The objective of this paper is two-fold: 1) present the innovative concepts that were evaluated for treating septage using aerobic co-digestion of thickened waste activated sludge (WAS) and domestic septage and only routing the digesters decant to the plant headworks, and 2) summarize and present the results of the full-scale pilot program of this co-digestion process that was conducted at the 1.20 million gallons per day (mgd) Rollin-Woodstock Wastewater Treatment Plant (WWTP) in Lenawee County, Michigan. The full-scale pilot program, which was conducted between October 2006 and January 2007, used the two existing aerobic digesters at the plant to accept, screen and co-digest domestic septage and WAS.In March 2006, the Lenawee County Drain Commissioner's Office retained Hazen and Sawyer, P.C. to conduct a septage acceptance study and complete a Project Plan report for the Central Lenawee Wastewater Treatment (WWTP) and the Rollin-Woodstock WWTP, located in Lenawee County, Michigan. The study was completed in June 2006, in response to the amendments to the State of Michigan Septage Waste Servicers law and due to the lack of septage receiving and treatment facilities within the County.Based on the evaluation results and the limited or lack of capacity in the liquid stream at the plants, treatment of septage through the sludge handling process (solids stream) was explored and recommended, using aerobic co-digestion process of thickened waste activated sludge (WAS) and domestic septage. Due to the lack of experience, operational data and basis of design for using this aerobic co-digestion process, the report also recommended a full-scale pilot program to be implemented at the Rollin-Woodstock WWTP by using two of the existing aerobic digesters, for accepting, screening and co-digesting domestic septage with WAS. The goals of the pilot program were to gather data on the septage characteristics that can be used for properly sizing the plant upgrades, and determine the optimum operational procedures and conditions for accepting septage at the plant.

This paper focuses on total suspended solids (TSS) in wastewater effluent, which can have two major negative impacts on ultraviolet (UV) disinfection. First, an increase in TSS may cause a decrease in the disinfection rate, especially at lower UV doses, and second, tailing of the doseresponse curve can be attributed to suspended solids. While the concentration of suspended solids (total weight measurement) is typically used to evaluate the potential performance of a UV disinfection system, correlations between suspended solids concentrations and UV performance are often inconsistent. Past research has shown that suspended solids size has an important impact on UV disinfection efficiency with larger particle size fractions, resulting in lower disinfection efficiency than smaller fractions. A summary of particle size distribution data from several different types of wastewater facilities will be presented, showing the relationship between type of biological treatment process and particle size distribution. Additionally, a case study of a wastewater treatment plant in Massard, Arkansas, will be described in which UV was selected as the disinfection process following a trickling filter, a biological treatment process that can produce solids with a high fraction of large particles. A discussion of the biological process improvements that were implemented to improve TSS conditions making such UV disinfection feasible is also provided.