WET WEATHER DISINFECTION ALONG THE MUDDY MISSOURI…PILOTING FOR SUCCESS

The Missouri Department of Natural Resources (MDNR) recently imposed fecal coliform effluent limitations requiring disinfection of wastewater treatment plant (WWTP) effluent discharged to the Missouri River. Three plants affected by these new regulations are the Birmingham, the Blue River, and the Westside WWTPs owned and operated by the City of Kansas City, Missouri (KCMO). This paper presents the results of a preliminary assessment study of two of the most common disinfection approaches currently used at WWTPs in the United States: chlorination and ultraviolet (UV) disinfection. Primary effluent and secondary effluent were used in bench scale tests to compare disinfection properties for dry and wet weather conditions.The chlorination study consisted of bench testing to develop chlorination dose response curves and to collect data on disinfection byproduct (DBP) formation. The results indicated that chlorination should effectively meet the new fecal coliform limits; however, the bench scale tests also confirmed that chlorination will generate DBPs, some of which are regulated by water quality standards (WQS). In fact, MDNR has already established water quality standards for some of the DBPs that were found during the testing, namely chloroform, bromodichloromethane, dibromochloromethane, bromoform, and vinyl chloride. The adverse side-effect of DBP formation is likely to make regulators frown upon chlorination as a method of disinfection; therefore, if chlorination is chosen, it is anticipated that NPDES permit limits for DBPs would need to be developed to protect the designated use of the receiving stream.The UV disinfection study consisted of a 30-day period of sampling and analysis for UV transmittance (UVT), during which measurements were taken during both dry-weather and wet-weather operating conditions. UVT measurements revealed different wet-weather responses for the Birmingham and Westside WWTPs (activated sludge plants) from those of the Blue River WWTP (a trickling filter plant). Transient UVT excursions at the Birmingham and Westside WWTPs appeared to stem from final clarifier suspended solids carryover during wet weather episodes; however, the effluent UVT from the Blue River WWTP appeared to mostly suffer from soluble UV- absorbing compounds.These studies did not eliminate either chlorination or UV as viable disinfection technologies for the three plants, especially during dry weather conditions. The low UVT excursions during wet weather episodes suggest that perhaps consideration should be given to a dual disinfection system that uses UV technology for dry weather flows and chlorination technology for wet weather flows.