Nitrate versus oxygen utilization rates in wastewater and activated sludge systems (original) (raw)
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Journal of Chemical Technology & Biotechnology, 2007
This paper provides a detailed investigation of the mass transfers involved in chemical oxygen demand (COD) and N removal in an intermittent aeration activated sludge plant, as described by the widely used ASM1 model. The model was calibrated and validated on a data set obtained during three intensive sampling campaigns. The mass transfers of COD and nitrogen were calculated with the calibrated model for every biodegradable variable of the model in each tank of the biological treatment. Only by making this balance can evaluation of the contribution of each reactor (anaerobic, anoxic and intermittently aerated) to carbon and nitrogen removal be done. It was pointed out that in such a plant (activated sludge under very low organic mass loading (F/M) ratios, sludge retention time of 30 days) operating at 20 • C, the contribution of the anoxic tank in the denitrification process is very low (only 17%). The oxygen transfer in this tank was also estimated and found partly responsible for the low denitrification efficiency. 2007 Society of Chemical Industry Keywords: activated sludge; ASM1; chemical oxygen demand and nitrogen removal; mass transfers; model simulation LIST OF SYMBOLS S S Readily biodegradable substrate (g COD m −3 ) X BH Active heterotrophic biomass (g COD m −3 ) S NH Soluble ammonia nitrogen (g N m −3 ) X BA Active autotrophic biomass (g COD m −3 ) X P Particulate products arising from biomass decay (g COD m −3 ) X S Slowly biodegradable substrate (g COD m −3 ) S NO Soluble nitrate nitrogen (g N m −3 ) X ND Slowly biodegradable organic nitrogen (g N m −3 ) S ND Soluble biodegradable organic nitrogen (g N m −3 ) S I Soluble inert COD (g COD m −3 ) X I Inert suspended organic matter (g COD m −3 ) i XB Mass of nitrogen per mass of COD in biomass (g N (g COD) −1 ) i XP Mass of nitrogen per mass of COD in products from biomass (g N (g COD) −1 ) Y A Yield for autotrophic biomass (g cell COD formed (g N oxidized) −1 ) Y H Yield for heterotrophic biomass (g cell COD formed (g COD oxidized) −1 ) µ A Maximum specific growth rate for autotrophic biomass (d −1 ) k L a Oxygen transfer coefficient (d −1 ) F/M Organic mass loading (kg BOD (kg MLVSS) −1 d −1 ) SRT Sludge retention time (d)
Effects of Oxygen Transport Limitation on Nitrification in the Activated Sludge Process
Journal of Water Pollution Control Federation, 1991
A pseudohomogeneous model of the nitrifying activated sludge process was developed to investigate the effects of mass-transport resistance and heterotrophic/nitrifier competition on the apparent relationship between dissolved oxygen (DO) concentration and nitrification. The kinetics of both heterotrophic carbon oxidation and autotrophic ammonia oxidation within activated sludge flocs were described by an interactive-type, multiple-substrate-limiting model. The values for the model's parameters were estimated from a series of experiments that were conducted in laboratory-scale nitrifying activated sludge reactors. Using the estimated parameter values, the model was used to simulate the effect of organic shock loads and steady-state nitrification efficiency over a typical range of mean cell retention times (MCRTs), DO concentrations, and levels of mass-transport resistance. Varying oxygen transport effects, MCRTs, and heterotrophic/autotrophic competition for DO can combine to give apparent nitrification-limiting DO concentrations that range from 0.5 mg/L to as high as 4.0 mg/L. Res.
A general kinetic model for biological nutrient removal activated sludge systems: Model evaluation
Biotechnology and Bioengineering, 2007
In this article, a kinetic model is developed and presented for biological nutrient removal (BNR) activated sludge (BNRAS) systems in general, but for external nitrification (EN) BNRAS (ENBNRAS) systems in particular. The model is based on the UCTPHO model, but includes some significant modifications, such as anoxic P uptake and associated denitrification by phosphorus accumulating organisms (PAOs). Some key features of the model are described and discussed before the model is presented. Model evaluation will be addressed in another article . PO 4 GroYes Á X NIT 17 Decay of nitrifiers b NIT Á X NIT 1248
Nitrogen Oxidation and Reduction in Aerated Single-Stage Activated Sludge Process
2003
This experiment was conducted in an aerated single-stage reactor at activated sludge concentration of about 3000 mg· dm -3 . The influence of hydraulic retention time on nitrogen removal from rural wastewater was investigated. Hydraulic retention times were 24 and 12 hours. Two series were performed at a constant C/N ratio of 3.5. Enhancement of carbon removal from 83.8% to 87.6% was observed. An increase of activated sludge loading from 0.11g COD · g -1 · d -1 to 0.25 g COD · g -1 · d -1 did not cause total inhibition of nitrifica- tion, but nitrification efficiency decreased from 78.8% to 55.5%. Higher nitrogen removal efficiency (52%) was achieved in series I at 24 hours reaction time in contrast to shorter one (42%). Despite the fully aerobic condition, denitrification was observed. The denitrification process was pos- sible due to the use of accessibility of high amount of readily bioassimilable organic compounds. Obtained denitrification efficiency at C/N ratio 3.5 was about 5...
Water Science & Technology, 2014
It is common practice in wastewater engineering to extend standard activated sludge models (ASMs) with extra process equations derived from batch experiments. However, such experiments have often been performed under conditions different from the ones normally found in wastewater treatment plants (WWTPs). As a consequence, these experiments might not be representative for full-scale performance, and unexpected behaviour may be observed when simulating WWTP models using the derived process equations. In this paper we want to highlight problems encountered using a simplified case study: a modified version of the Activated Sludge Model No. 1 (ASM1) is upgraded with nitrous oxide (N 2 O) formation by ammonia-oxidizing bacteria. Four different model structures have been implemented in the Benchmark Simulation Model No. 1 (BSM1). The results of the investigations revealed two typical difficulties: problems related to the overall mathematical model structure and problems related to the published set of parameter values. The paper describes the model implementation incompatibilities, the variability in parameter values and the difficulties of reaching similar conditions when simulating a full-scale activated sludge plant. Finally, the simulation results show large differences in oxygen uptake rates, nitritation rates and consequently the quantity of N 2 O emission (G N2O ) using the different models.
Oxygen inhibition of activated sludge denitrification
Water Research, 1999
ÐThe eect of dissolved oxygen (DO) on denitri®cation by activated sludge in a bench-scale sequencing batch reactor (SBR) was investigated over a range of 0.09 to 5.6 mg/l DO. Mixed liquor DO as low as 0.09 mg/l was found to signi®cantly inhibit denitri®cation, resulting in a rate decrease of 35%. On the other hand, some denitri®cation activity was observed with DO levels as high as 5.6 mg/l, although at only 4% of the anoxic rate. A one-parameter non-linear model was found to ®t results of signi®cant inhibition of denitri®cation at low DO and persistent denitri®cation at higher DO. A more complex two-parameter model predicted that denitri®cation would ®nally cease at a maximum liquid DO concentration when the activated sludge¯oc volume is completely aerobic. The rate of denitri®cation in the SBR activated sludge¯ocs was reduced by 85% when the mixed liquor DO was 2 mg/l (28% saturation), indicating that aerobic denitri®cation is more persistent in activated sludge than has been reported in dispersed cultures of bacteria in water and soil, probably due to limited oxygen diusion into the¯ocs.
Water research, 2008
Mixtures of methanol and acetate as carbon source were investigated in order to determine their capacity to enhance denitrification and for analysis of the microbial composition and carbon degradation activity in activated sludge from wastewater treatment plants. Laboratory batch reactors at 20 degrees C were used for nitrate uptake rate (NUR) measurements in order to investigate the anoxic activity, while single and mixed carbon substrates were added to activated sludge. Microautoradiography (MAR) in combination with fluorescence in situ hybridisation (FISH) were applied for microbial analysis during exposure to different carbon sources. The NUR increased with additions of a mixture of acetate and methanol compared with additions of a single carbon source. MAR-FISH measurements demonstrated that the probe-defined group of Azoarcus was the main group of bacteria utilising acetate and the only active group utilising methanol under anoxic conditions. The present study indicated an imp...