Anaerobic Digestion

By Sreeremya.S

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

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

Conventional Standard RateAnaerobic Digester

High Rate, Continuous-Flow,Stirred Tank Single Stage

Digester

Two-Stage Anaerobic Digester

Standard Rate AnaerobicDigester

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

Egg Shape AnaerobicDigester

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

High-Rate Anaerobic Digester

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.

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

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

Conditions Responsible for Upsetsand Unstable Anaerobic Digesters

• Conditions• Hydraulic overload• Organic overload• pH changes• Temperature• fluctuations• Toxicity• Large withdrawal of• sludge• Sudden changes

• ExampleOverpumping of dilute feed sludgeOverpumping of concentrated feedsludgeDrop in pH (<6.8) and loss of alkalinityOverpumping of feed sludgeSpecific inorganic and organic wastesExcess withdrawal of sludge and reducedretention timeRapid increase in nitrate ionconcentration

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.