Chlorine (Sodium Hypochlorite)

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A women using chlorine to disinfect water

Sodium hypochlorite is one form of chlorine used for water disinfection. It can be manufactured in most locations since it can be obtained through the electrolysis of salt water. Bottles can be purchased for household water treatment from many manufacturers in various sizes. Chlorine concentrations range from 0.5 to 10% and each product should have its own instructions for correct dosing of contaminated water. Liquid household bleach also contains sodium hypochlorite, and is widely available.

Chlorine forms hydrochloric acid when added to water which causes chemical reactions which deactivate contaminants and reacts through oxidization with micro-organisms, organic matter, manganese, iron and hydrogen sulphide.

Three things can happen when chlorine is added to water:

  1. Some chlorine reacts through oxidization with organic matter and the pathogens in the water and kills them. This portion is called consumed chlorine.
  2. Some chlorine reacts with other organic matter and forms new chlorine compounds. This portion is called combined chlorine.
  3. Excess chlorine that is not consumed or combined remains in the water. This portion is called free residual chlorine (FRC). The FRC helps prevent recontamination of the treated water.

Chlorine began to be widely used as a disinfectant in the early 1900’s. It revolutionized drinking water treatment and dramatically reduced the incidence of waterborne diseases. Chlorine remains the most widely used chemical for water disinfection in the United States.

Suitable conditions

Unlimited amounts of water can be processed at a time, depending on the amount of active chlorine.

Some users complain about the taste and odour that chlorine may cause in water. Chlorine reacts with organic matter naturally present in water to form by-products such as trihalomethanes (THMs), which are potentially cancer-causing. Lantagne et al. (2008)1 indicate that THM levels produced during household chlorination may fall below World Health Organization (WHO) guideline values.

The use of household bleach products for household water treatment is not recommended in the absence of ongoing quality control testing, due to poor quality control of most sodium hypochlorite bleaches. If household bleach is used, the concentration should be regularly tested and an appropriate dosage strategy (2 mg/l for waters of turbidity 0-10 NTU, 4 mg/L for water of turbidity 10-100 NTU) should be developed by the recommending organization.


Advantages Disadvantages
- Inexpensive

- Kills bacteria nad viruses effectively
- Residual chlorine protects water from recontamination
- Acceptability to users because of ease-of-use
- Proven health impact
- Scalability
- Low cost
- Plenty of testing and field assessments
- Local familiarity with product

- Lower disinfection effectiveness in turbid waters contaminated with organic and some inorganic compounds

- May not be effective at killing parasites
- Taste and odor are unacceptable to some
- Chlorine forms complex compounds with organic material which may be detrimental to health over time.
- Chlorine degrades over time.
- Contact time is required.
- Most users cannot determine the dosing quantity themselves; proper use requires simple instructions from the manufacturer
- Chemical dosage required varies with water quality
- Chlorine needs to be purchased continuously
- Chlorine can be hazardous if used improperly. Requires quality control process to ensure product reliability - Chlorine fumes and contact with skin are hazardous
- Concern about the potential long-term carcinogenic effects chlorination by-products
- Relatively short shelf life
- Uncertain concentration and shelf life, susceptibility to gaps in supply chain, high transportation costs, and difficulty in dispensing precise quantities


Highly effective for: Somewhat effective for: Not effective for:
- Bacteria
- Viruses

- Most Protozoa
- Helminths
- Ammonium

- Cryptosporidium oocysts

- Toxoplasma oocysts
- Turbidity
- Chemicals
- Taste, odour, color

Treatment process:
Chemical disinfection
Inlet water criteria:
Low Turbidity

Temperature: above 18º
pH between 5.5 and 7.5, disinfection
unreliable above a pH of 9

Construction, operations and maintenance

There are several different brands of chlorine products that have been manufactured specifically for household water treatment. Each product should have its own instructions for correct dosing and contact time.

Liquid household bleach products are also commonly used to disinfect drinking water. The strength of the product must be known to calculate how much bleach is needed to disinfect a given amount of water. Commercial bleach from the United States requires users to add 3-5 drops to 5 gallons of water. Some bleach bottles come with a top that dispenses by drops

The effectiveness of chlorine is affected by turbidity, organic matter, temperature and pH.

For high turbidity levels, the water should first be strained through a cloth or sedimented before adding chlorine. These processes will remove some of the suspended particles and improve the reaction between the chlorine and pathogens.

The manufacturer’s instructions for specific sodium hypochlorite products need to be followed. The required dose and contact time varies with water quality (e.g. turbidity, pH, temperature).

Use a 30 minute minimum contact time. If the pH is above 7.5, a higher FRC concentration of 0.6 mg/litre should be used and the contact time should be extended to 1 hour.

The contact time should be increased to 1 hour when the temperature is between 10˚ and 18˚C. It should be increased to two or more hours when the temperature falls below 10˚C.

Manufacturing

It can be manufactured in most locations since it can be obtained through the electrolysis of brine. Bottles can be purchased for household water treatment from many manufacturers in various sizes

Hypochlorite solution for household water treatment can be manufactured in four ways, in order of decreasing quality control: 1) chlorine gas injection into a stream of water, 2) dilution of higher concentration sodium hypochlorite solution, 3) dilution of calcium hypochlorite powder; and, 4) electrolytic generation of sodium hypochlorite from salt, water, and electricity. In all cases: 1) distilled or deionized water should be used for manufacturing; 2) the solution should be stabilized to pH over 11,9 to ensure adequate shelf- life of a minimum of 12 months; and, 3) each batch should be tested for chlorine concentration and pH. For national scale programs, it is recommended to work with a local company to manufacture the solution using high-quality processes

Skin and eye protection should be used when handling chlorine solutions. Work should be conducted in a well ventilated area or in the open air. Trained workers needed to produce and test the sodium hypochlorite.

Treatment Efficiency

With the appropriate dose, disinfection with chlorine will kill 100% of bacteria and viruses, but is not efficient enough to inactivate pathogenic parasites (e.g. Giardia, Cryptosporidium and helminths eggs).

A recent meta-analysis showed that chlorine reduces the risk of child diarrhea by 29%

Bacteria 2 Viruses 3 Protozoa4 Helminths
Resistance to Chlorine low5 moderate5 high5 high

Toxoplasma oocysts and cryptosporidium oocysts are highly resistant to chlorine disinfection 5). Chlorine alone should not be expected to inactivate these pathogens in drinking water.

Maintenance

Free residual chlorine protects against recontamination. Most users cannot determine the dosing quantity themselves; proper use requires simple instructions from the manufacturer. Users often use less than the recommended dose to save money.

Chlorine requires a supply chain, market availability and regular purchase. It also requires quality control process to ensure product reliability. Sourcing suitable plastic containers to manufacture chlorine solutions can sometimes be a challenge.

Chlorine should be stored in a cool, dark place in a closed container.

Estimated Lifespan

Chlorine deteriorates over time, especially in liquid form. Liquid chlorine products should be used within 3 months of being manufactured.

Suppliers

There are many producers of chlorine solutions all around the world.

Costs

Captial Cost Operation Cost Replacement Cost Estimated 5 years Cost Cost/liter treated
US$ 0 US$ 3/year US$ 0 US$ 15 US$ ~0.02

Note: Program, transportation and education costs are not included.

Field Experiences

The following project(s) utilize chlorine.

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RSR Project 810
Improved Water Sanitation and Hygiene


Manuals, videos, and links

References

  1. Lantagne et al. (2008)
  2. Bacteria include Burkholderia pseudomallei, Campylobacter jejuni, Escherichia coli, Salmonella typhi, Shigella dysenteriae, Shigella sonnei, Vibrio cholerae, Yersinia enterocolitica.
  3. Viruses include enteroviruses, adenoviruses, noroviruses, rotavirus.
  4. Protozoa includeEntamoeba histolytica, Giardia lamblia, Toxoplasma gondii, Cryptosporidium parvum.
  5. 5.0 5.1 5.2 5.3 CDC (2007)

Acknowledgements

This article is based on a factsheet from Centre for Affordable Water and Sanitation Technology (CAWST), which is gratefully acknowledged.

  • Centers for Disease Control and Prevention (CDC 2007). Effect of Chlorination on Inactivating Selected Pathogens. Available at:www.cdc.gov/safewater/about_pages/chlorinationtable.htm
  • Lantagne, D.S., Blount, B. C., Cardinali, F., and R. Quick (2008). Disinfection by-product formation and mitigation strategies in point-of-use chlorination of turbid and non-turbid waters in western Kenya. Journal of Water and Health, 06.1, 2008.
  • Luby, S., Agboatwalla, M., Razz, A. and J. Sobel (2001). A Low-Cost Intervention for Cleaner Drinking Water in Karachi, Pakistan. International Journal of Infectious Diseases; 5(3): 144-150.Cambodia.</cite>