Abstract
In the previous paper1), it was confirmed that the oxygen consumption during filtration (OCF) could be used as an index indicating the degree of purification of breeding water by filtration. By using OCF as an index in evaluating the degree of purification, the present studies have been carried out to investigate influences of the filtering velocity and the depth of sand layer on the ability of a filter to purify breeding water.
In general, the BOD data well fit the equation of a first-order reaction. Since OCF as well as BOD is due to biochemical oxidation, it may be possible that the OCF reaction fits the theory of chemical reaction. When compared the OCF data obtained by filtration of breeding water of Suma Aquarium with a first and second-order reaction, it was found that they fitted much better to a second-order reaction formula than to a first order reaction formula (Fig. 1, Table 1).
The time taken for the water passage through the sand layer is correlated directly with OCF (a degree of purification), and the relation of OCF to the depth of sand layer is not direct but only in appearance, since the time taken for filtration is proportion to the depth (Fig. 2).
From the results of the experiments carried out to know the factors which fit the OCF reaction to second-order reaction, it has been deduced that one of the factors may be the distribution of microorganisms in a filter. They are distributed more abundantly in the upper layer of the sand bed than in the lower layer. The ammount of them in each layer of a filter may be determined by the quantity of unoxidized organic materials in the water which flows through that layer. According to the facts mentioned above, the relation of OCF (y/mg/l) to the filtering velocity (Vcm/min) and to the sand depth (Dcm) are shown by the following formula. 1/y=1/C0+1/C02kD
were Co and k are constants, determined by the condition of breeding water.
