Next Generation Resource Recovery: Co-Digestion to Renewable Natural Gas (RNG) Pipeline Injection at the Des Moines WRF

The Des Moines Metropolitan Wastewater Regional Authority (WRA) is a national leader in transforming wastewater treatment into a revenue generating resource recovery operation. Recent upgrades to the anaerobic digestion facilities of municipal sludge and FOG and other organic wastes processed in the co-digestion process has more than doubled WRA’s biogas production, and this upward trend is expected to continue. WRA is moving ahead with plans to process up to 2,250 scfm of anaerobically digested biogas into pipeline quality renewable natural gas (RNG). This approach offers the opportunity to generate revenue from the sale of RNG to the local utility, as well as revenue from sale of Renewable Information Numbers (RINs) via compliance with EPA’s Renewable Fuel Standards for a D5 advanced biofuel credit. Final design is in process now with construction completion expected in 2017.CDM Smith will present this case study describing the biogas treatment technology alternatives considered, evaluation process, and recommended design criteria for new WRA facilities. Background information regarding RNG quality requirements, utility constraints and perspectives, financial expectations, and planned approach to sell RINs will be offered.Commercially available treatment technologies used to separate methane from carbon dioxide and other contaminants include pressure swing adsorption (PSA), liquid scrubbers, and membrane scrubbing systems. Technology Evaluation highlighted many differences between these technologies, including: ability to handle high hydrogen sulfide concentrations, methane capture efficiency, cost, ease of operation and maintenance (O&M), and ability to consistently meet product purity requirements.The presentation will include a discussion of the Bioenergy model developed by CDM Smith. This tool models various process conditions, financial scenarios, and technology options impacts on the revenue and anticipated payback of the project. A screenshot of one page of the Bioenergy model is included in Figure 1.Figure 1: Bioenergy ModelThe design documents are anticipated in September 2016 with construction starting in Q4 of 2016. CDM Smith will present the design documents, including the facility code modifications required to install the biogas treatment technology within an existing unclassified building. CDM Smith will also share lessons learned and negotiating tips for working with municipalities, gas utilities, and RIN partners for other wastewater agencies considering this approach to resource recovery.