On-site Sewage System Ultraviolet Disinfection Study

Effective on-site sewage treatment reduces the risk of disease transmission and human exposure to pathogens, which can occur through drinking water, surface water, and shellfish bed contamination. The ability of the soil to reduce pathogens in on-site sewage system effluent is impacted by a variety of factors such as soil texture and structure, depth and distance between point of discharge, and the nearest ground or surface water requiring protection. At some sites, the treatment capacity of soil or proximity of the soil dispersal component is insufficient. Disinfecting effluent before soil dispersal is a way to rectify this shortcoming.

Ultraviolet (UV) disinfection is the most commonly used disinfection method in Washington State and more than 6,000 UV disinfection (UVD) units are currently in use. Many members of the on-site wastewater industry and local health jurisdictions have expressed concerns that UVD units are ineffective and not functioning reliably under field conditions. Ongoing operational and installation problems and high maintenance costs associated with UVD units have been reported, raising questions about the need for their use, and whether costs outweigh the benefits of UV disinfection in the field.

Field Effectiveness of UVD Units in On-Site Sewage Treatment

The purpose of this study was to evaluate the effectiveness and reliability of UVD units in the field, and to determine if additional management measures are necessary to assure UVD unit performance on sites that are highly susceptible to microbial pollution. The evaluation work was carried out by staff within the Wastewater Management Section of the Washington State Department of Health in partnership with Tacoma-Pierce County Health Department, Thurston County Public Health and Social Services Department, and University of Washington.

Approximately one hundred on-site sewage systems with permitted UVD units were evaluated in Thurston and Pierce Counties for indicators of proper installation, maintenance, electrical, and physical status. Effluent samples from twenty-two UVD units were also collected and analyzed for microbial and physical quality in Thurston County. We compiled results from all of the systems evaluated and wrote a final report based on those results.

Final Report

Field Effectiveness of Ultraviolet Disinfection Units in On-site Sewage Treatment (PDF)


Appendix A - Additional Data Analysis (PDF)

Appendix B - Explanation of Less-Frequent Statistical Methods Used for Data Analysis (PDF)

Appendix G - Quality Assurance and Quality Control (PDF)

Appendix G - Sample Representativeness (PDF)

De-identified data available upon request.

Standard Operating Procedures

Standard Operating Procedures for the Collection of Fecal Coliform Bacteria Samples (PDF)

Standard Operating Procedures for Measuring Ultraviolet Transmittance (PDF)

Standard Operating Procedures for Measuring pH, Conductivity, and Temperature (PDF)

Standard Operating Procedures for Measuring Turbidity (PDF)

Standard Operating Procedures for Measuring Dissolved Oxygen (PDF)


Quality Assurance Project Plan (PDF)

More Resources

Ultraviolet Disinfection Fact Sheet, National Small Flows Clearinghouse (PDF)

Contact Meagan Jackson at meaganja@uw.edu to access the Master's Thesis.