Water used in homes and factories is classed as dirty water. Before it can be put back into the river, it has to be cleaned or treated. There are harmful bacteria in the water which can make people ill if they drink it. The water goes through many processes. There are four main categories: preliminary, primary, biological and final treatment.

Preliminary treatment

1.    Inlet screen

Used water is discharged into the sewer system, From there, it is pumped into the treatment works. The water is pumped along concrete channels to the inlet screen.

Concrete channels

The water then goes through an inlet screen. This is a large screen with 5mm holes in it. This screen removes larger solids which cannot pass through the holes. The solid stuff which is removed is called 'rag'. The waste water and rag are then pumped to the liceps.

2.    Liceps

Liceps are like large spin dryers. The waste water and rag spin round and round and the solid sewage or 'rag' is dried by the process. The rag is then put into a skip and taken to a rubbish dump. The waste water then goes into grit removal channels.

Liceps which dry the rag

3.    Grit removal channels

In the grit removal channels, solid particles smaller than 5mm sink to the bottom of the tank. This material settles into a layer. Every 2 weeks, the channels are washed out so that they don't get blocked.

Grit removal channels

Primary treatment

Primary tanks

The waste water is then pumped to the primary tanks. In most Waste Water Treatment Works, gravity takes the waste water to these tanks. However in some Waste Water Treatment Works the primary tanks are elevated so a pump is needed. There are usually four primary tanks and normally only two are in use. All four are used if the weather has been very wet.

All of the primary tanks have a half bridge scraper. The waste water stays in the primary tanks for 5 to 6 hours where the organic (solid stuff which can be broken down by bacteria) waste is allowed to sink to the bottom of the tank. The half bridge scrapers travel round the primary tank and scrape the sludge from the bottom of the tank into the centre of the tank. This organic waste or 'sludge' is then pumped away to the primary digesters for treatment.

Primary treatment tank (side view)	Primary treatment tank (top view)

Half bridge scraper

Biological treatment

Biological treatment lanes

The waste water is taken from the primary tanks into biological treatment lanes. Most Treatment Works have about 6 lanes. Each lane has diffusers in them. Air is pumped into the lanes so that the bacteria which live there can survive. The bacteria eat colloids and fats, which are tiny particles left in the water.

There are three zones in each lane. In Zone 1, more air is pumped through the diffusers. This is because there are more fats and colloids in the water for the bacteria to eat. 

Diffusers	Treatment lane, Zone 1

Some of the waste water now goes to a holding area where it can be used for future treatments.

This water is being kept for future treatments.

Final treatment

Final settlement tanks

These tanks are the last stage that waste water goes through. They look just like the primary tanks, but with no half bridge. The activated sludge settles to the bottom of the tank. The treated clear water, or effluent, flows over the radial weirs at the edge of the tank. The effluent flows into channels which take it back to the river. Samples are taken regularly to check that the effluent meets the required standards.

Sludge treatment

1. Primary digester

The sludge from the primary tanks is pumped into the primary digester. This sludge is the organic material which was allowed to sink to the bottom of the primary tanks. Solid organic waste is eaten by anaerobic bacteria, which live without air. The sludge stays in the primary digester for about 15 days and is heated to a temperature if 36°C. The solids are eaten and broken down by the bacteria. This removes the offensive smell.

Methane gas is made from sludge digestion. The methane is drawn off from the primary digester and stored in a gas holder. The gas is used to heat the primary digester.

2. Secondary digester

The digested sludge is then taken to the secondary digester where more of the organic solids are eaten.

The secondary digester

3. Press building

Next, the sludge is taken to a press building. In this building, the sludge is pressed to remove water. This makes a black cake-like mixture. The sludge is then deposited outside the building and then used on agricultural land as fertilizer.

Sludge