Abstract
In previous papers, the author determined the flocculation conditions of the puddled water of Kotoh area by Ca application on the basis of the hetero coagulation theory. The results were examined by a laboratory test using 8 flocculation aids. The agreement of the theoretical and experimental results was good and the puddled water was found to flocculate at conditions of Ca concentration >1 — 2 mmol/dm3 or below pH< 5. To realize the condition,gypsum application of lg/dm3 was considered to be the most effective among the aids tested. However,those results were confirmed only in ldm3 volume of the suspensions and it is still unknown that the treatment is effective on the actual conditions of a puddled paddy field. In this paper, gypsum application was examined in the test plots made on the two paddy fields of the area. The effect of basal fertilizer of usual rate on the flocculation was also tested. In addition,the effect of this treatment on the whole effluent load out of the paddy field was estimated. The findings were as follows ;(1)gypsum application rate of 30kg/10a was sufficient to decrease the suspended solid(SS) concentration under 0.1% in 12 hours. The criterion Ca concentration in the puddled water at rapid flocculation was 1—2 mmol/dm3 in the field test, which coincides well with the results of the laboratory test. (2)Basal fertilizer promoted flocculation to some extent and did not hinder the flocculation by gypsum. (3) The rate of 30kg/l0a gypsum was estimated to reduce 70—80% of the whole effluent of SS of the puddled water.
