Chlorine has been used in municipal wastewater treatment since the early 20th century but it wasn’t until the Federal Water Pollution Control Act of 1972 that regulations were put in place to establish standards for wastewater treatment and the quality of effluent discharged into the environment.
Residual chlorine can make its way into the environment via treated wastewater and can be toxic to both plant and animal life. In response, some facilities have installed dechlorination equipment to reduce residual chlorine levels.
The dechlorination process typically involves the use of a reducing chemical such as sodium bi-sulfite (NaHSO³) or sulfur dioxide (SO²). These chemicals require special provisions for handling, storage and emergency response training.
In the last step of wastewater treatment by a plant using chlorine treatment, sodium bi-sulfate is often dosed into the wastewater in a dechlorination basin. To properly manage residual chlorine, effluent is treated with sodium bi-sulfate which must be thoroughly mixed and have the correct amount of contact time to be effective. Precise dosing and control of the reducing chemical is crucial to achieve the desired result.
Regulations around residual chlorine accelerated the development and adoption of UV (ultraviolet light) for treating municipal wastewater. In 1986, the USEPA Design Manual: Municipal Wastewater Disinfection was published. It included comprehensive pilot data on UV systems and design guidelines for the application of this rapidly growing technology.
UV light is an effective physical treatment alternative that will not create by-products or discharge a chemical residual which can impact the environment.
UV treatment uses a fundamentally different process than chemical-based systems. Clarified wastewater, typically secondary effluent, is directed through open channels where it flows past a series of ultraviolet lights that are submerged in the effluent. As microorganisms in the water flow past the array of UV lamps, they are exposed to ultraviolet light at a wavelength of typically 254 nm (inactivation zone). These powerful photons in the UV-C range alter the genomic structure of microbes rendering the microbe unable to reproduce and therefore becomes non-infectious.
UV provides effective inactivation of microorganisms through physical treatment of water. The retention time required to achieve inactivation is usually in the range of a few seconds which reduces the physical footprint and capital costs of installation for UV systems.
Chlorine Treatment UV Treatment No Treatment By-products (DBP’s) No Chemical Residue No Chemical Spill Risk Effective Against Cryptosporidium and Giardia Well-Suited for Changing Regulations 
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Municipal UV water treatment systems are simple and straightforward to operate.
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UV light provides rapid, effective inactivation of microorganisms through a physical process.
