Chloramines, chlorine demand

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). 

Residuals of chloramine decline to a minimum value that is referred to as the breakpoint. When dosages exceed the breakpoint, free chloride residuals result. Breakpoint curves are unique for different water samples since the chlorine demand... 

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.)...

When no dechlorination chemical was added, the chlorine concentration decreased from 1.05 to 0.95 mg/L after 1000 feet (Fig. 2). This indicated that only a small amount (0.1 mg/L) of the chloramines dissipated through chlorine demand of paved surfaces. Sodium bisulfite, sodium sulfite, ascorbic acid, and sodium ascorbate neutralized all detectable chlorine to below 0.1 mg/L within 2 ft downstream of the mixing hose (approx 2 s). Sodium thiosulfate neutralized more than 80% of the chlorine within 2 ft. However, chlorine concentrations decreased below 0.1 mg/L (the discharge limit in most states) after about 500 ft (elapsed time 3 min, 2 s). Calcium thiosulfate neutralized 60% of the chlorine within 2 ft and neutralized 90% of the chlorine after 1000 ft (elapsed time 7 min, 10s). 

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. 

The chlorine demand is the difference between the chlorine added and the residual concentration after a designated reaction time of approximately 10 min. Total residual chlorine is determined by the oxidation of AA -diethyl-p-phenylenediamine (DPD) to produce a red-colored product Addition of iodine then catalyzes further reaction with chloramines, and it is possible to obtain concentration values for aU chloramines and free chlorine. 

Antimicrobial efficacy is also affected by demand in the cooling water system, specifically demand exerted by ammonia. Chlorine reacts with ammonia to form chloramines, which are not as efficacious as hypochlorous acid or the hypochlorite ion in microbiological control. Bromine reacts with ammonia to form bromamines. Unlike chloramines, bromamines are unstable and reform hypobromous acid. 

HOBr and HOCl are equal in microbicidal activity. However, since HOBr dissociates at a higher pH range than HOCl, HOBr is often used in place of HOCl in water systems operated at pH values >7, e.g. pH 8.5. Another benefit of the application of HOBr instead of HOCl are reduced corrosion rates. In the presence of amino groups containing substances HOBr forms (as does HOCl) N-bromo-amines or amides which are more effective than corresponding chlorine releasing chloramines. A disadvantage of HOBr is that it is very susceptible to oxidant demand. 

The primary use of anhydrous ammonia (ammonia gas) in water treatment is to combine with chlorine to form chloramines. Chloramines are used both as primary and secondary disinfectants. Use as a secondary disinfectant (residual in the distribution system) is more common. A typical treatment strategy is to use free chlorine to satisfy the USE PA regulatory CT requirements as a primary disinfectant. Ammonia is then added to combine with the free chlorine residual to form chloramines for use as the secondary distribution system disinfectant. The ammonia added is carefully controlled to ensure that all the free chlorine is combined and little free ammonia remains. This control is necessary because the presence of free chlorine can form regulated by-products. Free ammonia can increase the growth of nitrifying bacteria, thus causing residual demand that could lead to conditions that could violate the Total Coliform Rule. 

The chloramines are called combined available chlorine. Chlorination practice frequently provides for formation of combined available chlorine which, although a weaker disinfectant than free available chlorine, is more readily retained as a disinfectant throughout the water distribution system. Too much ammonia in water is considered undesirable because it exerts excess demand for chlorine. 

Effluents from the Annacis and Lulu Island STPs frequently contain higher levds of contaminants than permitted by the provincial government. For the Annacis plant permit, noncomphance is most apparent for Biochemical Oxygen Demand (BOD), toxicity, oil and grease, and dissolved oxygen. For example, in 1985 toxidty levels were exceeded 50 percent of the time for Annacis and 66.7 percent of the time for Lulu Island STPs. The toxic compotinds identified in municipal STP effluent include im-ionized ammonia, cyanide, sulfides, chlorine, chloramines, phenols, anionic siufactants, heavy metals, and organic compounds. Table P-5 provides a... 

---