Wastewater treatments generate a series of wastes that can be classified into two groups:
- Large solids: normally obtained in the pretreatments and, generally, are either incinerated or dumped into landfills.
- Sludge: generated in both primary and secondary treatments .
We may find sludge in liquid or semisolid form and containing between 0.25 and 12% (by weight) of solids, according to the preceding process. The sludge is subjected to treatment depending on their origin and pollutant or toxic load. These treatments aim to reduce the volume of these sediments. The most usual stages are:
- Thickening. Static or dynamic thickeners are used, depending on whether they have or not scrapers. These equipments are conical reservoirs in which the sludge is concentrated by natural sedimentation.
- Flotation. Certain sludge types are concentrated by a flotation process aided by the injection of air.
- Centrifugal process. It separates solids from liquids. When applied to very dilute sludge (1-2 % solids), this is concentrated.
The sludge can be stabilized by anaerobic or aerobic digestion.
To reduce the water content of the sludge the following processes are used:
- Dry beds: It consists in the disposal of the sludge to be dried on an outdoor surface provided with a good drainage system.
- Filtration: Vacuum filters and filter presses are used in order to achieve the dewatering of the sludge.
- Centrifugal process: It allows the separation of solids from liquids. When applied to less diluted sludge (5-10 % solids), this is dehydrated.
Finally, the sludge can be incinerated, agriculturally applied (composting or direct soil application), sent to a landfill or used to stabilize or fill land dumps.