Repurposing Infrastructure for Phosphorus Removal: Developing Design Guidelines for 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 side stream 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 side stream reactors. These side stream reactors focus on hydrolysis of a portion of return activated sludge (RAS) or mixed liquor (ML) to select for fermentative PAOs and drive phosphorus removal. This presentation will focus on results for the ongoing Water Research Foundation (WRF) project focused on developing practical considerations for S2EBPR design and operation (Project Number 4975). This project will yield guidelines for the design, operation, and implementation of a unique configuration to help remove phosphorus while maximizing the use of existing infrastructure. The re-purposing of existing infrastructure is a key benefit of S2EBPR configurations. When fermentative PAOs are the main driver for phosphorus removal, the selective pressure occurs in a side stream reactor typically conditioning RAS. Only 10 to 30% of the RAS is diverted to this side stream reactor, resulting in a relatively small reactor. Depending on the influent characteristics, this side stream reactor can ferment RAS, select for PAOs, and then this fermented flow is sent back to the existing aeration basins. This can result in a phosphorus removal upgrade the requires no modifications to the existing aeration basins, a smaller anaerobic side stream reactor, and often the side stream anaerobic reactor can be incorporated into existing, underutilized tankage for facilities. There are 14 full-scale demonstration facilities participating in the WRF project, and each facility has repurposed infrastructure in different ways. Some of the unique configurations being tested include:
- A Texas facility that utilized existing tanks to test ML fermentation to successfully achieve a 1 mg/L effluent limit with no new tanks (WRF Report 4827C) ###- A large midwestern facility that converted excess aeration basin capacity into a side stream reactor at a COD limited facility to achieve a 0.5 mg/L effluent limit
- A northeastern facility that utilized an abandoned gravity thickener tank to achieve a 0.3 mg/L effluent limit###- Danish facilities that utilize small RAS hydrolysis tanks with no mainstream selector zones to achieve phosphorus removal The facilities being tested, as well as past literature values, will be presented to observe trends in sizing and performance related to diversion rates of return activated sludge (RAS), hydraulic retention times of side stream reactors, and the ratio of biomass in the side stream reactor. One important concept is the idea of the fermenter biomass to total biomass in the system. This idea, first developed in Denmark, provides a design basis for S2EBPR that relates the time in the side stream anaerobic zone to the overall sludge retention time of the system. When well performing EBPR facilities are examined (effluent phosphorus below 0.75 mg/L), the generally fall above a curve developed for Danish facilities (Figure 1). Facilities that fall below this curve to not achieve consistent EBPR.