The wastewater treatment process explained

15 Dec 2018


If you are reading this then the chances are you are looking to install or upgrade your sewage treatment system and would like to understand the process involved in treating wastewater. Firstly, let’s start with answering the question “what is wastewater?”

Wastewater is water produced from the following:

  • Toilets
  • Sinks
  • Showers/baths
  • Washing machines
  • Dishwashers

It contains contaminants that are extremely harmful to the environment and therefore requires treatment before being released (into a ditch or stream).

The treatment process:

Different sewage treatment systems adopt different treatment processes, but broadly speaking the process can be defined in 3 stages as follows:

  1. Pre-treatment settlement – wastewater will usually enter a pre-treatment chamber in the treatment plant. In this chamber, solids will settle to the bottom and separate from the liquid. At this early first stage, the main focus is to separate the solids from the liquid, however, there will also be some growth of bacteria which will feed upon the contaminates in the water, although without the presence of oxygen, this is limited. The settled “sludge” will need removing normally once a year by a sewage disposal company with a tanker.
  2. Main Treatment – Once solids have settled, water then flows into the main treatment chamber. This is where the majority of the treatment happens. This chamber aims to encourage the growth of bacteria to further feed upon the contaminants in the water. Bacteria need 3 things: food, oxygen, and somewhere to live. Food comes in the form of contaminants in the water. Oxygen is supplied by an air blower, normally the bacteria live on some filter material inside the chamber (although it must be noted, in some systems the bacteria is freely living in the water and not housed on filter material, these systems are called “active sludge systems).
  3. Final settlement – the cleansed water then flows into a final settlement chamber where any suspended solids (fine particles) are allowed to settle. The settled solids (often dead bacteria) are then moved back to the primary chamber to join the settled sludge ready for removal.

If the plant is adequately sized and functioning well, the water will then be clean enough to discharge to a ditch or stream.

Additional Tertiary treatment.

Occasionally, in sensitive areas like a site of scientific special interest. The Environment agency requires very clean water to protect the environment. In this instance, it may be required to add tertiary treatment to further cleanse the water. Reed beds are often considered for this, bacteria from in the gravel that the reeds are planted in, and the reeds themselves feed upon the nutrients in the water. Other types of tertiary treatment are trickle filter beds like the Ecoflo. Very occasionally, it is required to kill off the bacteria that can be found in the cleaned water, in this instance an Ultraviolet lamp can be used, however, this isn’t common for domestic sewage.


To sum up, the treatment process is a natural biological process. We encourage it by providing oxygen and as a result, we prevent pollution to our environment. You can help your sewage treatment plant to produce good quality water by cutting back a little on using bleach and detergents and swapping your normal detergents to eco-friendly versions. This will aid the bacteria to treat your water and in doing so, being kind to the environment.

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