Energy from Residuals – Comparing Anaerobic Digestion to Drying/Gasification Model

This paper reports on results from a biosolids management study conducted for the Susquehanna Water Pollution Control Facility (SWPCF) operated by Lancaster Area Sewer Authority (LASA) of Pennsylvania. The facility currently co-settles waste activated sludge (WAS) with primary solids. The blended residuals are dewatered by a centrifuge and then are stabilized to Class B using quick lime. A portion of the final Class B product (25% TS in average) is land-applied on local farms by a contractor, and the remainder is landfilled at the Lancaster County Solid Waste Management Authority (LCSWMA) Landfill In 2008, approximately 7,800 wet tons were landfilled and almost same amount was land applied. Faced with re-negotiating their long term landfilling contract and the prospect of increasing costs, LASA wanted to investigate alternative treatment processes and end-uses, considering public health, environmental sustainability, flexibility, as well as cost-effectiveness. The study started with a broad list of alternatives for biosolids management. The list was narrowed down by a screening process involving a weighted scoring/ranking system to develop a “short list” for full evaluation. A “market-back” end-use analysis of the various biosolids products generated by each alternative helped to shape the ranking/scoring process in terms of public health and sustainability. An economic analysis was done for each of the “short-listed” alternatives (process trains) in order to determine the 20-year life cycle costs (LCC) of each. The top two alternatives based on the 20-year LCC, Anaerobic digestion with co-generation and gasification with power production, are substantially different with respect to energy generation, final products and approach to sustainability. This paper will explain the selection process step-by-step, while highlighting these differences.