Developing a Cutting Edge, Closed-Loop Organics-to-Energy Project – Roseville Case Study

The City of Roseville (City) is developing an Energy Recovery project to collect and digest food waste, or pre-processed source-separated organics (SSO) and fats, oil, and grease (FOG) at the Pleasant Grove Wastewater Treatment Plant (PGWWTP). The additional gas production from high strength waste (HSW), FOG and SSO will help drive a new digester gas (DG) upgrading project aimed at delivering renewable vehicle fuel to City’s solid waste collection fleet. Anaerobic digesters, which will be constructed as part of the PGWWTP Expansion project, are not only a proven process for stabilizing wastewater solids, but are also increasingly recognized as a preferred pathway for codigestion of organics that would otherwise be landfilled.The Energy Recovery Project began with a comprehensive analysis aimed at determining the best use of DG in terms of net present value (NPV). The various alternatives, the analysis methodology, and the results are outlined in this paper. DG utilization alternatives included engine-based combined heat and power (CHP) systems, microturbine-based CHP systems, direct sale to an adjacent power plant, and DG separation for use as vehicle fuel. All technologies were evaluated across a range of DG production rates, which assumed various levels of co-digestion of trucked organic waste. Alternatives were compared to a status quo alternative that assumed DG would be used to operate hot water boilers for digester heat, as needed, with excess DG being flared. Alternatives were ranked based on 20-year NPV. The key findings of the analysis are listed below:All alternatives benefitted from increased DG production from co-digestion of organic HSWUpgraded DG for use as vehicle fuel provides the greatest apparent NPV compared to CHP systems or direct sale to the adjacent power plantA hybrid microturbine and vehicle fuel project affords greater potential for funding opportunities since both standalone projects have grant and rebate possibilities