Disinfection of Water and Wastewater

The disinfection of potable water and wastewater provides a degree of protection from contact with pathogenic organisms including those causing cholera, polio, typhoid, hepatitis and a number of other bacterial, viral and parasitic diseases.  Disinfection is a process where a significant percentage of pathogenic organisms are killed or controlled.  As an individual pathogenic organism can be difficult to detect in a large volume of water or wastewater, disinfection efficacy is most often measured using “indicator organisms” that coexist in high quantities where pathogens are present.  The most common indicator organism used in the evaluation of drinking water is Total Coliform (TC), unless there is a reason to focus on a specific pathogen.  The most common indicator organism for wastewater evaluation is fecal coliform but there has been discussion regarding the use of Escherichia coli (E. coli) or Total Coliform.  As domestic wastewater contains approximately 1,000 times more indicator organisms than typical surface water, understanding wastewater disinfection will make it easier to understand water disinfection.

Wastewater Disinfection

There are a number of chemicals and processes that will disinfect wastewater, but none are universally applicable.  Most septic tanks discharge into various types of subsurface wastewater infiltration systems (SWIS), such as tile fields or leach fields.  These applications rely on the formation of a biomat at the gravel-soil interface where “biodegradation and filtration combine to limit the travel of pathogens.”¹  Aerobic treatment processes reduce pathogens, but not enough to qualify as a disinfection process.  “Chlorination/dechlorination has been the most widely used disinfection technology in the U.S.; ozonation and UV light are emerging technologies.”²  Each of these three methods have different considerations for the disinfection of wastewater.

Water Disinfection

Disinfection is usually the final stage in the water treatment process in order to limit the effects of organic material, suspended solids and other contaminants.  Like the disinfection of wastewater, the primary methods used for the disinfection of water in very small (25-500 people) and small (501-3,300 people) treatment systems are ozone, ultraviolet irradiation (UV) and chlorine.  There are numerous alternative disinfection processes that have been less widely used in small and very small water treatment systems, including chlorine dioxide, potassium permanganate, chloramines and peroxone (ozone/hydrogen peroxide).

Surface waters have been the focal point of water disinfection regulations since their inception, as groundwaters (like wells) have been historically considered to be free of microbiological contamination.  Current data indicates this to not be true.  Amendments to the Safe Drinking Water Act in 1996 mandate the development of regulations to require disinfection of groundwater “as necessary.”  While these regulations will apply to very small systems serving twenty-five people at least 60 days out of the year, the rules will not apply to private wells.  However, the EPA recommends that wells be tested at least once per year and disinfected as necessary.  While these proposed regulations have not yet been finalized, they will likely include; testing by each state, identification of contaminated water supplies, corrective action requiring disinfection and compliance monitoring.  The rules are currently scheduled to be implemented in July 2003.

Summary

The last 100 years have brought significant environmental advances.  At the beginning of the 20th Century, water and wastewater were treated by one principle, “the solution to pollution is dilution.”  But as population density increased, so did the spread of infectious disease.  Only by the use of science and technology have we been able to identify threats to public health and find ways to overcome them.

Driven partly by regulation, safe drinking water has now become commonplace.  Ongoing research will continue to make it safer, even in the light of increasing wastewater reuse.  Wastewater effluent limits also continue to evolve.  “Attainment of the disinfection guidelines can only be achieved by the disinfection process, which, from a disease prevention standpoint, is the most important unit process in the wastewater treatment system.”³

Disinfection of water and wastewater, primarily by chlorine, has played a large part in the reduction of waterborne diseases.  While new disinfection processes are constantly being developed, the industry cannot abandon proven technology.  This is of such importance that The Wall Street Journal cited U.S. Army Chemical Engineer, David A. Reed, as saying “until alternative technologies are more widely accepted, the country can’t do without chlorine.”⁴

References

1. National Small Flows Clearinghouse, Small Flows Quarterly. The Role of Biomats in Wastewater Treatment.  (Fall 2001).
2. Water Environment Federation. Wastewater Disinfection Manual of Practice FD-10.  (1996).
3. U.S. Environmental Protection Agency. Municipal Wastewater Disinfection Design Manual.  (1986).
4. Ann Davis, The Wall Street Journal. New Fears Heat Up Debate on Chemical Risks.  (May 30, 2002).