A Systemic Look at Energy Optimization – Balancing Aeration Demands and Digester Gas Utilization to Provide Energy Savings and Carbon Footprint Reduction

The Rivanna Water and Sewer Authority operates the 57 ML/d (15 mgd) Moores Creek WWTP in Charlottesville, Virginia. The facility is under construction and start-up to convert from a conventional nitrification activated sludge process to a five-stage enhanced nutrient removal process. Residuals from the treatment plant are thickened and stabilized via anaerobic digestion prior to dewatering. The existing facility relied on gas engine-driven aeration blowers equipped with a waste heat recovery system to provide hot water for digester and building heating. The engine-driven blowers routinely utilized natural gas to supplement digester gas to produce sufficient hot water for digester heating, resulting in excessive aeration and inefficient energy utilization. Four alternatives were evaluated to optimize digester gas utilization and improve aeration energy efficiency. The existing aeration blowers were replaced with high-speed direct-drive centrifugal blowers, and a combined heat and power (CHP) system for electrical power generation from digester gas was installed. The CHP/digester heating system will allow for seasonal digester gas optimization and provide substantial savings in purchased natural gas and electrical power while reducing the facility's carbon footprint. The energy improvements will also enhance nutrient removal performance by eliminating digester heating's dependency on operation of the aeration blowers.