Accommodating Process Variation in the Design and Sizing Of Thermal Drying Facilities

The City of Wilmington's James A. Loughlin Wastewater Treatment Plant liquid and solids treatment trains are currently being designed for upgrade and expansion to 16-mgd capacity. Class A residuals management alternatives were investigated as a part of the preliminary engineering phase due to concerns over the long-term viability, risks and economics of the current Class B land application program. These preliminary investigations resulted in the selection of thermal sludge drying as the preferred Class A residual management alternative.Specifically, this paper presents the following information related to the sizing and operating economics of the thermal drying facilities:Development of the thermal dryer sizing design basis using an influence diagram process design model coupled with a Monte' Carlo statistical simulation. This model develops a design basis for total dry sludge production, water evaporation rate, and thermal dryer utilization rate.Development of the thermal dryer economic cost model using an influence diagram model coupled with a Monté Carlo statistical simulation. This model examines both the incremental operating costs and the fully allocated costs for development of the thermal drying alternative as compared to the current Class B land application programThis approach for conducting the sizing and economic evaluations provides for the rapid and simultaneous consideration and evaluation of a combination of possible input variables on the key output performance criteria. Simulation results indicate that a thermal dryer with 4,000 kg per hour (8,800 lbs./hr) evaporative capacity would provide sufficient thermal drying capacity to eet process needs at the 16-mgd design flow condition. The economic evaluation simulation indicated that the incremental operating costs associated with thermal drying were less than the current Class B land application program operating costs, however, the fully allocated costs for thermal drying were greater than the current Class B land application program costs.