RESPIROMETRIC METHOD TO DETERMINE DENITRIFICATION ACTIVITY AND DENITRIFIER CONCENTRATION OF ACTIVATED SLUDGE
Date of Award
Summer 7-23-2026
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Civil, Construction, and Environmental Engineering
First Advisor
Daniel Zitomer
Second Advisor
Brooke Mayer
Third Advisor
Patrick McNamara
Abstract
This thesis presents a novel application of respirometry to simultaneously quantify denitrification kinetics and active denitrifying biomass concentrations in activated sludge from 10 water resource recovery facilities (WRRFs). Biomass samples were collected from a range of configurations, including facilities designed for nitrification only and others that incorporate denitrification processes (e.g. biological nutrient removal or oxidation ditch configurations). Values of the maximum specific growth rate (µmax), the Monod maximum specific substrate utilization rate constant (kmax), and the initial active denitrifying biomass (X0) concentration and fraction were determined from respirometer data. Significant inter-facility differences were observed for all four parameters (ANOVA, p < 0.001). Measured kinetic values were compared to literature data and design parameters from activated sludge models (ASM) and BioWin, revealing that municipal activated sludge exhibited lower denitrification µmax values than those commonly assumed in the models. Inter-facility parameter analysis for active biomass showed an active denitrifier biomass fraction range from approximately 6 to 29 percent of total volatile suspended solids (VSS), with an average value of approximately 15 ± 6.8 percent. Unlike the commonly reported specific denitrification rate (SDNR), which is normalized to total mixed liquor volatile suspended solids (MLVSS) and includes both active and nonactive VSS, the specific denitrification rates reported here were based exclusively on active denitrifying biomass. The average denitrifier kmax value was 1.48 ± 0.33 g NO3-N/g CODx-d considering all WRRF biomass samples. Tukey post-hoc groupings showed that WRRFs 4 and 8a consistently exhibited higher kinetic rates for kmax and µmax (Tukey Group a), whereas WRRF 5 exhibited significantly lower rates (Tukey Group e). Statistical groupings of kinetic parameters and biomass concentration did not align with activated sludge configurations. For example, some facilities designed only for nitrification (WRRFs 7 and 9; Tukey Group a) had significantly greater kmax values than facilities which incorporate denitrification processes (WRRFs 1, 2, and 5; Tukey Groups cd, de, and e, respectively). This finding suggested that plant-specific factors beyond activated sludge configuration influence denitrification kinetics and active biomass concentration and fraction. Respirometry coupled with microbial community analysis may further establish this novel methodology for quantifying kinetic parameters and active biomass.