(478g) Techno- Economic Analysis of Wastewater Biosolids Gasification

Wastewater treatment in the U.S. produces over eight million dry tons of sludge per year. Wastewater sludge in its raw form is a noxious dilute suspension of organic matter in up to 95% water.  Management and disposal is an expensive challenge for wastewater treatment plants (WWTPs), where the conventional approach to sludge management is to dewater the material to varying degrees and dispose by landfilling or application as a soil amendment. Such an approach requires energy and financial expense both in drying and ultimate disposal. Dried wastewater sludge has a lower heating value (LHV) close to that of low- rank coal. Thermochemical conversion of wastewater sludge to energy and/ or fuel product may allow for a more value- added approach to sludge management in addition to concentrating and stabilizing the waste stream.

            Many WWTPs currently valorize a portion of the energy in sludge via anaerobic digestion. The biological process allows bacteria to operate on dilute sludge in an anaerobic environment producing a combustible bio- gas. Anaerobic digestion is an established technology but allows for only about 20% recovery of the energy in sludge. Anaerobic digestion does not significantly reduce the amount of sludge requiring downstream disposal. Additionally, bio -gas contaminants, especially siloxanes, are a problem for downstream combustion apparatus.

            Thermochemical conversion (TCC) technologies subject sludge to chemical processes at elevated temperatures producing heat and/or combustible fuel products. TCCs applicable to wastewater treatment include direct combustion, pyrolysis, and gasification. TCC offers a great advantage over digestion by allowing nearly full utilization of sludge energy and a similarly great reduction in disposable waste volume. Each of these TCCs has been successfully applied to biomass.  Wastewater sludge however is fundamentally different from most biomass sources. Raw wastewater sludge may be up to 95% water and 30% ash (dry basis). Balancing moisture removal with reactor operating conditions is the most important consideration in whether TCCs can be net energy- and net economically-positive. Ash concentration and composition present certain challenges to reactor design. Additionally, wastewater sludge organic composition can be variable leading to uncertainty in fuel output.  

            This work reviews the current state of practice of various TCCs with application to sewage sludge. We discuss technical and economic challenges restricting greater implementation of TCC within the wastewater process. A thermodynamic model of a sludge gasification- electric generator system is built using performance characteristics of a generic, commercially available small biomass gasification platform. We perform energy and techno- economic analyses on sludge gasification to determine lower- limit WWTP capacity and as- delivered sludge moisture content for both air- blown and steam- blown gasification processes.