Driving a Public-Private Green Energy Partnership: How the Maxson WWTP Biogas Project Fuels the New 1-Gigawatt Combined Cycle Power Plant

The T.E. Maxson Wastewater Treatment Plant, located in Memphis, Tennessee, is a 265,000-cubic meter/day (70 MGD) primary and secondary treatment facility. The sludge produced from the WWTP operations is pumped to be processed in a nearby series of covered anaerobic lagoons prior to final on-site land disposal. Under current operations, all biogas generated within the anaerobic digestion process is withdrawn and combusted in a waste gas burner on-site, combusting and wasting this valuable potential resource.Directly adjacent to the T.E. Maxson lagoon facility, the Tennessee Valley Authority (TVA) has constructed and commissioned the new Allen Combined Cycle Plant (opened in the Spring of 2018), which is an approximately 1,100 MW combined cycle natural gas power plant. As part of the new innovative plant construction, the Allen Plant pursued multiple options for green power production, with the final plant including a 1MW solar array, along with the key piece of this project and abstract, an auxiliary boiler to be fueled by the biogas generated from the Maxson lagoon facility. The City of Memphis and TVA worked over multiple years to form a public-private partnership agreement for the long-term supply and utilization of the biogas for renewable energy, which is anticipated to produce about 5 MW of power. The agreement led to an innovative delivery mechanism where the City’s 25 million-dollar capital project to implement upgraded biogas collection, cleaning and hydrogen sulfide scrubbing and delivery to the TVA Allen Combined Cycle Plant is funded and delivered through an alternative delivery model with TVA whereby costs for the capital program are covered by TVA and recovered through the reduced cost of gas purchase over the contracts 20-year period. The agreement limits the City’s up-front capital expenses and risks and delivers the valuable biogas to TVA at a fraction of the Henry Hub Natural Gas traded value. Creating a true “win-win” partnership for both organizations.CDM Smith is the Engineer of record for the biogas improvements at the Maxson lagoon facility and will outline the approach for this innovative project ensuring beneficial utilization of biogas at TVA’s new power plant.The facility’s biogas handling system was designed to process and treat a maximum flow of 6,955 cubic meters/hour (4,100 scfm) with a hydrogen sulfide (H2S) concentration of 4,000 ppm produced from the lagoon(s) and convey the treated biogas to the Allen Combined Cycle Power Plant.The biogas improvements at the facility included upgraded blower buildings with new centrifugal multi-stage blowers, new candlestick waste gas burners, an innovative biological hydrogen sulfide removal system, moisture removal system, and a gas delivery pipeline onto TVA’s facility. The biological H2S system is a series of four large treatment vessels which utilize Thiobacillus genus of bacteria to oxidize the H2S into sulfuric acid and elemental sulfur, which provides cost effective treatment for high H2S loading applications. This presentation will include a discussion on the upgraded controls and biogas quality monitoring systems installed, as well as the operational and maintenance (O&M) of the process during early periods of operation.Construction of the improvements was completed in December of 2017 with the continued optimization of the facility occurring through the early months of 2018. The presentation will highlight lessons learned regarding startup of the biological H2S removal system, as well as an update on treatment performance and beneficial utilization of the biogas at the Allen Combined Cycle facility.