Disinfection is a process in which pathogens are destroyed or inactivated by physicochemical treatments. Disinfection is a final step for water treat ment; however, increasingly, wastewater treatment plants also apply disinfection in wastewater treatment because of the concern for pathogens in treated wastewater being discharged to a receiving water body or field. Pathogens in raw wastewater, to a large extent, have been either removed or inactivated among the trail of wastewater treatment processes. Nevertheless, there are still many opportunities for recontaminating treated wastewater because many treatment processes are conducted in open facilities outdoors. Other reasons for wastewater disinfection or use of disinfectants such as chlorine or its derivatives in wastewater treatment are oxidation of ammonia and of organic materials that contribute to BOD; destruction and control of ion-fixing and slime-forming bacteria; and destruction and control of filter flies, algae, and slime growth on trickling filters.

The mechanism of disinfection is said to be inactivation of enzymes of the pathogens by denaturing them. Without functional enzymes, microorganisms are destroyed or inactivated. In order to gain access to the enzymes in pathogen cells, the walls of the cells have to be penetrated by disinfectants or destroyed by thermal, chemical, or physical means. Chemical disinfectants, such as ozone, chlorines, and chlorine dioxide, work through oxidizing and reacting with cells of pathogens. Heat energy, irradiation, and ultrasound, even high pressure, perform their duties through physical destruction.

Chlorine and its derivatives are the most common disinfectants used in water and wastewater treatment. Chlorine in an aqueous solution hy-drolyzes to yield the following (Equation 5.5):

HOCl may be further hydrolyzed to yield the following (Equation 5.6): HOCl + H2O reversible > H3O + + OCl-

Both HOCl and OCl~ are disinfectants. Although chlorine is a very effective disinfectant, handling of it is inconvenient to say the least. Sodium hypochlorite (NaOCl) is often used in place of chlorine for disinfection of water and wastewater. Chlorinated lime (containing up to 70% CaOCl2 and 20% Ca(OH)2, as well as carbonate), also called bleaching powder, is also widely used in the same manner as NaOCl in applications. Calcium hypochlorite (Ca(OCl)2) is another chlorine derivative that is broadly employed in water and wastewater disinfection. Chlorine dioxide, an unsta ble gas, is also used in disinfection—it is often generated on-site through the following reaction (Equation 5.7):

2NaClO 2+Cl2 Kve^ible > 2ClO2 + 2NaCl

Another group of disinfectants belongs to strong oxidizing agents. Ozone (O3) is a particularly powerful but unstable oxidizing agent and is used extensively in Europe for both disinfection and removal of objectionable odor, state, and color. The popularity of ozone in Europe is also linked to its lack of residual products or by-products that might be harmful to human health. However, ozone is unstable and ozone-treated water does not have residual protection from recontamination like that treated with chlorine does; as a result, in the United States, chlorine and its derivatives are still dominant in water and wastewater disinfection. Hydrogen peroxide (H2O2) is an unstable liquid oxidizing agent; its oxidizing power is somehow not entirely related to its disinfecting power. It is believed that some bacteria can produce an enzyme called catalase that decomposes hydrogen peroxide into water and O2, thus rendering hydrogen peroxide harmless to those bacteria. Consequently, hydrogen peroxide is not a suitable disinfectant for any large-scale water or wastewater disinfection.

The disinfection or inactivation of microorganisms is, in a way, a physic-ochemical process that is not instantaneous. The rate of disinfection is believed to follow a first-order relationship called Chick's law (Equation 5.8):

Which can be integrated to result in the following (Equation 5.9):

N ° = N °0e-kt where N° is the number concentration of surviving microorganisms at time, t, and k is the rate constant.

Chick's law should be used as a rough estimation of the rate of disinfection in a practical application of disinfectants. This rate may be increased or decreased depending on the environmental factors, disinfectants, and microorganisms. Also, the concentration of disinfectants or dosage of disinfectants is not reflected in Equation 5.8 and 5.9. The temperature effect of rate of disinfection cannot be easily included in the rate constant, k, because temperature also affects certain reactions involved in disinfection in addition to disinfection rate. The value of pH can also exert influence on the disinfection rate, as well as reaction steps involved in certain disinfection processes. Extreme pH can inactivate microorganisms without disinfectants. Organic matters may interfere with disinfection processes by reacting with disinfectants or shielding microorganisms that attach to the surfaces of organic matters.

Nonchemical disinfection may also be used in lieu of chemicals. Thermal treatment is an effective method of inactivation of microorganisms; extended thermal treatment such as high-temperature steam or boiling water can achieve sterilization. But thermal treatment is an unlikely choice of disinfection method for wastewater treatment due to the enormous cost it entails. Ultraviolet irradiation has certain bactericidal effects, but its effectiveness is debatable; also, the presence of substances, including water, zaps the strength of ultraviolet irradiation. Gamma- and x-ray irradiation can inactivate certain species of bacteria; however, once again, this technology is impractical in disinfecting wastewater because the economical feasibility for treatment of a large volume of water dictates the selection of disinfection methods.

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