What happens to sewage in the UK?

I saw an article in the Channel Islands press covering a problem with sewage from a housing development.
Which got me thinking: what do you do with sewage on an island?

The simple answer is that some island systems give wastewater preliminary treatment to remove solids before discharging it through long sea outfalls.

Full conventional treatment is often more challenging on a small island because it requires significant land, energy, infrastructure investment, and a long-term solution for sludge disposal.

The trouble is that on this estate the sewage pipe is not very long - you could even say it’s a bit short…

Increased awareness

For decades this may not have been seen as much of a problem. But now we are far more aware of environmental impacts and, with the popularity of open-water swimming, it's hard to ignore.

Which got me wondering: what happens to sewage in the UK?
How do we get from what disappears down the drain to water clean enough to return to the environment?

The Treatment Plant

All the delights are pumped to a treatment plant, which is a multi-stage cleaning system designed to remove solids, organic waste, nutrients, and - where required - pathogens before water is discharged back into rivers or the sea.

1. Screening (removing the big stuff)

The first step removes large debris such as:
Wet wipes
Sanitary products
Plastics

2. Grit removal

The wastewater is slowed so dense inorganic particles settle out, such as:
Sand, Small stones, Coffee grounds etc

3. Primary settlement

The sewage then enters large settlement tanks. Here gravity separates the flow into:
Top: Fats, oils and grease (scum)
Middle: Partially clarified wastewater
Bottom: Heavy solids

4. Biological treatment

This is the clever bit, where microorganisms do most of the cleaning.
The partially treated water enters aeration tanks, where air is bubbled through continuously.
This creates oxygen-rich conditions that allow bacteria to multiply and consume dissolved organic waste.
This stage removes most of the organic pollution.

5. Final settlement

The biological (bacteria) solids are allowed to settle.
The clarified water is separated from the microbial sludge.
The residue goes to sludge treatment.

6. Tertiary treatment (where required)

More advanced plants add extra cleaning.
This is particularly important where treated water enters sensitive areas eg rivers, shellfish waters, or designated bathing areas.

7. Discharge

Once it meets regulatory standards, the treated effluent is discharged into:
Rivers, Estuaries + the sea

8. Sludge treatment (the solids)

The separated sludge (biosolids) is further processed and may then be used as agricultural fertiliser (where regulations permit).

Treated biosolids applied to farmland are generally considered low-risk.
That said, there is an active and growing debate about whether existing standards fully reflect what we now know about:
Micropollutants
Microplastics
Persistent fluorinated chemicals (PFAS)
Pharmaceutical residues

References

Why does Guernsey dump raw sewage into the sea?

https://www.bbc.co.uk/news/articles/cn0p19v0ge8o