Aerobic and Anaerobic
Reactor Configurations
Biochemical Environment
 Aerobic Conditions: Oxygen is used as
electron acceptor
 Anoxic Conditions: Nitrate is the electron
acceptor
 Anaerobic Conditions: absence of oxygen
and nitrate and organic compounds are
converted to biomass, CO2, CH4 and H2S.
Organic matter + H2O
CH4 + CO2 +
NH3 + H2S+ new cell
Why anaerobic Treatment
 Process stability
 Produced Methane can be used to produce energy
 Produced amount of excess sludge is about 10 % of





aerobic treatment. Hence, reduction of waste disposal
cost
Low nutrient requirement (BOD/N/P is 100/5/1 for
aerobic; 700/5/1 for anaerobic mo.s
No air supplementation, so lower operational cost
No off-gas air pollution
Biodegradation of aerobic non-biodegradable
Seasonal treatment is appropriate
Possible disadvantages of anaerobic
Treatment
 long startup
 Alkalinity should be sufficient
 Under mesophilic conditions, optimum
temperature is 35 ºC
 Nitrification not possible
 Low kinetic rates at low temperature
 If COD < 1000 mg/L anaerobic treatment is not
practical economically
 Effluent from anaerobic treatment is generally not
acceptable for direct discharge and aerobic
polishing step is needed.
wastewater
Waste ?
Aerobic
Bioreactor
Equilization
basin
Anaerobic
Bioreactor
Anaerobic Reactor Configuratios
Anaerobic Contact Stabilization:
CSTR with cell recyle under anaerobic conditions.
 Well-mixed reactor
 suspended growth
 Before settling tank a gas separator is used to make
settling easier
Advantages:
Simple and inexpensive
Disadvantages:
System is not stable for shock loading and toxic
compounds
Settling is problem
Anaerobic Filters
Attached growth systems
In this type of reactor, waste enters in the
bottom and flows through the rocks or plastic
media used for biomass immobilization.
Recirculation is used to dilute any toxic
compound in the influent.
The main limitations of reactor :
accumulation of solids in the packing material
(plugging). So, wastes containing high amount
of suspended solids are not suitable for A.F
channeling
cost of packing material
Table.1. Some Studies on the treatment of industrial
wastewaters using A.F.
Type of
Tempr.
wastewater (°C)
Organic
HRT (h)
Loading
Rate
(kg/m3.d)
Removal
efficiency
Sugar
industry
Distillery
wastes
Chemical
process
leachate
35-37
-
12-36
55%
35
15
72
90
37
12-15
22-30
80-90
37
0.2-0.7
30-40 d
90-96
Upflow Anaerobic Sludge Blanket Reactor
(UASB)
This type of reactor was developed to avoid the main problems of
the anaerobic filter.
Flow is in upward direction.
Biomass settles in the bottom usually in the forms of granule.
Advantages:
High biomass concentrations. Hence high organic loading rates can be
applied.
so excellent COD removals due to high biomass concentrations.
Compared to Anaerobic filter, wastewaters with higher suspended solid
concentrations can be applied.
Disadvantages:
Sludge granulation is complex and not fully understood process
Biomass escape at the effluent at higher loading rates hybrit reactors are
used to avoid this problem)
Fluidized and Expended Bed Reactors
gas
Effluent
ORP, pH
probes
Recirculation
pump
High Recycle ratios are used to
keep the particles in suspension
Water-jacketed
glass reactor
carrier
Glass
beads
wastewater
In these systems microorganisms
grow on small inert particles
such as fine sand or activated
carbon
The rate of liquid flow and the
resulting degree of expansion of
the bed (10-25%) determine
whether the reactor is a fluidized
or an expanded (less expansion)
bed reactor
limitation: high and uniform
upflow so high pumping cost
Table 2. Some treatability studies using UASB
Type of
Influent
wastewat COD
er
OLR
Temperat HRT (h)
(kg/m3/d) ure C
Beer
industry
Agricultu
ral waste
Slaughter
hause
Paper
industry
Sugar
industry
10001500
11 000
4.5-7
20-24
5
% COD
removal
Efficienc
y
75-80
2-5
30
48
70-65
20003500
1000
4
30
19
85
5
-
49
75
400060000
20-25
28-32
-
92-95
Aerobic Treatment Systems
 Activated sludge Process (CSTR with/wo
cell recycle)
 Contact Stabilization
 Oxidation Ditch
 Sequencing batch reactor (SBR)
 Extended Aeration
 Step feed
Contact stabilization
wastewater
mixing
Contact Basin
Secondary
clarifier
Stabilization Tank
Air
Sludge waste
Fill
SBR
 No settling tank, no sludge pumping
React
 Aerobic/anoxic/anaerobic cycles for
nutrient removal
 process flexibility for bulking
Settle
sludge
 Tolerant to shock loading
 No washout
Draw
Idle
Oxidation Ditch
Nitrification and denitrification is also possible
Typically operate in an extended aeration mode