Residual chlorine concentration

In the sodium borate solution containing bromide, when the pH 4 buffer is added before the potassium iodate solution, titrations give low total residual chlorine concentrations. This loss increases with the amount of stirring time between the addition of the reagents. Even for a stirring time of 10 seconds, there is a loss of about 17% of the total residual chlorine. If the solution were stirred for 30 min, 85% of the chlorine would have disappeared. The concentration of total residual chlorine determined by the reference methods does not change throughout the experiment. This implies that this loss of chlorine does not occur in the reaction vessel, but in the titration cell as a result of the analytical procedure. 

If potassium iodide is added first, and then the solution is stirred, acidified and titrated, the loss of residual chlorine is reduced, although still significant. The loss again increases monotonically with stirring for 20 min. However, for a stirring time of 1 minute or less, the loss is not detectable within the uncertainty of the analytical method. There is a loss of chlorine whether the sample is stirred in the titrator or on a stirrer, although the loss seems smaller in the latter case. For a stirring time of 20 min, only 24% of the residual chlorine is lost. Moreover, titrations performed at pH 2 and pH 4 yield the same residual chlorine concentrations. 

Free residual chlorine concentration leaving the treatment plant as finished water. 

Fig. 9.16. Temperature dependence of the residual chlorine concentration in the growing film triangles are experimental points and diamonds are simulation results.

Ammonia is combined with chlorine to purify some municipal and industrial water supplies. They act as delayed sterilizing agents and permit higher residual chlorine concentrations than does chlorine alone - without producing a chlorine taste and odor57. 

This chapter only discusses the applications of chlorination and chloramination in potable water treatment. In case the two processes are to be used for wastewater treatment, residual chlorine concentration in the plant effluent may become a regulatory issue (30). Selection of an alternative disinfectant becomes more important. New alternative disinfectants have been studied by Wang (19-25). Wang (35,36) also reported that UV is an effective process for dechlorination, dechloramination, or de-ozonation. 

Free chlorine is highly reactive and relatively unstable. Utilities using free chlorine for disinfection have been known to use secondary chlorination stations to maintain residual chlorine concentrations in the potable water distribution system as regulated by the Clean Water Act (CWA). One of the key concerns in using free chlorine for disinfection is that, under certain conditions, free chlorine may react with organic substances in water to form carcinogenic trihalomethanes (THMs) (1,2). 

When 12 tablets were placed across the flow of 100 gpm, the chlorine concentration decreased below the detection limit (0.1 mg/L) within 5 min. It remained below the detection limit even after 60 min. In the next test, initially a flow rate of 300 gpm was maintained and 16 tablets were placed across the flow. Within 5 min the chlorine concentration decreased to below detection limit. After 10 min, the flow rate was increased to 450 gpm. At this increased flow rate, the residual chlorine concentration increased to values of 0.6-0.8 mg/L, well above the detection limit of 0.1 mg/L (which is the allowable discharge hmit in many locations), within 25 min (Fig. 3). This indicates that the flow rate of chlorinated waters can significantly impact the efficiency of dechlorination operations. Higher flow rates may not provide sufficient contact time for dissolution of tablets into the stream. After approx 40 min, the number of tablets was increased to 20. This decreased the residual concentration to below detection limit within 5 min. The increase in the number of tablets probably provided an enhanced contact area and better dissolution of the tablets into the flow, resulting in a decrease in the residual chlorine concentrations. 

In summary, results from the test series indicated that, for a flow rate of up to 100 gpm for EBMUD water, one Dechlor tablet maintained the residual chlorine concentration below the detection limit for 45 min. An increase in the number of tablets increased the residual chlorine removal efficiency. However, an increase in flow rate to 450 gpm resulted in an increase in residual chlorine concentrations to above detection (and compliance) limits within 25 min, even in the presence of 16 tablets. Results also indicated that, when the flow rate was decreased (to 50 gpm) and the number of tablets increased (to 28), the DO concentration decreased significantly. 

In this test, water quality was analyzed along the flow path, upon contact with dechlorinating chemical. A flow rate of 100 gpm was maintained, and one or four tablets were placed across the flow. Samples were collected at the point of release and 40, 80, 120, and 160 ft downstream of the tablets. The residual chlorine concentration in the water decreased with distance (Fig. 4). One tablet was sufficient to remove chlorine to below 0.1 mg/L after 120 feet of travel under the test conditions. 

Sufficient gaseous chlorine is added to the water to leave a residual chlorine concentration of 0.1-0.2 mg/L, after the normal consumption of a part of the added chlorine in reactions with any dissolved or residual suspended matter in the supply has taken place [6]. A preferred contact time of 1-2 hr is recommended, but at least 20-30 min of contact time should be ensured before use. The residual chlorine content is necessary to maintain safe transmission of the treated water supply through a local piping system. 

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... 

Lamb (1978) introduced chlorine into a trout stream in Central New York to evaluate the toxicity to various stream organisms. Periphyton density was reduced only at high chlorine residuals fish were sensitive to low concentrations and total residual chlorine concentrations of less than 0.01 mg/1 caused catastrophic drift of sensitive stream insects. 

Together the three compounds contribute to what is known as the ree residual chlorine concentration. When chlorine is added, it reacts with organic matter and ammonia to form organochlorine compounds and chloramines [50.5]. So much chlorine has to be added that a break-point chlorination is obtained. Above the break-point the bactericidal effect is considered good and the viricidal effect moderate. 

Biological growth is promoted. Slime accumulates on the cell decks or the rate of chlorination needed to maintain a residual chlorine concentration is increased. (Amine or other organics leaking into the cooling water will have the same effect.)... 

Austenitic steels are sufficiently corrosion-resistant for chlorine concentrations below 3 mg/1, which usually occur if the waste water is chlorinated. The molybdenum-free grades are susceptible to crevice corrosion if they are in continuous contact with waters with a residual chlorine concentration of 3-5 mg/1. Austenitic steels are not suitable in chlorine discharge systems in which the chlorine concentration may reach several hundreds of mg/1. 

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