Introduction

The disinfection process is a key step in water treatment. In water treatment plants, this step is usually performed just before or after filtration. Pre-disinfection using chlorine dioxide and ozone can also be performed with careful consideration of the potential for chlorite, chlorate, and bromate formation.

Several major disinfection technologies have been evaluated and exist for water treatment plants. These technologies use chlorine and chloramines, ozone, chlorine dioxide, potassium permanganate, and UV radiation for disinfection.

Chlorination

Chlorine is a chemical that is often used in the disinfection of drinking water. The first use of chlorine in drinking water treatment was for the purpose of controlling taste and odor in the 1830s. At that time, it was believed that diseases were transmitted by odors. This misconception led to the chlorination of water even before the disinfection process was known.

Chlorine is currently used as the primary disinfectant in drinking water treatment. Other uses include taste and odor control, algae control, conditioning of filter media, iron and manganese removal, hydrogen sulfide removal, and color removal.

Chlorine is available in various forms, including elemental chlorine (gas or liquid), solid compounds such as calcium or sodium hypochlorite, and gaseous chlorine dioxide.

Chlorine Chemistry

Free Residual Chlorine

The many properties of chlorine make it an ideal disinfectant. Chlorine is highly soluble in water, so it is easy to use, easy to measure and control, has a stable residue, and is relatively inexpensive compared to other disinfectants. When chlorine gas is dissolved in water, it has a stable residue and is relatively inexpensive compared to other disinfectants. When chlorine gas is dissolved in water, it hydrolyzes according to the following equation to form hypochlorous acid.

HOCl+H++Cl–  →Cl2+H2O

HOCl is a weak acid that dissociates into hypochlorite ion according to the following equation.

A++OCL– →HOCl

Therefore, the sum of HOCl and OCl– is called residual free chlorine.

The relative concentrations of HOCl and OCl– vary with pH,​​water temperature, and chlorine concentration in the solution.

HOCl is a much stronger oxidizer than OCl–. HOCl is the dominant compound at pH below 6 and OCl– at pH above 7.5. Therefore, the disinfecting power of chlorine decreases with increasing pH. At pH around 9, residual free chlorine has little disinfecting power due to the high concentration of hypochlorite ion.

Free chlorine is also added to water in the form of hypochlorite salts (calcium and sodium hypochlorite). The decomposition reactions of calcium and magnesium hypochlorites are shown in the equations below.

2HOCl+Ca2++2OH–  →Ca(OCl)2+2H2O

HOCl+Na++OH– →NaOCl+H2O

As can be seen from these reactions, chlorine gas lowers the pH of water, while hypochlorites raise the pH. Therefore, in an equal amount of chlorine added to water with a low buffer capacity, higher disinfection efficiency is obtained using chlorine gas than hypochlorite.

Factors that affect the disinfection efficiency of chlorine and other disinfectants are: 1) type and nature of the disinfectant, 2) concentration of the disinfectant, 3) contact time with the disinfectant, 4) temperature, 5) type and concentration of the disinfectant, and 6) pH of the water.

Chloramines

The addition of ammonia to form a combined chlorine residual in drinking water treatment was first experimented with in 1917 in the United States. This practice continued for many years to control the formation of characteristic taste and odor compounds, algae and bacteria growth on the walls, overflows of treatment units and filters. The ability of chloramines to retain residual disinfectant for a longer period of time than free chlorine in the distribution network has also contributed to this. However, the current tendency to add ammonia is due to recent concerns about chlorination byproducts, particularly trihalomethanes (THMs). Combined chlorine (or chloramines) does not produce THMs and may therefore replace chlorination in many chlorination treatment systems.

Chlorine dioxide

Chlorine dioxide has been used for many years to bleach flour, paper and textiles. Its use in water treatment was primarily limited to taste and odor control and removal of iron and manganese in smaller systems. Since the mid-1970s, its use for disinfection has increased due to its lack of reaction with organic compounds and the lack of production of THMs, as well as the reduction of THM precursors.

Properties of Chlorine Dioxide

Chlorine dioxide is a greenish-yellow gas with a strong unpleasant odor that is stronger than that of chlorine. Chlorine dioxide is toxic to humans if inhaled. Its odor is detectable at concentrations above 0.1 ppm. It is an unstable gas and is explosive at concentrations above 10% by volume in air. Due to its instability, chlorine dioxide is always consumed immediately at the point of production. Some of the special properties of chlorine dioxide are:

• It is a more powerful deodorizer than chlorine, although it has a lower oxidation potential than chlorine.
• When prepared in the absence of excess free chlorine, it does not produce THMs or other chlorinated organic by-products. Some chlorine dioxide production methods create excess free chlorine conditions that result in the formation of chlorination by-products.
• If excess free chlorine is present, hypochlorous acid and bromine compounds are produced.
• If excess free chlorine is present, chlorite and chlorate ions can be slowly formed at pHs below 2 or above 11.
• Chlorine dioxide does not react with ammonia. Therefore, it is more stable than residual free chlorine.
• Chlorine dioxide can be used to pre-oxidize phenolic compounds and to remove iron and manganese ions from some organic compounds that are resistant to chlorination. A 30 to 40 percent reduction in THM precursors has been reported with chlorine dioxide.

Ozonation

Ozone is widely used in Europe for disinfection and taste and odor control. The use of ozone in the United States and Canada has increased in recent years due to increasing concerns about the formation of THMs during chlorination of drinking water. In addition to using ozone as a disinfectant, pre-ozonation is also used to 1) remove taste and odor, 2) remove color, 3) remove iron and manganese, 4) enhance NOM removal, and 5) oxidize organic matter.

Ozone is an unstable gas, so it must be produced at the point of use. In addition, ozone cannot be used as a secondary disinfectant because it cannot remain in the water for a long time. Due to its high oxidation potential, the CTcal value of ozone is short. Ozone is added to the water as a microflocculation aid before or in the rapid mixing unit. Studies have shown that pre-ozonation enhances coagulation-flocculation and improves the performance of the settling and clarification processes.

Potassium permanganate

Potassium permanganate is a strong oxidizer and is widely used for taste and odor control and manganese removal. Potassium permanganate is a weak disinfectant, so the contact time and concentration required for disinfection are very long. For example, at a residue of 2 mg/L, 24 hours are required for satisfactory disinfection. As a result, the use of potassium permanganate as a disinfectant is limited, especially for surface water treatment.

Ultraviolet radiation

The germicidal effects of UV have been discovered and recognized since the late 19th century. Even in ancient times, sunlight was used to purify water. The direct application of this technology in water treatment dates back to 1901, when mercury vapor lamps were introduced. Currently, UV technology is widely used in wastewater treatment due to improvements in lamp design, the absence of toxic chemical residues, excellent virucidal power, high reliability, and ease of operation.

The primary mechanism of inactivation of microorganisms by UV light is direct damage to the nucleic acids of the cell. When UV energy is absorbed by the genetic material (DNA) of microorganisms, structural changes occur that prevent the multiplication of microorganisms. The maximum UV absorption is at a wavelength of 250 to 255 nm, which is the optimal wavelength range for effective microbicidal activity. The effectiveness of UV radiation is a direct function of the amount of energy or dosage absorbed by the organism. The dosage is the product of the radiation intensity and the time the organism is exposed to the radiation.