Nitrite-shunt and biological phosphorus removal at low dissolved oxygen in a full-scale high-rate system at warm temperatures.

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 ( NO 3 -  +  NO 2 - ) concentrations occurs mainly via SND by operating the aerated zone at low DO, but still achieving near-complete ammonium ( NH 4 + ) 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.

Modeling quaternary ammonium compound inhibition of biological nutrient removal activated sludge.

Quaternary ammonium compounds (QACs) are surface-active organic compounds common in industrial cleaner formulations widely used in various sanitation applications. While acting as effective pathogenic biocides, QACs lack selective toxicity and often have poor target specificity. As a result, adverse effects on biological processes and thus the performance of biological nutrient removal (BNR) systems may be encountered when QACs enter wastewater treatment plants (WWTPs). Because of these impacts, there is motivation to screen wastewater influents for QACs and for process engineers to consider the inhibition effects of QACs on process evaluation and design of BNR plants. This paper introduces a mathematical model to describe the fate of QACs in a WWTP via biodegradation and bio-adsorption, and the inhibitory effect of QACs on nitrifiers and ordinary heterotrophic organisms. The model was incorporated as an add-on model in BioWin 5.3 and simulations of experimental systems were used for comparison of model results to measured data reported in the literature. The model was found to accurately predict the bulk phase concentration of QAC and the inhibition of nitrification with QAC concentrations ≥2 mg/L. This work provides a preliminary framework for simulation of BNR plants receiving inhibitory substances in the influent.

Effect of solids residence time on dynamic responses in chemical P removal.

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 pseudo-equilibrium 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.

The Effect of Solids Residence Time on Chemical Phosphorus Removal in Low Concentration Applications.

The effect of solids residence time (SRT) on steady state phosphorus (P) removal when striving for ultralow concentrations through metal salt addition was studied. Lab-scale continuous flow sequencing batch reactors (SBRs) were operated under high (6.4 mg P/L; 1.4 mol Fe/mol P) and low (3.4 mg P/L; 2.6 mol Fe/mol P) influent phosphate concentrations to characterize P removal. Residual P concentrations, particle size distribution, and microscopy analyses were determined over a range of SRTs. A majority of P removal (94% with 3.4 mg P/L; 83% with 6.4 mg P/L) occurred immediately after iron (Fe) addition with additional removal in the SBRs (3.3-4.8% with 3.4 mg P/L; 5.5-8.8% with 6.4 mg P/L). Soluble P uptake was higher for SRTs ≤ 7.4 days with 3.4 mg P/L and ≤ 14.3 days with 6.4 mg P/L. Normalized P uptake (µg P/mg total suspended solids [TSS]) decreased with SRT providing evidence that aging changed floc properties relevant to P removal. Floc size was found to have no distinguishable influence on P removal. However, changes in floc morphology were consistent with P removal trends.

The effect of solids residence time on phosphorus adsorption to hydrous ferric oxide floc.

The impact of solids residence time (SRT) on phosphate adsorption to hydrous ferric oxide (HFO) floc when striving for ultra-low P concentrations was characterized and an equilibrium model that describes the adsorption of P onto HFO floc of different ages was developed. The results showed that fresh HFO had a higher adsorption capacity in comparison to aged (2.8, 7.4, 10.8 and 22.8 days) HFO and contributed substantially to P removal at steady state. P adsorption onto HFO solids was determined to be best described by the Freundlich isotherm. P desorption from HFO solids was negligible supporting the hypothesis that chemisorption is the mechanism of P adsorption on HFO solids. A model that included the contribution of different classes of HFO solids (i.e. High, Low or Old, containing high concentration, low concentration or no active surface sites, respectively) to adsorption onto HFO from a sequencing batch reactor (SBR) system was found to adequately describe P adsorption onto HFO solids of different ages. From the model it was determined that the fractions of High and Low HFO decreased with SRT while the fraction of Old HFO increased with SRT. The transformation of High HFO to Low HFO did not limit the overall production of Old HFO and the fresh HFO solids contributed more to P removal at steady state than the aged solids.