United States Office of Water EPA 832-F-99-033 Environmental Protection Washington, D.C. September 1999 Agency Combined Sewer Overflow Technology Fact Sheet Alternative Disinfection Methods DESCRIPTION While chlorine disinfection is the most common method used to kill pathogenic microorganisms at Combined sewer overflows (CSOs) occur when wastewater treatment plants, this methodology flows exceed the hydraulic capacity of either the may not be feasible at all CSOs for several wastewater treatment plant (WWTP) or the reasons, including: collection system that transports the combined flow of storm water and sanitary sewage to the • CSOs occur intermittently and their flow WWTP. When an overflow occurs, the excess rate is highly variable, thus making it flows tend to be discharged into a receiving body difficult to regulate the addition of of water. CSOs typically discharge a variable disinfectant. mixture of raw sewage, industrial/commercial wastewater, polluted runoff, and scoured materials • CSOs have high suspended solids that build up in the collection system during dry concentrations. weather periods. These discharges contain a variety of pollutants that may adversely impact the • CSOs vary widely in temperature and receiving water body, including pathogenic bacterial composition. microorganisms, viruses, cysts, and chemical and floatable materials. Health risks associated with • Residual disinfectants from chlorine bacteria-laden water may result through dermal disinfection may be prohibited from contact with the discharge, or through ingestion of receiving waters. contaminated water or shellfish. • CSO outfalls are often located in remote Preliminary reduction of microorganisms and areas and thus may require automated bacteria may be accomplished through physical disinfection systems. reduction of solids in the wastewater, primarily through sedimentation, flotation, and filtration. In addition to these problems, the increased health Following solids reduction, most systems further and safety concerns regarding the use of chlorine reduce bacterial concentrations through to disinfect CSOs has prompted the development disinfection. Disinfection occurs as the of alternative disinfectants, which often pose wastewater is brought into contact with oxidizing fewer problems and hazards. Alternatives to chemicals (such as chlorine, bromine, ozone, chlorine have been developed and evaluated for hydrogen peroxide, and related compounds). continuous disinfection of wastewater discharges to small streams or sensitive water bodies, and are Chlorine has long been the disinfectant of choice now being considered for treatment of CSOs and for most disinfection systems. It offers reliable other episodic discharges. reduction of pathogenic microorganisms at reasonable operating costs. (See EPA's CSO This fact sheet addresses the use of chlorine Technology Fact Sheet 832-F-99-021, dioxide, ozonation, ultraviolet radiation, peracetic Disinfection-Chlorination, for more information). acid and Electron Beam Irradiation (E-Beam) to disinfection is to expose wastewater to a UV treat CSOs. lamp. Historically, most UV disinfection facilities have been designed to utilize Low Pressure Low Chlorine Dioxide Intensity UV lamps for disinfection. For example, low-pressure mercury arc lamps emit Studies have shown that chlorine dioxide is an approximately 90 percent of their light energy effective wastewater disinfectant, although its use around 254 nanometers. in the United States is limited. Chlorine dioxide is applied to wastewater as a gas that is generated UV disinfection works by penetrating the cell on-site using excess chlorine. Although it is walls of pathogenic organisms and structurally relatively easy and economical to produce altering their DNA, thus preventing cell chlorine dioxide is unstable and reactive and any replication and function. No hazardous chemicals transport is hazardous. are produced or released while treating CSOs with UV. Chlorine dioxide is effective at oxidizing phenols, but does not react with aquatic humus to produce Because UV is not a chemical disinfection trihalomethanes (THMs). However, any excess method, it disinfects without altering the physical chlorine remaining from the generation of or chemical properties of water. However, UV chlorine dioxide would react with THM efficiency is affected by suspended solids in the precursors and form THMs. Therefore, operators wastewater, which can scatter and absorb light. must be careful to use the correct amounts of Thus, UV disinfection is not effective in chlorine when generating chlorine dioxide. And wastewaters with a high TSS level. while chlorine dioxide will not react with wastewater to form chloramines, it can produce Peracetic Acid potentially toxic byproducts such as chlorite and chlorate. Peracetic acid (CH3COOOH) (PAA), also known as ethaneperoxoic acid, peroxyacetic acid, or actyl Ozonation hydroxide, is a very strong oxidant. Based on limited demonstration data for disinfection of Ozone is a strong oxidizer and is applied to secondary treatment plant effluent, peracetic acid wastewater as a gas. Its use in CSO treatment appears to be an effective disinfectant and should facilities for wastewater disinfection is relatively be evaluated further for treating CSOs. The new in the United States, and there are few equilibrium mixture of hydrogen peroxide and facilities currently using ozone for disinfection. acetic acid that produces PAA is too unstable and This can be potentially attributed to high initial explosive to transport, and so PAA must be capital costs associated with ozone generation produced on site. The decomposition of PAA equipment. Ozone is equal or superior to chlorine results in acetic acid, hydrogen peroxide and in "killing" power, but it does not cause the oxygen. formation of halogentated organics as does chlorination. Electron Beam Irradiation Ultraviolet (UV) Radiation Electron Beam Irradiation (E-Beam) uses a stream of high energy electrons that are directed into a UV radiation is one example of electromagnetic thin film of water or sludge. The electrons break radiation used for disinfection. UV disinfection apart water molecules and produce a large number incorporates the spectrum of light between 40 of highly reactive chemical species. There are a nanometers and 400 nanometers. Germicidal few reactive species formed during this process properties range between 200 and 300 and include oxidizing hydroxyl radicals, reducing nanometers, with 260 nanometers being the most aqueous electrons and hydrogen atoms. lethal. The primary method for utilizing UV APPLICABILITY As discussed above, Table 1 provides comparative data for chlorine dioxide, peracetic acid, and UV A brief summary illustrating the general radiation to help in determining which compounds applicability of chlorine dioxide, peracetic acid may be most advantageous for specific and UV radiation as alternative CSO disinfectants applications. The following sections summarize is provided in Table 1. While ozonation and the the advantages and disadvantages of using E-Beam process are discussed in this fact sheet as ozonation and E-Beam as alternative CSO potential alternative disinfectants for CSOs, they disinfectants. are not currently considered practical for CSO disinfection and thus they are not included in Ozonation: Advantages Table 1. Because ozone must be generated on-site and the amount generated is dependent on the C More powerful disinfectant than most chlorine demand, ozone is not currently considered compounds. practical for intermittent use in situations where the system would be frequently turned on and off C Inactivates most strains of bacteria and viruses or where there are wide fluctuations in flow rate and is noted for destroying chlorine-resistant and disinfection demand, such as in CSO strains of both. Highly effective for treatment applications. The E-Beam system was Cryptospiridium eradication. initially developed for the disinfection of municipal wastewater treatment plant sludge and C Will oxidize phenols with no negative the destruction of hazardous organic compounds, residuals such as trihalomethane production. and it has not been evaluated for CSO disinfection. EPA will continue to evaluate the E- C Does not produce a disinfection residual that Beam system as a promising innovative would prevent bacterial growth. technology for wastewater technology. C Degenerates into oxygen, which can elevate ADVANTAGES AND DISADVANTAGES oxygen levels in treated water. It does not alter pH of water. As discussed above, one of the primary reasons for seeking alternatives to chlorine disinfection of C Increases coagulation. CSOs is the growing concern over safety in handling gaseous chlorine and the possible toxic C Helps remove iron and manganese. side effects of treatment with chlorine. Studies on alternative disinfectants such as peracetic acid, C Has taste and odor control properties. ozone, E-Beam, and UV, have shown that these alternatives serve as good substitutes for chlorine C Requires short contact time because they produce no toxic byproducts. Although chlorine dioxide does produce Ozonation: Disadvantages byproducts and residuals, the limited use of chlorine dioxide in this country has made it C More costly than traditional chlorinated difficult to assess these byproducts for toxicity. disinfection techniques. The alternatives to chlorine for CSO disinfection C Forms nitric oxides and nitric acid which can are not problem-free, however, and their lead to corrosion.