Optimizing Anaerobic Digestion: Five Ways to Boost Methane Production – a Case Study of the Spring Branch WWTP

Faced with tighter operating budgets and limited funds for capital improvements, utilities across the United States are forced to look at innovative ways to increase treatment performance of existing processes while realizing and capitalizing on secondary benefits of that process. Facilities which utilize anaerobic digestion for solids stabilization are well equipped for such improvements. Co-digestion of high strength waste streams not only increases the stability of the digestion process, but also boosts digester gas production all while providing utilities a revenue stream through tipping fees. Furthermore, co-digestion of high-strength waste streams provides potential to obtain renewable energy credits, reduces greenhouse gas emissions and landfill usage, and offsets energy costs through the use of biogas for heating and/or production of energy. The Spring Branch Wastewater Treatment Plant (SBWWTP) is one such facility that has taken a step back and looked beyond their solids treatment process as simply a means for stabilization and subsequent disposal.Located in Huntsville, Alabama, the SBWWTP is a conventional activated sludge plant with a treatment capacity of 41 mgd and is operated by the City of Huntsville Water Pollution Control (WPC). Originally designed to operate using five anaerobic digesters (four primary units and one secondary unit), the facility has been forced to decommission two primary units and the secondary digester due to safety concerns. Presently, primary sludge and WAS is sent, unblended, to one of the two remaining digesters. Primary sludge bypasses gravity thickening units and is sent directly to the designated primary digester whereas secondary sludge is thickened in a holding tank prior to pumping to the anaerobic digester units. When received, high strength organic wastes in the form of fats, oils and grease (FOG) is blended with the WAS in the holding tank.WPC staff, with the assistance of Garver, has embarked on a facility wide evaluation to analyze the entire solids treatment train, from the point of sludge generation all the way to gas production. The goal of this evaluation is to develop a phased improvements plan to rehabilitate the existing solids treatment units (primary and secondary digesters, gravity thickeners, and gas handling equipment) and construct needed improvements to maximize the potential that anaerobic digestion with high-strength waste addition provides. This evaluation focuses on five ways the SBWWTP can improve and enhance the existing anaerobic digestion process: sludge thickening, sludge feeding, grease receiving and addition, digester mixing, and identifying and repairing leaks within the gas collection system. A FOG receiving station will be preliminarily designed and sized based on historical volumes and discussions with local haulers. A new methane generator, complete with gas conditioning equipment, will be evaluated and selected to capitalize on the increased gas production and transform the biogas into readily available energy.Upon completion of the recommended improvements, preliminary results indicate that gas production will increase by 40% under average day WWTP operating conditions and maximum grease disposal conditions. Under average grease day FOG volumes, gas production is anticipated to increase 11%. The revenue generated simply through tipping fees are projected to provide Huntsville WPC with an additional $750,000 per year. Additional revenue and savings, not yet quantified, will be realized by use of energy produced by the new methane generator. This evaluation and implementation plan will provide the Spring Branch WWTP a blueprint to go beyond the primary goal of wastewater treatment and epitomize the standard of a water resource recovery facility.