CHAPTER III
DESIGN OF FILTER TANKS
Filter tanks should be carefully designed and sized in order to efficiently handle waters from the rearing tanks of finfish and other aquatic organisms, over a given period of time. The bigger the filter tank, the more stable the water parameters in the breeding tank itself. However, in the culture of any aquatic organism the rearing equipment and facilities should be adequate in terms of their compatibility with the organisms and their economic feasibility. In order to design a filter tank, it is necessary to have a good understanding of the filtering capacity of the filter system, contamination and the quality of water used.
3.1 Norms for designing filter tanks
3.1.1 Oxidation rate of ammonia
The norms may be decided based on the oxidation rate of ammonia in the filter media, therefore determining the purifying capacity of the system. In this respect, the amount of oxidation of nitrogen by bacteria in the filter media is 0.21 mg a day per 10 g of filter media, whereas the total daily nitrogen produced by 100 g of fish is 50 mg. Bearing this in mind, the quantity of filter medium required is 25 times (50/2.1) the weight of the fish being cultured (30 times to be on the safe side) in order to remove all of the nitrogen produced in the water. In other words, about 30 tons of filter medium are needed for breeding 1 ton of fish. If the ratio of filter medium is 2.7, the volume of sand would be about 11 m3. The gap rate of sand grains is about 50 % so that the volume will effectively be about 20 m3 when placed in a filter tank. However, since the filtering capacity of a filter medium varies depending on its surface, this norm is subject to variation according to the size of filter medium such as sand, broken stones, etc., compared to other media of the same weight.
3.1.2 Dissolved oxygen reduction during filtration
When the rearing water flows through the filter tank, the level of dissolved oxygen will decrease due to bacteria oxidation in the filter medium. The level of oxygen consumption in the filter medium is calculated to determine the OCF (Oxygen Consumption Factor) to express the purifying capacity in the filter medium.
The depth of the sand bed, filtering rate and size of the sand grains, all of which affect the purifying capacity, have been studied in relation to the decrease of dissolved oxygen during the filtration process. As a result, the maximum purifying amount of any volume of a filter medium can be calculated by using the formula:
where:
Ym = purifying amount of a filter tank of W m2/minute (OCF mg/min)
W = surface area of the filter medium (m2)
V = filtering velocity (cm/min)
D = depth of the sand bed (cm)
G = coefficient of sand diameter (110 times of the reciprocal of the sand diameter in mm)
The above formula shows that the purifying capacity can be increased by enlarging the filter area rather than increasing the depth of the sand bed. Therefore, it is more effective to perform filtration uniformly in parallel than in serial.
On the other hand, the contamination of rearing water can be calculated from the load on the filter media (decrease of dissolved oxygen during filtration mg/min) when yellow porge is fed with mackerel flesh.
X = (B0.564 × 10-2) + 0.051 F
where:
X = load on filter media (mg/min)
B = weight of yellow porge (g)
F = daily mean weight of mackerel flesh given (g)
If the relation between the above Y and X is Y≥X, the purified amount and the amount of contamination are in equilibrium and rearing in this tank will perform well.
3.2 Factors affecting level of dissolved oxygen (DO)
3.2.1 Food level and DO consumption
When rearing fish like eel which tend to produce considerable organic matter, it is better to consider the purifying amount in terms of DO consumption rather than the oxidation rate of ammonia. For this reason, the DO consumption of the filter medium (DO consumption by filtration multiplied by the amount of water in circulation) and change in ammonia level has been studied while rearing eels reared with different amount of feed. The DO consumption of the filter medium corresponds to the amount of the feed given (Fig. 7).
When 100 g of feed is given, about 1000 mg of DO is consumed per hour for purification, and when 150 g is given over 1700 mg of DO is consumed. Therefore, DO consumption increases with the increase in feed input. The load on the filter medium is affected more by the amount of feed than the number of fish cultured. However, when 200 g of feed is given, the consumption of DO in the filter medium is almost the same as when 150 g is given. Therefore it may be considered that the purifying capacity of the filter medium is almost at its limit when 150 g of feed is provided. At 200 g the load is greater than the purifying capacity of the filter. This may be ascertained by the fact that ammonia accumulates remarkably in the water in which 200 g of feed is given. Thus the amount of feed has a certain relation with DO consumption in the filter medium. If the above observations are true, the DO consumption can be roughly estimated based on the level of feed input.
As shown in Figure 8, feeding level has a proportionate relation with the DO consumption of the filter medium. About 1/90 DO (g) is consumed per hour in the filter medium to purify the contamination produced by the feed.
For example, if the DO consumption is about 11 g/hr, the water quality does not deteriorate when 1 kg of feed is provided a day. A filter tank in which DO is consumed at about 1/90 of any amount of feed per hour may be used for stable rearing for about one month. Therefore, the volume of the filter medium needed for the removal of contamination caused by any amount of feed can be calculated from the purifying capacity (DO consumption) per unit surface area of the filter medium.