Energy recovery from anaerobic digestion as compared to energy recovered from close-coupled gasification

Abu-Orf, M., Goss, T., Davies, G., Stinson, B. and Hartz, F., AECOM Water, USA



During the summer of 2010, the Washington Suburban Sanitary Commission (WSSC) began a study comparing biosolids management options for the Seneca, MD and Piscataway, MD Waste Water Treatment Plants with a focus on energy recovery from residuals through the methods of drying followed by gasification and anaerobic digestion with combined heat and power generation.  Both plants currently manage their biosolids with lime stabilization and Class B beneficial use.  The study was set up as a three phase approach consisting of identifying and screening technologies, performing a detailed economic and noneconomic analysis of the short listed options and finally developing a concept design report for the selected option.  Samples from both plants were analyzed for both energy potential and digestibility leading to the finding that the Seneca sludge was not suitable for conventional mesophilic anaerobic digestion alone.  Several drying and gasification alternatives were screened but only close coupled systems were short listed for further analysis.  Several anaerobic digestion configurations and pretreatment technologies were also screened leading to a short list of conventional mesophilic anaerobic digestion at Piscataway WWTP; acid-gas phase digestion for both WWTPs; 2PAD™ technology for both WWTPs and a combined digestion facility at Piscataway with thermal hydrolysis pretreatment.  A detailed economic analysis led to drying and gasification along with a regional thermal hydrolysis and anaerobic digestion being the most favored options for the two plants under study.  Future work includes expanding the study to compare additional regional options in conjunction with other Commission three plants.

Key words

Anaerobic Digestion, Beneficial Reuse, Energy Recovery, Gasification, Thermal Hydrolysis, Solids Processing


Energy recovery from residuals and biosolids is one option to offset energy usage and reduce carbon footprint in public and private utilities treating domestic wastewater.  One established method of recovering energy from biosolids is to recover biogas from anaerobic digestion and produce energy using combined heat and power generation (CHP) technologies.  Another method of recovering energy from biosolids is to use dried biosolids as fuel source for energy generation through the processes of gasification and energy recovery systems.

Biosolids gasification produces a synthetic gas also known as syngas with approximately one third of the energy content as compared to biogas, however, the byproduct of gasification is an inert ash with 90% less mass for disposition than a dewatered cake produced after an anaerobic digestion process. Typically, the syngas is used to dry the biosolids providing an energy neutral drying system; however, excess energy may be available depending on plant conditions.  Biosolids energy recovery systems following gasification are newer technologies for the application of biosolids than is anaerobic digestion; however the demand to reduce energy costs and pressure on land application programs are paving the way for adapting dried biosolids gasification and energy recovery systems as a disposition method. The USEPA is encouraging green energy production and recovery, specifically through the use of anaerobic digestion to create digester gas that can be used in a CHP system. Using this, the Washington Suburban Sanitary Commission (WSSC) has obtained funding to assess the potential for installing green energy technologies at the Piscataway and/or Seneca wastewater treatment plants (WWTPs).

Comparing energy recovery via anaerobic digestion and combined heat and power (CHP) to drying followed by gasification and energy recovery started in August 2010 for two WSSC WWTPs; Seneca and Piscataway, MD.  The goal of the study is to determine the best solution(s) for beneficial use of the plant’s biosolids with a focus on tapping into the energy potential of biosolids.  The study was structured to consist of a three phase approach to indentify and screen potential technologies, evaluate short listed options using both economic and non economic metrics and finally developing conceptual design reports of the selected alternative for each plant.  In addition to studying individual plant approaches, regional solutions combining the sludges from both plants were also evaluated.

This paper presents the energy recovery potential from the screened alternatives and the amount of electricity that can be produced, where applicable.  The paper also presents the 20 year life cycle cost (LCC) analysis for each alternative.  Non-economic analysis including carbon footprint, odor potential, noise potential, process integration, and side stream recycle impact to enhanced nutrient removal treatment process is also presented for each alternative.

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