Lecture notes of Environmental Engineering-II as per Solapur university syllabus of TE CIVIL. Prepared by Prof S S Jahagirdar, Associate Professor, N K Orchid college of Engg and Technology, Solapur

1. L-24
Activated Sludge Process
Part-I
Environmental Engineering-II

3. ASP Flow sheet

4. • In a sewage (or industrial wastewater)
treatment plant, the activated sludge
process can be used for one or several of
the following purposes:
1. oxidizing carbonaceous matter: biological
matter.matter.
2. oxidizing nitrogenous matter: mainly
ammonium and nitrogen in biological
materials.
3. removing phosphate.

5. 4. driving off entrained gases carbon
dioxide, ammonia, nitrogen, etc.
5. generating a biological floc that is easy to
settle.
6. generating a liquor(wastewater) that is
low in dissolved or suspended material.low in dissolved or suspended material.

6. • Activated sludge plant involves:
1. wastewater aeration in the presence of a
microbial suspension,
2. solid-liquid separation following aeration,
3. discharge of clarified effluent,
4. wasting of excess biomass, and4. wasting of excess biomass, and
5. return of remaining biomass to the
aeration tank.

7. • In activated sludge process
wastewater containing organic matter
is aerated in an aeration basin in
which micro-organisms metabolize
the suspended and soluble organic
matter.matter.
• Part of organic matter is synthesized
into new cells and part is oxidized to
CO2 and water to derive energy.

8. • In activated sludge systems the new
cells formed in the reaction are
removed from the liquid stream in the
form of a flocculent sludge in settling
tanks.
• A part of this settled biomass,• A part of this settled biomass,
described as activated sludge is
returned to the aeration tank and the
remaining forms waste or excess
sludge.

9. • The general arrangement of an activated
sludge process for removing
carbonaceous pollution includes the
following items:
a) Aeration tank where air (or oxygen) is
injected in the mixed liquor.injected in the mixed liquor.
b) Settling tank (usually referred to as "final
clarifier" or "secondary settling tank") to
allow the biological flocs to settle, thus
separating the biological sludge from the
clear treated water.

10. Primary
treated waste

16. • There are a number of factors that affect the
performance of an activated sludge treatment
system. These include:
1. temperature
2. return rates
3. amount of oxygen available
4. amount of organic matter available4. amount of organic matter available
5. pH
6. waste flow rates
7. aeration time
8. wastewater toxicity

17. Design Criteria
1. HRT(Hydraulic Retention Time):- The
ratio volume of aeration basin to the flow
rate.
HRT in hrs = [V/Q] x 24HRT in hrs = [V/Q] x 24
V= volume in m3
Q= flow rate in m3/day

18. 2. Food to microorganisms ratio (F/M)
• The “food” in the ratio is the BOD
entering the process (BOD load).
• The “microorganisms” are the
activated sludge solids in the aerationactivated sludge solids in the aeration
tanks, which are measured as ppm or
mg/L of MLSS (Xt).
• F/M = (Qx BOD5)/ (V x MLSS)
• = Q.La/V.Xt

19. 3. Volumetric BOD loading
It is ratio of the BOD5 applied per unit
volume of aeration tank.
Volumetric Load = [Q x La ] / V
La = Influent BOD5 to aeration tankLa = Influent BOD5 to aeration tank

20. 4. Sludge age or Sludge retention time (SRT)
• SRT is denoted by θc and is defined by the
equation
θc = X / (∆ X/ ∆ t)
X= total microbial mass in the reactor
(∆ X/ ∆ t) = total quantity of solids withdrawn(∆ X/ ∆ t) = total quantity of solids withdrawn
daily, including solids deliberately wasted
and those in the effluent

21. Derrivation
• Refer Class note book

22. L-25
PART-II
L-25

23. Sludge Volume Index
• Sludge Volume Index (SVI) is a very
important indicator that determines your
control or rate of desludging on how much
sludge is to be returned to the aeration
basin and how much to take it out from the
system.
• It actually serves as a very important
empirical measurement that can be used
as a guide to maintain sufficient
concentration of activated sludge in the
aeration basin.

24. • SVI can actually be determined through
use of standard laboratory test methods to
come up with the results.
• Basically the procedure involves
measuring the Mixed Liquor Suspended
Solids (MLSS) value and also the sludgeSolids (MLSS) value and also the sludge
settling rate.
• A simple explanation on how it is carried
out can be summarized below with
accompanying images for easy reference
and better understanding:

25. 1. Obtain sample of mixed liquor from the pond
discharge pipeline and fill it to a 1 liter graduated
measuring cylinder until the 1.0 liter marking.
2. Allow it to settle for 30 minutes
3. After the time period, read the marking to determine
the volume occupied by the settled sludge and the
reading is expressed in terms of mL/L and this is
figure is known as the Vs value.figure is known as the Vs value.
4. Next, for MLSS, there are actually two approaches to
get the value. A conventional standard approach is by
filtering the sludge, drying it and then weigh the
second portion of the mixed liquid.

27. • Value of Sludge Volume Index can then be
calculated from the formula given here.
Whereby,
• SVI = Sludge Volume Index, mL/g• SVI = Sludge Volume Index, mL/g
SV = Volume of settled solids in one-liter
graduated transparent measuring cylinder after
30 minutes settling period, mL/L
MLSS = Mixed liquor Suspended Solids, ppm

28. • Typically a healthy sludge aeration pond
basin should have the value registered
within 80 to 150 mL/g.
• Sludge volume index is a quality indicator.
It reflects the settling quality of the sludge.
As the SVI increases, the sludge settlesAs the SVI increases, the sludge settles
slower, does not compact as well, and is
likely to result in more effluent suspended
solids.

29. Relation between SVI and
Recirculation rate
• Refer note book