Akron, Ohio – a high solids anaerobic digestion process replaces a successful In-Vessel composting operation

Abstract

For 26 years, the City of Akron had been managing their wastewater solids using the in-vessel Paygro Composting process. While the operations of the facility and the marketing of the finished compost was successful, the concern by the public over the odor generation, the steadily increasing operational costs and the need for a large capital investment to continue composting, as a result of the facility showing its years of wear from the operations, was the reason for the City to move forward with building a new 13,607 dry t (15,000 dry ton) anaerobic digestion (AD) plant. The new 13,607 dry t (15,000 dry ton) AD plant modeled the 4535 dry t (5000 dry ton) Schmack BioGas AG AD plant built in 2007, which was used to validate the operational process and understand the biogas generation prior to going full scale. With the success of the 4535 dry t (5000 dry ton) AD plant, the City in 2011 entered into a long term contract with KB BioEnergy (formerly KB Compost) to build and operate the new 13,607 dry t (15,000 dry ton) (AD) plant on the same grounds that the composting facility existed now known as the Akron Renewable Energy Facility. The $32M plant would use the BIOFerm Energy Systems/Schmack BioGas AD technology. Three 600 kW MWM all-in-one packages from 2-G Cenergy were purchased to convert the biogas to electricity. In addition, an indirect paddle dryer by Komline Sanderson was installed to further process the solids coming out of the AD plant to produce material suitable for market. Construction of the facility began in September of 2011 with the plant being operational by the end of 2013. To process the solids anaerobically, two 1000 cubic meter Eucos (horizontal plug flow tanks) and two 3000 cubic meter Coccus' (complete mix tanks) were constructed. One Euco and one Coccus act as a train capable of processing 6803 dry t (7500 dry tons) of solids. The feedstock consists of a liquid stream entering the Euco tanks at 5% solids and the solids stream where the dewatered sludge enters at 28–30% solids. Maintaining a temperature between 35°C and 37.8°C (95°F and 100°F) in the tanks, the material is slowly moved thru the Euco over the next 8 days where 50% of the biogas production occurs. The digested solids are then transferred to the Coccus where the retention time is approximately 20 days prior to being discharged to holding tanks. Material entering the holding tanks is between 9 and 11% solids. The digestate is then dewatered using centrifuges. This dewatered digestate cake is then the feedstock for the dryer where the solids enter at approximately 30% and leave at ≥92% solids. The biogas production from the process is approximately 11.3 m³/min (400 cfms). This biogas can be used to fuel the three 600 kW MWM engines or the thermal boilers used with the dryer or a combination of both depending on the need. With some of the biogas used in the thermal boilers, a biogas conditioning skid by Unison Solutions was installed that removes hydrogen sulfide, siloxanes if present and moisture. Normal operations result in the biogas being consumed by the engines with a power production of 1.2MW of electric capacity and 1.3MW of thermal capacity. An enclosed flare was installed to manage up to 17.0 m³/min (600 cfms) of biogas. After one year of operations, the 13,607 dry t (15,000 dry ton) AD plant has been able to process all the sludge generated by the Akron Water Reclamation Facility successfully. On average 12.0 m³/min (425 cfms) of biogas have been produced resulting in 1048 kWh generated or 87% design capacity. This paper will take a closer look at the operational numbers as well as the challenges that occurred along the way of commissioning the facility.