What is sewage treatment and why should we do it?
It’s easy to forget the nasties in human waste when the flush washes our urine and faeces away. It’s often only when things break that we take notice. Things went wrong with sewage on Scilly a long time ago and we’ve waited more quietly than we should have for these issues to be fixed. But what should it look like when it’s working?
There are a number of stages to waste water treatment. You can read about some of them in South West Water’s own words. For those really keen to know more this video summary is a good introduction to large sewage treatment works.
The stages are as follows:
Waste water enters the sewer system from sinks, toilets, dishwashers and external drains. It travels through a system of pipes to reach the treatment centre.
Preliminary: Sewage is screened (think sieving or straining) to remove debris and rubbish - such as sanitary towels, cotton buds, wet wipes, tights, and all sorts - and solids from human faeces. This process protects the treatment equipment from damage.
(This is where treatment in South West Water’s first proposal for St Mary’s in 2026 stopped.)
Primary: In this stage the remaining sewage sits in a tank. This allows the remaining solids to sink to the bottom, leaving a dirty liquid full of pathogens, fat and all sorts of other soluble nasties floating at the top. Sludge at the bottom is removed and the liquid progresses to:
Secondary: During secondary treatment, adding oxygen and/or biological treatments can filter up to 90% of the remaining suspended solids from the sewage. Without this stage, sewage discharged into waterways will reduce available oxygen and add numerous harmful substances into the environment.
(The Environment Agency shared an update with the IoS community on 14th July 2026, after which SWW quietly updated their project page to re-commit to secondary treatment. )
Tertiary: This stage of treatment is what we are campaigning for. This final stage used in UK sewage treatment seeks to reduce and disinfect more of the remaining bacteria, and reduce phosphorous and nitrate levels significantly. All of these substances can be catastrophic for the sensitive habitats in our water. UV Disinfection is a form of tertiary treatment.
Quaternary: EU countries have committed to deliver this gold-standard level of sewage treatment by 2045 in large treatment works. Despite increasing pressure, the UK has not committed to incorporating this into water legislation and did not adopt the updated Urban Waste Water Treatment Directive. This treatment stage addresses the presence of micro-pollutants like hormones, pharmaceuticals, forever-chemicals like PFAs, and pesticides (and the list goes on).
Legal expectations
Water legislation first became applicable to the Isles of Scilly in 2020. However, on the mainland water companies were expected to provide secondary treatment at minimum for agglomerations (or populations) with a ‘population equivalent, or p.e’ of 2,000 or more by 2005. Scilly is over 20 years behind.
Page 10 of the UK’s implementation document of the Urban Waste Water Treatment Directive overrides guidance documents that suggest the limit for secondary treatment is 10,000, not 2,000 P.E. UK Government lowered this limit when they abolished ‘less sensitive areas’ (or areas with high dispersion) after recognising the harm caused by the lack of secondary sewage treatment to UK waters. Long sea outfall pipes could be 200 metres, 2000 metres, or 2000 miles long, but discharging untreated sewage into the environment is still illegal and immoral.
What is a population equivalent?
St Mary’s has a permanent population of over 1500, but this number almost triples during holiday season. It increases even further during events such as the annual World Pilot Gig Championships, but population and ‘population equivalent’ differ. One person has a population equivalent of 1, but offices, restaurants, cafes, bars, hotels and livestock all have their own population equivalents too. The government determines the formula water companies should use, but water companies calculate their own P.E. for an agglomeration. St Mary’s is easily over 2,000 P.E. and lawfully requires secondary treatment at minimum. This was recognised by South West Water in their PR19 and PR24 plan documents, yet inexplicably, it took community pressure to compel SWW to reconsider a screening-only plant.
South West Water have not published the population equivalent they are using for their proposals, but the Environment Agency acknowledge that the legal minimum expected treatment is secondary, unless there is a robust case, they say, to disregard the law.
We feel there is no robust case for the acceptance of routine and avoidable sewage pollution on the Isles of Scilly, or anywhere else.