In the previous reports
1)2), it was confirmed that oxygen consumption during filtration (OCF) could be used as an index indicating the degree of purification of breeding water by filtration, and that a considerably better fit of the OCF data could be obtained by using a formula of second-order reaction.
In order to find out the relation between grain gize of the sand used for a filter and its ability to purify breeding water, in the present studies OCF of the breeding water of Suma Aquarium was determined using 3 experimental filters with sand of different grain sizes (Fig. 1).
The results obtained are as follows.
1. There is no correlation between the size of the sand grains and the constant (
C0) of a second-order equation fitted to the OCF reaction (Fig. 3, Table 2).
2. The velocity constant (
K) of a fitted equation is in proportion to the grain-size coef-ficient (
G) of sand (Fig. 2), (Fig. 4, Table 4).
3. According to the facts mentioned above, when
V, D, W, and
G denote filtering velocity (cm/min), sand depth (cm), filtering area (m
2) and grain-size coefficient respectively, OCF per unit time (
Y mg/min) is shown by the following formula,
Y=10
W/1/
C0V+1/
C02αGD where C
O and
α are constants determined in response to the condition of water.
4. According to a previous research
3) Suma Aquarium was one of the aquaria in which amount of fish per volume of water was considerably large. Therefore, the condition of breeding water of Suma Aquarium seems to be the safe limit to keeping marine fish healthily for a long time. When mean values of
CO and α, obtained from many experiments on breeding water of Suma Aquarium, are applied to this formula, it may be said that Y indicates a maximum OCF obtained by filtration of breeding water with the condition of the safe limit. So, the following formula can be established,
YM=10
W/0.70/
V+0.95×10
3/
GD where
YM indicates a maximum degree of purification (OCF).
From this equation, it can be deduced that when the filtering velocity in a filter is small and the grain-size coefficient of its sand is large, purification of water is accomplished mostly in its upper layer (Table 5).
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