Introduction

Disinfection is the selective destruction of pathogenic organisms, whereas sterilization implies the complete destruction of all microorganisms. Disinfection is used during wastewater treatment in order to reduce pathogens to an acceptable level, which helps in the control of diseases caused by contaminated water and food-stuffs by bacteria and other microorganisms.

Methods used for wastewater treatment may be physical (e.g., heat), chemical (e.g., chlorine and its derivates, ozone, hydrogen peroxide, and colloidal silver), mechanical (e.g., sedimentation and filtration), or radiative (e.g., electromagnetic and acoustic). The efficiency of a disinfecting agent depends on several variables such as the contact time with pathogens in the sample, their concentration, temperature, number, and the nature of the liquid in which the pathogens are suspended. One of the main variables is the contact time. In general, the longer the contact time with a fixed concentration of disinfectant, the higher the mortality. First-order kinetics is usually followed. This relationship is known as the Chick equation:

dt where Nt = number of microorganisms at time t, and k = constant (in s_1). If No is the number of microorganism at t = 0, the integrated equation is:

It is common to find deviations from first-order kinetics caused by the resistance of subpopulations (within a mixed population) to the disinfecting agent or to the presence of protecting factors in the medium, both of which interfere with pathogen destruction. This effect can be evaluated by assuming that the mortality rate under different conditions follows the relationship:

where m is a constant. If m < 1, the mortality rate decreases with time, while if m > 1, it increases with time. The value of m can be determined by plotting the equation:

A brief overview of the disinfectants that will be used in this experiment is now given. More extensive discussions are given in Chapter 10.

Hypochlorite

When chlorine gas is dissolved in water it reacts as follows:

When sodium hypochlorite is used, the reaction is:

The HOC1/OC1" ratio depends on pH. As discussed in Chapter 10, the amount of chlorine present in the sample is the available free chlorine. Hypochlor-ous acid is a better disinfectant agent than its anion. Pathogenic bacteria are easily attacked, although resistance increases in the order: vegetative bacteria < protozoan cysts < helminth eggs.

The presence of compounds that react with chlorine boosts the required quantity for adequate disinfection. In addition, toxic secondary products can be formed, and therefore new disinfectants have been developed.

Ozone

Ozone is an unstable gas that decomposes rapidly to form oxygen. As a result, it is synthesized in situ. Its production methods are reviewed in Chapter 10. This disinfecting agent inactivates pathogens, oxidizes Fe(II) and manganese (II) ions, combats wastewater odors and color, and oxidizes refractory organics and trihalomethanes. Its efficiency is not influenced by pH and does not interfere with ammonia. In general it does not leave residual compounds in water.

Ozone is a better oxidizing agent than chlorine; its minimum concentration is 0.1 mg/L for bacteria inactivation and 0.5 g/L for protozoan cysts under appropriate exposure times. Its disinfecting action is due to the production of free radicals, which affect cellular permeability, enzymatic activity and the DNA.

Ultraviolet Light

Ultraviolet radiation is an efficient bactericide and virucide, as it reacts with DNA at 260 nm, although the time for treatment depends on the type of microorganism present. Low-pressure mercury arc lamps are typically used, as they generate 85% monochromatic radiation at 253.7 nm, which is within the optimum range for germicide action.

Suspended particles and colored substances may interfere with the process; that is why UV disinfection is especially useful for the treatment of potable water. It has the advantage that it does not form toxic secondary products. Challenges include a practical difficulty for the determination of the optimum dose; the lamps require constant maintenance, and its cost is higher than that of a chlorine treatment.

Titanium dioxide +UV

Catalytic disinfection is a modern alternative that involves the use of water-soluble or water-suspen-dable sensitizers to generate transient oxygen species.

Solar radiation promotes coliform and virus pho-tocatalytic disinfection. This involves oxidizing reactions in which electron removal and the generation of hydroxyl radicals attack biological molecules at rates controlled by their diffusion through the walls and membranes of microorganisms.

In the present experiment, the disinfection treatments discussed above are applied to wastewater samples containing mixed populations of microorganisms, and the surviving colonies are counted.

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