Conference Agenda

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. Water above 374°C and 22.1 MPa becomes supercritical, a special state where organic solubility increases, and oxidation processes are accelerated. Supercritical Water Oxidation (SCWO) is a promising technology that converts organic material (biosolids) to inert gases, minerals and water.

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, 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.

Orange County Sanitation District (OC San) teamed with 374Water, Hazen and Sawyer, and Merrell Brothers to demonstrate this technology at their site. The demonstration unit was developed based on a Research and Development (R&D) unit installed at Duke University which has a capacity of one wet tons per day. The demonstration unit has capacity of 6 wet metric tons per day and manufactured by 374Water Inc., ready to be installed. The design of the demonstration project is completed by Hazen. The research team, including OC San, 374Water, Hazen and Sawyer, Yorke Engineering, Alliance Testing worked together to develop a rigorous test plan to support permitting, design and operations. This presentation will focus on the findings from the research unit located at Duke University and the test plan prepared to identify the emissions from SCWO process. It will also discuss the challenges and advantages of the technology. The information will benefit utilities that seeks sustainable solution for their biosolids management considering recent concerns on PFAS compounds.

King County Wastewater Treatment Division’s (WTD) West Point Treatment Plant (West Point), adjacent to Discovery Park on Puget Sound in Seattle, Washington, receives flows ranging from 60 to 440 million gallons per day (mgd). West Point treats combined wastewater produced by a mix of residential, commercial, and industrial customers. Solids are treated using BFP thickening, anaerobic mesophilic digestion, and centrifuge dewatering to produce Class B biosolids beneficially used through the King County Loop™ program.

Digestion at West Point is approaching capacity constraints and major assets are aging. Solids capacity solutions must address a broad range of needs and challenges, including affordability, equity, reliability and resiliency, on a highly constrained site footprint.

The study was executed in a systematic manner using a combination of economic and non-economic analyses. A vision statement developed collaboratively by the consultant/WTD team established project goals and informed decision-making criteria. Our team first evaluated the universe of solids handling options to determine the best method of addressing capacity constraints at West Point. Technologies included digestion pre-treatment, anaerobic digestion, dual digestion, composting, chemical stabilization, thermal drying, and thermal decomposition. Technologies were screened using a pass/fail analysis applying criteria such as technology maturity, end uses, technology footprint and state of the industry. A short-list of technologies passing the pass/fail step were carried forward for subsequent evaluation.

Six alternatives were retained, each including anaerobic digestion as the core stabilization process. Alternatives were categorized based on desirable outcomes each might achieve (e.g., maximizing existing assets, Class A biosolids requirements, emerging contaminant risk mitigation, or nitrogen site impacts). Capital costs, life cycle costs, carbon footprint, impacts to construction sequencing, process reliability during construction, etc. were evaluated. Secondary technical, environmental, and social evaluation criteria were developed and applied in a series of workshops using collaboration tools such as electronic polling and interactive sensitivity analyses. The interactive sensitivity analyses allowed the team to change criteria weights, to pose risk scenarios and see their outcomes, to better understand different priorities across the organization, and ultimately to reach consensus.