a. Chemical (chlorination, ozonation, and acid and alkaline treatments)
b. Physical (heating, UV irradiation, filtration, and settling)
c. Radiation (electromagentic and acoustic)
CHLORINE DOSAGE (mg/1)
8-15 mg/l (secondary effluent) CONTACT TIME (min)
>30 minutes (peak hourly flow) CHLORINATION TANK FLOW CONFIGURATION PF
MAXIMUM CHLORINE RESIDUALS 0.1-0.5 mg/l (undiluted effluent)
Disinfection is the selective destruction of disease-causing organisms. Disinfection of effluents prior to discharge insures that bacteria, viruses, and amoebic cysts are reduced to acceptable levels. Many means can accomplish the dis infection of effluents: chemical agents, physical agents, mechanical means, and radiation.
The most common chemical agents used in disinfection are chlorine and its compounds. Ozone is highly effective but it does not leave residual. Acids and alkalies are sometimes used since pH >11 or pH <3 are toxic to most bacteria. Bromine, iodine, phenols, alcohols, and hydrogen peroxide are other common chemical disinfection agents. Heat and light (especially UV light) are effective physical disinfection agents. However, using heat and UV light to disinfect large quantities of effluents is cost prohibitive. The presence of suspended matter in effluents can also reduce the efficacy of UV radiation.
Preliminary and primary treatment processes used in wastewater treatment (e.g., coarse and fine screens, grit chambers, and primary clarifiers) are capable of removing or destroying a large number of bacteria (Metcalf and Eddy, Inc. 1991). Up to 75% of the bacteria in incoming wastewater can be removed or destroyed by the settling mechanism alone. However, removal and destruction of bacteria are the by-products instead of the primary functions of these treatment processes. Wastewater treatment facilities can use electromagnetic, acoustic, or particle radiation to disinfect water, wastewater, and sludge. However, these applications are limited due to the high costs involved.
Bacteria and viruses are removed or killed by disinfection and sterilization. Numerous disinfection and sterilization techniques are available. Tables 8.2.1 and 8.2.2 in Chapter 8 compare their effectiveness, advantages, and disadvantages. This section discusses disinfection with chlorine since it is the most common disinfectant used.
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