Sludge Treatment

Urban or industrial water treatment generates liquid, pasty or solid waste that needs to be characterized in order to minimize its environmental impact.

The bulk of these by-products is sludge, a generic term for the insoluble or particulate phase, or the phase rendered insoluble by appropriate pollution treatment.

Manufacturers, local authorities and their representatives must therefore adopt a logic of qualitative and quantitative waste reduction. To do this, they must first identify their waste, assess its potential to harm the environment and evaluate its suitability for treatment.

In this first article, we will examine the physical, chemical and biological characteristics of sludge, depending on the type of industry and the type of water treatment applied, as well as the means of reducing the volume (quantity) and odour or microbiological nuisance (quality) of sludge. These means typically involve liquid-solid phase separation by natural or mechanical thickening, dewatering by centrifugal decantation or filtration, drying by evaporation, and finally stabilization and hygienization to reduce nuisances.

The final disposal of sludge will be the subject of a second article [G 1 451] . It is highly dependent on the legislation in force, but in all countries, the tendency is to valorize the energy potential of sludge by anaerobic digestion or thermal degradation, and to recycle the organic or mineral matter in agriculture or industry.

Toxic or simply undesirable sludge is preferably eliminated by thermal oxidation. In this case, the organic fraction is generally destroyed in the gas phase in a high-temperature incinerator, but can also be treated directly in the liquid state by pressurized oxidation. However, landfill, now reclassified as a "waste storage facility" (ISD), will remain a preferred option for many sludge and waste products, provided that its design and operation are beyond reproach.