How do Waste Water Treatment Plants Work?


Once we have used water in daily activities such as cleaning, washing, irrigation, drinking, etc., there is a remaining water volume that went out of our homes.

But, where does this used water go?

Well, this question has more than one answer and depends on where we live- in the city or in the country. Is there a stream nearby?

If we take a sample of this used water, we will find that generally parameters such as fecal coliforms, alkalis, temperature, smell, color, pH and cloudiness don’t meet the regulations to be used anywhere.

The first thing we have to do to make this water fit to be poured into a current or onto land through the construction of a waste water treatment plant.

Water Processes

Usually the great majority of these kinds of plants have the following processes:

Pre-Treatment: for elimination of those elements that can cause maintenance and operation problems. Pre-treatment may have three steps:

  1. Grilles: big solids and clothes elimination.
  2. Grease removal: grease and oil elimination through flotation.
  3. Sand removal: solid separation. Solid density must be higher than water density.

Primary Treatment: for the elimination of part of the suspended solids and part of the organic matter. Waste water after this process still has big quantities of BOD and organic matter. This process is materialized with:

  1. Primary Sedimentation
  2. Sieving

Secondary Treatment: for biodegradable organic matter elimination. We can choose between a variety of methods:

· Activated Sludge

· Anaerobic Reactors

· Fixed Bed Reactors

· Pool Systems

We finish this process with a Secondary Sedimentation.

Advanced Treatment: this treatment is used only if it is necessary to eliminate nutrients (Phosphorus and Nitrogen Salts), toxics, or organic matter excess. Here we can do:

  1. Chemical Coagulation
  2. Flocculation
  3. Sedimentation
  4. Filtration
  5. Activated Carbon

Sludge Treatment: for water content and sludge organic matter reduction. Sludge is set up for reuse or final evacuation.

The next Figure shows an example of the processes sequence:

This Figure shows an example of the processes sequence:


We can classify treatment methods in:

· Unit Physical Operations: physical forces are predominant. They are: Smoothing, Mixing, Flocculation, Sedimentation, Flotation, Gasses Transfer, and Filtration.

· Unit Chemical Processes: contaminants conversion or elimination is achieved with the addition of chemical products. They are: Precipitation, Adsorption, and Disinfection.

· Unit Biological Processes: contaminants elimination is achieved through biological activity.