Anaerobic digestion

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anaerobic digestion,types of anaerobic digesters,factors influencing anaerobic digestion

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<ul><li> 1. Anaerobic DigestionBy Sreeremya.S</li></ul> <p> 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, qC, 1/ When the bacteria are removed from thereactor faster than they can grow, HRT islimited. For methanogenic bacteria, 3 to 5days at 35C 4. Conventional Standard RateAnaerobic Digester 5. High Rate, Continuous-Flow,Stirred Tank Single StageDigester 6. Two-Stage Anaerobic Digester 7. Standard Rate AnaerobicDigester 8. Tank Design Cylindrical: 20~125 ft in diameter, &gt; 25 ft deep, conicalbottom with a min. slope of 1 vertical to 4 horizontal;awaffle shape bottom to minimize grit accumulationand to reduce the need for digester cleaning Rectangular: rarely used due to poor mixing and deadzones Egg-shape: eliminate cleaning, better mixing, bettercontrol of the scum layer, and smaller land arearequirements; steel or reinforced concrete; due tobetter mixing devices, the use may not be justified. Provide ways of cleaning out grit and scum Door at ground level 9. Egg Shape AnaerobicDigester 10. Mixing Device Gas injection Unconfined: collect gas at the top of the digesters, compress thegas, and then discharge the gas through a pattern of bottomdiffusers 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 andthen discharge through confined tubes; suitable for fixed, floating, orgas holder covers Mechanical stirring: low speed turbine or mixers, suitable for fixedor floating covers Mechanical pumping: propeller-type pumps mounted in internal orexternal draft tubes or axial flow or centrifugal pumps and pipinginstalled 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 gasholders or at high pressure in pressure vessels. Used as fuel for boiler and internal combustion engines,which are in turn used for pumping wastewater, operatingblowers, 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: &gt; 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 Upsetsand Unstable Anaerobic Digesters Conditions Hydraulic overload Organic overload pH changes Temperature fluctuations Toxicity Large withdrawal of sludge Sudden changes ExampleOverpumping of dilute feedsludgeOverpumping of concentratedfeedsludgeDrop in pH (</p>