Biological treatment, activated sludge

1. FUNDAMENTALS OF ENVIRONMENTAL SCIENCE AND
ENGINEERING
Lecturer: Assoc. Prof. Nevin Yağcı
Environmental Engineering Department (140611)
yagcin@itu.edu.tr
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2. 6th week
The Biological Basis of
Wastewater Treatment
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3. Content
• Biological wastewater treatment
• Role of microorganisms
• Types of biological treatment processes
• Aerobic process
• Anaerobic Processes
• Nutrient Removal
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4. 3

5. Biological wastewater treatment
• In the early years of the twentieth century the method of
biological treatment was devised, and now forms the basis of
wastewater treatment worldwide.
• It simply involves confining naturally occurring bacteria at
very much higher concentrations in tanks.
• The bacteria remove small organic carbon molecules by
‘eating’ them.
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6. Biological wastewater treatment
• As a result, the bacteria grow, and the wastewater is cleansed.
• The treated wastewater or effluent can then be discharged to
receiving waters – normally a river or the sea.
• These bacteria, together with some protozoa and other
microbes, are collectively referred to as activated sludge.
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7. Biological wastewater treatment
Objectives of biological treatment
• Reduce organic content
• Removal/reduction of nutrients
• Removal/inactivation of pathogenic microbes
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8. Role of microorganisms
Convert the colloidal and dissolved carbonaceous organic
matters into various gas and cell tissue.
The resulting cells can be removed by settling.
Bacteria eat organic compounds and
nutrients present in the wastewater
Nitrogen
Phosphate
Various gases (like CO2)
H2O
New bacteria
Bacteria convert C, N, P and etc. into
gases and new bacteria 7

9. What are bacteria?
Bacteria are
microscopic living
organisms, usually
one-celled, that can be
found everywhere.
Reproduce very fast
Some are good, some
bad.. (majorty are
harmless or even
beneficial)
First observed by Anton Van
Leeuwenhoek in 1676
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10. What are bacteria?
Usually reproduce
asexually using the
process of binary
fission
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11. Role of microorganisms
Biological Metabolism
Metabolism is the set of chemical reactions that happen in the
cells of living organisms to sustain life.
Metabolism is usually divided into two categories.
Catabolism breaks down organic matter, for example to harvest
energy in cellular respiration.
Anabolism uses energy to construct components of cells such
as proteins and nucleic acids.
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12. Aerobic process
A. Suspended – growth
- Activated sludge process
- Aerated lagoon
- Aerobic digestion
B. Attached growth
- Trickling filters
- Rotating biological Reactors
- Packed-bed reactors
Activated sludge is commonly used aerobic biological
treatment process
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13. Aerobic process
Activated sludge process
• Activated sludge – sludge
particles produced in
wastewater by the growth of
organisms in aeration tanks.
• The term ‘activated’ comes
from the fact that the particles
teem with bacteria, fungi, and
protozoa.
• Activated sludge process –
speeds up waste
decomposition which is very
slow in the nature.
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14. Aerobic process
Activated sludge process
Activated sludge is added to
wastewater, and the mixture is
aerated and agitated. After a
certain amount of time, the
activated sludge is allowed to
settle out by sedimentation
and is disposed of (wasted) or
reused (returned to the
aeration tank)
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15. Aerobic process
Activated sludge process
Basic Components of Activated
Sludge Systems
1. A reactor: the microorganisms are
kept in suspension and aerated
2. Liquid – solid separation: usually
in sedimentation tank
3. A sludge recycle system for
returning solids from
the liquid-solid separation unit
back to the reactor
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16. Types of biological treatment processes
1. Aerobic process
2. Anoxic processes
3. Anaerobic processes
---------------------------------------------------------------------------------
Aerobic – a condition where oxygen is present
Anoxic & Anaerobic – a condition where oxygen is not present
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17. Aerobic process
Activated sludge process
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18. Anaerobic Processes
Degradation of the organic load to gaseous products (mainly CH4
and CO2)
Reactions occur in anaerobic treatment
Organic load  soluble organic material
acid producing bacteria
volatile fatty acid, CO2, H2
methane producing bacteria
CH4, CO2
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19. Nutrient Removal
Wastewater may contain high levels of
the nutrients nitrogen and phosphorus.
Excessive release to the environment
can lead to a build up of nutrients,
called eutrophication, which can in turn
encourage the overgrowth of
weeds, algae, and cyanobacteria (blue-
green algae).
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20. Nutrient Removal
Nitrogen removal
Nitrification:
Biological oxidation of nitrogen
from ammonia to nitrate
Denitrification: The reduction
of nitrate to nitrogen gas
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21. Nutrient Removal
Nitrogen removal
Nitrification: from ammonia to nitrate
In the first step of nitrification, ammonia-oxidizing bacteria oxidize
ammonia to nitrite according to equation below.
NH3 + O2 → NO2
- + 3H+ + 2e-
In the second step of the process, nitrite-oxidizing bacteria oxidize
nitrite to nitrate according to equation below.
NO2
- + H2O → NO3
- + 2H+ +2e-
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22. Nutrient Removal
Nitrogen removal
Denitrification: from nitrate to nitrogen gas
6 NO3
- + 2 CH3OH → 6 NO2
- + 2 CO2 + 4 H2O (Step 1)
6 NO2
- + 3 CH3OH → 3 N2 + 3 CO2 + 3 H2O +6 OH- (Step 2)
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23. Nutrient Removal
Nitrogen removal
The removal of nitrogen is effected through the
biological oxidation of nitrogen
from ammonia to nitrate (nitrification), followed
by denitrification, the reduction of nitrate to nitrogen gas.
Nitrogen gas is released to the atmosphere and thus removed
from the water.
Nitrification  Denitrification
NH3 → NO3
- NO3- → N2
aerobic condition anoxic condition 22

24. Anoxic reactors
(denitrification)
(no aeration)
Aerobic reactors
(nitrification)
(aeration required)
Nitrogen removal system
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25. Nutrient Removal
Phosphorus removal
Phosphorus can be removed biologically (enhanced biological
phosphorus removal – EBPR) or chemically.
Phosphorus Removal
Chemical Biological (BEPR)
*Traditional *N+P removal
*Well known *Cheap
*Chemical sludge production
*Expensive
*Supplement to EBPR
*Reliable
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26. Nutrient Removal
Chemical P removal
Chemical precipitation is used to remove the inorganic forms of
phosphate by the addition of a coagulant and a mixing of
wastewater and coagulant.
The multivalent metal ions most commonly used are calcium,
aluminium and iron.
M PO MPO
 
  
3
4
3
4
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27. Nutrient Removal
Biological P removal
In this process, specific bacteria, called polyphosphate
accumulating organisms (PAOs), are selectively enriched and
accumulate large quantities of phosphorus within their cells (up
to 20 percent of their mass).
Sequential anaerobic–aerobic operation of activated-sludge
processes is applied to achieve enhanced biological phosphorus
removal (EBPR)
Anaerobic Aerobic
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28. Nutrient Removal
P removal mechanism
Energy
PO4-P
*Soluble
organics
O2
CO2+H2O
energy
PO4-P
settling
anaerobic aerobic
Storage material (PHB)
Polyphosphate volutine granules
VFAs (like acetic acid) *
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29. ANAEROBIC
TANK
AEROBIC
TANK
SETTLER
INFLUEN
T Effluent
Excess sludge
Returned sludge
Biological phosphate removal
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