Alternate: Demonstration of Supercritical Water Oxidation to Diversify End Products, Destroy PFAS and Reduce Air Emissions

Utilities have been investigating new technologies to diversify their end products from water reclamation facilities (WRRFs) due to strict regulations around land application of biosolids and recent concerns on presence and accumulation of poly-fluoroalkyl substances (PFAS) in biosolids. Supercritical Water Oxidation (SCWO) is a promising technology that converts organic material (biosolids) to inert gases, minerals and water and transforms materials into elemental forms. The technology is also identified by Environmental Protection Agency as an innovative technology to destruct PFAS. The SCWO process provides rapid destruction of a wide variety of organic species normally considered to be refractory or difficult to breakdown under conventional means such as incineration. Water above 374 °C and 22.1 MPa becomes supercritical, a special state where organic solubility increases, and oxidation processes are accelerated. Most hydrocarbons and oxygenated hydrocarbons are converted to CO2 and H2O. Nitrogen in the feed is mainly converted to N2. SCWO has been recently shown to destroy hazardous substances such as halogenated compounds including PFAS. Studies showed a greater than 99% reduction of the total PFAS identified in a targeted compound analysis, including perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) (Krause et al, 2022). As a destructive technology, SCWO is proposed as an alternative to incineration and other combustion processes and could be a permanent solution for PFAS-laden biosolids or solid matrices. However, reliability and operability and performance at commercial scale has not been proven yet. Also, additional investigation of reaction byproducts: water, solids and air emissions need to be conducted for a complete assessment of SCWO's potential as a safe and effective technology. This project aims to provide information for adapting the technology at full scale for public utilities and the engineering community and better understanding to the quality of byproducts. Figure 1 shows the process flow diagram for the technology, and process outputs. Orange County Sanitation District (OCSan) teamed with 374Water, Hazen and Sawyer, and Merrell Brothers to demonstrate the first commercial unit AirSCWO-6 at their Plant 1 for processing six (6) wet metric tons per day. OC San currently produces 250,000 wet tons of biosolids annually that must be treated, stored, and transported to appropriate beneficial use locations. The commercial unit was developed based on a Research and Development (R&D) prototype unit installed at Duke University which has a capacity of one (1) wet tons per day. This first unit was funded by Bill and Melinda Gates Foundation. The commercial unit is manufactured by Merrell Brothers based on 374Water Inc. design, and ready to be installed. SCWO aims to eliminate the combustion products and therefore reduce the air emissions. Still, OC San is in South Coast Air Quality Management District (SCAQMD) and an air permit (research permit) is required to install this unit and conduct the project. To characterize SCWO air quality impacts and to obtain necessary data for a research permit from SCAQMD, the research team, including OC San (Utility), 374Water (a private firm-technology provider), Hazen and Sawyer (a consulting firm), Yorke Engineering (permitting support), Alliance Testing (a source testing firm) worked together to develop a rigorous test plan. Since the prototype unit at Duke University is the only one in operation, air sampling was conducted at Duke by Alliance Testing. There are some differences between prototype and commercial unit, and questions were raised whether emission data between the two units will be comparable. The main difference between these two units was identified as the capacity rather than the process, inputs or operations. Hence, the results from prototype unit were considered as being representative. Table 1 presents the parameters included in the test plan. SCWO aims to eliminate the combustion products and therefore reduce the air emissions. Still, OC San is in South Coast Air Quality Management District (SCAQMD) and an air permit (research permit) is required to install this unit and conduct the project. To characterize SCWO air quality impacts and to obtain necessary data for a research permit from SCAQMD, the research team, including OC San (Utility), 374Water (a private firm-technology provider), Hazen and Sawyer (a consulting firm), Yorke Engineering (permitting support), Alliance Testing (a source testing firm) worked together to develop a rigorous test plan. Since the prototype unit at Duke University is the only one in operation, air sampling was conducted at Duke by Alliance Testing. There are some differences between prototype and commercial unit, and questions were raised whether emission data between the two units will be comparable. The main difference between these two units was identified as the capacity rather than the process, inputs or operations. Hence, the results from ptototype unit were considered as being representative. Table 1 presents the parameters included in the test plan. Testing was performed with biosolids transported from OCSan over the three phases of the unit operation, including start-up phase (when the system warms up and when organics are fed to the system), the initial warmup phase(when co-fuel is being pumped into the unit, and the temperature increases) and the steady state phase(when biosolids were fed to the system and were treated at a steady state). The results were gathered for all the parameters listed in Table 1. Emission calculations were conducted for criteria pollutants, toxic contaminants and greenhouse gas emissions. The results indicated that the air emissions from the demonstration unit will not exceed the threshold values and therefore vented gas from SCWO unit will not pose any health risk. The design for integrating the commercial unit with the plant for this project was completed by Hazen and evaluation of construction bids is underway. Figure 2 shows a process flow diagram of the facility once installed at Plant 1 of OCSan, including controlled screening and dewatering skid for consistent 12-15% TS feed to the AirSCWO-6 commercial unit. Our full paper and presentation will provide detailed information on SCWO process, pretreatment requirements, process inputs and outputs. The air emission results from vented gas will be presented in detail along with further information on process efficiency for organics destruction. The information will benefit utilities that seeks sustainable solution for their biosolids management considering recent concerns on PFAS compounds.