The biological treatment of waste involves the intervention of a range of micro-organisms to change the shape of waste or extract certain components. It mostly applies to waste containing a significant proportion of organic materials enabling the development of active micro-organisms, for example:
- green waste (gardens and green areas),
- sludge from wastewater treatment plants, grease and human waste,
- organic waste generated by food processing industries and agriculture,
- putrescible waste, paper, cardboard and small green waste,
- the fermentable fraction of household waste,
- water contaminated by hydrocarbons,
- soil contaminated by hydrocarbons.
The main processes are as follows:
- composting: this is the transformation of fermentable materials by micro-organisms combined with oxygen, resulting in a release of steam and carbon dioxide; the completed decomposition process leaves a residue made up of stabilised organic material and mineral substances: the compost. Quality compost can constitute an excellent soil conditioner. This quality largely depends on the quality of incoming waste.
There are two major treatment categories:
- slow composting: after crushing, mixed waste is regrouped in the form of windrows, which are regularly turned,
- accelerated composting via air blowing or permanent waste stirring.
- methanisation: this is the transformation of fermentable materials by micro-organisms within an oxygen-deficient atmosphere. A combustible gas (biogas) is produced, as is a residue generally used as organic conditioner after maturation by composting. Biogas can be recovered in the form of heat or electricity production, via a specific generating unit. This technique is used specifically on waste containing a lot of water and easily biodegradable.
- biological treatment of wastewater: the removal of organic carbonaceous and nitrogenous pollution, when sufficiently biodegradable, is mostly the result of biological treatment processes. It is carried out in a reactor where purifying micro-organisms are in contact with the water to be treated. The process is carried out in three crucial successive stages:
- adsorption and absorption of the soluble and colloidal pollutants of the wastewater by the bacterial cells,
- biochemical oxidation and enzymatic degradation of the materials thus retained,
- self-destruction of the cellular materials. Generally referred to as “secondary treatments”, biological processes are normally implemented downstream of the pre-treatments guaranteeing the separation of dense and unwanted bulky materials or after primary or physical-chemical settling treatment.
- biological treatment of contaminated soil: the compounds found on contaminated sites are mainly of organic origin; as soil is seldom sterile, the existing microflora, when in contact with organic pollution, tries to metabolise these pollutants, depending on the chemical and physical-chemical conditions of its environment. Biological treatment techniques associated with contaminated sites therefore consist of using and stimulating existing micro-organisms to enhance this natural phenomenon in order to destroy the pollutants as quickly as possible.