Factors that affect anaerobic sludge digestion
- 2% TS sludge has twice the water volume of 4% TS sludge.
□ Additional water pumped to a digester requires additional heat energy to maintain
the digester temperature. - Additional water lowers the detention time in the digester. Pumping 4% TS sludge
provides double the detention time compared to sludge pumped at 2% TS.
Mixing of an anaerobic digester has two important functions
- Distribution of food throughout the digester
□ Aids in maintaining uniform temperature throughout the digester
The most effective way to feed an anaerobic digester.
Add raw sludge in small amounts at frequent intervals
Commonly used anaerobic digester process control tests
□ pH
* Volatile acid / Alkalinity ratio
* Digester gas percent CO2
The optimum range for anaerobic digester control parameters.
□ pH: 6.8 - 7.2
□ Volatile acid / Alkalinity’ ratio: 0.1 - 0.5
* Digester gas percent: 30 - 35%
The overall purpose of anaerobic digestion.
- To reduce the solids content and volume of the sludge
- To biologically stabilize the sludge to reduce nuisance odors, pathogens, and
attraction of insects and animals.
Anaerobic digester temperature maximum daily adjustment.
No more than 1 °F per day is recommended
Energy savings provided by anaerobic digester co-generation equipment.
- Methane produced by the digester is the fuel for an electrical generator engine that
supplements the electrical demand of the treatment plant. - Exhaust heat from the generator engine is passed through a heat exchanger to help
maintain the digester temperature.
Symptoms that indicate an anaerobic digester is approaching “sour” condition
- Total alkalinity of the sludge decreases (first indication)
- Volatile acid content of the sludge increases (first indication)
- Methane gas production decreases, while CO2 content of gas increases
- pH drops below 6.0 and possible foaming (final indication of “sour” or upset
digestion)
Measure of the effectiveness or completeness of anaerobic sludge digestion.
Volatile solids reduction, typically at least 38%
Functions of a secondary anaerobic digester
- Provides time for solids to concentrate and a clear supernate to develop
- Provides source of “seed sludge” for digester start-up or recovery
- Provides for additional gas storage
- Provides some additional digestion
Loading to an anaerobic digester is optimum when the ratio is 1:10
Pounds of volatile solids added per day per pound of volatile solids under digestion
Gasses that make up anaerobic digester gas.
- Methane (approx. 70%) and carbon dioxide (approx. 30%)
- Hydrogen, nitrogen and hydrogen sulfide gases are also present in minor amounts
Anaerobic digester control parameter that is the most sensitive to digester conditions and
responds first when the digester is approaching upset conditions.
Volatile acid / alkalinity ratio
Anaerobic digester control parameter that is the least sensitive to digester conditions, and
responds last when the digester is approaching upset conditions.
Digester pH
Equipment associated with a typical anaerobic sludge digester.
A. Provides fire and explosion protection
B. Removes condensed water vapor from digester gas
C. Handles excess or waste digester gas
D. Protects pipes and tank from over or under pressurization
E. Heat transfer to digester sludge
F. Source of heat for digester
A. Flame arrester
B. Water trap
C. Waste gas burner (or perhaps co-generation equipment)
D. Vacuum/pressure relief valve
E. Heat exchanger equipment
F. Boiler (or exhaust gas heat recovery from co-generation equipment)
Actions that may be taken to correct an anaerobic digester that is on the verge of going
“sour.
- Add seed sludge
- Increase the mixing time or mixing intensity
- Maintain constant digester temperature control
□ Add sodium bicarbonate or other form of alkalinity to adjust the pH to the normal
range (Other forms of supplemental alkalinity used to raise digester pH include
anhydrous ammonia, caustic soda and soda ash)
Processes used to thicken and condition sludge prior to digestion.
Gravity thickening - Air floatation thickening - Centrifuge thickening
Poor anaerobic sludge digestion may be indicated by these symptoms.
□ Foul (putrid) odor
* Low pH
□ Sludge attracts flies and rodents
Characteristics of well digested sludge from an anaerobic digester.
- Black color and “musty” or “hot asphalt” odor
□ pH 6.5 - 7.5 - Total solids of2- 4%
□ Volatile solids of 50- 60%
Normal gas pressure in an anaerobic digester.
Maintained at 8 - 12 inches of water column pressure (0.35 - 0.43 psi)
The source of water that accumulates in anaerobic digester gas piping.
Condensation of water vapor from the digester gas
Supernate from an anaerobic digester can cause treatment plant problems due to these
characteristics.
□ High levels of ammonia and phosphate. Ammonia can add large oxygen demand to an
aeration basin that is nitrifying, and ammonia and phosphate can add to the demand on
nutrient removal treatment processes.
* Zero D.O. and high oxygen demand
* High concentration of total solids (1,000 - 2,000 mg/L) <
The explosive danger of anaerobic digester gas is greatly increased when this occurs.
When digester gas is mixed with air. Digester gas becomes explosive at5- 20% of the
total volume of air.
