Wastewater Treatment Plant UV Disinfection System Improvement Project

Disinfection is considered to be the primary mechanism for the activation and destruction of pathogenic organisms to prevent the spread of waterborne diseases to downstream users and the environment. It is important that wastewater be adequately treated prior to disinfection in order for any disinfectant to be effective. The City switched from chlorine to UV for disinfection of the treated effluent prior to discharge into the Siuslaw River in 1999.

In UV wastewater disinfection technology, UV light is used for the disinfection of bacteria, viruses, molds, algae and other microorganisms. UV light disinfects by penetrating microorganisms and destroying their genetic material (DNA and RNA). DNA and RNA plays an important role in organisms’ functions and reproduction; destroying the DNA prevents the organism from growing and multiplying. In wastewater treatment, one microorganism that is of specific importance is fecal coliform. Fecal coliforms are used as indicator organisms for fecal contamination. If fecal coliform is present in a wastewater treatment plant effluent, then it can be assumed that other harmful forms of microorganisms are too. A high level of fecal coliform can also mean that the UV disinfection system is not functioning properly.

An UV disinfection system is not simply a lamp inside a channel. The four basic parts of a UV system include the lamp, quartz sleeve, reactor and ballast. The ballast controls the UV bulbs which are housed in a quartz sleeve. The area that contains the bulbs is the reactor. The system that the City utilizes is a parallel contact type reactor where the UV lamps are in the quartz sleeves are submerged into the wastewater effluent which is parallel to the direction of flow and weirs are used to control the level of wastewater.

Contact UV disinfection systems are designed to ensure that all microbes receive sufficient exposure to the UV light. Based on the hydraulic properties of water, the reactor needs to be optimized to guide the flow in a manner to maximize contact time and turbulence. Along with hydraulic properties, UV transmittance (UVT) is a measurement used to determine how much UV light passes through a sample of water compared to distilled water. Dissolved organics, suspended particulates, color, and certain UV light absorbing metals are the main absorbers of UV light.