Friday, 18 May 2018

Living the dream: a visit to Mogden sewage treatment works

I was accosted by a train nutter on Monday. My quiet reading on the finer points of fermentation was interrupted by someone keen to tell me about the gaps in railway tracks. Not something I was desperate to learn more about, but it's not like I had a choice. Fortunately I soon changed at Clapham Junction. Whilst my new friend was heading home I was spending my free time visiting a sewage works. Which did give me a little twinge of doubt about which of us was the sane one.

Mogden sewage treatment works holds a special place in the hearts, or possibly wallets, of brewers as its name is immortalised in the Mogden formula, the way in which effluent costs are calculated.

It was built on a farm in the 1930s to replace 28 smaller sewage works. It now deals with over two million peoples worth of sewage. It was originally surrounded by fields but now is surrounded by housing so controlling the smell and mosquitoes is a key concern!

As is often the case I can't remember the name of the speaker but I think it's fair to say he was a big cheese in big jobs. We got told all about the sewage treatment process and of course about the famous formula:

Here it is

Bask in its glory
The cost is calculated on the conveyancing of the effluent, the volume, the biological treatment cost and the amount of suspended solids. To reduce the effluent costs reduce the volume, Chemical Oxygen Demand, and suspended solids.

The situation is complicated slightly by the fact that as different sewage works deal with different issues there are in fact many Mogden formulas:

But I feel confident that being at Mogden sewage works we were seeing the original and best.

Sewage is treated in several stages:

First it is screened to remove solids like wet wipes, traffic cones and dead goldfish. This forms "rag" which is taken off in lorries.

Grit is then removed in a process that sounded a bit like how a brewery whirlpool operates.

Primary treatment next takes place in a settling tank.

These have a sloping floor on which the sludge settles from which it can be scraped off and pumped away for further treatment, whilst the liquid goes to aeration treatment.

In final settling tanks live and dead microbes settle out as more sludge which is sent for treatment and water which can be sent to the river.

The sludge is thickened with a polymer so there's less to pump and it goes to anaerobic digesters linked to Combined Heat and Power generators which make a Gigawatt a week.

I'll have to check with Alanis Morissette but it may well have been ironic
that one of the khazis at the sewage works was out of order. 
After digestion the sludge is de-watered in centrifuges and goes to farmers.

And here's a heron
After the tour it was on to the networking.

The Pride was drinking well

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