Abstract
Discharge from paddy fields after puddled is regarded as one of causes of water pollution of rivers and lakes. The problem is considered to be most serious on Uso-river basin in Shiga prefecture. The author investigated a practical method to clarify the puddled water by gypsum addition to it. A critical coagulation condition of the puddled water was estimated on the basis of the Hetero coagulation theory. The calculated results showed the Ca ion concentration greater than 2m mol// will flocculate the puddled water. The estimated value agreed well with experimental results. Then the most favorable flocculation aid was searched among eight soil amendments and fertilyzers. Gypsum was found to be the best aid because it showed most effective flocculation and moderate pH change. The optimum gypsum application rate was examined in test plots made on two paddy fields of the area. The rate of 30 kg/lOa gypsum deceased the suspended soild concentration below 0.1% in 12 hours. The critical Ca ion concentration at rapid flocculation was found to be ranging from 1 to 2m mol/Z, which agreed with the estimated critical concentration. The rate of 30kg/lOa gypsum was estimated to reduce 70 to 80% of the whole effluents of the suspended solids of the puddled water.
