Disinfection...

Disinfection refers to the selective destruction of disease - causing organisms. All the organisms are not destroyed during the process. In the field of wastewater treatment, the three categories of human enteric organisms of the greatest consequence in producing disease are bacteria, viruses, and amoebic cysts. Diseases caused by waterborne bacteria include typhoid, cholera, paratyphoid, and bacillary dysentery; diseases caused by waterborne viruses include poliomyelitis and infectious hepatitis. Disinfection is most commonly accomplished by the use of chemical agents, physical agents, mechanical means, and radiation.

Chemical agents that have been used as disinfectants include (1) chlorine and its compounds, (2) bromine, (3) iodine, (4) ozone, (5) phenol and phenolic compounds, (6) alcohols, (7) heavy metals, (8) soaps and detergents, (9) hydrogen peroxide, and (10) alkalies and acids.

"Flow - Diagram of Ozone Generator"...

There are three methods normally encountered in the production of ozone ; (1) electrical discharge, (2) electrolysis of perchloric acid and (3) ultraviolet exposure of oxygen. The first is the only practical method of large-scale ozone production currently in use. However, only 10 % of the energy supplied to a discharge gap ozonator is used to make ozone. The rest is dissipated as heat, light, and sound. When a clean, dry, oxygen-rich gas is fed to the ozone generator and an efficient means of heat removal is used, then the production of ozone is governed by the following relationships :

"Production of Ozone"...

Physical disinfectants that can be used are heat and light. Heating water to the boiling point, will destroy the major disease - producing nonspore - forming bacteria. Sunlight is also a good disinfectant. In particular, ultraviolet radiation can be used.

Bacteria and other organisms are also removed by mechanical means during wastewater treatment (screen, grit chamber, primary sedimentation, chemical sedimentation, aeration tank). The major types of radiation are electromagnetic, acoustic, and particle. Gamma rays are emitted from radioisotopes. Because of their penetration power, gamma rays have been used to disinfect wastewater.

Mechanism of Disinfection...

Four mechanisms that have been proposed to explain the action of disinfectants are ; (1) Damage to the cell wall (Damage or destruction of the cell wall will result in cell lysis and death. Some agents, such as penicillin, inhibit the synthesis of the bacteria cell wall.), (2) Alteration of cell permeability (Agents such as phenolic compounds and detergents alter the permeability of the cytoplasmic membrane. These substances destroy the selective permeability of the membrane and allow vital nutrients, such as N and P, to escape.), (3) Alteration of the colloidal nature of the protoplasm (Heat, radiation, and highly acidic or alkaline agents alter the colloidal nature of the protoplasm. Heat will coagulate the cell protein and acids or bases will denature proteins, producing a lethal effect.) and (4) Inhibition of enzyme activity (Oxidizing agents, such as chlorine, can alter the chemical arrangements of enzymes and deactivate the enzymes).

Contact Time...

Perhaps one of the most important variables in the disinfection process is contact time. It has been observed that for a given concentration of disinfectant, the longer the contact time, the greater the kill. This observation was first formalized in the literature by Chick.

"Chick's Law"...

On the design of chlorine contact tank, to calculate the volume, hydraulic retention time is being most commonly selected as 30 minutes.

Disinfection with Chlorine...

The most common chlorine compounds used in wastewater treatment plants are chlorine gas, calcium hypochlorite, sodium hypochlorite, and chlorine dioxide. Calcium and sodium hypochlorite are most often used in small treatment plants, where simplicity and safety are far more important than cost. Sodium hypochlorite is often used at large treatment plants, primarily for reasons of safety as influenced by local conditions. Because chlorine dioxide does not react with ammonia, it is also used in a number of treatment facilities.

When chlorine in the form of Cl₂ gas is added to water, two reactions take place ; hydrolysis and ionization:

"Hydrolysis and Ionization"...

Breakpoint Chlorination...

The maintenance of a residual (combined or free) for the purpose of wastewater disinfection is complicated by the fact that free chlorine not only reacts with ammonia but also is a strong oxidizing agent. As chlorine is added, readily oxidizable substances, such as Fe⁺², Mn⁺², H₂S, and organic matter, react with chlorine and reduce most of it to the chloride ion. After meeting this immediate demand, the chlorine continues to react with ammonia to form chloramines. With continued addition of chlorine, most of the chloramines will be oxidized at the break point. Continued addition of chlorine past the break point will result in a directly proportional increase in the free available chlorine.

"Fraction of Free Chlorine vs pH"...
"Breakpoint Chlorination"...

Hypochlorus Acid Chemistry...

All chlorinating agents used in pool applications react with water to produce hypochlorus acid. Hypochlorus acid is the primary compound responsible for disinfection in pools. Disinfection is the removal of infectious agents such as potentially pathogenic microorganisms, bacteria, viruses, and algae. Hypochlorus acid (HOCl) is the most active form of chlorine in pool water. At higher pH s the active hypochlorus acid is tremendously reduced. Its ability to kill is also greatly reduced by high Total Dissolved Solids (TDS) and high Cyanuric Acid levels. Fresh water should be introduced periodically to lower high TDS and Cyanuric levels, allowing the chlorine to achieve a better kill factor. This is extremely important in regions where heat, rain, heavy bathing loads, and ultraviolet rays are excessive. The pH value should be greater than 7. However, it is inadvisable to allow the pH to exceed 7.8, unless total alkalinity and/or calcium hardness are adjusted to offset the higher pH. It can also interfere with the operation of the filtration system and the precipitation of minerals will most likely occur.

pH	Percent of active HOCl ( % )	Percent of inactive OCl ( % )
5.0	100	0
6.0	96	4
7.0	75	25
7.2	66	34
7.5	48	52
7.8	33	67
8.0	22	78

Free Available Chlorine (FAC) : The sum total of hypochlorus acid (HOCl) and the hypochlorite anion (OCl). The acid form (HOCl) is responsible for disinfection (kill factor) while the hypochlorite anion (OCl) is a poor disinfectant.
Combined Available Chlorine (CAC) : Chloramine (CAC) is the chemical combination of (FAC) with ammonia, nitrogen compounds and/or organic compounds.
Total Available Chlorine (TAC) : The sum of (FAC) and (CAC).
Breakpoint Chlorination : Breakpoint chlorination is the active process to chemically convert (oxidize) chloramine and ammonia nitrogen compounds to inert nitrogen gases by adding sufficient (FAC). Breakpoint is achieved by adding 10 times the (CAC) in added (FAC).

TAC - FAC = CAC

CAC * 10 = Breakpoint chlorination in ppm