Southwest Environmental Limited

A Trip Around a Sewage Treatment Works

This is a write up about a pleasant tour around a small and out dated sewage treatment works. We were granted access as we were undertaking some Odour Reporting for a developer adjacent to the site. Below is the layout of the site. Scroll down! Below this layout image the "tour" continues. Location of Wessex Water Sewage Treatment Works.

Site Plan

The Inlet

First up on the tour, is perhaps the smelliest part of the sewage treatment works. The Inlet. This is where raw sewage flows in to the sewage treatment works. In this works there was a pumped inlet (flowing as pictured), and a gravity inlet which is out of site.

The smell was quite bad, but not as bad as you might expect. The pumped inflow had a flow rate of approx. 10l/s.

The Macerator

Next up was a macerator. This smashes and mashes up all of the sewage to make sure there are no lumps. Note the small red skip which has to be emptied with shovel from time to time. Lots of wet wipes in here.

The Balance Tank

The Balance Tank. The most innocuous, but also most damaging piece of equipment on site. Effluent flows in here and is held back (in terms of flow) to prevent too much water rushing through the sewage treatment works.

The worrying bit is that if this flow is exceeded, the whole works is bypassed and primary sewage ends up going straight in the river.  You will note long pipe labeled "Storm O/f".  Storm Over flow that is. . .raw sewage straight in the river.

This is why there is a push for better surface water drainage on sites at the moment, when rain water goes down the drains it causes sewage works to overflow.

Primary Settlement

Next up are the primary settlement tanks, there were round ones and square ones at this works. The round ones being more recent, and the preferred shape.

Here the raw sewage effluent is allowed to settle. The sludge rises and the liquid falls . . .

Trickle Beds

. . . the liquid is then passed to perhaps the most recognisable piece of the works, the trickle beds. Here the waster is trickled on to a course gravel material, allowing for aeration and further breakdown by bacteria.

The spinning part is pushed around by the water pressure there is no motor.

Humus Tanks

This is a circular humus tank. This is more settlement, and these are sometimes called final settlement.

Sludge Tanks

Sludge from the primary and final settlement tanks in pumped in to a large tank for storage. This is then collected by tanker, where it can go for further dewatering and landfill, or can be spread on land as a fertilizer.

If you look carefully in the bottom of the picture you will see tomatoes growing. Lots of them. 5 points if you can guess how they got there.

Effluent Sampling Point

The last bit of the works is the final effluent sampling point. This where the cleanest water is, right before it is discharged to a river, brook or stream.

I hope you found this tour interesting. I did when I did it, and I hope to get a tour around a newer site some time soon.