Energy (and Cost) Implications Associated with Dewatering Technology Selection for Thermal Drying Applications

Increasingly biosolids generators are seeking treatment and processing alternatives which have improved pathogen reduction over conventional stabilization technologies (e.g., aerobic and /or anaerobic digestion, or liquid lime treatment) and result in a “Class A” biosolids product with significantly lower pathogen density. Furthermore, biosolids generators are seeking a product with other characteristics which differentiate their product from the liquid and dewatered cake products associated with most conventional “Class B” treatment technologies. As a result, biosolids treatment by thermal drying is being viewed as a viable alternative which results in a product with both the reduced pathogen densities associated with Class A biosolids and a differentiated product which can be marketed to a variety of outlets.During the thermal drying process dewatered biosolids are exposed to a heat source and the remaining internally and externally bound water is evaporated by either conduction or convection. Thermal drying unit processes are typically rated by evaporative capacity (i.e., mass of water evaporated per unit time) and the dry solids throughput becomes a function of the dewatered cake solids content in the feed and the final product solids content. Dewatering unit processes are typically rated by dry solids throughput capacity; however, different dewatering unit processes will typically produce dewatered cake with different cake solids content even with the same feedstock. Therefore, the design and operation of the thermal drying unit process and the dewatering unit process must be considered a matched pair.Furthermore, thermal energy consumption in the drying unit process is directly related to the water content of the dewatered residuals to be treated and the dry solids mass throughput. Excess water content in the dewatered cake, in addition to requiring additional evaporative capacity, also results in a significant operating cost as a result of purchased fuel (e.g., natural gas, fuel oil, etc.). Therefore, the selection of a dewatering unit process technology can have significant lifecycle cost implications which should also be considered when designing a dewatering and thermal drying facility.This paper will examine the impact of dewatering technology selection on both energy and capital and operating costs for a combined dewatering and thermal drying process.