Water used for drinking and cooking should be free of pathogenic, i.e., disease causing, microorganisms that cause such illnesses as typhoid fever, dysentery, cholera, gastroenteritis, and similar disorders. Whether a person contracts these diseases from water depends on the type of pathogen, the number of organisms in the water (density), the strength of the organism (virulence), the volume of water ingested, and the susceptibility of the individual. Purification of drinking water containing pathogenic microorganisms requires specific treatment called disinfection. Although several methods eliminate disease-causing microorganisms in water, chlorination is the most commonly used especially in the United States.
Chlorination is effective against many pathogenic bacteria, but at normal dosage rates it does not kill or inactivate all viruses, cysts, or worms. When used in combination with filtration, chlorination is an excellent way to disinfect drinking water supplies and create a barrier to the spread of waterborne pathogens. Chlorine is the primary disinfectant used in the United States. In order to be effective, the chlorine must be given time to react with the microorganisms, chlorine must be able to interact with the organisms, and if possible, all other demands for the chlorine should be removed from the water. Disinfection reduces pathogenic microorganisms in the water to levels designated safe by public health standards. This prevents the transmission of disease.
An effective disinfection system kills or neutralizes all pathogens in the water. It is automatic, simply maintained, safe, and inexpensive. An ideal system treats all the water and provides residual (long term) disinfection. Chemicals should be easily stored and not make the water unpalatable. State and federal governments require public water supplies to be biologically safe. The U.S. Environmental Protection Agency (EPA) recently proposed expanded regulations to increase the protection provided by public water systems. Water supply operators will be directed to disinfect and, if necessary, filter the water to prevent contamination from Giardia lamblia, coliform bacteria, viruses, heterotrophic bacteria, turbidity, and Legionella.
Private systems, while not federally regulated, also are vulnerable to biological contamination from sewage, improper well construction, and poor-quality water sources. Since more than 43 million people in the United States rely on private wells for drinking water, maintaining biologically safe water is a major concern. Chlorine readily combines with chemicals dissolved in water, microorganisms, small animals, plant material, tastes, odors, and colors. These components "use up" chlorine and comprise the chlorine demand of the treatment system. It is important to add sufficient chlorine to the water to meet the chlorine demand and provide residual disinfection.
The chlorine that does not combine with other components in the water is free (residual) chlorine, and the breakpoint is the point at which free chlorine is available for continuous disinfection. An ideal system supplies free chlorine at a concentration of 0.2-0.5 mg/L, and the maximum allowable concentration is 4 mg/L. Simple test kits, most commonly the DPD colorimetric test kit (so called because diethyl phenylene diamine produces the color reaction), are available for testing breakpoint and chlorine residual in private systems. The kit must test free chlorine, not just total chlorine. We also recommend monitoring the ORP (Oxidation Reduction Potential) of the water. Paper - Use of ORP Monitoring for Disinfection University of California and YSI.
The CT concept was developed specifically for surface water, with the assumption that water suppliers would be trying to inactivate both Giardia and Viruses. The CT required to provide 3 log inactivation of Giardia is at least enough to provide the required 4 log inactivation of viruses. The EPA just set the standard for Giardia, but this should also address any issue with Viruses.
If a well tests positive for E. coli Bacteria, it is more likely to test positive for viruses and other waterborne pathogens as well. It seems reasonable, therefore, that the disinfection of a well that tests positive for Coliform Bacteria should be effective in inactivating Viruses, but it may be wise to provide a multiple barrier approach that includes disinfection followed by ultrafiltration.
Ideally, we should design the chlorination system to inactivate not just total coliform bacteria, but a more rigorous organism, like Giardia. The time required depends on the temperature and the pH of the water. Chlorine works best in water with a low pH and a high temperature. The concentration and contact time (CT) required to inactivate Giardia using chlorine is approximated by the following formula.
This formula was developed by Peter Martin, an associate Engineer with the Contra Costa Water District, and was published in an AWWA Journal (AWWA 85:12:12 Dec 1993). The CT concept was developed specifically for surface water, with the assumption that water suppliers would be trying to inactivate Giardia.
This method is calculated using regression equations developed by Smith et al. (1995). The equations can be found in Appendix E of the US EPA Disinfection Profiling and Benchmarking Guidance Manual (EPA 815-R-99-013), August 1999, and the calculation is for the inactivation of Giardia.
CT = (0.353L)* (12.006+e^(2.46-(0.073T))+(0.125*X) + (0.389(pH))); where e = 2.7183, the base for the natural logarithm.
Log Inactivation Desired (L)
Free Chlorine (X) - mg/L
Water Temperature (T) - Celsius
pH- Units
CT = (0.361L)*(-2.261+e^(2.69-(0.065T)+(0.111X)+(0.361(pH))); where e = 2.7183, the base for the natural logarithm.
The PDFs below use this formula to solve for any desired parameter.
EPA Guidance Manual LT1ESWTR Disinfection Profiling and Benchmarking
Standards and Guidelines for Municipal Waterworks, Wastewater and Storm Drainage Systems (2012)
The CT Method: A Reference Guide
The next question is how does the configuration of the tank effect the calculation? The contact time (the “T” in “CT”) is affected by baffling factor for the tank or contact chamber. Unfortunately, it is not as easy as dividing the storage volume by the flow rate. In order to count at all, there must be a separate inlet and outlet to the tank, widely separated, ideally with baffles to lengthen the flow path which would also increase the contact time. Even with this provided, the volume would then be discounted by one of the following baffling factors.
To evaluate the actual CT achieved by a proposed design, we need to know the free chlorine concentration (X) (mg/L), short circuiting factor for the baffling condition (Fsc), minimum volume of storage V (m3) , and the maximum pumping rate through the chamber (Q) (m3/min).
The formula for CT “achieved” is:
CT achieved = C * Fsc * (V/Q)
C = 0.2 mg/L
Fsc = 0.5 (Reservoir with Average baffling)
V = 1 m3 or 264 gallons
Q = 0.02 m3/min or 5.28 gallons per minute
CT (achieved) = 0.2 * 0.5* (1/0.02) = 5 mg*min/L, if the “Required” CT was greater than this number this would mean the system failed to meet the design criteria. To meet this requirement, we can change a number of variables.
