Disinfection Methods

DISINFECTION METHOD 

The advantages and dis-advantages of using different disinfectants for 

drinking water treatment are explained. Due to the wide variation of system sizes, water quality, and dosages applied, some of these advantages and disadvantages may not apply to all systems.

CHLORINE

Advantages 

- Oxidation of soluble iron, manganese, and sulfides

- Enhancement of color removal

- Enhancement of taste and odor

- Eventual enhancement of coagulation and filtration of particulate contaminants

- Effective biocide

- Easiest and least expensive disinfection method, regardless of system size

- Most widely used for disinfection , and, therefore, the best known disinfectant

- Available as calcium and sodium hypochlorite. 

These solutions are more advantageous for smaller systems than chlorine gas because they are easier, safer, and need less equipment compared to chlorine gas

- Residual provision 

Disadvantages

- Eventual deterioration in coagulation/fi ltration of dissolved organic substances

- Formation of halogen-substituted byproducts

- Eventual taste and odor problems, depending on water quality and dosage

- Hazardous corrosive gas

- Special leak container and scrubber facilities required for chlorine gas

- Sodium and calcium hypochlorite are more ex-pensive than chlorine gas

- Degradation of sodium hypochlorite over time and with exposure to light

- Sodium hypochlorite corrosion anility

- Special storage requirement for calcium hypochlorite (must be cool, dry place because of its reaction with moisture and heat)

- Eventual precipitation in a calcium hypochlorite solution due to impurities and consequent need of antiscalant 

- Formation of the byproduct chlorate at higher concentrations of hypochlorite solutions (unstable) solutions

- Less effi ciency at high pH

- Formation of oxygenated byproducts that are biodegradable and which can enhance subsequent biological growth if the chlorine residual is not maintained.

- Release of constituents bound in the distribution system (e.g., arsenic) by changing the redox state.

GENERATION

Chlorination may be prepared using chlorine gas or other chlorinated compounds in liquid or solid form. Chlorine gas can be generated by a number of processes including the electrolysis of alkaline brine or hydrochloric acid, the reaction between sodium chloride and nitric acid, or the oxidation of hydrochloric acid. Since chlorine is a stable compound, chlorine gas, sodium hypochlorite, and calcium hypochlorite are typically produced off-site by a chemical manufacturer.

PRIMARY USES

The primary use of chlorination is disinfection. Chlorine also serves as an oxidizing agent for taste and odor control, algal growth prevention, maintenance of a clear filter media, iron and manganese removal, hydrogen sulfide destruction, color removal, maintainance of the water quality at the distribution systems, and coagulation improvement.

INACTIVATION EFFICIENCY

Chlorine disinfection is extremely effective for bacteria, highly effective for viruses and less effective for protozoa, such as Giardia cysts and Cryptosporidium oocysts, being the last one highly resistant to chlorine.

BYPRODUCT FORMATION

When added to the water, free chlorine reacts with NOM and bromide to form DBPs, primarily THMs, some haloacetic acids (HAAs), and others.

 POINT OF APPLICATION

Chlorine can be applied at different treatment stages: in the raw water storage, pre-coagulation/post-raw water storage, pre-sedimentation/post-coagulation, post-sedimentation/pre-filtration, post-filtration (disinfection), or in the distribution system.

SPECIAL CONSIDERATIONS

Due to its oxidation and corrosion power, special storage and handling of chlorine should be considered in the treatment plant planning.