Last month, the BBC released a documentary on ‘The Secret Science of Sewage’, which explored one of the most advanced sewage works in the UK.
Based in the Eastern outskirts of Birmingham, biologist George McGavin, was granted access to Minworth Sewage Works, a plant run by Severn Trent Water.
It receives wastewater from 1.7 million people based in Birmingham and the Black Country and aims for the water to become clean and safe enough to be sent back into the environment.
The treatment plant pumps up to 12,500 litres per second, which equates to 1 billion litres of wastewater per day.
Another aim, arising from scientific research, is the potential for sewage to be used as a renewable energy resource and also possibly contain medical secrets that could transform healthcare.
The main resources discovered from sewage in the documentary were:
Dried solids are extracted from wastewater and is turned into foods for plants.
These solids are rich in nitrogen & phosphorus, that can be taken out onto land and use as a soil conditioner for certain types of crops
McGavin and scientists also discussed in depth how human waste could be transformed into power.
In the documentary, McGavin showed us that these excrements could even power his mobile phone.
How does the treatment plant work?
Stage 1: Screens
The first point of sewage filtration. The sewage is brought in from the inflow and is brought to the screening station. Inside these screens are suctioned sieves that are constantly rotating.
The sewage flows through these screens. Pieces of debris like plastic are collected onto its tracks and fall into a collection bin below. Anything liquid like or small enough, pass through the 6mm size holes in the sieves.
A large issue that can damage these screens is wet wipes being flushed. They don’t break up like toilet paper and shreds into smaller pieces, which can block the holes on the sieves.
Wet wipes have only become an issue in the last 20 years. Now, nearly 15 billion are used in the UK every year, many get flushed, contributing to 75% of all sewer pipe blockages.
Stage 2: Settlement Tanks
The water from the screens is then deposited into a settlement tank. There are 22 large circular tanks at the plant. These tanks allow the water to ‘settle’ by keeping the movement to a minimum.
A bar holds a scum trap, that is constantly moving on top of the water, and slowly collects all the fat and grease that has risen to the top. Scaper hooks will paddle under the water, scraping the sludge that has sunk to the bottom of the tanks into a well.
The much cleaner sewage water is then filtered over the tanks baffle and is channelled onto the next stage of treatment.
Stage 3: (For remaining sludge) Biodigesters
The sludge from the settlement tanks is piped across the plant to 16 giant silos called biodigesters.
Acting like pressure cookers, conditions inside the biodigesters are engineered to force a bacterial reaction in the sludge, by reaching temperatures up to 40 degrees, causing methane gas to be released.
The sludge is held in these biodigesters for about 20-30 days. During this time, inside, special bacteria are added to the sludge. The atmosphere inside the digesters is airless, and along with the temperature, that causes the bacteria to start feeding on the proteins and carbs in the sludge.
From this, a methane-rich biogas is released and rises up in the digesters. This is piped off and then injected into the National Grid, supplementing our supply of natural gas we drill for in the North Sea and import.
From this, the Minworth Sewage Works plant can power and supply gas to 28,000 homes per day.
George McGavin said ‘So the person frying their egg in the morning, might not be aware that what their cooking with is a gas flame that has been produced by what they may have deposited in the toilet a couple of weeks before?!’
The dried solids that are extracted from the biodigesters is turns into foods for plants. To make it safe from pathogens, the sludge goes through an advanced called thermal hydrolysis. It’s heated to 160 degrees and pasteurised.
The dried solids are rich in nitrogen, phosphorus, that can be taken out onto land and use as a soil conditioner for certain types of crops.
Stage 3 (For water only) Activated Tanks
There are six large pools on the treatment plant. Huge green pipes pump air into the sewage water. The pumped oxygen gives life to trillions of naturally occurring microorganisms in the sewage that eat the tiny human waste particles, cleaning the water.
These microorganisms can make the water clean in as little as 5 hours. Removing these microelements of human wastewater makes the water safe enough to return to the environment.
The documentary all in all was an interesting watch. The topic of sewage can be such a taboo topic, however, with current sources of power being unsustainable, its comforting to know that research is progressing drastically, making the future for renewable resources clearer.
If you would like to watch the documentary yourself, click here & for more information on the Secret Science of Sewage, click here.