Chlorine is a very effective disinfectant for water and the most common method used for disinfection of municipal drinking water worldwide. It is nor-mally obtained by adding liquid sodium hypochlo-rite (NaOCl) to the water, but solid calcium hypochlorite (Ca(OCl)2) mixed into the water or pure chlorine gas (Cl2) may also be used.1,4 All these compounds are strong oxidizing agents and have the ability to break up organic molecules.
As for ozone, there is a need for a certain contact time to achieve the necessary effect. This includes time for dissociation in water, time for diffusion through cell walls and time to inactivate selected enzymes. Chlorine concentrations of 0.2–0.5 mg/l with 20–30 min contact time or 3–5 mg/l for 1–5 minutes have been reported.16 To kill parasites in wastewater, typical values of free residual chlorine concentration of 1–3 mg/l with a contact time of 10–15 minutes have been reported.17 As for ozone, the residual concentration after a certain retention time is also of interest, and overdosing must be avoided. As an example the minimum residual con-centration of chlorine in the drinking water supply in Norway is 0.02 mg/l after 30 minutes contact time.
Methods for achieving this contact time are as for ozone: use of a retention tank, or to add it at the start of the transfer pipe, so a natural retention time is achieved when the water is flowing through the pipeline. However, a higher retention time is needed for chlorine than for ozone.
Water containing free chlorine is very toxic for fish. Concentrations of chlorine should not exceed 3–5 μg/l, although for shorter periods of up to 30 minutes concentrations up to 0.05 mg/l can be tol-erated by most species.18When disinfecting a tank or other equipment with chlorine, it is important that enough clean water is used to wash away the chlorine residues produced. Therefore chlorine is not normally used for disinfection of inlet water for aquaculture facilities. If chlorine is to be used, a method for dechlorination must also be included which can, for instance, be use of aeration, UV light, activated carbon, or reducing agents such as Na2S2O3 or Na2SO3.17 However, this will be a sub-optimal method for fish farming purposes, because of the costs.
Effluent water could be disinfected with chlorine, but here also it might be necessary to dechlorinate before discharge to the recipient water body because it might be toxic for the fish it contains. If effluent with a high content of chlorine were to be discharged into recipients with high content of organic substances, chlorinated organic substances might be created: trihalomethanes (THMs) are found in drinking water which has been disinfected by chlorination; chloroform is the most common THM and its presence is well correlated with the dosage of chlorine.
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