David Beeson, February 2023
When one of my granddaughters arrived for a half-term stay she brought along a homework project – to make a webpage of the biological processes of cleaning waste water. So, what is a grandfather to do? Take the 13-year-old (and her 11-year-old sister) to the local treatment works!
Fullerton Works is owned by Southern Water and do not usually allow visits … but, if you do not ask, the manager cannot say yes. And, he did say, “ I‘ll show you around.”
The site processes the water from the Andover town area. A population of around 50 000. But, the surrounding countryside has indirect inputs via their septic tanks, with tankers collecting the effluent and bringing it directly to the site. So, the place processes the waste water from around 70 000 people.
So, what is in the input? It is 70% water, plus urine, faeces, washing up and showering chemicals and anything else that reaches the domestic and commercial drains.
In many places in Britain, it is permissible to discharge untreated input into the sea or larger rivers – despite this being very negative. However, with the local River Test subject to stringent controls that should not occur. The waste needs to be processed thoroughly and the end product almost to drinking water standard. (No, it is not drinkable without further filtering and chlorine treatment.)
The site is gently sloping, so the watery materials can flow naturally down towards the river.
The first stage is to pass the fluid through a filter. This removes larger objects that should never be in the drains, including wet wipes. The input is slightly smelly (it was a cool day!), very watery and carrying undissolved sediments. Initial sedimentation brought out stones, grit and quantities of yellow sweetcorn. I guess from this view, sweetcorn must be considered a slimming product!
Stage two: huge, slow-flow through sedimentation tanks allow the faeces’ solid materials to settle. There is a series of tanks to optimise the removal of the organic solids. The sediment is scraped away and passed to the anaerobic digesters in stage 5.
Stage four. Bacterial especially will inevitably start to break down any organic materials – urine, food fibre and other input materials such as detergents. Now vast, deep beds of clinker, gravel and sand holding bacterial and protozoans get to work. These are aerobic tanks through which the fluid is sprayed and percolates. The organisms take in the organics and digest them, releasing carbon dioxide and some ammonia-like gases.
Birds enjoy the site, picking off worms, insect larvae and other life. So, our waste feeds into the wild food chains.
The organic materials are being taken out of the water and used in microorganism metabolism and growth, so the fluid that escapes the bottom of this biological filter will require settling to remove this debris.
You’ve guessed already that there are a series of these biological filters until the water is clear and can be released into the River Test. Continuous monitoring of the outflow occurs and if something has gone wrong the water can be poured onto a grassy field to further ‘polish’ it.
Stage five. The deposited debris is passed into a vast closed anaerobic biodigester. Here the bacterial breakdown the dead organisms and fibre from the faeces into gases like methane and carbon dioxide. The methane is burned to make electricity that drives the works. Finally, after days of digestion, the remaining sludge is dried by centrifugation into pellets and these are used as an agricultural fertilizer and soil improver. Any extracted water is passed back to the start of the whole process.
Only four humans work the site, but they enjoy the company of resident rabbits, badgers, deer and foxes.
There are alternative systems for processing our water-carried wastes. The main one is when air is pumped through the materials to encourage its aerobic breakdown – the activated sludge process. Sometimes, the final water can be passed through a reed bed.
Humans generate the organic wastes but we rely on mainly microorganisms to convert the water suitable to be released back into the system. And, London’s residents (including King Charles) drink water that has passed through six sewage works.
It is too easy to ‘write off’ bacteria and other microorganisms as irrelevant, yet they process this water plus dead and decaying other organic materials in the environment. As an ex-microbiological biochemist, I am always stunned by how these minuscule organisms can be used to human benefit.
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2 thoughts on “The Andover Sewage Treatment Works”
Very interesting. Would be a great venue for school visits!
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indeed. I used to take groups round but Southern Water took over and changed the rules ….