Water Environment Research • 1–12, 2019 RESEARCH ARTICLE 1 Department of Civil and Environmental Engineering, University of South Florida, Tampa, Florida 2 Hillsborough County Public Utilities, Tampa, Florida Received 23 July 2019; Revised 5 September 2019; Accepted 6 September 2019 University-Utility Partnership between the University of South Florida (USF) and Hillsborough County Public Utilities Additional Supporting Information may be found in the online version of this article. Correspondence to: Jeffrey A. Cunningham, Department of Civil and Environmental Engineering, University of South Florida, Tampa, FL. Email: cunning@usf.edu * WEF Fellow DOI: 10.1002/wer.1239 © 2019 Water Environment Federation Mass fluxes of nitrogen and phosphorus through water reclamation facilities: Case study of biological nutrient removal, aerobic sludge digestion, and sidestream recycle Hélène Kassouf , 1 Andrés García Parra , 1 Luke Mulford , 2 Gita Iranipour , 2 Sarina J. Ergas , 1 * Jeffrey A. Cunningham 1 • Abstract At water reclamation facilities, recycling of nutrients (nitrogen and phosphorus) from solids‐handling processes to the mainstream treatment process can have det- rimental 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 side- stream. 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 enter- ing 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 bio- logical phosphorus removal (EBPR) process. Some measurements suggest that fer- mentation 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 phospho- rus through a combination of EBPR and alum addition. Although the fluxes meas- ured 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 con- sider (a) removal or recovery of phosphorus from their sidestreams and (b) return- ing sidestreams downstream of fermentation basins to avoid inhibition of EBPR processes. © 2019 Water Environment Federation • Practitioner points • Sidestreams from aerobic digestion can represent significant sources of phospho- rus to mainstream wastewater treatment. • Recycle of nitrate in aerobic digestion sidestreams can interfere with enhanced bi- ological phosphorus removal (EBPR) during mainstream treatment. • Fermentation of return activated sludge (RAS) can support EBPR, even under short average hydraulic residence times (minutes).