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.