Environmental engineering

1. UNIT –III
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THEORY & DESIGN OF SECONDARY TREATMENT UNITS:
 Introduction to unit operations and processes for
secondary treatment.
 Principles of biological treatments,
 important microorganisms in waste water & their
importance in waste water treatment systems,
 bacterial growth, general growth pattern, growth in
terms of bacterial numbers and bacterial mass.
 Kinetics of biological growth, cell growth, substrate
limited growth, cell growth and substrate utilization,
effect of endogenous metabolism.

2. UNIT OPERATIONS AND UNITPROCESSES
FOR SEWAGE TREATMENT
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 Physical Unit Operations
 Involves physical change such as separation,
evaporation, filtration, sedimentation, adsorption,
absorption etc.
 Chemical Unit Processes
 Involves chemical reactions such as neutralization,
coagulation, etc.
 Biological Unit Processes
 ASP
 Oxidation pond
 Stabilization pond
 Biological filtration

3. CLASSIFICATION OF
SEWAGE TREATMENT
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 Preliminary Treatment
 Primary Treatment
 Secondary Treatment
 Tertiary Treatment

4. BIOLOGICAL PRINCIPAL
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 Under proper environment condition, the soluble
organic substances of the waste are completely
destroyed by biological oxidation.
 The end products of the metabolism are either
gas or liquid.

5. BIOLOGICAL
UNIT PROCESSES
 Classification of biological treatment:
 Aerobic processes
 Anaerobic processes
 Anoxic processes
 Facultative processes
Further subdivided:
 Suspended growth processes
 Attached growth processes
 Combined growth processes 6

6. MICROORGANISM IN ACTIVATED SLUDGE PROCESS
Create a very rich environment
for growth of a diverse
microbial community

7. MICROOROGANISMS IN TRICKLING FILTER

8. BACTERIALGROWTH
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 Log and exponential growth phase
 Declining or retarded growth phase
 Endogenous growth phase or death phase

9. OXYGEN REQUIREMENT IN AEROBIC PROCESSES
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 Oxygen requirement will be equal to the amount
that would be required to remove all the BOD by
oxidation for the fraction of BOD removed by
sludge wasting.
C5H7NO2 + 5O2 5CO2+2H2O+NH3
113(Cells) 5(32)
kg O2/kg cells= 160/113=1.42
Therefore ,
BOD of cells=1.42(mass of cells)

10.  Theoretical oxygen requirement-
kg O2/day=(total mass of BOD
utilized
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kg/day-1.42(mass of organisms wasted,
kg/day)
Kg O2/day =Q(SO-S)/F-1.42(QWXR)
XR=micro-organism conc. in the waste.
F=conversion factor for converting BOD5 to
BODL=(1/1.470)=0.68
This allows only for carbonaceous BOD removal.

11. TREATMENTS
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 Activated sludge processes
 Trickling filter process

12. ACTIVATED SLUDGE PROCESSES
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 Activated sludge- is the sludge which is obtained
by settling sewage in presence of abundant
oxygen.
 RESULT=
 OM present in sewage is oxidised
 Suspended and colloidal matter coagulate and
form flocculent masses which are readily
settleble.

13. WORKING
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 The organism oxidize a portion of the OM present
in the sewage to CO2 and H2O and other end
products and energy
 Synthesize the other portion OM and convert into
new microbial cell tissue using energy.
 Endogenous respiration.

14. OPERATION AND UNITS
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 Mixing regime.
 Plug flow.
 Completely mixed flow.
 Flow scheme

15. METHODS OF AERATION
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 Diffused air aeration
 Mechanical aeration
 Combined aeration

16. LOADING RATE OF AERATION TANK
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 Hydraulic retention time.
 Volumetric BOD loading.
 Food to micro-organism ratio.
 Mean cell residence time.

17. SLUDGE PRODUCTION AND PROCESS CONTROL
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 Rate of sludge production.
 Sludge volume index.

18. TYPES OF ACTIVATED SLUDGE PROCESSES
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 Conventional Activated sludge processes
 Tapered aeration process
 Step aeration process

19. OPERATIONAL DIFFICULTIES
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 Rising sludge
 Bulking of sludge

20.  Advantages
o Clear sparkling effluent of high quality.
o Process requires small area of land.
o There is freedom from fly and odour nuisance.
o Process is highly efficient. Removal of SS,BOD
and bacteria around 90% each.
o Low cost of installation than trickling filters.
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21.  Disadvantages
o If there is sudden increase in the quantity of
sewage, effluent of poor quality is obtained.
o Operation cost of the process is high.
o Large quantity of wet sludge is obtained at the
end process.
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22. THANK YOU
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