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Activated Sludge Processes CE - 370

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Basic Process The basic AS process consists of The basic AS process consists of A reactor in which the microorganisms responsible for treatment are kept in suspension and aerated A reactor in which the microorganisms responsible for treatment are kept in suspension and aerated Liquid-solids separation, usually sedimentation tank Liquid-solids separation, usually sedimentation tank A recycle system for returning solids removed from the liquid-solids separation unit back to the reactor A recycle system for returning solids removed from the liquid-solids separation unit back to the reactor Important feature of the AS process is: Important feature of the AS process is: Formation of flocculent settleable solids that can be removed by gravity settling Formation of flocculent settleable solids that can be removed by gravity settling

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Activated Sludge process utilizes: Activated Sludge process utilizes: Fluidized microorganisms Fluidized microorganisms Mixed growth microorganisms Mixed growth microorganisms Aerobic conditions Aerobic conditions Microorganisms Microorganisms Use organic materials in wastewater as substrates Use organic materials in wastewater as substrates Thus, they remove organic materials by microbial respiration and synthesis Thus, they remove organic materials by microbial respiration and synthesis MLSS MLSS Ranges between 2000 and 4000 mg/l Ranges between 2000 and 4000 mg/l Flows Flows Feed wastewater (Q) Feed wastewater (Q) Waste activated sludge (Q w ) Waste activated sludge (Q w ) Recycled activated sludge (R) Recycled activated sludge (R) Prior to entering aeration tank Prior to entering aeration tank OR immediately after entering OR immediately after entering

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Oxygen Supply Oxygen Supply Diffused compressed air Diffused compressed air Mechanical surface aeration Mechanical surface aeration Pure oxygen Pure oxygen Purposes of aeration Purposes of aeration Provides oxygen required for aerobic bio-oxidation Provides oxygen required for aerobic bio-oxidation Provides sufficient mixing for adequate contact between activated sludge and organic substances Provides sufficient mixing for adequate contact between activated sludge and organic substances In order to maintain the desired MLSS in the aeration tank, R/Q ratio must be calculated In order to maintain the desired MLSS in the aeration tank, R/Q ratio must be calculated

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Calculate (R / Q) Ratio Calculate the Sludge Density Index (SDI) Calculate the Sludge Density Index (SDI) Sample MLSS from downstream of aeration tank Sample MLSS from downstream of aeration tank Determine SS in MLSS Determine SS in MLSS Place 1 liter of the MLSS in 1-liter graduate cylinder Place 1 liter of the MLSS in 1-liter graduate cylinder Settle the sludge for 30 minutes Settle the sludge for 30 minutes Measure volume occupied by settled sludge Measure volume occupied by settled sludge Compute SS in settled sludge in mg/l Compute SS in settled sludge in mg/l SS represents SDI SS represents SDI The test approximates the settling that occurs in final clarifier The test approximates the settling that occurs in final clarifier If SDI = 10,000 mg/l and MLSS must be 2,500 mg/l If SDI = 10,000 mg/l and MLSS must be 2,500 mg/l Then, Q(0) + R(10,000) = (Q+R)(2500) Then, Q(0) + R(10,000) = (Q+R)(2500) R/Q = (2500)/(7500) = (1/3) = 33.3 % R/Q = (2500)/(7500) = (1/3) = 33.3 % So, R is 33.3% of feed wastewater (Q) So, R is 33.3% of feed wastewater (Q)

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Sludge Volume Index (SVI) = 1/ SDI Sludge Volume Index (SVI) = 1/ SDI Is the volume in ml occupied by 1 gram of settled activated sludge Is the volume in ml occupied by 1 gram of settled activated sludge It is a measure of settling characteristics of sludge It is a measure of settling characteristics of sludge Is between 50 and 150 ml/gm, if process is operated properly Is between 50 and 150 ml/gm, if process is operated properly Why Q w ? Why Q w ? Microbes utilize organic substances for respiration and synthesis of new cells Microbes utilize organic substances for respiration and synthesis of new cells The net cell production (Q w ) must be removed from the system to maintain constant MLSS The net cell production (Q w ) must be removed from the system to maintain constant MLSS Qw is usually 1 to 6 % of feed wastewater flowrate (Q) Qw is usually 1 to 6 % of feed wastewater flowrate (Q)

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Common organic materials in municipal wastewater are: Common organic materials in municipal wastewater are: Carbohydrates (C, H, O0 Carbohydrates (C, H, O0 Fats (C, H, O) Fats (C, H, O) Proteins (C, H, O, N, S, P) Proteins (C, H, O, N, S, P) Urea (C, H, O, N) Urea (C, H, O, N) Soaps (C, H, O) Soaps (C, H, O) Detergents (C, H, O, P) Detergents (C, H, O, P) Traces of Traces of Pesticides Pesticides Herbicides Herbicides Other agricultural chemicals Other agricultural chemicals Activated sludge can be represented by: Activated sludge can be represented by: C 5 H 7 O 2 N C 5 H 7 O 2 N Has a molecular weight of 113 Has a molecular weight of 113

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Design To design of AS, the following must be determined: To design of AS, the following must be determined: Volume of reactor Volume of reactor Number of basins Number of basins Dimensions of each basin Dimensions of each basin Volume of reactor is determined from: Volume of reactor is determined from: Kinetic relationships Kinetic relationships Space loading relationships Space loading relationships Empirical relationships Empirical relationships Sludge production per day (X w ), kg/day Sludge production per day (X w ), kg/day Oxygen required per day (O r ), kg/day Oxygen required per day (O r ), kg/day Final clarifier Final clarifier Number of basins Number of basins

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Biological Kinetics 1. Michaelis – Menten Concept 1. Michaelis – Menten Concept (1/X)(ds/dt) = specific rate of substrate utilization (1/X)(ds/dt) = specific rate of substrate utilization (ds/dt) = rate of substrate utilization (ds/dt) = rate of substrate utilization k s = maximum rate of substrate utilization k s = maximum rate of substrate utilization K m = substrate concentration when the rate of utilization is half maximum rate K m = substrate concentration when the rate of utilization is half maximum rate S = substrate concentration S = substrate concentration

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If S is very large, Km can be neglected, therefore S cancels out and the reaction is zero order in substrate. K is the rate constant for zero- order reaction. If S is very large, Km can be neglected, therefore S cancels out and the reaction is zero order in substrate. K is the rate constant for zero- order reaction. If S is relatively small, it can be neglected in the denominator and the reaction is first-order in substrate. K is the rate constant for the first-order reaction If S is relatively small, it can be neglected in the denominator and the reaction is first-order in substrate. K is the rate constant for the first-order reaction

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Rearrange and integrate Equation (2) Rearrange and integrate Equation (2) X = average cell mass concentration during the biochemical reaction, that is X = (X 0 + X t )/2 X = average cell mass concentration during the biochemical reaction, that is X = (X 0 + X t )/2 S t = substrate concentration at time t S t = substrate concentration at time t S 0 = substrate concentration at time t = 0 S 0 = substrate concentration at time t = 0

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Rearrange and integrate Equation (3) Rearrange and integrate Equation (3) X = average cell mass concentration during the biochemical reaction, that is X = (X 0 + X t )/2 X = average cell mass concentration during the biochemical reaction, that is X = (X 0 + X t )/2 S t = substrate concentration at time t S t = substrate concentration at time t S 0 = substrate concentration at time t = 0 S 0 = substrate concentration at time t = 0

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Equations (4) and (5) are in the form of Equations (4) and (5) are in the form of y = mx + b y = mx + b Plotting S t on y-axis versus Xt on the x-axis on arithmetical paper produce a straight line with a slope of –K Plotting S t on y-axis versus Xt on the x-axis on arithmetical paper produce a straight line with a slope of –K Plotting S t on y-axis versus Xt on the x-axis on semilog paper produce a straight line with a slope of -K Plotting S t on y-axis versus Xt on the x-axis on semilog paper produce a straight line with a slope of -K The substrate could be The substrate could be The BOD 5 The BOD 5 Biodegradable part of COD Biodegradable part of COD Biodegradable fraction of TOC Biodegradable fraction of TOC Biodegradable of any other organic matter Biodegradable of any other organic matter

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Rate Constant, K Rate Constant, K Depends on the specific wastewater Depends on the specific wastewater For domestic wastewater, it ranges between 0.1 to 1.25 liter/(gram MLSS)(hr) using BOD 5 For domestic wastewater, it ranges between 0.1 to 1.25 liter/(gram MLSS)(hr) using BOD 5 Should be determined using lab-scale or pilot-scale studies Should be determined using lab-scale or pilot-scale studies In the absence of studies, K between 0.1 and 0.4 liter/(gram MLSS)(hr) is recommended In the absence of studies, K between 0.1 and 0.4 liter/(gram MLSS)(hr) is recommended

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Example on Biochemical Kinetics

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Food to Microorganism Ratio (F/M) F/M ratio is equal to the specific rate of substrate utilization (1/X)(dS/dt) F/M ratio is equal to the specific rate of substrate utilization (1/X)(dS/dt) The units of F/M ratio are (mass substrate) / (mass microbes) (time) The units of F/M ratio are (mass substrate) / (mass microbes) (time) (kg BOD 5 /kg MLVSS-day) (kg BOD 5 /kg MLVSS-day)

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Mean Cell Residence Time ( c ) It is defined as: It is defined as: X = active biological solids in the reactor X = active biological solids in the reactor X = active biological solids in the waste activated sludge flow X = active biological solids in the waste activated sludge flow Units of c is days Units of c is days Mean cell residence time is sometimes referred to as sludge age Mean cell residence time is sometimes referred to as sludge age

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F/M Ratio and c Both parameters are used characterize the performance of the activated sludge process Both parameters are used characterize the performance of the activated sludge process A high F/M ratio and a low c produce filamentous growth that have poor settling characteristics A high F/M ratio and a low c produce filamentous growth that have poor settling characteristics A low F/M ratio and a high c can cause the biological solids to undergo excessive endogenous degradation and cell dispersion A low F/M ratio and a high c can cause the biological solids to undergo excessive endogenous degradation and cell dispersion For municipal wastewater For municipal wastewater c should be at least 3 to 4 days c should be at least 3 to 4 days If nitrification is required, c should be at least 10 days If nitrification is required, c should be at least 10 days

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F/M Ratio and c Relationship between c and F/M ratio can be derived by starting with the equation of cell production, as follows: Relationship between c and F/M ratio can be derived by starting with the equation of cell production, as follows: ( X/ t) = rate of cell production, mass/time ( X/ t) = rate of cell production, mass/time Y = cell yield coefficient, mass cell created/mass substrate removed Y = cell yield coefficient, mass cell created/mass substrate removed k e = endogenous decay, mass cells/(total mass cells) (time) k e = endogenous decay, mass cells/(total mass cells) (time) X = average cell concentration, mass X = average cell concentration, mass

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F/M Ratio and c Divide by X Divide by X c is the average time a cell remains in the system, thus c is the average time a cell remains in the system, thus

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F/M Ratio and c The F/M ratio is the rate of substrate removal per unit weight of the cells, thus The F/M ratio is the rate of substrate removal per unit weight of the cells, thus Thus Thus

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F/M Ratio and c Since F/M was also expressed as: Since F/M was also expressed as: Then, Then,

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Types of Reactors Plug-flow reactors Plug-flow reactors Dispersed plug-flow reactors Dispersed plug-flow reactors Completely-mixed reactors Completely-mixed reactors

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Plug-flow and Dispersed-flow Reactors In plug-flow reactors, there is negligible diffusion along the flow path through the reactor In plug-flow reactors, there is negligible diffusion along the flow path through the reactor In dispersed-flow reactors, there is significant diffusion along the flow path through the reactor In dispersed-flow reactors, there is significant diffusion along the flow path through the reactor Both types of reactors are used in conventional and tapered aeration activated sludge Both types of reactors are used in conventional and tapered aeration activated sludge

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Conventional Activated Sludge Rectangular aeration tank Rectangular aeration tank F/M = 0.2 to 0.4 (kg BOD 5 /kg MLSS-day) F/M = 0.2 to 0.4 (kg BOD 5 /kg MLSS-day) Space loading = 0.3 to 0.6 (kg BOD 5 /day-m 3 ) Space loading = 0.3 to 0.6 (kg BOD 5 /day-m 3 ) c = 5 to 15 (days) c = 5 to 15 (days) Retention time (aeration tank) = 4 to 8 (hours) Retention time (aeration tank) = 4 to 8 (hours) MLSS = 1500 to 3000 (mg/l) MLSS = 1500 to 3000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.0 Recycle ratio (R/Q) = 0.25 to 1.0 Plug-flow and Dispersed-flow Plug-flow and Dispersed-flow BOD removal = 85 to 95 (%) BOD removal = 85 to 95 (%)

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Tapered Aeration It is a modification of the conventional process It is a modification of the conventional process F/M = 0.2 to 0.4 (kg BOD 5 /kg MLSS-day) F/M = 0.2 to 0.4 (kg BOD 5 /kg MLSS-day) Space loading = 0.3 to 0.6 (kg BOD 5 /day-m 3 ) Space loading = 0.3 to 0.6 (kg BOD 5 /day-m 3 ) c = 5 to 15 (days) c = 5 to 15 (days) Retention time (aeration tank) = 4 to 8 (hours) Retention time (aeration tank) = 4 to 8 (hours) MLSS = 1500 to 3000 (mg/l) MLSS = 1500 to 3000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.0 Recycle ratio (R/Q) = 0.25 to 1.0 Plug-flow and Dispersed-flow Plug-flow and Dispersed-flow BOD removal = 85 to 95 (%) BOD removal = 85 to 95 (%)

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Oxygen Demand versus Reactor Length for Municipal Wastewater O 2 Demand/total O 2 for entire reactor (%) Quarter of Reactor 35 1 st 26 2 nd 20 3 rd 194th

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Performance is the detention time for the plug-flow reactor is the detention time for the plug-flow reactor The volume of the plug-flow or dispersed-flow reactor is given by: The volume of the plug-flow or dispersed-flow reactor is given by:

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Completely Mixed Reactors Usually circular and square aeration tanks Usually circular and square aeration tanks F/M = 0.1 to 0.6 (kg BOD 5 /kg MLSS-day) F/M = 0.1 to 0.6 (kg BOD 5 /kg MLSS-day) Space loading = 0.8 to 2.0 (kg BOD 5 /day-m 3 ) Space loading = 0.8 to 2.0 (kg BOD 5 /day-m 3 ) c = 5 to 30 (days) c = 5 to 30 (days) Retention time (aeration tank) = 3 to 6 (hours) Retention time (aeration tank) = 3 to 6 (hours) MLSS = 2500 to 4000 (mg/l) MLSS = 2500 to 4000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.5 Recycle ratio (R/Q) = 0.25 to 1.5 Completely mixed Completely mixed BOD removal = 85 to 95 (%) BOD removal = 85 to 95 (%)

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Design Parameters The retention time and reactor volume for completely mixed reactors can be determined by: The retention time and reactor volume for completely mixed reactors can be determined by:

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Process Modifications Objective of modifications Objective of modifications Modifications Modifications Step aeration Step aeration Modified aeration Modified aeration Contact stabilization Contact stabilization High-rate aeration High-rate aeration Extended aeration Extended aeration

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Objectives of Modification Several modifications of the activated sludge process were made to attain a particular or design objective

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Step Aeration It was developed to even out the oxygen demand of the MLSS throughout the length of the reactor It was developed to even out the oxygen demand of the MLSS throughout the length of the reactor It uses plug-flow and dispersed plug-flow reactors with step inputs of the feed flow (Q) It uses plug-flow and dispersed plug-flow reactors with step inputs of the feed flow (Q) Design Parameters Design Parameters = 3-5 hrs; c = 5-15 days; R/Q = 25-75%; MLSS = mg/l; BOD 5 and SS removal = 85-95%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3 = 3-5 hrs; c = 5-15 days; R/Q = 25-75%; MLSS = mg/l; BOD 5 and SS removal = 85-95%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3

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Modified Aeration Designed to provide a lower degree of treatment than the other activated sludge processes Designed to provide a lower degree of treatment than the other activated sludge processes It uses plug-flow and dispersed plug-flow reactors It uses plug-flow and dispersed plug-flow reactors Design Parameters Design Parameters = hrs; c = days; R/Q = 5-15%; MLSS = mg/l; BOD 5 and SS removal = 60-75%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3 = hrs; c = days; R/Q = 5-15%; MLSS = mg/l; BOD 5 and SS removal = 60-75%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3

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Contact Stabilization Designed to provide two reactors, one for the sorption of organic matter and for the bio- oxidation of the sorbed materials Designed to provide two reactors, one for the sorption of organic matter and for the bio- oxidation of the sorbed materials It uses plug-flow and dispersed plug-flow reactors It uses plug-flow and dispersed plug-flow reactors Design Parameters Design Parameters = hrs; c = 5-15 days; R/Q = %; MLSS = mg/l; BOD 5 and SS removal = 80-90%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3 = hrs; c = 5-15 days; R/Q = %; MLSS = mg/l; BOD 5 and SS removal = 80-90%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3

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High-Rate Aeration Designed to provide a lower degree of treatment than the other activated sludge processes Designed to provide a lower degree of treatment than the other activated sludge processes It uses completely mixed reactor It uses completely mixed reactor Design Parameters Design Parameters = 2-4 hrs; c = 5-10 days; R/Q = %; MLSS = mg/l; BOD 5 and SS removal = 75-90%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3 = 2-4 hrs; c = 5-10 days; R/Q = %; MLSS = mg/l; BOD 5 and SS removal = 75-90%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3

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Extended Aeration Designed to minimize waste activated sludge production by providing a large endogenous decay of the sludge mass Designed to minimize waste activated sludge production by providing a large endogenous decay of the sludge mass It uses plug-flow and dispersed plug-flow reactors It uses plug-flow and dispersed plug-flow reactors Design Parameters Design Parameters = hrs; c = days; R/Q = %; MLSS = mg/l; BOD 5 and SS removal = 75-95%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3 = hrs; c = days; R/Q = %; MLSS = mg/l; BOD 5 and SS removal = 75-95%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3

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Pure Oxygen Process Designed to reduce retention time, decrease the amount of waste activated sludge, increase sludge settling characteristics and reduce land requirement Designed to reduce retention time, decrease the amount of waste activated sludge, increase sludge settling characteristics and reduce land requirement It uses completely mixed reactors It uses completely mixed reactors Design Parameters Design Parameters = 1-3 hrs; c = 8-20 days; R/Q = 25-50%; MLSS = mg/l; BOD 5 and SS removal = 85-95%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3 = 1-3 hrs; c = 8-20 days; R/Q = 25-50%; MLSS = mg/l; BOD 5 and SS removal = 85-95%; F/M = kg/kg-day; space loading = kg BOD 5 /day-m 3

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Effect of Temperature on Growth Rate Arrhenius relationship Arrhenius relationship K 1 = reaction rate constant at temperature T 1 K 1 = reaction rate constant at temperature T 1 K 2 = reaction rate constant at temperature T 2 K 2 = reaction rate constant at temperature T 2 = temperature correction coefficient = temperature correction coefficient T 1 = temperature of MLSS for K 1 T 1 = temperature of MLSS for K 1 T 2 = temperature of MLSS for K 2 T 2 = temperature of MLSS for K 2

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Effect of Temperature on Endogenous Degradation Rate Constant (k e ) The relationship The relationship k e1 = endogenous degradation rate constant at temperature T 1 k e1 = endogenous degradation rate constant at temperature T 1 ke 2 = endogenous degradation rate constant at temperature T 2 ke 2 = endogenous degradation rate constant at temperature T 2 = temperature correction coefficient = temperature correction coefficient T 1 = temperature of MLSS for k e1 T 1 = temperature of MLSS for k e1 T 2 = temperature of MLSS for k e2 T 2 = temperature of MLSS for k e2

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Other Kinetic Relationships 2. The Monod Equation 2. The Monod Equation = growth rate constant, time-1 = growth rate constant, time-1 max = maximum growth rate constant, time-1 max = maximum growth rate constant, time-1 S = substrate concentration in solution S = substrate concentration in solution K s = substrate concentration when the growth rate constant is half the maximum rate constant. K s = substrate concentration when the growth rate constant is half the maximum rate constant.

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Monod observed that the microbial growth is represented by: Monod observed that the microbial growth is represented by: dX/dt = rate of cell production dX/dt = rate of cell production X = number or mass of microbes present X = number or mass of microbes present = growth rate constant = growth rate constant

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Generalized substrate consumption and biomass growth with time.

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