Probabilistic Modeling of Two-Stage Biological Nitrogen Removal Process: Formulation of Control Strategy for Enhanced Process Certainty By Ayanangshu Dey (Ayan) & B S Magbanua Jr., Ph.D, PE Department of Civil & Environmental Engineering Mississippi State University
Introduction Two-stage Biological Nitrogen Removal Process: Nitrification in aerobic reactor Conversion of NH 3 -N to NO 2 -N/NO 3 -N by autotrophic nitrifiers De-nitrification in anoxic reactor Conversion of NO 2 -N/ NO 3 -N by heterotrophic denitrifiers to N 2 COD reduction in aerobic reactor Reduction of COD by heretotrophs Ratio of available organic carbon to TKN needs to be adjusted as necessary for simultaneous occurrence of all the three processes
The Simulation Model Influent Anoxic Reactor Aerobic Reactor Secondary Clarifier Internal Recycle Return Sludge Effluent Waste Sludge Wasted Sludge
Process Features Certain features of Two-stage Biological Nitrogen Removal Process are, Heterotrophs - prolific growth potential, tend to outnumber the heterotrophic denitrifiers in using available organic carbon Anoxic reactor Upstream of aerobic reactor to facilitate growth of denitrifiers with the organic carbon source in the influent High internal recycle rate -(from aerobic reactor to anoxic) to return NO 3 -N for denitrification Nitrifiers - slow growers, certain minimum SRT should be provided for their growth in the system
Operating and Process Parameters Operating Parameters, - Solids residence time, SRT (θ X ) - Internal Recycle Ratio (R 1 ) - Anoxic Time Fraction (θ 1 ) - Recycle Ratio (R) - Hydraulic Retention time, HRT ( ) Kinetic and Stoichiometric Parameters, - max. specific growth rate for heterotrophic biomass (µ H ) - half-saturation coefficient for heterotrophic biomass (K S ) - max. specific growth rate for autotrophic biomass (µ A ) - yield coefficient for autotrophic biomass (Y A ) - max. specific hydrolysis rate (k h ), etc.
Influent Characteristics Component a ASM1 Symbol Concentration mg/l b Soluble inert organic material S I 0 Readily biodegradable substrate S S 160 Particulate inert organic material X I 30 Slowly biodegradable substrate X S 240 Non-biodegradable particulates from cell decay X D 0 Free and unionized ammonia S NH 25 Soluble biodegradable organic nitrogen S ND 6.5 Particulate biodegradable organic nitrogen X ND 8.5 Nitrate and nitrite S NO 0 a Typical values (Grady et al., 1999). Active biomass was absent in the influent. b Expressed as COD for organics and as N for various nitrogen species.
Operating conditions for simulation Parameter a Range studied b Default Value Process Configuration θ 1 1 hr. 8 hr. 3 hr. R 1 θ 2 hr. - 24 hr. 12 hr. θ X 1.0 5.0 4.0 5 day 30 day 15 day Complete mix type anoxic and aerobic reactors in series followed by clarifier for biomass separation with sludge wasting and recirculation systems R 0.25 to 3.0 0.5 a Parameter symbols: θ 1 anoxic hydraulic retention time, R 1 internal recycle ratio, θ total hydraulic retention time, θ X solids residence time, and R sludge recycle ratio b Selected ranges typical of a range of TSBNR process configuration (Rittman, 2001)
Process Parameters in Simulation
The Simulation Model Influent Anoxic Reactor Aerobic Reactor Secondary Clarifier Internal Recycle Return Sludge Effluent Waste Sludge Wasted Sludge
Model Simulation Analysis of Two-Stage Biological Nitrogen Removal process by Activated Sludge Model 1 in GPS-X (Hydromantis, Inc., Canada) Dissolved oxygen (DO) level - 0 mg/l in anoxic reactor - 2 mg/l DO in aerobic reactor Overall HRT - 12 hr. Anoxic to aerobic HRT ratio - 1:3 Discrete simulation - with default values of operating and process parameters Stochastic simulation - with 1,000 random combinations of 15 process parameters with log-normal PDFs (Cox, 2004)
Target Nitrogen Concentration In Total Nitrogen discharge standards for different countries are, California (USA) (meeting US EPA s primary drinking water standard) German Wastewater Discharge Ordinance (2002), - 18 mg/l for plants with >10,000 PE - 13 mg/l for plants with >100,000 PE EU Directive for Wastewater Discharge (EU, 1998) - 1 for between 10,000 and 100,000 PE New Chinese 1A discharge standard - 1 Two limits set for this simulation study - and for all nitrogen species
Stochastic and Discrete Simulations Efflue uent TN, mg/l 20 5 2 No significant difference in process certainty for achieving target effluent S TN and COD levels between SRTs of 15 day and 20 day Effluent COD, mg/l 1 200 50 20 5 2 0.5 0.2 0.05 Stochastic, 15 day Stochastic, 20 day Discrete, 15 day Discrete, 20 day Certainty predicted by discrete simulation, 96% for effluent COD (20 mg/l) 40-42% for effluent STN concentration (5, ) 0.02 0.01 0.001 0.005 0.02 0.1 0.3 0.5 0.7 0.9 Cumulative Probability 0.98 0.995 0.999
Effect of Solids Residence Time on Eff. Nitrogen 0.995 Certainty of achieving treat tment target 0.950 0.700 0.400 0.100 0.010 0.001 0.995 0.950 0.700 0.400 0.100 0.010 Target effluent S NO concentration Target effluent S TN concentration Significant rise in certainty of meeting effluent S TN and S NO concentrations for increase of SRT (θ X ) from 5 day to 15 day Marginal improvement between 15 and 20 day SRT, and No appreciable rise beyond 20 day SRT Not any significant reduction in S NO concentrations between 15 and 20 day 0.001 5 10 15 20 25 30 Solids Retention Time, day
Effect of Anoxic HRT on Effluent Nitrogen 0.995 Certainty of achieving treatm ment target 0.950 0.700 0.400 Consistent process certainty for anoxic HRT (θ 1 ) in range of 2 hr. to 4 hr. (total HRT 12 hr.) Target effluent S concentration Increase or decrease of anoxic HRT showed drop in confidence level of attaining stipulated effluent S TN 0.100 Target effluent S NO concentration 0.010 0.001 0.995 0.950 0.700 0.400 0.100 0.010 Target effluent TN concentration Increase in certainty for target S NO concentrations signifying predominance of anoxic over aerobic condition as anoxic HRT is increased 0.001 1 2 3 4 5 6 7 8 Hydraulic Retention Time, hr.
Effect of Internal Recycle Ratio on Eff. Nitrogen Certainty of achieving trea tment target 0.95 0.70 0.40 0.10 0.01 0.95 0.70 0.40 0.10 Target effluent S NO concentration Target effluent S TN concentration Certainty for attaining target S NO concentration is increased up to 200% internal recycle (R 1 ) Marginal increase in target S TN level for increase of R 1 from 200% to 300% (for target) No appreciable change beyond 300% S NH concentrations unaffected by R 1 0.01 1.0 2.0 3.0 4.0 5.0 Internal Recycle Ratio
Effect of Overall HRT on Effluent Nitrogen 0.995 Certainty of achieving trea atment target 0.950 0.700 0.400 No effect of total HRT on certainty of achieving treatment targets for various N-species (S NH, S NO, S TN ) 0.100 Target effluent S NO concentration NH NO TN 0.010 0.001 0.995 0.950 0.700 0.400 0.100 Target effluent TN concentration or COD 0.010 0.001 5 10 15 20 Hydraulic Retention Time, hr.
Effect of Overall Recycle Ratio on Eff. Nitrogen 0.995 Certainty of achieving treatm ment target 0.950 0.700 0.400 0.100 0.010 0.001 0.995 0.950 0.700 0.400 0.100 0.010 Target effluent S NO concentration Target effluent S TN concentration Certainty for attaining target S NO and S TN concentration () does not show variation with increase in overall recycle (R) S NH and COD concentrations unaffected by increase in R value 0.001 1.0 2.0 3.0 Recycle Ratio
Conclusions Optimized conditions for two-stage BNR Process to maximize process certainty for attaining treatment targets, Solids Residence Time 15-20 days Internal Recycle ratio 4.0 or 400%) Anoxic HRT 2-4 hrs. (with process HRT of 12 hr.) Overall Recycle ratio and HRT do not have any apparent effect on the process (HRT 12 Hr., and R 0.5) BCOD: TKN ratio 10 (may require addition of external carbon source)
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