BIOLOGICAL TREATMENT
Objective
remove or reduce the concentration of organic and
inorganic compounds
transform (i.e., oxidize) dissolved and particulate
biodegradable constituents into acceptable end products
capture and incorporate suspended and nonsettleble
colloids to a biological floc or biofilm
transform or remove N, P
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
Microbial metabolisms require :
• source of energy : Light, chemical oxidation rxn.
• carbon for the synthesis of new cellular
material : organic matter, CO2
• nutrients : N, P, S, K, Ca and Mg
“Heterotrophs” : organisms that use organic C
“Autotrophs”
Classification
: organisms that use CO2
Energy source
Heterotrophic :
- Photoheterotrophic
- Chemoheterotrophic
Light
org. Ox. - Red rxn.**
Autotrophic :
- Photoautotrophic
- Chemoautotrophic
Light
inorg.Ox.– Red. Rxn**
** used in wastewater treatment
Type of microbial metabolism
- Emphasize on chemoheterotrophic organisms
1) . Respiratory metabolism
- organisms generate energy by enzyme mediated etransfer from an e- donor to an external e- acceptor
2) Fermentative metabolism
- does not involve e- acceptor
Environment
e- acceptor
Process
Aerobic
O2
Aerobic metabolism
Anaerobic
NO3-
Denitrification
(Anoxic process)
SO42-
Sulfate reduction
CO2
Methanogenesis
Types of biological treatment processes
1. Aerobic process
2. Anoxic processes
5 major groups
3. Anaerobic processes
4. Combined processes
5. Pond or lagoon processes
Individual process is further subdevided :
- Depends on method of use
• Suspended-growth processes
• Attached-growth processes
• Combination
Suspended growth processes:
The microorganisms responsible for treatment
are maintained suspension by appropriate
mixing methods.
Attached – growth processes :
The microorganisms are attached to some inert
medium, such as rock, slag or specially designed
ceramic or plastic materials.
(also called Fixed-film processes)
AEROBIC PROCESSES
A. Suspended – growth
- Activated sludge process
- Aerated lagoon
- Aerobic digestion
B. Attached growth
- Trickling filters
- Rotating biological Reactors
- Packed-bed reactors
C. Combination suspended / attached growth
- Trickling filter/activated sludge
A. SUSPENDED GROWTH PROCESS
a). Activated sludge process
Important feature :
convert soluble and insoluble organic and inorganic
1.
compounds
into
flocculent settable microbial suspension
2.
Remove the biomass by gravity settling
(solid – liquid separation)
Process description
1.
Org. Matters
Reactor (suspension of bacteria)
(oxidation and synthesis)
Organic matters (C,O,H,N,S) + O2 + nutrients
bacteria
CO2 + NH3 + C5H7NO2 + other end products
(new cells)
5 O2 + C5H7NO2
bacteria
5CO2 + 2H2O + NH3 +energy
“Endogenous respiration”
2.
After a specific period of time
Mixture of
new + old cells
Settling tank
Treated water
Cells (sludge)
Reactor
waste
Cells are recycled to maintain the desired
concentration of organism in reactor
3 Basic components
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
Types of reactors
i. Plug flow activated sludge
ii. Complete-mix activated-sludge reactor
iii. Sequencing batch reactor
i. Plug flow activated sludge
- Uses long and narrow aeration basins
- Mixing regime approaches plug flow
flow
influent
clarifier
Recycled sludge
Beginning of aeration basin
- High rate of O2
Utilization
- Oxidation of org.
- synthesis of cells
End of basin
- O2 utilization rate
approaches the
endogenous level
- Oxidation of new
cells
Plug flow system
ii. Complete mix activated sludge
Influent + recycled sludge are introduced
to the aeration basin
Contents are mixed completely
Separation of solid – liquid
In sedimentation tank
Complete mix sludge system
iii. Sequencing batch reactor
Single vessel is used to accommodate
All of the unit processes
Processes are timed sequences
1.
2.
3.
4.
Fill
Aeration
Settle
Decant
b). Aerated lagoons
- An aerated lagoon is a basin of significant
Depth e.g. 8-16 ft (2.4 – 4.9 m) deep
- Oxygenation
- 2 types
mechanical aeration
diffused aeration
induced surface aeration
Aerobic lagoon
Facultative lagoon
(Aerobic-Anaerobic lagoon)
- Aerated lagoon systems can be multiple basins
i.e. Aerobic Facultative settling
• Aerobic lagoon
- dissolved O2 and suspended solid are
maintained uniformly throughout the basin
- no solid recycle
• Facultative lagoon
- O2 is maintained in the upper liquid layer
- only a portion of solids is maintained in
suspension
-The processes’ design depends on
o equipment
o Available area
o site specific condition and
o treatment needs
B. ATTACHED GROWTH PROCESSES
a). Trickling filter
- A bed of a highly permeable medium to which
microorganisms are attached
- Medium can be rock, slag or plastic
Process description
Trickling filter system
Trickling filter system
Lakeside Equipment
Corporation
b). Rotating biological contractors
- A series of closely spaced circular disk of
polystyrene or polyvinyl chloride
- Biological growths is attached and grown on
the surface for 1-4 mm.
-The disks are submerged in wastewater (40%
submerged) and rotated slowly through it
(1-3 rpm).
Rotating biological contractor.
Process description
As the contractor rotates
When exposed to air the attached biomass
absorbs air
and when immersed the microorganisms
absorb the organic load.
Resulting in BOD removal
Selection of aerobic treatments
Several factors affecting the choice of processes
1. OPERATING CHARACTERISTICS
System
Resistance to
shock loads of
organics or toxics
Sensitivity to
intermittent
operations
Degree of
skill
needed
Lagoons
Maximum
Minimum
Minimum
Trickling
filters
Moderate
Moderate
Moderate
Activated
Minimum
Maximum
Maximum
(b) COST CONSIDERATIONS
System
Land needed
Initial costs
Operating
costs
Lagoons
Maximum
Minimum
Minimum
Trickling
filters
Moderate
Moderate
Moderate
Activated
Minimum
Maximum
Maximum
Adapted from Rich, 1980.
The considerations for the RBC systems are similar to those
of trickling filters.
ANAEROBIC PROCESSES
The anaerobic treatment
- 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
Insoluble org.
(protiens, lipid)
Hydrolytic and
Fermentative bacteria
Org. Material
(soluble)
Volatile fatty acids,
CO2, H2
CH4 producing bac.
CH4
CO2, H2
cells
cells
Acid producing bac.
cells
Endogenous met.
Cell lysis
anaerobic digestion process