Beneficial Utilization of Digester Gas – Making Sustainable Choices for Onsite Energy Production

Anaerobic digestion is commonly utilized for wastewater residuals stabilization and the resultant methane rich digester gas stream is commonly utilized for digestion process heating. It is estimated that of the 16,000 centralized wastewater treatment facilities in the United States approximately 3,500 utilize anaerobic digestion for residuals stabilization and only about two percent (∼70) of those facilities are currently utilizing digester gas to produce electricity (WE&T, January 2008, pg. 34). Increasingly, wastewater treatment facilities are examining digester gas beneficial use projects for energy recovery that transcend the current, and most common, practice of capturing heat energy for process heating and flaring surplus digester gas.Methodologies utilized for evaluating digester gas beneficial utilization projects must account for a wide range of site specific operational criteria in determining the quantity of usable energy that can be extracted from the digester gas while simultaneously balancing the process heating demands which are essential to anaerobic digestion process stability and the production of the digester gas energy resource. These on-site criteria include primary and secondary sludge mass fractions, digester residence time, seasonal heating demands, and local electrical energy costs. Additionally, utilities are faced with choices for digester gas pretreatment, and utilization equipment which must consider site-specific digester gas availability and quality in light of overall project economic factors. This paper presents results from digester gas utilization studies conducted for facilities ranging in size from 15-MGD to 55-MGD covering a wide range of site specific operational criteria which influence utility choices in considering on-site energy production from digester gas.Specifically, this paper provides overviews of the following issues which impact the choices utilities make when considering a digester gas energy recovery project:Digester Gas Production EstimationEnergy Recovery Equipment ChoicesDigester Gas Quality and Treatment AlternativesEnvironmental and Climate Change BenefitsLastly, case study summaries will be presented for several evaluations conducted at three treatment facilities.