The Water Research Foundation

Guidelines for Optimizing Nutrient Removal Plant Performance

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Path to Optimization

Water Resource Recovery Facility

Water Resource Recovery Facilities (WRRF) can be divided into five types. The type is typically reflective of the size of the WRRF and the permit requirements. Review WRRF definitions.

Secondary treatment

Secondary treatment refers to WRRFs designed to meet secondary treatment standards as defined in the Clean Water Act (CWA). These standards are reflected in terms of five-day biochemical oxygen demand (BOD), total suspended solids (TSS) removal, and pH. No nutrient removal is achieved.

Conventional nutrient removal (CNR)

Nutrient removal achieved by conventional treatment processes. Biological nutrient removal (BNR) is achieved by modifying the aeration basin configuration for nitrogen (N) and/or phosphorus (P) removal without external carbon (C) addition. Chemical addition to a primary clarifier or secondary process for P removal are examples of CNR processes.

Tertiary nutrient removal (TNR)

Nutrient removal with a combination of modified secondary and tertiary treatment processes. These processes include enhanced particle removal (using processes such as granular, media, or microfiltration) and chemical addition (such as carbon for denitrification or metal salts for P removal). TNR typically requires more than one anoxic zone for enhanced denitrification, either in the secondary treatment process or in a combined denitrification and tertiary filtration process.

Small non-mechanical process

Small-system WRRFs serving small or transient communities with space available for low-rate treatment processes (e.g., septic systems, lagoons), which require minimal mechanical equipment (e.g., screening, pumping).

Small mechanical process

Small communities such as towns, new residential developments, transient communities, and other communities with limited space available can use mechanical treatment facilities to meet permit requirements. These facilities are typically smaller versions of secondary, CNR, or even TNR WRRFs. The optimization strategies for small mechanical processes will therefore be similar to those for large WRRFs.

Please select the existing treatment process (secondary, conventional nutrient removal, or tertiary nutrient removal). Review definitions for more information. Select one of the options below.

Not sure where to start?

Your selections:

Potential Strategies

Biological Nutrient Removal

Chemical Use

Small Mechanical

Small Non-Mechanical

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Neethling, J., M. Falk, and E. Evans. 2023. Guidelines for Optimizing Nutrient Removal Plant Performance. Project 4973. Denver, CO: The Water Research Foundation.

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WRF 4973 Guide - Chapter 5. See Neethling et al. 2023 above.