Primary Treatments of Wastewater

Framing: the primary treatments

The objective of so-called primary wastewater treatments is to remove the “coarser” component of the effluent: sands, oils and grease, a substantial portion of larger suspended solids. This is usually a non-self-contained stage of treatment, which in fact should reduce the pollutant load and facilitate subsequent interventions.

We usually speak of “primary treatment” (and then secondary and tertiary) only in the case of municipal wastewater, i.e., in the context of public sewage treatment plants: in this case, European and national standards provide a very clear sequence of treatments and related technologies to support them (what is not always the case in the case of industrial wastewater).

Here is a list of the main primary treatments.

• Desanding: these are steps involved in the removal of heavy substances such as sands and stones, respectively.
• De-oiling: this is the separation of oils or hydrocarbons by taking advantage of their lower specific gravity than that of water.
• Screening: a process to separate solid substances such as debris, solid wastes, foams and sands by means of banded or mesh screens. This can be done with more or less fine grids.
• Primary sedimentation: this is the aggregation of suspended particles in the effluent and their subsequent settling to the bottom of the catch basin.
• Other types of primary treatment: interventions with Imhoff tanks, degreasers and desanders fall into this categorization.