Abstract
This report describes our attempt to purify an irrigation pond by means of running the water repeatedly through fallow paddy fields that are adjoining the pond and were not expected to produce rice profitably. The pond, Kameike, locates in a rural area in Takamatsu city, and is approximately 7, 000 m3 in volume. The experiment was carried out in three phases: the summer and winter of fiscal 1993 and the summer of fiscal 1994.
During the experimental period, a lot of water with pollutants flowed from outside of the pond. Furthermore, bottom sediment of the pond posed significant influence on the purification. Affected these factors, the pond could not be purified enough. However, inflow quantities of T-N and T-P were reduced in the pond before outflowing. And we believe that the purification method we introduced certainly prevented the pond to be further polluted.
In the experiments in 1993, the pump suction inlet which was installed in the pond drew up sediment together with water; the quantity of SS in the pond increased even higher than the amount before the experiments. Therefore, we altered the suction inlet so as to restrain the device from drawing up sediment. Consequently, SS was reduced in the following experiment.
Yet, assessment of pollutants moving in the pond implied that certain factors, in addition to the pump's suck and internal production, have increased the amount of pollutants throughout the experimental period. We supposed that sediment resuspended in the water and dissolved nutrient released from the bottom might have swelled pollutants.
In order to affirm our supposition, we carried out a laboratory experiment using sample of sediment. In this experiment, significant release of nitrogen from the sample was observed. This result indicates that release of nitrogen partly caused increase of nitrogen on the pond.
It seems that pollution with the bottom sediment resuspended in the water and dissolved nutrient released from the bottom is inevitable, as long as sediment accumulates. Given the effect of bottom sediment, this purification method would bring a better result with a dredge.
