宮永 一彦

Recently, excess sludge from wastewater treatment plant has been one of the serious problems. They are disposed by landfill, incineration, bio-gas production and so on. However, these conventional methods contain drawbacks such as short of landfill, dioxin emissions or inefficient production. A countermeasure against the problems is to minimize the sludge production in the wastewater treatment process. In this study, a biochemical sludge-reduction system was proposed. The microbes were experienced in the aerobic and anaerobic conditions alternately. By the change of microbial growth condition, it is thought that obligate aerobic and anaerobic microbes are to be dead, resulting in their solubilization under alternated anaerobic and aerobic conditions, respectively. The concentrations of organic carbonaceous and nitrogenous substrates and mixed liquor suspended solid (MLSS) were more greatly reduced under the alternate aerobic-anaerobic conditions than under the continuous aerobic or anaerobic conditions. The MLSS mainly consisted of protein (more than 50%w/w) and the amount of protein in the suspension fluctuated during the cultivation without any additions of substrates. These results indicate that the microbial solubilization and utilization of protein lysate seem to take place in this system. So far, however, the effects of growth condition on the microbial viabilities in the solubilization have not been investigated. To clarify them, carboxyfluorescein diacetate (CFDA) that was substrate of intracellular esterase was used as an indicator of microbial viability. As a further experiment, the relationships between sludge solubilization, bacterial viabilities and substrate removal are to be investigated in the continuous wastewater treatment process.

To simulate microbiologically influenced corrosion (MIC) of carbon steel in the subterranean environment, artificial soil consisting of silica sand, microbes, and medium was used. Sulfate-reducing bacteria (SRB) and activated sludge from a municipal wastewater plant were used as model microbes. Incubation was carried out under aerobic and anaerobic conditions. Corroded mean depth (CMD) under aerobic conditions with activated sludge inoculation reached 34.1 ~tm and was 28 times as much as that under anaerobic conditions after incubation for 56 d. Sole existence of SRB didn't accelerate carbon steel corrosion, while addition of activated sludge accelerated the corrosion. The influence of water content in artificial soil on carbon steel MIC was analyzed. Model soils of silica sand with 100, 60 and 20% water content in the voids of the soils were prepared. The model soils were inoculated with activated sludge. The existence of air space and the uneven distribution of medium on the carbon steel coupons accelerated corrosion. The heterogeneity that resulted from bacteria had the same effects as the uneven distribution of the medium.

Carbon steel coupons were exposed to nutritionally-poor synthetic wastewater inoculated with activated sludge from a municipal waste water plant. Mass loss of the coupons was proportional to the incubation time, and reached 70.4 (mg/cm 2) after incubation for 140 d. The observed mass loss was 5 times as much as that under sterile conditions. Heterogeneous distribution of the dissolved oxygen (DO) concentration on the surface of the steel plate was observed. The average roughness of the metal surface observed after 112 d of incubation was 4.27 ~tm. Carbon steel coupons covered with artificial biofilm, which consisted of 0.6% agarose and diluted activated sludge (150 mg/1), were exposed to synthetic wastewater. When only one side of the coupon was covered with the artificial biofilm, mass loss of the steel coupon reached 0.46 mg/cm2/day. On the other hand, complete insulation of both sides with the artificial biofilm resulted in carbon steel corrosion of 0.16 mg/cm2/day. Complete consumption of the oxygen from the biofilm delayed of the steel oxidation. On the other hand, pH decrease and sluggish decrease of DO in the film were observed when only one side of the steel coupon was covered with the artificial biofilm. These observations indicated that complete polarization due to one side covering by biofilm on the steel surface enhances the microbiologically influenced corrosion (MIC) of carbon steel.