1. Activated Sludge Processes

2. 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

4. 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

5. 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

6. 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)

7. 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)

8. 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

9. 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

10. 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

12. 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

13. 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

14. 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

15. 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

16. 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

18. Example on Biochemical Kinetics

21. 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)

22. 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

23. 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

24. 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

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

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

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

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

29. 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

31. 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 (%)

33. 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 (%)

35. 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

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

37. 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 (%)

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

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

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

42. 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

44. 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

45. 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

47. 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

48. 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

50. 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

52. 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

53. 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

54. 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.

55. 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

57. Generalized substrate consumption and biomass growth with time.