Sewage PlantHave you ever wondered about the workings of a sewage plant. What happens to your water when you flush it down the toilet or it runs down the drain - you're about to find out.

The treatment of sewage is a multi-step process that takes as many solids from the water as possible before the water once again enters the environment and our homes. However, no two sewage plants work in the same way, although they do use similar techniques.

There are three main treatments of sewage:

Primary treatment: sewage is held in a large container for a period of time that allows large heavy solids to sink to the bottom and all liquids and oils remain on the top. Anything that is floating or the sunken solids are then removed.

Secondary treatment: this removes any dissolved matter in the water. This process is done by water born micro-organisms in a controlled environment. These micro-organisms are then removed from the water before tertiary treatment.

Tertiary treatment: this is anything that is over and above the tasks of primary and secondary treatment. This treatment allows water to be placed back into protected eco systems such as rivers and lakes.

Along with these three main processes there are other smaller treatments that have to take place in order to make the water safe.

1. Screening: this is a screen of bars and mesh used to remove any large items and rubbish such as cans, rags etc. If these are not removed, then blockages in sewage pipe systems and the treatment plants become blocked - meaning that the system is inefficient and the water has not been cleaned to the proper standards. All of the collected waste is then disposed of either at landfill sites or incinerated.

2. Grit Removal: large plants may have a grit chamber that allows grit, broken glass and stones to settle at the bottom in order to be removed.

3. Fat and grease removal: large plants have a system whereby the surface fat is collected by skimmers; sometimes air blowers are used at the bottom that lets fat from the bottom of the tank rise, it then becomes froth at the top allowing it to be collected by skimmers.

Does a sewage plant smell?

A smell coming from a sewage plant usually means that it is in the early stages of treatment; the most common cause of these smells is the gases given off - usually hydrogen sulphide. Large sewage plants tend to use carbon reactors or small amounts of chlorine to diffuse the smell. Other methods are also used, including the addition of iron salts to the treatment. This helps control the levels of hydrogen sulphide given out.

This post was written for www.biopro.ie, by Nathan Baldwin working for a septic tanks installation expert.