CHAPTER II
WATER QUALITY CONTROL
2.1 Water purification system
2.1.1 Water purification
The process of water purification in ponds, rivers, reservoirs or elsewhere is mainly a biological one carried out by microorganisms and/or unicellular green algae which absorb, assimilate and eliminate ammonia. What kind of system can be established in indoor ponds or recirculating units in which the micro-organisms and/or unicellular green algae are not found?
Generally rivers do not become contaminated under natural conditions, as they benefit from auto-purification. Polluted waters, due to sewage and other contaminants influx, gradually regain their original quality level as it flows downstream. This phenomena is attributable to the presence of micro-organisms attached on the surface of gravel, stones and other substrates. Bearing in mind the above natural process, a similar process can be reproduced in indoor and recirculating tanks to purify the rearing water.
In indoor tanks, the side walls, bottom and other rearing structure, all act as “purifiers”, while the filter medium in the recirculating tanks carries out the same function that sand and gravel achieves in rivers. The water quality in indoor rearing systems deteriorates due to the presence of excretions as well as unconsumed feed. The distance of water flow in rivers is equivalent to the staying time in indoor tank water or filter media.
2.1.2 Filtering micro-organisms
The micro-organisms involved in water depuration may be grouped into several categories depending on their activity.
In aquaculture activities, the activity of oxidizing bacteria is considered to contribute significantly towards the quality of the rearing medium. Heterotrophic bacteria decompose and convert organic substances into their inorganic form, while nitrifying bacteria oxidize the ammonia produced either by the heterotrophic bacteria during the conversion of inorganic substances and/or excreted by the cultured organisms, into the low-toxic nitrate. The latter bacteria require oxygen as the oxidant agent. As a result of their bio-chemical activity they can be divided into ammonia-oxidizing bacteria which generate nitrous acid through the oxidation of ammonia, and nitrous-acid-oxidizing bacteria forming nitrate by oxidizing the nitrous acid.
2.1.3 Auto purification
In auto purification, anaerobic decomposition occurs through which inorganic substances are decomposed in the absence of oxygen. The bacteria involved in this process are known as “anaerobic bacteria”. These bacteria can be divided into two groups: facultative anaerobes which tolerate a wide range of oxygen concentration and compulsory anaerobes which will not develop when oxygen is present.
The reduction of nitrate is very important in anaerobic decomposition. Nitrate develops into nitrogen gas via the nitrous acid stage and this process is called “denitrification”. Denitrification also comprises a process of "oxidized denitrification, that is, the development of hydroxyl amine formed at the ammonia-oxidation stage into a nitrogen gas. Figure 1 describes the nitrification process carried out by the above-mentioned bacteria.
Figure 1. Nitrification of nitrogen
Figure 1.
The intensity of purification is determined by the number of the bacteria and level of their activity. The filter media provides the growth substratum for the bacteria.
2.2 Critical factors in water purification
The main factors involved in the water purification process are the formation of bacteria on the filter media, a series of ecological factors which favour bacteria propagation, and the type of filter media.
2.2.1 Maturation of the filter media
The level of heterotrophic bacteria, whose nutrition source consist of organic substance, is usually high in newly-made filter tanks used for the purification of water. On the contrary, ammonia or nitrous acid oxidizing bacteria are scarcely found. At this stage, the accumulation of organic substances in the filter will induce a rapid increase in the number of heterotrophic bacteria. After about 7 days, the number of ammonia oxidizing bacteria will increase and nitrous acid will tend to accumulated in rearing water. The presence of nitrous acid then triggers of the prolification of the nitrous acid bacteria which will transform the nitrous acid into nitric acid.
At this stage the purification ability of the filter medium increases considerably. This initial series of events are usually referred to as the “maturation or activation of the filter medium”. At least a two-month period is required for the bacteria to develop in a stable fashion. The quality of the rearing water in the process is shown in Figure 2.
The disappearance of nitrous acid from the rearing water is usually used as a criteria for identifying the degree and maturity of the filter.
As shown in Figure 2, a satisfying level of bacteria formation and water quality stability usually occurs after 40–60 days from the initial activation of the filter. The period required for filter activation is not always the same as it is strongly related to the load of the filter media. Bacteria oxidation usually does not stabilizes before the complete maturation of the filter media. Therefore, an abrupt increase in their number may result in the deterioration of the water quality. It is therefore important to carefully regulate water volume, rearing density and the amount of feed administrated, so that the optimum load of the filter media is always maintained.
2.2.2 Bacteria formation in the filter media
To estimate the number of bacteria in the filter medium, sand grains are placed at the bottom of a 70 l glass water tank and rearing conducted by the application of the circulating/filtration method.
At this stage, the total number of heterotrophic bacteria is calculated and the following results have been obtained (Fig. 3): 10 total heterotrophic bacteria per g of filter media, and 105 ammonia oxidizing bacteria per g of filter media, and 104 nitrous acid oxidizing bacteria per g of filter media. The number of bacteria in a mature filter media vary according to the nature of the filter media itself. Infact the measured value is based on the unit weight and not unit surface area of the filter media. Therefore, the number of bacteria per gram in a sand-based filter will be higher than the number calculated from a stone-based filter, as the surface area per gram of filter media increase with particle of small diameter.
Figure 2.
Figure 2. Rearing water quality variations during the process of filter activation.
Figure 3.
Heterotrophic bacteria
Ammonia oxidizing bacteria
Nitrous acid oxidizing bacteria
Figure 3. Bacteria count per gram of filter medium.
A 50 cm long filter tank is used to investigate the number of bacteria at different layers of a filter media.
As shown in Figure 4 the upper and lower layers may contain almost the same number of bacteria and it has been shown that water purification is conducted in uniformity. However, the level of purification and the bacteria number will increased in the surface layer if composed of small particles such as sand grains.
A number of researchers in various countries have calculated the maximum value of bacteria per unit weight of a filter media (Table 1).
Table 1. Maximum number of bacteria per unit weight of filter media
Filter media Limitation value of bacteria
(per g filtered media)
Limestone
D=3.5 cm Total heterotrophic bacteria Ammonia oxidizing bacteria Nitrous acid oxidizing bacteria
10,000,000 100,000 10,000
2.2.3 Filter media quality and bacteria number
In the selection of filter media, it is important to know whether the quality and form of filter media is responsible for the number of the attaching bacteria.
As shown in Figure 5 the number of bacteria per unit surface area does not vary between different filter media. Therefore, the filter media of any quality can be used, unless harmful substance are releases into the water.
2.2.4 Conditions favourable for bacteria activity
Bearing in mind that the number of bacteria is not dependent on the filter material used, a filter media with a large surface area per volume may be more desirable to one with a small one, as the number of bacteria is higher. The purifying ability of the filter media is not simply determined by the number of bacteria, but also on the activity of bacteria.
Figure 4.
Heterotrophic bacteria
Ammonia oxidizing bacteria
Nitrous acid oxidizing bacteria
Figure 4. Bacteria count per gram of filter medium (a= top; b= middle; c= bottom layer).
Figure 5.
Limestone
Beehive shaped hexagonal column of vinyl chloride plate
Piled Saran fibre
Rough entanglement of yarn-shaped polyprofilyn
Compressed mesh-shaped polyethylene film
Figure 5. Relationship between type of filter medium and bacteria count.
Oxygen concentration. The activity of bacteria is dependent on the level of oxygen saturation, and therefore the rearing water has to be supplied with a sufficient level. Generally, nitrifying bacteria are mainly aerobic and therefore require a high oxygen supply. According to the results of research and experimental trials, the optimal volume of air blowing is 2–3 times the volume of circulating water. In this case, the concentration of oxygen should be in the range of 6–8 mg/l. The concentration of oxygen tend to increase when a proper contact is made between the circulating water and the filter media. The filter media with large surface area per volume is liable to clogging, which may decrease the activity of bacteria.
Water purification (i.e. ammonia oxidation) in the filter media is carried out by the active mud trapped between the spaces of the filter media particles. The active mud occupies one fortieth of total filter media. The daily oxidation of ammonia in the filter media is 0.5 mg ammonia per 1 ml of active mud. The above mentioned purification process is possible, only when sufficient leve