Sustainability Incorporated into the Solids Handling Improvements of the Douglas L. Smith Middle Basin Wastewater Treatment Plant

The expansion of the Douglas L. Smith Middle Basin Wastewater Treatment Plant (Middle Basin WWTP) in Johnson County, Kansas to 14.5 million gallons per day (mgd) average capacity will require the construction of a fourth anaerobic digester for treatment of primary and waste activated sludges. Additionally, Johnson County Wastewater (JCW) is planning to truck thickened sludge from the Blue River Main WWTP to the Middle Basin WWTP for processing until solids handling facilities are constructed at that plant in the future. JCW is also interested inproviding an environmentally friendly approach for the treatment of fats, oils, and grease (FOG) wastes from local restaurants and industrial sources. Accepting Blue River Main WWTP sludge and FOG wastes from local contract waste haulers will impart additional loadings that were incorporated into the design of the expanded solids processing facilities.A comprehensive evaluation of systems needed for the additional loadings identified opportunities to incorporate sustainable approaches based on the ability to produce power from the digester gas and reduce the WWTP's greenhouse gas emissions. In addition to upgrading the digester gas-fired hot water boilers, the project includes the installation of two 1-megawatt (MW) cogeneration units that will provide the majority of the electrical power required by the WWTP. The viability of cogeneration was based on a detailed economic analysis that demonstrated the payback period to be less than the 20 year limit established by JCW. Keys to the positive economic analysis included use of high-efficiency packaged cogeneration units and the acceptance of FOG waste from commercial haulers. Anaerobic digestion of FOG wastes produces substantial quantities of digester gas, but the characteristics of this material vary significantly with the source, efficiency of FOG waste pretreatment processes, and the frequency of pretreatment process cleaning. This paper presents the results of the sampling and analyses program used to characterize the FOG waste for the Middle Basin WWTP solids handling improvements, describes the state-of-the-art FOG waste handling facility designed for this WWTP, and summarizes the cogeneration economic analysis that supports the installation of the two cogeneration units.