UV Disinfection of Wastewater

Domestic (human) wastewater may contain a range of pathogenic microorganisms (bacteria, protozoa, viruses), with these pathogens usually present at only low concentrations. However, low concentrations of pathogenic microorganisms can cause sickness, with the likelihood and severity of sickness influenced by the concentration, infectious dose, and virulence of an individual microbe.

Direct laboratory analysis for pathogens is challenging due to their low concentrations. However, domestic wastewater contains very high concentrations of faecal indicator bacteria such as Escherichia coli, faecal coliforms, and enterococci, typically millions per litre. These bacteria may not be pathogenic, but they can indicate the possible presence of pathogens. Acceptable concentrations of faecal indicator bacteria are commonly stipulated in resource consent conditions, with these conditions designed to protect public health.

Ultraviolet (UV) disinfection is commonly used as the final treatment process in WwTPs. The effectiveness of UV disinfection is strongly influenced by the total suspended solids (TSS) concentration in the wastewater, and the ability of light to pass through the liquid (UV transmittance, or %UVT). The lower the %UVT, the more light is absorbed by compounds in the wastewater, the greater the intensity of light required to reach, and deactivate, the microbes, and the greater the power requirement. TSS can shield microbes from UV light, allowing the microbes to pass through a UV system intact. Therefore, the higher the level of treatment prior to a UV disinfection system, the smaller a UV system would need to be to achieve the required level of disinfection.

An optimal UV disinfection system is designed to provide an effective UV “dose” to the wastewater. UV dose (mJ/cm2) is the product of the intensity of light received by the microbes (mW/cm2) and retention time (seconds). The susceptibility of microorganisms to UV light varies from microbe to microbe.