Chlorine dosage

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). 

There are three basic terms used in the chlorination process chlorine demand, chlorine dosage and chlorine residual. Chlorine demand is the amount of chlorine which will reduced or consumed in the process of oxidizing impurities in the water. Chlorine dosage is the amount of chlorine fed into the water. Chlorine residual is the amount of chlorine still remaining in water after oxidation takes place. For example, if a water has 2.0 ppm chlorine demand and is fed into the water in a chlorine dosage of 5.0 ppm, the chlorine residual would be 3.0 ppm. 

Table 3 gives recommended ranges of chlorine dosages for disinfection of various wastewaters. Recommended minimum bactericidal chlorine residuals are given in Table 4. Data in Table 4 are based on water temperatures between 20 C to 25 C after a 10-minute contact for free chlorine and a 60 minute contact for combined available chlorine. 

The minimum residuals required for cyst destruction and inactivation of viruses are much greater. Although chlorine residuals in Table 4 are generally adequate, surface waters from polluted waterways are usually treated with much heavier chlorine dosages. Ordinary chlorination will destroy all strains of coli, aerogenes, pyocyaneae, typhsa, and dysenteria. 

The effectiveness of a disinfectant also depends upon the age of the microorganism. For example, young bacteria can easily be killed, while old bacteria are resistant. As the bacterium ages, a polysaccharide sheath is developed around the cell wall this contributes to the resistance against disinfectants. For example, when using 2.0 mg/L of applied chlorine dosage, for bacterial cultures of about 10 days old, it takes 30 min of contact time to produce the same reduction as for young cultures of about one day old dosed with one minute of contact time. In the extreme case are the bacterial spores they are, indeed, very resistant and many of the chemical disinfectants normally nsed have little or no effect on them. 

Breakpoint reactions. Figure 17.1 shows the status of chlorine residual as a function of chlorine dosage. From zero chlorine applied at the beginning to point A, the applied chlorine is immediately consumed. This consumption is caused by reducing species snch as Fe Mn, H2S, and NO2. The reactions of these substances on HOCl have been discussed previously. As shown, no chlorine residual is produced before point A. 

In waters and wastewaters, organic amines and their decomposition products such as ammonia may be present, hi addition, ammonia may be purposely added for chloramine formation to produce chlorine residuals in distribution systems. Also, other organic snbstances snch as organic amides may be present as well. Thus, from point A to B, chloro-organic compounds and organic chloramines are formed. Ammonia will be converted to monochloramine at this range of chlorine dosage. 

Beyond point B, the chloro-organic compounds and organic chloramines break down. Also, at this range of chlorine dosage, the monochloramine starts to convert to the dichloramine, but, at the same time, it also decomposes into the nitrogen gas and, possibly, other gases as well. These decomposition reactions were addressed previonsly. 

Note If superchlorination is to be practiced to ensure complete disinfection and it is also desired to have long-lasting chlorine residuals, then ammonia should be added after superchlorination to bring back the chlorine dosage to the point of maximum monochloramine formation. 

Figure 17.2 shows the threshold odor as a function of pH and the concentration of chlorine dosage. Figure 17.2a uses a concentration of 0.2 mg/L and, at a pH of 9.0, the maximum threshold odor concentration is around 28 /tg/L. When the pH is reduced to 8.0 this threshold worsens to around 20 /tg/L, and when the pH is further reduced to 7.0, the threshold concentration becomes worst at around 13 /xg/L. Thus, chlorination at acidic conditions would produce very bad odors compared to chlorination at high pH values. This is very unfortunate, because HOCl predominates at the lower pH range, which is the effective range of disinfection. 

Important parameters to be considered in the design of chlorination unit operations facilities shonld include chlorine feeders, dosage control, chlorine injection and initial mixing, contact time and chlorine dosage, and maintenance of self-cleaning velocities throngh the chlorine contact tank. Each of these will be discussed in succession. 

Contact time and chlorine dosage. The two most important parameters used in the design of chlorine contact tanks is the contact time and dosage of chlorine. These parameters have already been discussed the equation is given by the universal law of disinfection. Equation (17.2). Figure 17.12 shows a contact tank used to disinfect treated sewage. 

The water is treated with UV radiation for primary disinfection, then chlorinated for secondary disinfection. An applied chlorine dosage of only about 1 mg/L is necessary. The entire water treatment system is housed in a 2.97 m (32 fU) building. The UV disinfection system consists of six irradiation chambers, two control cabinets with alarms, chart recorders, relays, hour-run meters, lamp and power on-lights, six thermostats, electrical door interlocks, mimic diagrams, and six UV intensity monitors measuring the total UV output. Each irradiation chamber contains one 2.5-kW mercury vapor, medium-pressure arc tube, generating UV radiation at 253.7 nm. 

Chlorine dosage is the amount of chlorine required to oxidize the target substance to be treated (such as water, wastewater, sludge, or septage) plus the desired chlorine residual. The target substance to be treated is termed chlorine demand. Usually the chlorine dosage is computed as mg/L concentration and the chlorine feed system set at the equivalent Ib/d feed rate. 

Given a desired chlorine residual of 300 mg/L and a chlorine demand of 800 mg/L, the chlorine dosage and resulting feed rate (for 12,000 gal/d throughput) are computed as follows ... 

Figure 2 shows how residual chlorine affects coliform number. The curves show the most probable number (MPN) of coliforms remaining after 30 min of chlorine contact in a well-designed chlorine contact tank. These results should not be considered as being exact. Table 1 lists chlorine dosages often used for disinfection of raw and partially-treated sewage. 

Obtain typical operating data for the chlorination system being studied. For example (a) type of efQuent = activated sludge (b) peak plant flow = 5 MGD (c) volume of chlorine contact chamber (CCC), v = 13,926 ft (d) chlorine dosage = 6 mg/L (e) chlorine residual = 1 mg/L. 

For wastewater treatment, the recommended chlorine dosage for disinfection purposes should produce a chlorine residual of 0.5-1 mg/L after a specified contact time. Effective contact time of not less than 15 min at peak flow is recommended. Practical chlorine dosages recommended for wastewater disinfection and odor control are presented in below ... 

The required input data include (a) chlorine contact tank influent flow, MGD (b) peak flow, MGD and (c) average flow, MGD. The design parameters include (a) contact time at maximum flow, min (b) length-to-width ratio (c) number of chlorine contact tanks (d) chlorine dosage, mg/L. 

The fourth step in design is to select chlorine dosage according to the recommended chlorine dosages in this section, and then calculate chlorine requirements ... 

Use chlorine residual analyzer to monitor and control the chlorine dosage automatically. 

Batch No. Volume of sludge chlorinated Ch used SS of sludge before treatement (mg/L) Dry sludge treated Chlorine % Dosage of dry (mg/L) solids ... 

The slurry was treated at a rate of approx 2.5 L/s (40 gpm). The weighted average of the chlorine dosage was 830 mg/L or 10% of the dry weight of the sludge chlorinated. The chlorine dosage was adjusted to produce a pH of 2.3-2.8 in the chlorinated slurry. The pH of the unchlorinated slurry indicates that considerable nitrification had occurred on some batches before treatment. 

Chlorine dosages vary from 700 to 3000 mg/L, depending on the solids content of the septage and the amount of chlorine-demanding substances present. (9). These substances include ammonia, amino acids, proteins, carbonaceous material, and hydrogen sulfide. The Babylon, New York septage treatment facility uses about 0.6 kg C JL influent (5 lb per 1000 gal). 

The chlorine dosage is approx 0.7 kg C JL influent (6 Ib/lOOO gal) for septage with a suspended solids concentration of 1.2%. The chlorine demand varies in proportion to the... 

Chlorination system size = to treat daily septage volume within 4-6 h Chlorine dosage = 0.7 kg/L for 1.2% TS, chlorine demand varies directly with TS... 

CD chlorine dosage, mg/L CR chlorine requirement, Ib/d CT contact time at maximum flow, min CTL contact tank length, ft CTW contact tank width, ft PCR peak chlorine requirements, Ib/d average flow, MGD Qp peak flow, MGD RLW length-to-width ratio SA surface area, ft ... 

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