In the spring of 2017, a sewage pipe in the fields just outside of Barrow burst … twice. Since then, I’ve often wondered where it was coming from and going to, so I decided to investigate and came across some very interesting facts. What do you think is the most unusual thing that Severn Trent have found in their sewers? Read on, to find out.
Our sewage runs underground from our toilets, showers, sinks and dishwashers to the sewage treatment works in Flesh Hovel Lane via the pumping station on Bridge Street. Over eight bathfuls of muddy brown sewage per second arrive at the main inlet pipe. This sewage is screened to remove any large items – these have included false teeth, tennis balls and once even a motorbike broken down into parts!
The sewage flow is slowed to allow sand and grit to sink to the bottom where it’s removed, washed and recycled for use by other industries, for example in road building. The sewage then flows down long channels into what’s called the primary settlement tank, where seagulls often scavenge for undigested pieces of sweetcorn or seeds (we’ll come back to this primary tank later). A sewage sample is tested for chemicals. These include those that are expected to be there in small doses (like ammonia) and which are treated onsite; but also a number of other, more unusual, chemicals which need to be treated differently.
The sewage sits in these tanks for several hours as solids (like poo) settle to the umm… bottom. Cooking fats and other oils float to the surface and are removed by a big metal arm skimming slowly round. What’s left in the tank is called ‘sludge’ and is like thick but horribly smelly melted chocolate. It’s removed by opening a valve at the bottom of the tank and is transported to a ‘digestor’ at one of the larger main sites, such as Wanlip. Here the sludge is heated to 35 °C which allows billions of ‘good’ bacteria to anaerobically digest and break down the poo. As it does this, it produces a biogas which is burned in a generator producing enough hot water and electricity to power a school. When digestion is complete, water is taken out and the sludge looks like an enormous chocolate cake (ugh!); ‘bad’ bacteria are allowed to die off and then this cake is sold to local farmers for natural fertiliser.
Now, back to that primary tank, with the solids removed, the water is much clearer, but still unclean. It is moved over to the filter beds where it is filtered through stone media (working in a similar way to a coffee percolator) and then put through ‘humus tanks’ where, like the primary settlement tanks, any finer particles are allowed to settle. The clearer liquid then flows over from the top of the humus tanks and leaves the site to go into the local watercourse which is monitored 24/7 to ensure anything leaving the site is compliant with Severn Trent’s discharge consent which is issued by the Environment Agency.
All this in under 24 hours!