COUPLING MESOPHILIC ANAEROBIC DIGESTIONAND HEAT DRYING: AN ENERGY EFFICIENT MEANS TO FLEXIBILITY IN BIOSOLIDS MANAGEMENT

Conventional anaerobic digestion is commonly used by many wastewater treatment facilities. The process reduces the amount of solids for further processing, reduces the odor potential of the treated biosolids, and produces a renewable energy byproduct – digester gas. Several process variations have been proposed to upgrade the performance achievable from anaerobic digestion, but even if “Class A” pathogen reduction standards are met, beneficial use of the liquid or dewatered biosolids is limited to bulk land application.Heat drying transforms conventionally digested biosolids into an attractive “Class A” product that can be economically transported to distant points of use as a soil conditioner, an ingredient in a fertilizer blend, or an alternative fuel. But heat drying requires substantial energy, typically provided by fossil fuel. The combination of digestion and drying is a synergistic pairing. Digestion provides digester gas for the drying process and, in return, waste heat recovered from drying provides process heating for the digesters.A mass and energy balance was performed for a typical 40 MGD wastewater plant to show how digester gas could be used to provide much of the energy needed for an add-on heat drying process. Heat drying would produce Class A biosolids for use as fertilizer or fuel, or reduce the amount of biosolids for disposal. The amount of energy for drying that can be provided by digester gas depends on the efficiency of mechanical dewatering prior to drying, the volatile solids content of the solids and gas production, and the operation schedule for the dryer. For a typical facility with 25 percent dry solids to the dryer, over 80 percent of the energy requirements can be provided from digester gas with continuous operation of the dryer and digester heating will be provided from heat recovered from the dryer system.