Autogenous Biochar Production from Municipal Wastewater Biosolids Feedstock

Autogenous Biochar Production from Municipal Wastewater Biosolids Feedstock at the Ephrata Borough Authority WWTP #1 in Lancaster County, Pennsylvania, USA Lead Authors: Charles Winslow, GHD, charles.winslow@ghd.com Stan Chilson, GHD, stan.chilson@ghd.com Key Words; Pyrolysis, Volatile Organic Compounds (VOC), Biological Contamination, Synthesis Gas, Carbon Fraction, Carbon Dioxide, Greenhouse Gas, Biodryer, Dried Biosolids, Biochar EXECUTIVE SUMMARY The Ephrata Borough Authority (EBA) owns and operates two municipal wastewater treatment plants in Lancaster County, Pennsylvania. The biosolids produced from the 4 MGD WWTP #1 are unstabilized and currently disposed of at local landfills. As a result of rising landfill costs, reduced landfill availability, and concern about future PFAS regulations associated with municipal wastewater biosolids, the Authority engaged GHD to design a new innovative, carbon negative biosolids handling and processing system to produce biochar, a nutrient and carbon rich material with no detectable biological contaminants or PFAS compounds in the final end product, as well as resale value in several markets. This presentation evaluates EBA's decision making process in selecting biodrying coupled with pyrolysis versus other alternatives under consideration. The presentation also includes a summary discussion of pyrolysis fundamentals, including mass and energy balances. THE CHALLENGE One of the challenges many municipalities face is the prospect of doing something new. EBA was confronted with the challenge of upgrading their existing biosolids processing and handling system in the context of uncertain financial and regulatory factors. While they had an outlet for the anaerobically digested, unstabilized biosolids produced at their WWTP #1, they were concerned about this outlet's long-term viability and cost. With an eye on the future and the goal of producing higher quality biosolids and minimizing the amount of material for disposal, they began the process of evaluating their options. Ultimately two upgrade options were selected for final consideration. The first option was conversion of the existing digesters to a two-phase anaerobic digestion (TPAD) system consisting of both thermophilic and mesophilic digesters, which was capable of producing Class A biosolids. The second option was to scrap digestion altogether and to design a new biodrying and pyrolysis system capable of producing Class A biochar. Facing a similar scenario, many in our industry would arguably select the more well-known and established TPAD technology because familiarity breeds a certain comfort level, and our industry is generally risk adverse. THE SOLUTION EBA is a forward-thinking municipality, and they realized that the biodrying and pyrolysis system alternative offers a number of advantages over the two-phase anaerobic digestion option, including lifecycle cost savings and a more favorable construction sequence and schedule; however, if EBA selected this option they would be only the second operating biodrying and pyrolysis facility processing municipal wastewater biosolids in the USA. There are five known biodryer installations operating on wastewater biosolids only feedstock worldwide at the time of this writing. Biodryers are a tried and proven yet relatively new technology that produces Class A biosolids. The biodrying option does not require the addition of any new tanks, thus allowing for repurposing the existing digesters as liquid storage tanks. The biodryer solution also eliminates the need to recycle high strength nutrient loads, as required with TPAD, negating the need for side-stream treatment and mitigating potential impacts on the liquid side of the treatment plant. Coupling a pyrolysis reactor on the back end of the biodryers further reduces the amount of end product by another 60% on a mass basis, and produces a high-quality biochar, which is of such exceptional quality that it is no longer considered a biosolid, and that testing has shown to be devoid of PFAS compounds. More importantly, the inclusion of a pyrolysis reactor allows for the combined system to run autogenously, negating the need for external fuel sources after it is brought to temperature. The Authority was excited about the prospect of bringing this innovative technology to Ephrata, but conventional wisdom would suggest that the TPAD solution was the safer option. However, that being the case, the EBA made the bold move of selecting a new biodrying and pyrolysis process because of the long-term benefits in the face of regulatory uncertainty. PROJECT SUMMARY The project scope consists of new dewatering, biodrying, and pyrolysis equipment housed in a new building, as well as conversion of the existing primary and secondary digesters to aerated and covered sludge storage, including a biofilter for odor control. Construction is anticipated to begin by the end of 2021. At design the new solids handling system will process 8,400 dry lb per day of undigested biosolids through what is best described as a continuous / batch process. Centrifuges will be utilized to process liquid biosolids to produce 22% to 24% total solids (TS) dewatered cake. Dewatered sludge cake is fed to each biodryer drum and, once filled, the biodryer undergoes a batch process lasting 48 to 56 hours. At the conclusion of the biodrying process the material is about 80% dry and classified as a Class A biosolid. The dried material is stored in a hopper, which is used to continuously feed a pyrolysis reactor. The pyrolysis reactor strips the volatile organic compounds from the solid mass and further dries the material to about 95%, producing nutrient- and carbon-rich biochar in the process. The syngas resulting from the pyrolysis reaction is combusted to produce heat energy that is recovered and used to kickstart the biodrying process and maintain the pyrolysis reaction. The new carbon negative biodrying and pyrolysis system will operate autogenously after initial startup and heat up, requiring no external fossil fuel to maintain system temperature and operation. The amount of biochar produced represents a 90% reduction by mass relative to dewatered cake entering the biodryers. Biochar is a marketable product with no detectable PFAS, which Ephrata plans to sell for a small profit at the completion of the project. CONCLUSION The situation facing EBA is one that many municipalities will likely be confronted with in coming years as the disposal of municipal biosolids comes under increasing scrutiny. It is always a challenge to do something new because many municipalities hesitate at the thought of being an early adopter of a relatively new technology. In general, our industry is very risk adverse and, arguably, needs to become risk smart to evolve. The story of EBA and their evaluation and selection of innovative biodrying and pyrolysis technology can serve as a model and an inspiration to others considering a similar journey.