2. ANAEROBIC DIGESTION Anaerobic digestion is a series ofmicrobiologicalprocessesthat convert organic compounds to methane. Anaerobic digestion con take place only in an air tight chamber kept at a temperature not lower than 20° C. The maximum temperature for a digester is 65°C: such a high temperature reduces processing time and digester dimensions, but the digestion is more prone to upset, so it requires close control.
3. The digestion takes place thanks to four different types of bacteria which operated at different stages of the digestion process:
4. HYDROLYTIC BACTERIA The hydrolytic bacteria break down complex organic waste into sugars and amino acids.
5. Hydrolytic bacteria form a variety of reduced end-products from the fermentation of a given substrate. One fundamental question which arises, concerns the metabolic features which control carbon and electron flow to a given reduced end-product during pure culture, and mixed methanogenic cultures of hydrolytic bacteria.
6. FERMENTATIVE BACTERIA The fermentative bacteria turn sugars and amino acids into organic acids.
7. Fermentation bacteria are anaerobic, but use organic molecules as their final electron acceptor to produce fermentation end-products. Bacteria are a large group of unicellular or multi-cellular organisms which can tolerate different temperatures. While most bacteria have a temperature optimum of between 20 to 30ºC, there are some which prefer higher temperatures (50 to 55ºC) and those with colder temperature optima (15 to 20ºC).
8. ACIDOGENIC MICRO ORGANISMS The acidogenic micro organism converted organic acids into hydrogen, carbon dioxide and acetic acid.
9. Acidogenesis is a biological reaction where simple monomers are converted into volatile fatty acids ; is one of the main reactions of this stage, in this, the intermediary metabolites produced are metabolized to acetate, hydrogen and carbonic gas by the groups of bacteria.
11. Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are common in wetlands, where they are responsible for marsh gas, and in the guts of animals such as ruminants and humans, where they are responsible for the methane content of flatulence.
16. Heating the waste reduces retention time to less than 20 days. It can be used for large manure volumes with concentration of solids from 3% to 10%.
17. PLUG-FLOW Plug Flow is a simple model of the velocity profile of a fluid flowing in a pipe. In plug flow, the velocity of the fluid is assumed to be constant across any cross-section of the pipe perpendicular to the axis of the pipe. The plug flow model assumes there is no boundar layer adjacent to the inner wall of the pipe.
18. The manure remeins in the digester for 20-30 days. It is mosty used for ruminant manure which contains a solid concentration of 11% to 13%.
19. TPAD Temperature-phased anaerobic digester (TPAD) has a potential to enhance digestion of the lignocellulose present in the solids and improve biogas production.
20. It consist of a two-staged reactor, which first operates at a high temperature (57°C), then at a lower temperature (35° C) in order to optimize methane recovery from the manure by enhancing the digestion of lignocellulosic solids -50% of them usually passes thorough anaerobic digester undigested.
21. Anaerobic digester is very important because produce methane for industrial purposes is an effective , reduce greenhause gases, global warming, good for enviromental and producing energy from waste.
