Civil wastewater treatment plants sizing

During the last ten years, the various human activities increased the production of waste water or discharge water, hence the increase to treatment processes before delivering to the nature.The reason why these processes are required is due to the fact that the quantity of pollutions became higher than the self-cleaning capacity of the environment.Therefore, it is essential to provide waste treatment systems that simulate the biological processes that happen in nature, by making them quicker and more effective thanks to the technology employed.In order to size a wastewater treatment plant it is fundamental to understand first: what is a treatment process?

The Unique Environmental Paper – D. lgs. 152/06 (www.camera.it/parlam/leggi/deleghe/06152dl.htm) fortunately comes to rescue us because the article n. 74 shows a list of definitions such as:

  • Appropriate treatment: the civil wastewater treatment through a process, that is a disposal system which, after the discharge, guarantees the compliance of water body receptors to the specific quality targets that is in accordance with the requirements of the third-party of this act;
  • Primary treatment: the wastewater treatment that involves the sedimentation of the suspended solids through physical and/or chemical-physical processes and/or other types. After the processes, before the discharge, the BOD5 of treated water will be reduced at least by 20% and all the suspended solids by 50%;
  • Secondary treatment: the wastewater treatment through a process that leads to the biological treatment with secondary sedimentation, or through another process in which the requirements of the third-party of this act of the table n. 1 of the attachment n. 5 are respected;

Once understood what a treatment plant is and by how many phases is made up, we need to identify the number of users that will use it.

In the civil treatment field we talk about Population Equivalent (P.E.).

This is a word used for a specific usage level in terms of uniformity and comparability with the civil users. It is a conventional concept based on an average intake of a typical user equal to 60 g/BOD5 per inhabitant (D.Lgs. 152/2006 art. 74), but extremely useful because it allows to easily compare various usage levels that can also be disparate, by expressing every single user with its load of “Population Equivalent”.

Finally, we need to identify the receptor body in which deliver the treated waste water. The D.Lgs. 152/06 (art. 103 -104-105-106-107) classifies the final discharges as follows:

  • Discharges on the ground
  • Underground discharges
  • Discharges in surface water
  • Discharges in sewerages

As a result, depending on the discharge final disposal, various system solutions can be provided. These solutions, as treatment performance, reach the limit concentrations shown in Table 3 (discharge in surface water and in public sewerage) and in Table 4 (discharge in the ground) of the Unique Environmental Paper D. lgs. 152/06 (Part III – Attachment 5).

The images down below show a summary of the most commonly used standard plants depending on the type of final discharge.

P.N.: the plant sequences shown in the image are purely indicative. Please always refer to the current local laws in this area that can provide other solutions)