Full-Scale Biosolids Drying and Pyrolysis for PFAS Destruction and Organic Nutrient Recovery

Overview: Pyrolysis is the thermal decomposition of organic matter at elevated temperatures (250-750 °C) in an inert atmosphere (i.e., in the absence of oxygen). During pyrolysis, volatiles including contaminants of emerging concern (CECs) such as PFAS are driven off into a high heating value gas phase that can be harnessed for biosolids drying or other onsite energy use. The remaining solids are a carbon-rich biochar with high nutrient content suitable for direct land application or blending into high value fertilizer product. Pyrolysis represents a critical tool to sustainably manage biosolids and support effective resource recovery for nutrients and energy. Pyrolysis will become increasingly valuable as biosolids management options are expected to be further constrained by state and national policies including landfilling bans, land application restrictions, competition for compost capacity, and federal PFAS regulations. Located in Rialto, CA, Rialto Bioenergy Facility (RBF) provides an important case study as the largest biosolids pyrolysis facility constructed in North America. It is designed and permitted to process 300 tons per day (TPD) (equivalent to 75 dry tons per day, DTPD) of dewatered biosolids from municipal WWTPs. The facility is owned by Anaergia, who is responsible for design, construction, financing, operations, and maintenance of the facility, as well as feedstock management and marketing and distribution of the biochar end-products. Low temperature belt dryers and a subsequent Anaergia-developed and -patented high-temperature pyrolysis process converts Class B Biosolids into a biochar product (Class A) for use as a fertilizer. The facility's drying operations currently operate continuously, twenty-four (24) hours per day and seven (7) days per week. The proprietary pyrolysis system will begin commissioning in Q1 2023 with commissioning data and lessons learned to date available for conference date of May 2023. The goals of this plant include: -Maximizing biochar value by eliminating PFAS and other contaminants of emerging concern (CECs) while maximizing carbon and nutrient yield -Maximizing heat recovery for autothermal drying -Meeting South Coast Air Quality Management District (SCAQMD) permitting limits -Demonstrating successful full-scale operation of biosolids pyrolysis Process Description: Feed Handling: RBF receives up to 300 tons per day of dewatered biosolids from municipal WWTP's in Southern California, which are transported to the facility via end-dump truck. The offloading area at RBF includes one (1) below-grade reception bin dedicated for biosolids receiving. The bins are vented to an odor control system and piston pumps are used to transfer the biosolids to a storage silo, prior to pumping to the belt dryers for processing. Drying: The dewatered biosolids are dried to 90% total solids (TS) in two (2) 5-tph evaporation capacity belt dryers. While the pyrolysis system is in construction, dryers currently recover heat from CHP engines on site; the dryers will ultimately recover heat from the pyrolyzer via thermal oil. Dryer foul air passes through an ammonia scrubber, regenerative thermal oxidizer (RTO), and SOx scrubber to meet SCAQMD requirements before being released to atmosphere. Dried product recycle is used in the dryers to produce a granular Class A dried biosolids product. From the dryers, the dried biosolids are pneumatically conveyed to a nitrogen blanketed biosolids storage silo with truck loading, or conveyed to the pyrolysis system for carbonization. Pyrolysis: Biosolids from the dryer are forwarded to a 3.5 tph indirectly heated pyrolysis unit where the biosolids are converted in the absence of oxygen to pyrogas and biochar. Biosolids are processed at temperatures up to 1600 degrees F with adjustable residence time to maximize fixed carbon yield while eliminating PFAS and other CECs (see Table 1). Out of the pyrolyzer, the biochar is cooled to below 40 degrees C before being forwarded to bagging or storage. Pyrogas generated via this process is sent through heat traced pipe directly to the thermal oxidizer. Thermal Oxidizer, Heat Recovery, & Flue Gas Cleaning: The pyrogas from the pyrolyzer is processed directly in a multi-stage low-NOx thermal oxidizer (TO) with >2 seconds residence time. The TO is used to eliminate volatile organic compounds (VOCs) and contaminants of concern in the pyrogas, release their available heat while keeping NOx limts below 200 ppmvd@3%O2. Downstream of the TO, flue gas passes through a thermal oil heat exchanger from which recovered heat is directed to the dryers to achieve energy neutral operation. Flue gas is subsequently treated via Selective Catalytic Reduction (SCR), SOx scrubbing, and mist/particle elimination. This system provides best available control technology (BACT) and is permitted in SCAQMD among the nation's most stringent with NOx <30 ppmvd@3%O2 and SOx <1ppmvd@3%O2. Downstream of the flue gas cleaning equipment, induced draft fans are used to motivate the pyrogas and flue gas through the system while maintaining a slight vacuum through the entire system to eliminate any fugitive emissions. Biochar End Use: Carbon- and nutrient-rich biochar produced at RBF will be transported approximately 15 miles to a local Anaergia-owned fertilizer blending facility. The product is a nutrient-balanced fertilizer with high levels of nitrogen, phosphorus, and potassium, with significantly lower carbon intensity than traditional fertilizers. As a carbon-negative product, carbon removal credits from biochar production will be sold on the voluntary market. This approach maximizes the value that can be realized from the biochar product, allows direct to consumer sales to commercial agricultural operations, and facilitates direct replacement of conventional commercial fertilizers, reducing barrier to entry for end-users. As the remaining mass is approximately one-eighth to one-tenth that the inbound biosolids, the biochar may be much more readily managed. Research conducted by Anaergia indicates that crops perform equally as well when fertilized with blended biochar product as with conventional commercial fertilizers. Numerous studies indicate additional environmental benefits from use of biochar for agriculture, which may be referenced in the presentation. Benefits for Biosolids Generators: In addition to environmental benefits associated with pyrolysis and biochar use, RBF pyrolysis provides operational benefits to biosolids generators. RBF is contracted to receive biosolids from municipal generators throughout Southern California. RBF provides municipalities with a Class A biosolids management outlet and can offer guaranteed and/or fail-safe capacity. As a regional biosolids disposal option, RBF allows municipalities to achieve reduced vehicle mileage associated with hauling of biosolids to farther-flung alternative outlets, as well as improved service reliability and hauler availability resulting from reduced trip time. Pyrolysis at RBF offers destruction of PFAS and other CECs, providing 'future-proof' compliance to WRRFs for anticipated regulations pertaining to biosolids management and land application. Lessons Learned: Lessons learned from the design, permitting, construction, and commissioning phases of North America's largest biosolids pyrolysis facility will be shared. Data presented will focus on air emissions, mass and energy balances, and analysis confirming PFAS and CEC destruction through the operating pyrolysis system. Discussion may include favorable applications and various delivery approaches.