Abstract
In modeling studies, estimation of microorganisms kinetic parameters set is a key element for proper model operation and predictability. Nitrification process is very often, a crucial element of the wastewater treatment systems as bacteria responsible for ammonium and nitrite oxidation are slow growing microorganisms, making whole nitrification process vulnerable to external factors i.e. temperature, inhibition and load fluctuations. Growth and decay rate of nitrifiers decide about amount nitrifying biomass in the wastewater treatment plants, thus the nitrification efficiency. Paper presents analysis of the decay rate (ba) estimation methodology based on respirometric assays measuring the oxygen uptake rate (OUR). Evaluation of this simple and cheap method was made based on decay estimation tests performed on sludge samples from side-stream partial nitritation reactor treating reject water from digested sludge dewatering. Database obtained from these tests were analyzed to evaluate the impact of respirometric assay duration on calculated decay rate values. 11 time ranges were selected for the performed analysis. Calculated ba values were compared showing the optimal test duration between 5–6 hours, while test shorter than 2 hours resulted in unsatisfactory ba outcome.
Highlights
1.1 The idea of decay rateIn activated sludge models (ASMs), decay rate is a part of the function describing net microbial growth rate presented in equation 1 [1]
In case of autotrophic, nitrifying biomass, a simplified version of this function describes the relationship between the biomass yield coefficient (YA, g COD/g NH4-N), decay rate, solids residence time (SRT, d) and the oxidized ammonium load (NH4,ox, g NH4-N/d) and relates those parameters to amount of biomass produced daily: XA
During the initial 2 hours of each respirometric assay, very low values of oxygen uptake rate (OUR) were noticed probably due to a bacterial lag phase after starvation period
Summary
In activated sludge models (ASMs), decay rate (ba, d-1) is a part of the function describing net microbial growth rate presented in equation 1 [1]. In case of autotrophic, nitrifying biomass, a simplified version (omitting inhibition and operational conditions) of this function describes the relationship between the biomass yield coefficient (YA, g COD/g NH4-N), decay rate, solids residence time (SRT, d) and the oxidized ammonium load (NH4,ox, g NH4-N/d) and relates those parameters to amount of biomass produced daily: XA. To measure the rate of nitrifying biomass loss, its growth must be minimalized Due to these requirements, during the decay rate estimation test, activated sludge sample is kept in starvation conditions without substrate addition. Some explanations can be found in the methodology of that test which was performed on pure cultures of nitrifiers feed with synthetic wastewater Such conditions are significantly different from observed in activated sludge observed decay rate can be lower due to no protozoa grazing or neglected impact of toxic substances causing bacterial death or enhancing cell lysis.
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