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**Activated Sludge Processes**

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**Basic Process The basic AS process consists of**

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

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**Activated Sludge process utilizes:**

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

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**Oxygen Supply Purposes of aeration**

Diffused compressed air Mechanical surface aeration Pure oxygen Purposes of aeration Provides oxygen required for aerobic bio-oxidation 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

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**Calculate (R / Q) Ratio Calculate the Sludge Density Index (SDI)**

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

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**Sludge Volume Index (SVI) = 1/ SDI**

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

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**Common organic materials in municipal wastewater are:**

Carbohydrates (C, H, O0 Fats (C, H, O) Proteins (C, H, O, N, S, P) Urea (C, H, O, N) Soaps (C, H, O) Detergents (C, H, O, P) Traces of Pesticides Herbicides Other agricultural chemicals Activated sludge can be represented by: C5H7O2N Has a molecular weight of 113

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**Design To design of AS, the following must be determined:**

Volume of reactor Number of basins Dimensions of each basin Volume of reactor is determined from: Kinetic relationships Space loading relationships Empirical relationships Sludge production per day (Xw), kg/day Oxygen required per day (Or), kg/day Final clarifier

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**Biological Kinetics 1. Michaelis – Menten Concept**

(1/X)(ds/dt) = specific rate of substrate utilization (ds/dt) = rate of substrate utilization ks = maximum rate of substrate utilization Km = substrate concentration when the rate of utilization is half maximum rate 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 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)**

X = average cell mass concentration during the biochemical reaction, that is X = (X0 + Xt)/2 St = substrate concentration at time t S0 = substrate concentration at time t = 0

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**Rearrange and integrate Equation (3)**

X = average cell mass concentration during the biochemical reaction, that is X = (X0 + Xt)/2 St = substrate concentration at time t S0 = substrate concentration at time t = 0

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**Equations (4) and (5) are in the form of**

y = mx + b Plotting St on y-axis versus Xt on the x-axis on arithmetical paper produce a straight line with a slope of –K Plotting St 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 BOD5 Biodegradable part of COD Biodegradable fraction of TOC Biodegradable of any other organic matter

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**Rate Constant, K Depends on the specific wastewater**

For domestic wastewater, it ranges between 0.1 to 1.25 liter/(gram MLSS)(hr) using BOD5 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

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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) The units of F/M ratio are (mass substrate) / (mass microbes) (time) (kg BOD5/kg MLVSS-day)

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**Mean Cell Residence Time (c)**

It is defined as: X = active biological solids in the reactor X = active biological solids in the waste activated sludge flow Units of c is days 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 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 For municipal wastewater c should be at least 3 to 4 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: (X/t) = rate of cell production, mass/time Y = cell yield coefficient, mass cell created/mass substrate removed ke = endogenous decay, mass cells/(total mass cells) (time) X = average cell concentration, mass

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**F/M Ratio and c Divide by X**

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 Thus

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

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**Types of Reactors Plug-flow reactors Dispersed plug-flow 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 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

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**Conventional Activated Sludge**

Rectangular aeration tank F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day) Space loading = 0.3 to 0.6 (kg BOD5/day-m3) c = 5 to 15 (days) Retention time (aeration tank) = 4 to 8 (hours) MLSS = 1500 to 3000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.0 Plug-flow and Dispersed-flow BOD removal = 85 to 95 (%)

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**Tapered Aeration It is a modification of the conventional process**

F/M = 0.2 to 0.4 (kg BOD5/kg MLSS-day) Space loading = 0.3 to 0.6 (kg BOD5/day-m3) c = 5 to 15 (days) Retention time (aeration tank) = 4 to 8 (hours) MLSS = 1500 to 3000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.0 Plug-flow and Dispersed-flow BOD removal = 85 to 95 (%)

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**Oxygen Demand versus Reactor Length for Municipal Wastewater**

O2 Demand/total O2 for entire reactor (%) Quarter of Reactor 35 1st 26 2nd 20 3rd 19 4th

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Performance is the detention time for the plug-flow reactor 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 F/M = 0.1 to 0.6 (kg BOD5/kg MLSS-day) Space loading = 0.8 to 2.0 (kg BOD5/day-m3) c = 5 to 30 (days) Retention time (aeration tank) = 3 to 6 (hours) MLSS = 2500 to 4000 (mg/l) Recycle ratio (R/Q) = 0.25 to 1.5 Completely mixed 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:

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**Process Modifications**

Objective of modifications Modifications Step aeration Modified aeration Contact stabilization High-rate 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 uses plug-flow and dispersed plug-flow reactors with step inputs of the feed flow (Q) Design Parameters = 3-5 hrs; c = 5-15 days; R/Q = 25-75%; MLSS = mg/l; BOD5 and SS removal = 85-95%; F/M = kg/kg-day; space loading = kg BOD5/day-m3

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Modified Aeration Designed to provide a lower degree of treatment than the other activated sludge processes It uses plug-flow and dispersed plug-flow reactors Design Parameters = hrs; c = days; R/Q = 5-15%; MLSS = mg/l; BOD5 and SS removal = 60-75%; F/M = kg/kg-day; space loading = kg BOD5/day-m3

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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 It uses plug-flow and dispersed plug-flow reactors Design Parameters = hrs; c = 5-15 days; R/Q = %; MLSS = mg/l; BOD5 and SS removal = 80-90%; F/M = kg/kg-day; space loading = kg BOD5/day-m3

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High-Rate Aeration Designed to provide a lower degree of treatment than the other activated sludge processes It uses completely mixed reactor Design Parameters = 2-4 hrs; c = 5-10 days; R/Q = %; MLSS = mg/l; BOD5 and SS removal = 75-90%; F/M = kg/kg-day; space loading = kg BOD5/day-m3

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Extended Aeration 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 Design Parameters = hrs; c = days; R/Q = %; MLSS = mg/l; BOD5 and SS removal = 75-95%; F/M = kg/kg-day; space loading = kg BOD5/day-m3

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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 It uses completely mixed reactors Design Parameters = 1-3 hrs; c = 8-20 days; R/Q = 25-50%; MLSS = mg/l; BOD5 and SS removal = 85-95%; F/M = kg/kg-day; space loading = kg BOD5/day-m3

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**Effect of Temperature on Growth Rate**

Arrhenius relationship K1 = reaction rate constant at temperature T1 K2 = reaction rate constant at temperature T2 = temperature correction coefficient T1 = temperature of MLSS for K1 T2 = temperature of MLSS for K2

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**Effect of Temperature on Endogenous Degradation Rate Constant (ke)**

The relationship ke1 = endogenous degradation rate constant at temperature T1 ke2 = endogenous degradation rate constant at temperature T2 = temperature correction coefficient T1 = temperature of MLSS for ke1 T2 = temperature of MLSS for ke2

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**Other Kinetic Relationships**

2. The Monod Equation = growth rate constant, time-1 max = maximum growth rate constant, time-1 S = substrate concentration in solution Ks = 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:**

dX/dt = rate of cell production X = number or mass of microbes present = growth rate constant

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**Generalized substrate consumption and biomass growth with time.**

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