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How to use chlorine correctly so that by-products are not formed?

by:BOQU     2023-03-20
The use of chlorine for water treatment has a long history since it was first used in Jersey City in 1908. Although chlorine is used to produce drinking water, chlorination is not a simple process, especially considering that chlorine itself is toxic. Recently, we have paid great attention to the correct use of chlorine so that by-products are not formed. Wastewater The main problem with wastewater is the removal of pathogenic microorganisms. Second, wastewater discharge should not contain compounds that have an impact on environmental health. While residual chlorine is desirable in drinking water treatment, it is undesirable in wastewater treatment because chlorine discharge into the natural environment may have adverse effects on wildlife. Chlorine does this by inactivating viruses. Chlorine can also oxidize and degrade soluble pollutants, such as agricultural or pharmaceutical compounds. However, chlorination may also lead to the formation of by-products (DBP) by reacting with organic compounds. In wastewater, a high concentration of organic components leads to a higher potential for DBP formation compared to drinking water. Chlorine form Chlorine can exist as free chlorine or chloramines. Both groups can oxidize the compound and serve their purpose, however, more chloramine is required to meet the same chlorine demand. The measurement of free chlorine takes into account the ionic and protonated forms of chlorine in OCl- and HOCl in water, respectively. Of the two, HOCl (hypochlorous acid) is the stronger oxidizing agent and dominates at pH below 7.5. Chloramines include monochlorine, dichlorine and trichlorine and are formed by the reaction of chlorine and ammonia at various pHs. Above pH 6, monochloramine dominates. Chloramine measurements include all three forms, known as combined chlorine. Total chlorine is a measure of free and mixed chlorine. Unlike chlorine, chloramines do not degrade over time. The amount of chlorine added equals the amount of chlorine in the residue plus the amount of chlorine consumed in the treated water plus the amount of chlorine that reacts with ammonia to form chloramines. By-products Organic molecules not previously removed become precursors of DBP and may be harmful to human health. Free chlorine reacts to form compounds such as trihalomethanes (THMs) and haloacetic acids (HAAs), two groups of halogenated compounds that have adverse effects on human health. Chloramines form less DBP than free chlorine due to their lower oxidation potential, but they react to form N-nitrosodimethylamine (NDMA), a potent carcinogen. Ways to prevent DBP formation are to remove known precursors, avoid overchlorination, or choose another method. In general, chlorine and chloramines are cost-effective methods, but if DBP removal is required, other methods will be cost-competitive. Since UV light does not introduce halogens into the system, halogenated DBPs including THM and HAA are not produced during UV light. Other by-products may be formed, such as photodegradation of nitrate to nitrite. However, UV products are produced in concentrations that do not cause human health concerns. In fact, UV light has the potential to degrade DBP and is also effective in degrading chloramines. Chlorine Monitoring Chlorine demand is difficult to predict because it depends on water quality and the specific distribution of chlorine forms. Continuous monitoring provides control over the chlorine process. Off-line tests for chlorine rely on the indicator chemical N,N-1,4-diethyl-1,4-phenylenediamine sulfate, known as DPD, which can be used either colorimetrically or titrimetrically. On-line measurement of free chlorine uses the amperometric method, which relies on conducting electricity and membranes. Additionally, the measurement of the oxidation-reduction potential (ORP) provides information on the oxidative capacity of a chlorine dose, which is altered due to the speciation between HOCl and OCl- and the reaction with chloramines.
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