CODCr (465±147 mg L(-1); total BOD of 207±68 mg L(-1)) at the influent was reduced (70±14 mg L(-1); total BOD of 9±2 mg L(-1)) at the DHS effluent under the conditions of an overall hydraulic retention time of 12 h, a recirculation ratio of 2, and a low-sewage temperature of 7.0±2.8 °C. A microbial analysis revealed that sulfate-reducing bacteria contributed to the degradation of organic matter in the UASB reactor even in low temperatures. The utilized sulfur-redox reaction is applicable for low-strength wastewater treatment under low-temperature conditions.

Changes of microbial characteristics of retained sludge during low-temperature operation of an EGSB reactor for low-strength wastewater treatment

In this study, a lab scale EGSB reactor was operated for 400 days to investigate the influence of temperature-decrease on the microbial characteristic of retained sludge. The EGSB reactor was started-up at 15 degrees C seeding with 20 degrees C-grown granular sludge. The influent COD of synthetic wastewater was set at 0.6-0.8 gCOD/L. The process-temperature was stepwise reduced from 15 degrees C to 5 degrees C during 400 days operation. Decrease of temperature of the reactor from 15 degrees C to 10 degrees C caused the decline of COD removal efficiency. However, continuous operation of the EGSB reactor led the efficient treatment of wastewater (70% of COD removal, 50% of methane recovery) at 10 degrees C. We confirmed that the both acetate-fed and hydrogen-fed methanogenic activities of retained sludge clearly increased under 15 to 20 degrees C. Changes of microbial profiles of methanogenic bacteria were analyzed by 16S rDNA-targeted DGGE analysis and cloning. It shows that genus Methanospirillum as hydrogen-utilizing methanogen proliferated due to low temperature operation of the reactor. On the other hand, genus Methanosaeta presented in abundance as acetoclastic-methanogen throughout the experiment.

High-rate treatment of molasses wastewater by combination of an acidification reactor and a USSB reactor.

A combination of an acidification reactor and an up-flow staged sludge bed (USSB) reactor was applied for treatment of molasses wastewater containing a large amount of organic compounds and sulfate. The USSB reactor had three gas-solid separators (GSS) along the height of the reactor. The combined system was continuously operated at mesophilic temperature over 400 days. In the acidification reactor, acid formation and sulfate reduction were effectively carried out. The sugars contained in the influent wastewater were mostly acidified into acetate, propionate, and n-butyrate. In addition, 10–30% of influent sulfur was removed from the acidification reactor by means of sulfate reduction followed by stripping of hydrogen sulfide. The USSB achieved a high organic loading rate (OLR) of 30 kgCOD m−3 day−1 with 82% COD removal. Vigorous biogas production was observed at a rate of 15 Nm3 biogas m−3 reactor day−1. The produced biogas, including hydrogen sulfide, was removed from the wastewater mostly via the GSS. The GSS provided a moderate superficial biogas flux and low sulfide concentration in the sludge bed, resulting in the prevention of sludge washout and sulfide inhibition of methanogens. By advantages of this feature, the USSB may have been responsible for achieving sufficient retention (approximately 60 gVSS L−1) of the granular sludge with high methanogenic activity (0.88 gCOD gVSS−1 day−1 for acetate and as high as 2.6 gCOD gVSS−1 day−1 for H2/CO2). Analysis of the microbial community revealed that sugar-degrading acid-forming bacteria proliferated in the sludge of the USSB as well as the acidification reactor at high OLR conditions.

Protection of biomass from snail overgrazing in a trickling filter using sponge media as a biomass carrier: down-flow hanging sponge system

This study investigated down-flow hanging sponge (DHS) technology as a promising trickling filter (TF) using sponge media as a biomass carrier with an emphasis on protection of the biomass against macrofauna overgrazing. A pilot-scale DHS reactor fed with low-strength municipal sewage was operated under ambient temperature conditions for 1 year at a sewage treatment plant in Bangkok, Thailand. The results showed that snails (macrofauna) were present on the surface of the sponge media, but could not enter into it, because the sponge media with smaller pores physically protected the biomass from the snails. As a result, the sponge media maintained a dense biomass, with an average value of 22.3 gVSS/L sponge (58.1 gTSS/L sponge) on day 370. The snails could graze biomass on the surface of the sponge media. The DHS reactor process performance was also successful. The DHS reactor requires neither chemical treatments nor specific operations such as flooding for snail control. Overall, the results of this study indicate that the DHS reactor is able to protect biomass from snail overgrazing.

Development of a down-flow hanging sponge reactor for the treatment of low strength sewage

The process performance of a down-flow hanging sponge (DHS) reactor for treating low strength sewage (biochemical oxygen demand (BOD) 20-50 mg/L) was investigated in Bangkok, Thailand. The hydraulic retention time (HRT) was set at 4 h during the start-up period and was reduced to 1.5 h in a stepwise manner. Throughout the 300-day operational period, the DHS reactor shows high performance with respect to the removal of total suspended solid (>90% total suspended solid removal efficiency). No clogging of sponge media was observed in response to the self-digestion phenomena of the biofilm. At a HRT of 1.5 h, the BOD removal efficiency was sufficiently high (about 85%). The pathogen Escherichia coli and other coliform bacteria were removed almost completely as well (removal was 99.4% and 98.1%, respectively). Regarding the retained sludge activity measurement, the nitrite oxidation rate was higher than the ammonium oxidation rate (0.031 and 0.022 gram of nitrogen per gram of volatile suspended solids per day, respectively). In the 300 days of operation, the amount of excess sludge production was negligible. Thus, no sludge treatment system is required. Introduction of the DHS system in developing countries is recommended because this system requires a relatively small area, and has low electricity consumption and operation costs.

Treatment of low-strength wastewater in an anaerobic down-flow hanging sponge (AnDHS) reactor at low temperature

The process performance of a novel anaerobic down-flow hanging sponge (AnDHS) reactor for the treatment of low strength wastewater was investigated. A lab-scale experiment was conducted in which 300–400 mgCOD L−1 of artificial wastewater was fed in over 600 days. The reactor exhibited sufficient performance: 70–90% of total COD removal, and 60–90% of methane recovery were maintained at 20°C, with a hydraulic retention time (HRT) of 2 h. It was possible to maintain COD removal by extending the HRT to 4 h at 15°C and 10 h at 10°C. With regard to the wastewater feed, one-pass mode (without effluent recirculation) gave better performance in COD removal as compared with recirculation mode. The results of batch feeding experiments using single substrates (such as acetate, propionate or sucrose) indicated that acetate degradation was more strongly affected by decreasing operational temperature. In addition, the AnDHS reactor system had no significant problems related to sludge retention such as massive loss of sludge throughout the experiment. Microbial structure analysis of the retained sludge with respect to the domain Archaeal 16S rRNA gene showed the proliferation of relatives of both the acetate-utilizing genus Methanosaeta and the hydrogen-utilizing genus Methanolinea.

Influence of sugar content of wastewater on the microbial characteristics of granular sludge developed at 20°C in the anaerobic granular sludge bed reactor

The influence of the sugar content of wastewater on changes in the characteristics of the retained sludge was investigated by using two lab-scale granular sludge bed reactors at 20°C. Both reactors were inoculated with granular sludge grown at 20°C and were fed with synthetic wastewater containing sucrose and volatile fatty acids (VFAs). On day 70, the sucrose content of the wastewater was changed to 90% (based on wastewater COD value) for the first reactor and 0% (VFA 90%) for the second. After this change in feed composition, the COD removal efficiency became about 91% for the sucrose-fed reactor and 95% for the VFA-fed reactor. The growth yield (Yg) of the sucrose-fed sludge increased more than that of the VFA-fed sludge. Consequently, deterioration of the settleability of the sucrose-fed sludge was observed. The sucrose-degrading activity of the retained sludge obtained from the sucrose-fed reactor increased significantly from 3.7 g COD g VSS− 1 day− 1 on day 62 to 36.8 g COD g VSS− 1 day− 1 on day 230, in accordance with the predominant growth of sugar-degrading bacteria – namely, Lactococcus, Clostridium and Chloroflexi – in the retained sludge. The excessive growth of these sugar-degrading bacteria in the retained sludge caused unstable process performance in the sucrose-fed reactor at 20°C.