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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.
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.
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 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.
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 . . .
. . . 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.
This is a circular humus tank. This is more settlement, and these
are sometimes called final settlement.
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.
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.
The below section was written by a machine.
Understanding the internal mechanics of a Sewage Treatment Works (STW) is essential for effective environmental planning and infrastructure design. Modern wastewater treatment relies on a sequence of physical, biological, and chemical processes to ensure that effluent meets stringent discharge standards before returning to the environment. This technical overview explores the stages involved in converting domestic and industrial waste into treated water suitable for discharge into sensitive catchments.
The process begins at the inlet works, where large debris is removed via mechanical screening. Following this, the wastewater enters primary settlement tanks. Here, the flow rate is significantly reduced, allowing heavier organic solids to settle to the bottom as sludge, while oils and greases float to the surface for removal. This stage is a critical precursor to more advanced treatments, and its efficiency directly impacts the potential for volatile emissions, often requiring a detailed odour impact assessment to manage the impact on nearby sensitive receptors.
The heart of the treatment process occurs in the secondary stage, where biological organisms break down dissolved organic matter. This is typically achieved through one of two main methods:
In regions with high ecological sensitivity, such as the Somerset Levels, tertiary treatment is necessary to remove specific nutrients like phosphorus and nitrogen. This stage is vital for compliance with nutrient neutrality mandates. Measures may include chemical dosing for phosphate precipitation or the use of Integrated Constructed Wetlands (ICW) to provide a natural final polishing stage. Accurate mapping of these discharge points is a requirement for any Nutrient Neutrality Assessment and Mitigation Strategy (NNAMS) to ensure local water bodies are protected from eutrophication.
For developers, the proximity and capacity of local sewage infrastructure can represent a significant site constraint. Integrating treatment considerations early into a Flood Risk Assessment (FRA) or a broader Environmental Impact Assessment (EIA) ensures that both foul and surface water strategies are robust. At Southwest Environmental Limited, we leverage over 15 years of industry experience to provide the technical authority required to navigate these infrastructure challenges. With 900+ projects delivered across the UK, we offer professional insight (IES/FGS) to secure project viability and regulatory approval.