I Didn't Know That: Toilet Water Cleaning
Visit the Becton Sewage Treatment Works—one of Europe's biggest plants—which processes sewage from 3.4 million Londoners into water clean enough to return to the Thames.
Transcript
NARRATOR:
Waste from London's toilets, sinks and gutters is carried away by a network of inter-connecting ever larger pipes designed and built by sewage superstar and pioneer Joseph Bazalgette. It all ends up here - a sewage treatment works.
RICHARD AMBROSE:
This is Becton Sewage Treatment Works in the East of London and its Europe's biggest. It was originally part of Joseph Bazalgette's scheme, but I tell you what, sewage treatment has really moved on since his day.
NARRATOR:
In the 19th Century, sewage was simply stored here in tanks, then pumped out to the river at high tide. So you'll be pleased to know today's process is a lot more refined and takes just 4 hours for raw sewage to be clean enough to return to the river.
JONNY PHILLIPS:
Every day this treatment works handles the sewage from 3.4 million Londoners. Now that's an astonishing volume, it equates to 34 Olympic swimming pools every hour.
NARRATOR:
So how does it treat the by-products of our late night beer sessions and trips to the local curry house?
RICHARD AMBROSE:
The first process happens here at the inlet works using these screens. Large lumps like bricks, wood, rags, are raked out.
NARRATOR:
Once collected, these large objects will all be washed and sent to a landfill site nearby.
RICHARD AMBROSE:
They've even had half a car here.
NARRATOR:
Meanwhile, the remaining sewage passes onto the next part of the process, into these primary settlement tanks.
JONNY PHILLIPS:
Now very basically it enters at that end and flows relatively slowly this way.
NARRATOR:
Over the top of each tank is a moving bridge, which has two blades to remove solid organic material from the sewage. The top blade skims scum from the surface, while a second submerged blade dredges the heavier settled sludge from the bottom of the tank. All this solid sludge is then sent for further processing.
JONNY PHILLIPS:
The remaining sewage then flows down this way towards the secondary biological treatment process.
RICHARD AMBROSE:
And here it is. In these tanks the sewage is mixed in with billions upon billions of bacteria so all the dissolved materials that still remain can be bio-degraded.
NARRATOR:
And pumping air from below at very high pressures causes the sewage to be saturated with dissolved oxygen and this accelerates the biological breakdown.
RICHARD AMBROSE:
It's brilliant.
NARRATOR:
Finally, the treated effluent is tested to ensure it's clean enough to go back into the Thames. Meanwhile, remember all of the solids which were skimmed off earlier? They're taken to a processing plant.
RICHARD AMBROSE:
All sludges are brought to this filter press where it's de-watered and the resulting sludge cake is incinerated and that provides 75% of this entire work's power needs.