The Clean Water Act contains requirements to set water quality standards for all contaminants, including bacterial contaminants, in surface waters. Wastewater treatment cannot be expected to remove all of the bacterial contaminants, and as a result, many organisms remain in the waste stream. Therefore, disinfection of treated wastewaters is designed to reduce bacterial numbers to a safe level prior to discharge from the treatment plant.
Growing public awareness and concern for controlling water pollution led to enactment of the Federal Water Pollution Control Act Amendments of 1972. As amended in 1977, and later in 1987, this law became commonly known as the Clean Water Act. The Act established the basic structure for regulating discharges of pollutants into the waters of the United States. It gave EPA the authority to implement pollution control programs such as setting wastewater standards for discharge to water of the United States. The Clean Water Act also continued requirements to set water quality standards for all contaminants in surface waters. The Act made it unlawful for any person to discharge any pollutant from a point source into navigable waters, unless a permit was obtained under its provisions. In the U.S., federal, state, and local authorities have always been involved in the governing of wastewater treatment systems primarily through legislation, enforcement, and guidance.
Secondary and even tertiary wastewater treatment cannot be expected to remove all of the contaminants in the influent wastewater, and as a result, many organisms remain in the waste stream. While the majority of these organisms are not pathogenic, pathogens must be assumed to be present in the effluent. The purpose of disinfection of treated wastewater is to prevent the spread of waterborne diseases and thereby protecting public health. The organisms of concern in domestic wastewater include enteric bacteria such as Escherichia coli, Leptospira, Salmonella, Shigella, and Vibrio cholerae; viruses including Enteroviruses (72 types, e.g., polio) Gastroenteritis, meningitis, Hepatitis A, Norwalk agent, and Rotavirus; and also protozoan cysts which include Balantidium coli, Cryptosporidium, Entamoeba histolytica Amebiasis, Giardia lamblia, Helminths, and Ascaris lumbricoides.
Disinfection is treatment for the destruction of all pathogenic organisms while sterilization is the destruction of all microorganisms. While disinfection indicates the destruction of disease causing microorganisms, no attempt is made in wastewater treatment to obtain sterilization. Disinfection procedures which are applied to wastewaters are designed such that bacterial numbers are reduced to a safe level.
A variety of physical or chemical methods are capable of reducing the numbers of and or destroying microorganisms. One of the earliest methods of disinfection was simply filtering the effluent thru slow-rate intermittent sand filters. However, this did not remove all of the pathogens. Later efforts included the addition of chemicals such as halogens, most notably chlorine. Chlorine is an excellent disinfecting chemical, is available in gaseous, liquid, and solid forms, and until recently has been the disinfection method of choice. Maintenance of chlorination systems was relatively easy as it required only the maintenance of pumps and adjustment of application rates in the use of gaseous or liquid chlorine, and the periodic replacement of solid chlorine tablets. However, the fact that chlorine even at low concentrations is toxic to fish and other biota as well as the possibility that potentially harmful chlorinated by-products may be formed has made chlorination less favored as the disinfectant in wastewater treatment. As a result, there has been an increased use of ultraviolet light and ozone as a disinfectant in wastewater disinfection. Operation and maintenance for a chlorine disinfection system should include following all manufacturer recommendations and testing and calibrating equipment as recommended by the manufacturer. Regularly disassemble and clean system components, including meters and floats, such as every three to six months. Inspect and clean valves and springs annually. For those systems which include metering pumps, maintain pumps on a regular basis.
As discussed earlier, chlorination by-products have led to increased use of UV radiation for disinfection treated wastewater. The effectiveness of a UV disinfection system depends on the characteristics of the wastewater, the intensity of UV radiation, the amount of time the microorganisms are exposed to the radiation, and the reactor configuration. Disinfection success in any system is directly related to the concentration of colloidal and particulate constituents in the wastewater. Therefore it is imperative to reduce the level of suspended solids and turbidity prior to the UV process. It is also critical to implement a proper operation and maintenance program for a UV disinfection system. This is to ensure that sufficient UV radiation is transmitted to the organisms to inactivate them. All surfaces between the UV radiation and the organisms must be clean. Ballasts, lamps, and the reactor must be functioning properly. Inadequate cleaning is one of the most common causes of ineffective UV systems. The quartz sleeves or Teflon tubes must be cleaned regularly and common cleaning methods include mechanical wipers, ultrasonic baths, or chemicals such as citric acid or commercially available cleaning solutions. Cleaning frequency is site-specific depending on the characteristics of the treated wastewater.