Disinfection for Small Systems (See endnote 26)
A small system can be a septic system, sand filter, or a system that serves an individual house or group of houses. If wastewater enters any body of water, it must be disinfected. Wastewater can readily contaminate water used for drinking or recreation and may contain bacteria, protozoa, helminths, and viruses, all of which can cause disease in humans. Disinfecting the wastewater will only be effective if the fluid had been adequately treated prior to disinfection to reduce the suspended solids and the BOD.
Although ultraviolet radiation can and is being utilized in some areas, there are safety considerations to be concerned about, frequent maintenance is absolutely necessary, and the amount of contaminants in the effluent can affect the final product. The advantage of ultraviolet radiation is that if it is working properly it can effectively inactivate most viruses, bacteria, and spores, and even some cysts. The disadvantages are that the low dosage may not effectively inactivate some of the viruses, spores, and cysts, may not work effectively with increased levels of turbidity and total suspended solids, and may require large numbers of ultraviolet lamps.
Chlorine is most usually used and most practical for disinfecting wastewater effluent. Chlorine oxidizes the cellular material of the bacteria. Chlorine can come in many forms. The advantage of chlorine is that it is reliable and effective against a large number of pathogenic organisms, more cost-effective than ultraviolet or ozone disinfection, leaves a chlorine residual that can keep on killing the microorganisms that are still existing in the effluent (depending on the dosage), and dosing rates are easily controlled. The disadvantage of the chlorine residual is that it can be toxic to aquatic life, highly corrosive and toxic to people, create hazardous compounds when reacting with some organic matter in the effluent, and may be unstable under certain conditions. Some organisms are quite chlorine resistant and it may require more than normal contact time to destroy them.