This document summarizes different types of anaerobic digestion systems. It describes single-stage digesters that are unmixed and unheated with retention times of 30-60 days, and high rate single-stage systems that are mixed and heated with shorter retention times of 15 days or less. It also discusses two-stage systems where a second stage can function for solids separation and additional digestion.

1. Anaerobic Digestion
By Sreeremya.S

2. Anaerobic Digestion
• Single-Stage Digester with a Floating Cover
• (Standard Rate)
• Contents Unmixed and Unheated
• Td = 30 to 60 days
• Single-Stage High Rate Digestion System
• Contents Mixed and Heated
• Td = 15 days or less
• Two-Stage Anaerobic Digestion System
• Second Stage Can Function as a Solids Separation Phase
• Additional Digestion Can also occur in this Second Stage

3. Conventional Single Stage Process
• Sludge age = HRT, q
C,μ 1/μ
• When the bacteria are removed from the
reactor faster than they can grow, HRT is
limited. For methanogenic bacteria, 3 to 5
days at 35°C

4. Conventional Standard Rate
Anaerobic Digester

5. High Rate, Continuous-Flow,
Stirred Tank Single Stage
Digester

6. Two-Stage Anaerobic Digester

7. Standard Rate Anaerobic
Digester

8. Tank Design
• Cylindrical: 20~125 ft in diameter, > 25 ft deep, conical
bottom with a min. slope of 1 vertical to 4 horizontal;
a“waffle” shape bottom to minimize grit accumulation
and to reduce the need for digester cleaning
• Rectangular: rarely used due to poor mixing and dead
zones
• Egg-shape: eliminate cleaning, better mixing, better
control of the scum layer, and smaller land area
requirements; steel or reinforced concrete; due to
better mixing devices, the use may not be justified.
• Provide ways of cleaning out grit and scum
• Door at ground level

9. Egg Shape Anaerobic
Digester

10. Mixing Device
• Gas injection
• Unconfined: collect gas at the top of the digesters, compress the
gas, and then discharge the gas through a pattern of bottom
diffusers or through a series of radially placed top-mounted lances;
suitable for fixed, floating, or gas holder covers
• Confined: collect gas at the top of the digesters, compress gas and
then discharge through confined tubes; suitable for fixed, floating, or
gas holder covers
• Mechanical stirring: low speed turbine or mixers, suitable for fixed
or floating covers
• Mechanical pumping: propeller-type pumps mounted in internal or
external draft tubes or axial flow or centrifugal pumps and piping
installed externally, suitable for fixed covers

11. High-Rate Anaerobic Digester

12. Gas Collection
• Collect inside floating or fixed cover.
• Gas and air must not be allowed to mix.
• Need flame traps, pressure/vacuum relief valves.
• Floating cover: used for single-stage digesters or in the
• second stage of two-stage digesters, must allow the volume
• of the digester to change without allowing air to enter the
• digester.
• Fixed cover: gas stored at low pressure in external gas
holders or at high pressure in pressure vessels.
• Used as fuel for boiler and internal combustion engines,
which are in turn used for pumping wastewater, operating
blowers, and generating electricity.

13. Startup
• Fill with primary sedimentation tank effluent
(make sure it is anaerobic)
• 2. Heat it up (with heat exchanger)
• 3. Get tanker truck full of good digester seed
(15% of
• volume for seed)
• 4. Start feeding very slowly ®monitor very closely
• Stimulation
• Possible by addition of trace metals such as Fe,
Co, and

14. Performance Assessment
• 1. VS Reduction: > 50%
• 2. Gas Production: 12~17 ft3 gas/lb VS destroyed
• 3. Gas Composition: CH4 - 65~75%; CO2 - 30~35%
• 4. Gas Traces: N2, H2S (500~1000 ppm), H2
• If N2 increases, there is an air leak.
• H2S: form acid and lead to corrosion; keep down by adding FeCl2
• (FeCl2 + HS- ® FeS(s) + HCl)
• H2S(g)
• «H2S(aq)
• HSHeavy
• metal + S2-
• (aq)
• «Heavy metal sulfide

15. Conditions Responsible for Upsets
and Unstable Anaerobic Digesters
• Conditions
• Hydraulic overload
• Organic overload
• pH changes
• Temperature
• fluctuations
• Toxicity
• Large withdrawal of
• sludge
• Sudden changes
• Example
Overpumping of dilute feed
sludge
Overpumping of concentrated
feed
sludge
Drop in pH (<6.8) and loss of
alkalinity
Overpumping of feed sludge
Specific inorganic and organic
wastes
Excess withdrawal of sludge and
reduced
retention time
Rapid increase in nitrate ion
concentration

16. To Conclude
• There are a large number of different Biomethanation
• systems researched and developed.
• Each has its relative merits, strengths, weaknesses and
• limitations.
• Selection of the appropriate system will depend
largely
• upon the waste material to be processed, available
space,
• capital and operating costs, relative importance of
• energy production/pollution prevention, and so on.