Behind the scenes at Portswood Wastewater Treatment Works

If you are one of the 85,000 people living in the areas around Portswood, then your sewage comes to Southern Water’s Portswood Wastewater Treatment Works.

The plant on Belgrave Road treats sewage before passing it out into the River Itchen - and the Echo got to look around to see what goes on behind the scenes.

Our reporter was shown around by John Penicud, managing director for wastewater.

He explained the network relies on gravity, meaning all waste naturally runs downhill into the site without being pumped.

This can be the cause of most of the odour – which locals have criticised the plant for in the past – as less rain in the summer means slower movement in the pipes.

Any chunks bigger than 6 millimetres are screened out and put into skips. (Image: Newsquest)As sewage enters the plant, inlets work to take out anything bigger than 6 millimetres, which get kept behind on the sorting steps.

READ MORE: Foul sewage stench spreads to Bitterne Park - and residents have had enough

Water is sent down the flumes, while small chunks go into a skip of “grit” and "rags" which are later recycled as industrial compost used to plug landfills.

A skip full of what Southern Water call rags - bits of fluff, hair and streaky objects filtered out from the water. (Image: Newsquest)Bigger items that get caught in the sewers must be removed by hand - like a log which was present on our visit.

Bigger items that come down the sewers, like this log, are be removed by hand. (Image: Newsquest) Due to the reliance on gravity, sometimes sewage has already "gone septic" - meaning bacteria has already started to act - by the time it arrives at the plant.

This releases hydrogen sulphite which gives off a rotten smell that locals might recognise.

READ MORE: £2.5 million project to improve water quality in the New Forest complete

Southern Water are tackling this by dosing the water with iron salt, which breaks down the smelly chemical.

“The odours are definitely the biggest complaint,” said Mr Penicud said.

“The changes to the process that we are making has led to about a 50 per cent reduction in complaints this year.

“These changes are making the site as resilient as possible to make us a good neighbour.”

The water flow is then slowed down in the primary tanks which allows the solids to settle on top of the liquids – creating a layer of foam-like fats that can be scrapped off.

One of the Portswood plant's primary tanks, where liquids are separated from smaller solids. (Image: Newsquest) Biological treatment is next, where bacteria bugs are used in a natural process to attack diseases in the water.

A blade on top of the tank smashes and unsettles the water, oxygenating it and allowing the bugs to do their work.

Flow is then slowed again, allowing the final bigger bits to settle. 

The spinning bridge slows the flow of water down, allowing final tiny solids to settle, and cleaner water flows out of the side of this tank. (Image: Newsquest) It then goes straight into the River Itchen as cleaner water, Mr Penicud said.