Anaerobic digestion is one of the older processes used for sludge stabilization. In this process, the decomposition of organic and inorganic matter occurs in the absence of molecular oxygen.
In anaerobic digestion, the organic matter contained in the mixture of primary and secondary sludge is converted biologically (under anerobic conditions) into methane (CH4) and carbon dioxide (CO2). The digestion is carried out in mesophilic (35°C) and thermophilic conditions (55°C).
The process is performed in a fully closed reactor; the sludge is introduced continuously or intermittently and is retained in it for variable periods of time.
The stabilized sludge that is continuously and intermittently extracted is not putrescible and its content in pathogenic organisms is low.
Currently, there are two types of digesters: low and high load. In the process of low-load digestion, the content of the digester in not heated or mixed, generally. The retention time varies between 30 and 60 days.
On the other hand, in the high-load digestion, the content of the digester is heated and completely mixed. The required retention time is 15 days or less.
The main applications of the anaerobic digestion are still being the stabilization of concentrated sludge produced in wastewater treatment and of certain industrial waste. However, it has recently been demonstrated, that diluted inorganic waste can be treated anaerobically.
Since the last century the demand for energy has been increasing due to the progress and excessive demand of resources by humanity. For this reason, alternatives for obtaining energy have been searched like anaerobic digestion of solid waste (DARSO). Likewise, due to the great problem that countries face due to the disposal of solid waste, alternatives have been sought to accelerate its processes of degradation and stabilization and thus reduce environmental risks.
Anaerobic digestion is a process that takes place naturally at solid waste disposal sites. However, it is a slow process, therefore, anaerobic digesters are necessary to allow the loading and unloading of materials, as well as having a device for the collection of the gas produced, which can be stored in a gasometer to be used or burned in an incinerator.
The main characteristic of a digester is its size, which is determined by three independent variables, 1) concentration of unpleasant solids, 2) feed rate of solids and 3) Retention time of the solids in the digester.