Chlorine dioxide is generated by mixing acid or chlorine gas and sodium chlorite then mixed with ejector water as an entrained gas to form a solution that is applied to the process. Because it cannot be compressed and liquefied for transportation, it is generated on site close to its intended use. It is a strong oxidant and disinfectant across a wide pH range for both water and wastewater and does not react with ammonia to become a weaker disinfectant, important for plants where the water has a high ammonia content, often resulting in lower operating costs. Chlorine dioxide is frequently used in water treatment plants as a primary disinfectant early in the treatment process to prevent the formation of THMs.
Comprised of three oxygen atoms (O3), ozone is a very strong oxidant. It deteriorates rapidly to oxygen and is usually generated on site using either air or pure oxygen. Ozone does not produce disinfection byproducts (DBPs) and can be used as a primary disinfectant for water treatment to reduce THMs and DBPs. Ozone also is used for taste, odor and color control in potable water treatment, as well as Fe/Mn removal when THMs are a concern. Ozone can also be used to remove micropollutants including pesticides at disinfection dosages. Ozonation is typically not used for primary disinfection of wastewater effluent with high levels of suspended solids (SS), biochemical oxygen demand (BOD), chemical oxygen demand, or total organic carbon since the cost of treatment can be relatively high in capital and in power intensiveness.
Ultraviolet Light (UV)
Ultraviolet light energy at 254 nm wavelength is absorbed by the DNA of a microorganism, stopping the reproductive process and rendering it non-infective and microbiologically dead. UV systems operate at varying pressure and output, depending on application, target pathogen and water quality involved.
UV has most recently been used to treat wastewater effluent since regulations require more stringent chlorine discharge limits for various receiving streams. UV is effective in removing chlorine-resistant pathogens from drinking water including Cryptosporidium, Giardia and various viruses that have proven to be resistant to traditional disinfection methods such as chlorine and filtration.
Peracetic Acid (PAA)
Peracetic acid (CH3CO3H - also known as peroxyacetic acid, or PAA) is a liquid that functions as a strong oxidizing agent, has an acrid odor and can also be used as a disinfectant. PAA is generally commercially available as an equilibrium mixture of 12-15% peracetic acid and 18-23 % hydrogen peroxide. PAA is available in 330-gallon totes and in bulk, requires stainless steel piping, and is administered using a metering pump.
Since PAA is a highly effective bactericide, does not form DBPs, has minimal dependency on pH and does not leave a residual, it has received significant consideration for the disinfection of wastewater effluent.
With increasing pollution and resulting regulations, it is becoming increasingly difficult to achieve all of the requirements for safe disinfection with one treatment alone. Today, a more layered approach for disinfection with multiple technologies used together is often necessary. For example, a surface water potable water treatment facility may use chlorine dioxide at the head of the plant to gain disinfection credit without creating chlorine byproducts, then apply chlorine to the finished water to maintain a residual through the distribution system.
With a well-defined treatment process goal and experienced, knowledgeable partners working together, developing the most effective, simple and budget-conscious disinfection plan can be a painless process.