At water reclamation facilities, recycling of nutrients (nitrogen and phosphorus) from solids‐handling processes to the mainstream treatment process can have detrimental effects on biological nutrient removal systems. In this study, mass fluxes of nitrogen and phosphorus were quantified through the treatment trains at the Northwest Regional Water Reclamation Facility (NWRWRF) and the adjoining Biosolids Management Facility (BMF), which receives sludge from several water reclamation facilities in Hillsborough County, Florida. The driving objectives were to determine (a) whether the return stream from BMF to NWRWRF (i.e., the “sidestream”) represents a significant source of nitrogen and phosphorus to NWRWRF, and (b) whether the sidestream return from BMF is interfering with biological nutrient removal processes at NWRWRF. We determined that nearly half of the overall phosphorus flux into NWRWRF is recycled from the BMF sidestream. This leads to an increased cost of treatment, for example, for alum used in phosphorus removal at NWRWRF. In contrast to phosphorus, the flux of nitrogen from BMF to NWRWRF is small (~3%) compared with the flux of nitrogen entering NWRWRF in raw wastewater. However, nitrogen in the sidestream is mostly in the form of nitrate, which prevents anaerobic conditions from developing in the fermentation basin at NWRWRF, and thereby interferes with the enhanced biological phosphorus removal (EBPR) process. Some measurements suggest that fermentation and release of phosphorus may occur in the return activated sludge line (despite the relatively short residence time in that line), which supports EBPR and may partially compensate for anoxic (denitrifying) conditions in the fermentation basin. Therefore, overall, NWRWRF is able to meet its permit limits for phosphorus through a combination of EBPR and alum addition. Although the fluxes measured here are particular to the treatment systems under consideration, the general trends observed are likely to apply to many similar facilities that employ biological nutrient removal, aerobic digestion, and sidestream recycle, particularly those with regional biosolids management facilities. We recommend that such facilities consider (a) removal or recovery of phosphorus from their sidestreams and (b) returning sidestreams downstream of fermentation basins to avoid inhibition of EBPR processes. Practitioner points Sidestreams from aerobic digestion can represent significant sources of phosphorus to mainstream wastewater treatment. Recycle of nitrate in aerobic digestion sidestreams can interfere with enhanced biological phosphorus removal (EBPR) during mainstream treatment. Fermentation of return activated sludge (RAS) can support EBPR, even under short average hydraulic residence times (minutes).

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