Biological treatments of solid waste

Whatever the medium or material to which it applies, biological treatment consists in the vast majority of cases in using microorganisms under controlled physico-chemical conditions, with the aim of inducing them to carry out the reactions or processes desired by the operator implementing the treatment. Some treatments use higher organisms (e.g. phytoremediation of polluted soils using plants). Others use enzymes (catalysts for biochemical reactions) or cell fractions or extracts. But treatments using microorganisms are by far the most widespread in practice. This article will therefore focus on this type of treatment.

All microorganisms make up what biologists call the protista kingdom. They are often referred to as lower organisms, as they are unicellular organisms and their level of organization is therefore less complex than that of higher plant or animal organisms. However, they are extremely diverse and versatile, and possess astonishing capacities for adaptation. These characteristics make them of great interest in waste treatment, as they give microorganisms the ability to develop multiple activities under variable conditions such as those encountered in environments as complex and heterogeneous as solid waste.

The principle of biological treatment is therefore to stimulate and direct the activity of microorganisms to multiply within the waste and interact with its constituent matter, in order to transform it as desired by the operator. Depending on the treatment objectives, these transformations may involve the degradation (mineralization) of the most progressive constituents of the waste. The residual material obtained after treatment is then more stable (less progressive) than the original waste. It can therefore be stored or used, while controlling the nuisances (odors, etc.) or health and environmental risks that untreated waste could have generated. But under other operating conditions, biotransformations of the material can lead to the formation of high value-added end products (e.g. methane), or give the treated material new properties (e.g. agronomic value of compost). Biological treatments can therefore target both "reactive" objectives (the waste is considered a potential source of nuisance and negative impacts, which the treatment aims to reduce) and "pro-active" objectives (the waste is considered a resource, which the treatment aims to valorize in terms of material and/or energy potential).

The microorganisms involved are generally indigenous to the waste, i.e. present in the waste in question from the outset. The conditions for implementing biological processes for organic biomass waste are therefore based on a "biostimulation" approach. Biostimulation consists in selecting the parameters that create the most favorable...