ANAEROBIC DIGESTION

ANAEROBIC DIGESTION

Anaerobic digestion is a series of microbiological processes that 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.

The digestion takes place thanks to four different types of bacteria which operated at

different stages of the digestion process:

HYDROLYTIC BACTERIA

The hydrolytic bacteria break down complex organic waste into sugars and amino acids.

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.

FERMENTATIVE BACTERIA The fermentative bacteria turn sugars

and amino acids into organic acids.

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).

ACIDOGENIC MICRO ORGANISMS

The acidogenic micro organism converted organic acids into hydrogen, carbon dioxide and acetic acid.

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.

METHANOGENIC BACTERIA

Produce biogas from acetic acid, carbon dioxide and acetate.

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.

There a three traditional main types of digester , and more recently developed one which is promising but not yet largery tested:

- They have different building;- Operating and maintaining cost;- Each one can digester manure with a

specific total solid content.

COVERED LAGOON

It consist of a manure storage lagoon with an air-tight impermeable floating cover of industrial fabric that traps the gas produced during decomposition of the manure and a suction pipe which extracts the gas for use.

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%.

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.

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%.

TPAD

Temperature-phased anaerobic digester (TPAD) has a potential to enhance digestion of the lignocellulose present in the solids and improve biogas production.

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.

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.