MICROBIOLOGICAL PATHWAY OF ANAEROBIC DIGESTION
Extracellular
Enzymes
COMPLEX
ORGANICS
SOLUBLE
ORGANICS
Hydrolysis
Carbohydrates
Proteins
Lipids
Phosphorylated Organics
ORGANIC
ACIDS
Acid Producers
Glucose
Amino Acids
Fatty Acids
PO4 -3
METHANE
CO2
Methanogens
Acetic
Propionic
Lactic + Cell
Cells
Stabilized Organics
Figure by MIT OCW.
Adapted from: WEF. "Wastewater Treatment Plant Design. Water Environment Federation." Alexandria,
Virginia, 2003.
50
6.0
40
CO2 in Digester Gas, Percentage
Limits of Normal
Anaerobic Treatment
pH = 6.2
6.8
6.6
7.2
7.0
30
7.4
8.0
6.4
20
8.2
7.8
7.6
10
8.6
8.4
8.8
0
250
500
1000
2500
5000
10,000
25,000
Bicarbonate Concentration as CaCO3, mg/L
Relationship between pH, bicarbonate concentration, and carbon dioxide concentration at 35oC and 1 atm.
Figure by MIT OCW.
Adapted from: Sawyer, C. N., P. L. McCarty, and G. F. Parkin. Chemistry for Environmental Engineering
and Science. 5th ed. Boston, MA: McGraw-Hill, 2003.
SUBSTANCE
MODERATELY INHIBITORY
CONCENTRATION, mg/L
STRONGLY INHIBITORY
CONCENTRATION, mg/L
Na+
3500 - 5500
8000
K+
2500 - 4500
12000
Ca++
2500 - 4500
8000
Mg++
1000 - 1500
3000
Ammonia-nitrogen (pH dependent)
1500 - 3000
3000
200
200
Sulfide (un-ionized gas)
0.5 (soluble)
Copper (Cu)
Chromium VI (Cr)
Chromium III
Nickel (Ni)
Zinc (Zn)
50-70 (total)
3.0 (soluble)
200-250 (total)
180-420 (total)
2.0 (soluble)
30.0 (total)
1.0 (soluble)
Toxic and Inhibitory Concentrations of Selected Inorganic Materials in Anaerobic Digestion
Figure by MIT OCW.
Adapted from: WEF. Wastewater Treatment Plant Design. Water Environment Federation, Alexandria,
Virginia, 2003.
LOW-RATE ANAEROBIC DIGESTION
Digester gas
SCUM
Feed sludge
SUPERNATANT
Effluent to
treatment plant
ACTIVE LAYER
STABILIZED SOLIDS
Digested solids
Figure by MIT OCW.
Adapted from: WEF. Wastewater Treatment Plant Design. Water Environment Federation, Alexandria,
Virginia, 2003.
Section Through an Anaerobic Digester with a Floating Cover
Gas dome
Gas line
Overflow
Freeboard
Roller
Hatch
Composition roof hatch
Supernatant
withdrawal
Gas line
Side water
depth (swd)
Ceiling plate (1/4 in.)
(6.5 mm)
Landing bracket
Supernatant gas
Sludge inlet pipe
Sludge to recycle
Sludge in
Sludge in
Concrete support
Digested sludge out
6
1
Figure by MIT OCW.
Adapted from: Reynolds, T. D., and P. A. Richards. Unit Operations and Processes in
Environmental Engineering. 2nd ed. Boston, MA: PWS Publishing Company, 1996, p. 582.
TYPES OF MIXING SYSTEMS FOR ANAEROBIC DIGESTERS
Gas
Gas
Compressor
A
Gas recycle & draft tube
Gas
C
Drive
B
Gas recycle & gas injection
Drive
Gas
D
Impeller & draft tube
Compressor
Impeller
Figure by MIT OCW.
Adapted from: Reynolds, T. D., and P. A. Richards. Unit Operations and Processes in
Environmental Engineering. 2nd ed. Boston, MA: PWS Publishing Company, 1996, p. 584.
EGG-SHAPED ANAEROBIC DIGESTER
Mixer
Feed
Hopper
Elevator
Platform
21 m
Tower
Supernatant
withdrawal
Pipes
Pipe area
Platform to other
digesters
Scum door
20 m
DIA.
Piping to
withdrawal
bottom sludge
Gas
spargers
o
37
8.8 m
Figure by MIT OCW.
Mixing Systems For Egg-Shaped Anaerobic Digesters
Foam
Separator
Alternate biosolids
wasting to storage tank
Scam Breaker
Screw-impeller sludge
mixing pump with
draft-tube assembly
Raw sludge
influent
Heat
exchanger
Digested
biosolids
overflow
Gas injection
lines
To gas utilization
Solids recirculation
header
Gas header
Nozzle for mixing
Unconfined gas mixing
Mechanical draft-tube mixing
Pumped recirculation mixing
Figure by MIT OCW.
Adapted from: WEF. Wastewater Treatment Plant Design. Water Environment Federation, Alexandria,
Virginia, 2003.
Polymer
Static Box
Upper belt
Sludge feed
Alignment roller
Gravity drainage zone
Belt tension roller
Wedge zone
Belt wash
Lower belt
High pressure zone
Cake discharge
belt conveyor
Filtrate & wash-water collection
A
Figure by MIT OCW.
Adapted from: Hammer, M. J., and M. J. Hammer, Jr. Water and Wastewater Technology.
5th ed. Upper Saddle River, NJ: Prentice-Hall, Inc., 2004. Figure 11-56.