The impact of solids residence time (SRT) on the dynamics of phosphorus (P) removal by hydrous ferric oxide (HFO) floc was characterized through experimental and modeling studies. Three abiotic process conditions were considered in systems operated over a range of SRTs (~3 to 27 days): uptake in sequencing batch reactors (SBRs) under (a) constant and (b) dynamic P loading conditions, and (c) uptake in batch sorption tests with preformed HFO solids. P removal under all conditions was characterized by an initial period of fast removal followed by a period of slower removal until pseudoequilibrium was reached. The initial removal rate increased with increasing P concentrations and was attributed to a larger concentration gradient between soluble-and adsorbed-phase concentrations. A kinetic model was developed and found to describe the dynamic behavior of P adsorption onto HFO floc under all conditions tested. A consistent mass transfer rate coefficient (k) was found to describe mass transfer over a range of SRTs for low initial P concentrations. At elevated SRTs (23-27 days) and elevated influent P concentrations, k values were found to deviate from those estimated at reduced SRTs. Differences in process mixing conditions were reflected in the estimated rate coefficients (k). Integration of the kinetic model with existing equilibrium models in wastewater process simulators will improve the ability to predict P uptake onto HFO floc under dynamic loading conditions in water resource recovery facilities. Models that consider the kinetics of P uptake will be particularly relevant for facilities that are required to achieve ultralow P concentrations.
• Practitioner points• This work provides a kinetic model that can be integrated with existing equilibrium models in wastewater process simulators to improve the ability to predict P uptake onto HFO floc under dynamic loading conditions. • This research can be used to assist WRRFs to achieve ultralow effluent P requirements.
This study presents results from the City of St. Petersburg's (Florida) Southwest Water Reclamation Facility. This high‐rate BNR plant (SRT ~ 5 days; HRT < 8 hr) achieves combined bioP and shortcut simultaneous nitrification and denitrification (SND) via nitrite in a simple BNR configuration—an anaerobic–aerobic (A/O) process without mixed liquor recycle and a 25% unaerated fraction. N removal to low effluent and nitrate and nitrite (
NO3- +
NO2-) concentrations occurs mainly via SND by operating the aerated zone at low DO, but still achieving near‐complete ammonium (
NH4+) removal. Despite the low DO operation, very good bioP performance is achieved. Full‐scale performance data and detailed bench‐scale testing were conducted to assess the nitrogen and phosphorus removal at low DO conditions. Full‐scale results showed that the plant achieves effluent total inorganic nitrogen (TIN) and total phosphorus (TP) concentrations of approximately 2.0 mgN/L and 0.5 mgP/L, respectively, at an average influent C:N ratio of 7:1 mgCOD:mgN.
Practitioner points
Simple anaerobic–aerobic (A/O) process demonstrated combined N and P removal
Ammonia oxidation was not hampered by low DO (<0.5 mg/L) operation
Low DO (<0.5 mg/L) operation sustained SND via nitrite pathway in a high‐rate process (HRT < 6 hr)
P uptake was demonstrated at low DO which counters to the widely held understanding that high DO (>1.5 mg/L) is necessary
Heterotrophic consumption of nitrite at low DO was the key to the out‐selection of nitrite‐oxidizing bacteria
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