What is Anaerobic Digestion?


Anaerobic Digestion (AD) is a process used to break down “organic” based waste materials by bacterial micro-organisms (“Bugs”), in large sealed tanks, in the absence of oxygen. The organic material for the proposed unit is pre-processed and then fed into the ‘fermentation’ process which is carried out in very large agitated tanks to produce “Bio-Gas”, Digestate and Waste Water.

The “Bio-Gas” that is produced contains Carbon Dioxide, highly flammable Methane gas and other gases, including toxic Hydrogen Sulphide (the “Rotten Egg” smell) and Ammonia. “Bio-Gas” is processed into fuel to produce electricity and heat in a Combined Heat and Power (CHP) engine unit. The “Bio-Gas” can also be further refined to improve the quality and made into Bio-Methane. This is then sold and injected into the public gas supply distribution system.

If the “Bio-Gas” cannot be processed, then it can be burnt in a flare system, a large chimney on site and released to the atmosphere. What is left after this process is known as Digestate, which is left over indigestible material and dead micro-organisms but rich in nutrients and so it can be used as fertiliser. The volume of digestate will be around 90-95% of what was fed into the digester the remainder is waste water that is released into the local water course.