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Tests on erosion of the cohesive sediments in the Chikugo estuary in Japan were conducted. The results indicate that the decrease in the rate of erosion at constant shear stresses is caused rapidly 20 to 30 minutes after the start of tests and that tue decrease is caused by the increase in the solid fraction and the yield value of the seaiment remaining uneroded due to shear stress. In order to simulate the erosion process of sediments, a model in which the increase in the yield value of sediment is assumed to be proportional to the integration value of the absolute value of the sediment velocity gradient with respect to time was developed. The simulation based on this model is shown to be available to estimate the amount of sediments eroded in the erosional process of cohesive sediments.

The removal of coliform bacteria, enterococcus bacteria, and coliphages in two sewage treatment plants, one using the activated sludge process and the other using a high-rate trickling filter, was investigated over a period of one year. Coliform and enterococcus bacteria were removed with equal efficiency by the two plants, but coliphages were removed more efficiently by the activated sludge process. Experiments on the mechanism of removal revealed that it was mainly due to adsorption on the activated sludge and on the slime in the trickling filter. Die-off of the micro-organisms seemed to play a minor role in the reduction in counts. The treated sewage was disinfected by chlorination prior to discharge into the receiving water. No coliforms were detected in the chlorinated effluents when they had chlorine residuals in the range of 0 to 1.521 mg/l. However, enterococci were detected when chlorine residuals dropped below 0.598 mg/l. Coliphages proved to be the most resistant organisms and they were generally detected throughout the range of chlorine residuals encountered.

The fundamental behaviour of suspended sediments and muds was investigated using an annular flume with the condition of cyclically changed shear stress. The results from two series of experiments showed that particle sizes which participate in the variation of suspended sediments were restricted to smaller sizes of the material. It was also found that the cyclically steady state was finally attained with respect to the suspended sediments as the tidal cycle repeated. In that state the suspended sediments were subject to three phenomena: erosion; deposition with dispersed state; and deposition with flocculation. A simple model on the suspended sediments in an estuary was developed and its behaviour in a model estuary was simulated by using the fluxes of erosion and deposition in the experiment. That model well described the variation of the suspended sediments and muds.

The diffusion coefficients for glucose and oxygen through the microbial aggregate were measured, and then the dependency on the biochemical factors was investigated. Kinetic and diffusion studies were carried out experimentally for this purpose. Both coefficients were found to be dependent not only on the bacteria concentration but also on the C/N ratio of microbial aggregate at the high bacteria concentration, when the temperature was 20 +/- 2 degrees C. On the other hand, these values were considered to be independent of both factors at the low bacteria concentration, when the temperature was 20 +/- 2 degrees C. In other words, both coefficients were approximately 86-95% of the values in water at the high bacteria concentration and nearly 100% of the values in water at the low bacteria concentration. It was also found that they were dependent on the temperature.