Breakpoint chlorination

During superchlorination or shock treatment, ammonium ion is oxidized to nitrogen by breakpoint chlorination which is represented by the simplified reaction sequence... 

In reahty the chemistry of breakpoint chlorination is much more complex and has been modeled by computer (21). Conversion of NH/ to monochloramine is rapid and causes an essentially linear increase in CAC with chlorine dosage. Further addition of chlorine results in formation of unstable dichloramine which decomposes to N2 thereby causing a reduction in CAC (22). At breakpoint, the process is essentially complete, and further addition of chlorine causes an equivalent linear increase in free available chlorine. Small concentrations of combined chlorine remaining beyond breakpoint are due primarily to organic chloramines. Breakpoint occurs slightly above the theoretical C1 N ratio (1.75 vs 1.5) because of competitive oxidation of NH/ to nitrate ion. Organic matter consumes chlorine and its oxidation also increases the breakpoint chlorine demand. Cyanuric acid does not interfere with breakpoint chlorination (23). 

Some nitrate is also formed, thus the HOCl/NH stoichiometry is greater than theoretical, ie, - 1.7. This reaction, commonly called breakpoint chlorination, involves intermediate formation of unstable dichloramine and has been modeled kinetically (28). Hypobromous acid also oxidizes ammonia via the breakpoint reaction (29). The reaction is virtually quantitative in the presence of excess HOBr. In the case of chlorine, Htde or no decomposition of NH occurs until essentially complete conversion to monochloramine. In contrast, oxidation of NH commences immediately with HOBr because equihbrium concentrations of NH2Br and NHBr2 are formed initially. As a result, the typical hump in the breakpoint curve is much lower than in the case of chlorine. 

This reaction is called the breakpoint chlorination the combined chlorine level, which was rising as more chlorine was added, now drops suddenly. Free chlorine then rises without a corresponding rise in combined chlorine. This indicates that the pool pollution has been successfully oxidized by chlorine. 

Figure 10.2. Idealized total available chlorine curve (or breakpoint chlorination curve).

A simplified scheme of breakpoint chlorination is given in Figure 11.13. 

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.)...

A typical treatment of the blowdown includes coagulation with ferric salts and an organic polymer, followed by a breakpoint chlorination. However a combined biological treatment of this wastewater (after some physico-chemical pre-treatment) together with coke-plant effluents also has been suggested [2, 3 ]. 

Alternatives most frequently considered for taste and odor removal include breakpoint chlorination, aeration, ozonation, and oxidation with chlorine dioxide or potassium permanganate. None of these technologies have been found to approach the activated carbon adsorption process iri terms of effective treatment for this particular water quality problem. Another alternative is sorption onto other solids such as bleaching clays, synthetic resins or manganese dioxide. A brief summary of the advantages, disadvantages and cost factors associated with adsorption and alternative treatments for removal of tastes and odors... 

Fig. 2. Graphical representation of the breakpoint chlorination reaction. The straight line at the left shows that chlorine residual is proportional to dosage in pure water. When impurities are present, they exert a chlorine demand (US EPA).

Prior to 1976, the Baxter plant practiced breakpoint chlorination at the raw water basin and maintained free chlorine in the distribution system. A total of 96 h of free chlorine contact time was typically achieved. 

Chlorine is also used in advanced wastewater treatment (AWT) for nitrogen removal, through a process known as breakpoint chlorination. For nitrogen removal, enough chlorine is added to the wastewater to convert all the ammonium nitrogen to nitrogen gas. To do this, about 10 mg/L of chlorine must be added per mg/L of ammonia nitrogen in the wastewater—about 40 or 50 times more chlorine than normally used in a wastewater plant for disinfection only. 

In breakpoint chlorination, about 10 mg/L of chlorine must be added for each mg/L of ammonia nitrogen present in the wastewater. Studies show that better pretreatment will... 

Usually, a base chemical is added to the breakpoint process to neutralize some of the acidity resulting from the chlorine addition. The base requirements depend on wastewater alkalinity, individual treatment processes used before breakpoint chlorination, as well as effluent pH or alkalinity restrictions by regulatory agencies. 

Another consideration in breakpoint chlorination is dechlorination to remove the chlorine residual from the final effluents before it is discharged. Very often, dechlorination using sulfur dioxide or activated carbon may be needed when the breakpoint chlorination process is used. A new dechlorination technology has been introduced in another chapter of this handbook series (2). UV dechlorination is recommended by Wang (45). 

In most cases, control of breakpoint chlorination requires the use of accurate and reliable automatic equipment to reduce the need for manual process control by operators. However, the operator must give special attention to this equipment and monitoring devices in order to ensure their proper operation. Table 3 indicates how the common process shortcomings can be compensated and improved. Table 4 is a wastewater chlorination process trouble-shooting guide for use by practicing environmental engineers. 

A water with an alkalinity of 82.5 mg/liter as CaCO and a pH of 8,0 is treated with Cbfg) to oxidize to nitrogen gas (breakpoint chlorination). If the initial... 

An experiment is to be conducted on breakpoint chlorination (the oxidation of ammonia by chlorine) in which it is desired to maintain the pH constant within 0.5 units of the initial pH of 8. Make the assumption that all of the ammonia is in the NH4 form (a reasonable assumption, since the pffg of NH4 is 9.3 and the pH of interest is pH 8). The breakpoint reaction between CI2 and NH3 proceeds as follows ... 

The maximum amount of ammonia that will be used in the experiments is 12.5 mg as N/liter (0.89 X 10 moles/liter). Select an appropriate acid-conjugate base pair and determine the concentration of it that will control the pH to within 0.5 units of 8.0 during the reaction. Neglect ionic strength effects the temperature = 25°C. Determine the buffer intensity of this solution. (Note There is a detailed discussion of the breakpoint chlorination reaction in Chapter 7.)... 

Saunier and Selleck examined this model and experimentally determined the rate constants for each of the reactions in Table 7-10. They concluded that the model fits breakpoint chlorination results. They suggested, but did not prove, that hydroxylamine (NHaOH) and possibly hydrazine (N2H4) could be intermediates in the breakpoint reaction— perhaps the hypothetical NOH of Wei and Morris. 

A. T. Palin, "Water Disinfection—Chemical Aspects and Analytical Control," in Disinfection, J. D. Johnson, ed., Ann Arbor Science, Ann Arbor, Mich., 1975. I. W. Wei and J. C. Morris, "Eiynamics of Breakpoint Chlorination," in Chemistry of Water Supply, Treatment and Distribution, A. J. Rubin ed., Ann Arbor Science, Ann Arbor, Mich., 1974. 

B. Saunier and R. E. Selleck, "Kinetics of Breakpoint Chlorination and of Disinfection," Sanitary Engineering Research Laboratory Report 76-2, University of California, Berkeley, May 1976. 

An FAC residual is regarded by the US Environmental Protection Agency as a sign of adequate disinfection. The problem encountered in ensuring that water leaving the treatment plant has been treated with enough chlorine to leave an FAC residual is illustrated by breakpoint chlorination. In breakpoint chlorination, dissolved chlorine is added to the water in a stepwise manner to determine the chlorine demand and to allow for the formation of chloramines. 

Hgure 2 Idealized breakpoint chlorination curve showing chlorine residual as a function of chlorine dose for a water sample at pH 7 that contains only ammonia at a concentration of 1 mg nitrogen (as NH3) per liter. 

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