Systemic Energy Optimization: Balancing Aeration Demands and Digester Gas Utilization

Authors:

  • Joe Rohrbacher, P.E., Bryan Lisk, P.E., Christopher Szoch, Jennifer Whitaker, P.E., Robert C. Wichser, Ph.D., P.E., BCEE and Tho

Figure 1 – Seasonal Digester Gas Optimization

Figure 2 – Seasonal CHP Power Production vs. Purchased Electrical Power

Project Background

The Rivanna Water and Sewer Authority operates the 15 mgd Moores Creek WWTP in Charlottesville, VA. 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 and further processed at an off-site composting facility. Process aeration is provided by a coarse bubble diffused aeration system supplied by two electric motor-driven and two gas-engine driven positive displacement (PD) blowers. The gas engines can run off natural gas or digester gas and are equipped with a waste heat recovery system to provide hot water for digester and building heating. The existing engine-driven blowers routinely utilize natural gas to supplement digester gas in order to produce sufficient hot water for digester heating, resulting in excessive aeration and inefficient energy utilization.

Objective

The need to replace the existing gas engines coupled with energy cost volatility and the RWSA’s proactive approach to environmental sustainability lead to re-evaluation of the use of digester gas at the Moores Creek WWTP. The objective of the energy efficiency improvements project was to optimize digester gas utilization and improve aeration energy efficiency to reduce energy costs and carbon footprint associated with operation of the Moores Creek WWTP.

Methodology

Four alternatives were evaluated for the utilization of digester gas at the Moores Creek WWTP. Capital and 20-year net present worth costs were developed. The impacts of potential renewable energy credits and carbon taxes were also evaluated. Greenhouse gas (GHG) emissions were calculated and the potential carbon footprint of each alternative was compared.

Results and Conclusions

The selected alternative includes replacing the existing blowers with four high-speed centrifugal blowers and installation of a combined heat and power (CHP) system for electrical power generation from digester gas. Recovered heat will be used for digester heating with supplemental heating provided by a new hot water boiler. The CHP system includes an internal combustion engine-generator (ICE-G) with an engine jacket and exhaust heat recovery system. APG-Neuros blowers were installed at the Moores Creek WWTP in Spring 2010 and are currently in operation. The CHP system, supplied by GE-Jenbacher, is currently being installed and is scheduled for start-up in early 2011.

The CHP/digester heating system allows for seasonal digester gas optimization. The most economical operation is to send most of the digester gas to the hot water boiler for digester heating during December through March and direct all digester gas to the CHP system from June through September (Figure 1). Excess electrical power beyond that required for aeration would be available from the CHP system from May through October under this seasonal operating protocol (Figure 2). The CHP system is expected to provide more electrical power than required for aeration on an annual average basis, resulting in a net “credit” in aeration energy costs.

The net present cost of the selected alternative was nearly 25% lower than the next lowest option, and potential renewable energy credits or carbon taxes may result in a 50% net present worth savings. The selected alternative also has the lowest potential greenhouse gas emissions. The energy efficiency improvements at the Moores Creek WWTP will provide substantial savings in purchased natural gas and electrical power while providing an economically and environmentally sustainable alternative energy product (digester gas). The energy improvements will also enhance nutrient removal performance by eliminating digester heating’s dependency on operation of the aeration blowers.

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