Chlorine dosing

Most fish is still caught at sea and must be cooled soon after it is taken on board, and kept cold until it can be sold, frozen or otherwise processed [45]. The general practice is to put the fish into refrigerated sea water tanks, kept down to 0°C by direct expansion coils or a remote shell-and-tube evaporator. The sea water must be clean and maybe chlorine dosed. At this condition, fish can be kept for up to four days. 

During an on-line clean, the quality of the clean will normally be enhanced by the use of an initial chlorine dosing procedure at a pH of 7.2 to 7.6. This practice is to be recommended, irrespective of whether the current cooling system maintenance biocide program employs chlorine. With severely fouled cooling systems, it may be necessary to provide many hundreds of ppm of HOC1 in order to satisfy the oxidation demand. This in turn may require the use of a temporary, supplementary corrosion inhibitor, and almost certainly the use of some antifoam. 

Figure 16.30. (a) Absorbance spectra of coastal seawater chlorinated at varying chlorine doses, (b) Differential absorbance spectra of coastal seawater chlorinated at varying reaction chlorine doses. Reaction time 168h (Fabbricino and Korshin, 2005). 

Several studies were carried out on various germs and in different waters. They were performed in comparison with conventional disinfection processes using chlorine dosing or UV lamps. Several microorganisms were studied including Legionella... 

Figure 6.8 shows an evaluation performed on a sea microorganism, Marine bacteria H40. Once again, such bacteria are inactivated faster (four time) with Diacell -generated disinfectants than with the same level of disinfectant introduced with conventional chlorine dosing. 

Because chlorine is needed to produce the chloramines, higher chlorine doses are required to treat the same amount of water. 

Example 17.13 Specify the chlorine dosing system to be used for a wastewater treatment plant processing an average of 25,000 mVd of wastewater. The peaking factor is 3.0 and the regulatory agency requires a dosage of 20.0 mg/L. 

Example 17.15 A chlorine disinfection study was conducted to determine the constants of Equation (17.2). For a log 2 removal efficiency, the value of m is found to be 0.35 and the value of k is found to be 100. Calculate the contact time if the regulatory agency requires a chlorine dose of 20 mg/L. 

Figure 11.13. General scheme of breakpoint chlorination difference between total residual Cl and chlorine dose reflects chlorine demand, primarily from ammonium and amines. Before breakpoint, most Cl is in combined forms, primarily mono- and dichloramine after the breakpoint, the combined residual consists of slow-reacting organic chloramines. Added Cl remains in free form after the breakpoint. Sharpness of breakpoint and minimum observed Cl concentration depend on pH, temperature, and time of reaction. Loss of residual Cl at breakpoint is caused by oxidation of di- and trichloramines to Nj according to reactions 33a and 33b and other reactions. (Adapted from Brezonik, 1994.)...

The major parameters used to control the septage chlorination process are treated septage color, effluent pH, and effluent chlorine residual. The chlorine dose can be adjusted until the effluent stream is a hght buff color with a pH of 2-2.5, and a chlorine residual of 150-200 mg/L (43). 

The formation of CBPs is influenced by pH, temperature, ammonia, carbonate alkalinity, chlorine dose, contact time, removal of natural organic matter before chlorine application, etc. Moreover, the composition of these mixtures may change seasonally resulting... 

Jenner H.A., Polman H.J.G. and van Wijck R., 2004. Four years experience with a new chlorine dosing regime against macro fouling, VGB Powertech, 84 28-30. 

At a pH of 7.5, near neutral conditions, the hypochlorous acid will be roughly 50% dissociated and disinfection will still be quite effective. However, in disinfections of somewhat alkaline water supplies, the decreased hydrogen ion concentration will tend to depress the concentration of undissociated hypochlorous acid. At a pH of 10 or higher, hypochlorous acid would be present almost entirely (ca. 99.7%) as hypochlorite anion (e.g., see Section 15.5.2). This would substantially decrease the effectiveness of chlorine disinfection under these conditions [6], which could be remedied by pH adjustment of the supply before disinfection, by a higher chlorine dose rate, or by a longer contact time before water use. However, in practice, any source of supply above a pH of 8.5 would be adjusted to 8.5 before disinfection for other reasons. 

Chlorine dose the amount of chlorine originally used ... 

Chlorine demand the amount used up i.e., the difference between the chlorine dose and the chlorine residual ... 

Figure 5 shows the relationship between chlorine dose, chlorine demand and chlorine residual. 

A chlorine dose of 1 mg/Iiter as CIj satisfactorily disinfects a water at pH 7.0, What dose of chlorine would be required to achieve the same disinfection efficiency if the pH of the water was 8.5 Assume that HOCl is 100 times more effective as a disinfectant than OCl and that the temperature is 25°C. 

Chloramine formation and oxidation of ammonia by chlorine combine to create a unique dose-residual curve for the addition of chlorine to ammonia-containing solutions (Fig. 7-24). As the chlorine dose increases, the chlorine residual at first rises to a maximum at a [CI2] dose to [NH3] molar ratio of about 1.0. As the chlorine dose is increased further, the chlorine residual falls to a value close to zero. The chlorine dose corresponding to this minimum is called the "breakpoint" dose, and it occurs at a molar ratio of 1.5 1 to 2 1, depending upon solution conditions. The primary reaction that causes the residual chlorine concentration to decrease and thus to form the breakpoint is the breakpoint reaction, which can be represented as... 

As the chlorine dose is increased in excess of that required to produce the breakpoint, the increase in residual chlorine will be approximately equal to the excess. If we were to examine the nature of the chlorine residual after the breakpoint, we would find that it is largely free chlorine with some combined chlorine. 

Fig. 7-25. Chlorine residual as a function of chlorine dose/initial ammonia ratio at near-neutral pH..

Fig. 7-30. Chlorine dose-residual curves for lactose oxidation.

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