Diversifying Carbon for Phosphorus Removal: Understanding RAS Fermentation and S2EBPR

Management of the global phosphorus cycle is crucial for long-term sustainability. Water resource recovery facilities (WRRFs) play a critical role in in the phosphorus control and balance in many watersheds throughout North America, and the discharge of soluble and other bioavailable forms of phosphorus from WRRFs has a global impact on soluble-nutrient balances in the water cycle.
Enhanced biological phosphorus removal (EBPR) is one key process applied to remove phosphorus from the liquid discharges at WRRFs. Through EBPR, phosphorus is concentrated from the wastewater into the biosolids stream, where it can be recovered in biosolids or extracted as a fertilizer product. Recycling phosphorus in this way helps close the phosphorus cycle without discharging phosphorus into waterbodies. Over the past 50 years, EBPR has evolved from an unexplained phenomenon to a widely adopted process that is simultaneously praised for its efficiency and environmental soundness and, cursed for its instability. Understanding of the process has never been complete, but recent advances in the research and interests in alternative processes at the facility mean the time has come to enhance water-sector understanding with the latest knowledge of EBPR ecology, functionality, and design. One of the key innovations for phosphorus removal has been the adoption of sidestream EBPR (S2EBPR) concepts into the design of EBPR facilities. These concepts focus on fermentative activity of phosphate accumulation organisms (PAOs) to drive EBPR, often in sidestream reactors. These sidestream reactors focus on hydrolysis of a portion of return activated sludge (RAS) or mixed liquor (ML) to select for PAOs and drive phosphorus removal. Conventional design EBPR focused on providing volatile fatty acids (VFAs) to PAOs in an influent anaerobic zone. As an alternative, RAS fermentation diverts a portion of RAS flow to a longer hydraulic retention time (HRT) sidestream reactor. In this sidestream reactor, biomass and particulate COD is fermented to generate the VFA required for PAO metabolism. In a strict RAS fermentation reactor, this results is selection of PAO metabolism without direct use of influent readily biodegradable COD (rbCOD) or VFA. In a carbon focused wastewater paradigm, this provides increased flexibility for COD diversion and capture; use of influent rbCOD for nitrogen removal; and potentially more stable phosphorus removal due to a lack of sensitivity to influent COD characteristics.
This presentation will present the most recent results from Water Research Foundation project 4975, which is focused on developing practical guidelines for fermentative phosphorus removal processes. An overview of the project and concepts will be presented, with a focus on carbon balance and carbon sources for EBPR. Three case studies will then be presented, each with a different focus: •RAS fermentation and the importance of effluent nitrate (Figure 1) •RAS fermentation with carbon addition and the approach to developing an EBPR carbon balance (Figure 2) •Mixed liquor fermentation and impacts of low DO operation (Figure 3) Attendees will be exposed to operational experience, as well as take home messages about designing for fermentation processes in EBPR systems. Example design curves for a facility will be presented (Figure 4) with an explanation of the design approach for RAS fermentation.