Inhibitory action of mercury to anaerobic digestion was investigated by batch experiments.
At first, the role of sulfate-reducing process in inhibition of anaerobic digestion by mercury was studied to confirm the previous suggestion. It is found that sulfate-reducing process is much more tolerable to mercury than both acidogenesis and methanogenesis and it has an ability to make mercury nontoxic by producing a lot of sulfide as a result of its own metabolism. Owing to these properties described above, it is concluded that this process may greatly contributed to protect all the process in glucose degradation from toxicity of mercury and aid them in recovering from inhibition caused by mercury.
Sensitivity of methanogenesis to inhibitory action of mercury was also studied in detail using three typical intermediates of glucose degradation or usual anaerobic digestion, i.e. acetate, propionate, and n-butyrate. It is concluded that the tolerability to toxicity of mercury (or recoverability from inhibition by mercury) is higher, as the following order, at the process of degradation of n-butyrate, that of acetate, that of propionate, and at methanogenesis from hydrogen, that from acetate.

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A few attempts for chromium to form a ferrite (ferritization) with iron(II) sulfate more tightly and to be removed from wastewater simultaneously were examined.
The treatment of the wastewater containing chromium less than 10mg·l^-1 was possible by the ferritization under the standard conditions, i.e., at pH of 9.0, temperature of 65 °C, reaction time of 60min and Fe(II) of 5g·l^-1. In the case of the wastewater containing chromium more than 10mg ·l^-1, the treatment was impossible by the standard conditions but possible by different conditions, i.e., at reaction time of 90min, Fe(II) of 10g·l^-1 and addition of aluminium of 0.1g·l^-1. After the treatment of 25mgCr·l^-1 solution, it was found that aluminium restrained the dissolution of chromium from the ferrite. In the case of solution of more than 50mgCr·l^-1, it was better to lower pH to 8.0, but it was hard to prevent chromium from dissolving by alteration of the conditions and even by the addition of aluminium. The best method to lower the amount of chromium dissolving was to lower drying temperature to less than 70°C.
A counterplan for unsuccessfully produced ferrite and a utilization of ferrite sludge were also proposed in this paper.

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Fundamental functions of activated sludge are classified into the three : adsorption, sedimentation and separation function. Usually, water quality takes a bad turn by a fall of sedimentation and separation function rather than an adsorption function. Also the authors describe mainly in this paper about the separation function.
Firstly plant functions took a general view on an estimation space. So it was appointed that operating estimation varied from plant to plant and the estimation differed remarkably from optimum estimation. Secondly the aspect of separation was observed about various concentration. Consequently it was found to exist a critical separation concentration. Using this critical concentration, it was possible to evaluate the activated sludge separation function.
The following conclusions are obtained : (1) a clear distinction must be drawn between a plant and a sludge separation function, and the conventional indices are not fit for the activated sludge separation function, (2) the new separation index is proposed, and also the automatic measuring device of this index has been developed, (3) by applying this method to actual plants, it is confirmed that estimation by the new index is very useful.

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The effects of porous riverbeds on biological self-purification capacity by attached biofilm was investigated using an experimental open channel. The self-purification capacity was estimated by activity of nitrification. Hydrodynamic characteristics in porous beds were measured by means of chloride tracer method. Ammonium nitrogen was used as main substrate. A mathematical model was developed and simulated the purification process in porous riverbeds.
It was made clear that the surface region (a depth of two particles) of porous riverbeds played a major role in purification activity in channel due to vigorous mass transfer (substrate supply from surface flow) in that surface region. Also, the model simulated the effects of nitrifying rate constant and depth of porous beds on purification capacity.
Porous riverbeds provide increased surface area for biofilm and turbulence in surface flow which increases biofilm activity. In actual water channels, mearly placing 3 to 4 layers of gravel or rock would contribute greately to increased biological self-purification activity.

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