Since last Autumn we have been putting on regular events visiting places of interest in and around Bristol. Our most recent trip was to GENeco, part of Wessex Water, a multi-award winning business which processes both sewage and food waste.
I knew this visit required sturdy footwear and a high-vis jacket but I hadn’t fully appreciated that we’d be walking around this big outdoors site seeing the different processes transforming raw sewage into treated waste water, bio gas and compostables.
Yup, we saw plenty of wet wipes swirling around in the untreated water before they were raked out by vast machines (about 20 tonnes of wipes are removed each week).
Plus watched fat balls (compressed fat created from our digestive systems, cooking fat and any other fats you might find in sewage) bobbing around on the top of the primary sedimentation chambers.
It definitely wasn’t for the tender-stomached or delicate-nosed amongst us.
But wow was it an interesting tour. Here’s what was going on.
All the sewage from Bristol to Bournmouth and the South West as far as Devon is processed by GENeco. It’s quite extra-ordinary. The sewage pipes deep under our feet lead all the way out to the Avonmouth plant where it is scooped up via Archimedes screws and sifted to get rid of problem items such as wet wipes. Other industrial waste (like farm and cleaning waste) is blended at this stage and together it goes through a rag and grit and primary sedimentation process to separate the sludge from the liquid portion.
The liquid is channelled into big ‘swimming pools’ and aerated for biological treatment. My chemistry knowledge was too limited to understand the exact process but the result is that further sludge settles on the swimming pool floor, leaving apparently quite clear water. There’s a few more processes for testing the chemical content of this clear water effluent. But once it’s passed these tests it is either put back into the water cycle or used as grey water in local industry.
The sludge portion (polite term for our poo) is treated via a process called anaerobic digestion. For this, the poo sludge is pumped into digester silos where anaerobic micro-organisms feast on it to produce methane-rich biogas.
The biogas is either used to generate renewable electricity or it is converted in GENeco’s gas to grid plant to enriched biomethane that is injected into the gas grid. At this stage it can be used as fuel in GENeco’s Bio-Bee and other vehicles or to supply local homes. (If you're interested, Bristol Energy has a My Green Gas renewable tariff which is 100% green electricity and 15% green gas from the GENeco plant).
The solid by-product of the poo munchers' digestion process is used as a nutrient-rich and sustainable biofertiliser for farms.
It’s not just sewage. At the same depot that receives our water waste, GENeco processes all the food waste that we put in our brown food recycling bins. Plus it collects commercial food waste in its Bio-Bee, a biomethane-powered refuse truck.
We didn’t get to see the food digestion process in action as it mostly takes place inside large digester silos similar to the sewage sludge silos. However we did peek inside one of the big warehouses where the waste is dumped and separated from those bio-degradable bin liners (and other plastic) which undergo their own processing. The remaining food waste is put through a pasteurising process, then fed into the digesters where those poo micro-organisms set about producing methane-rich biogas and fertiliser, this time from our food.
All in all, GENeco treats over 700,000 tonnes of varied waste per annum, including food, liquid and solid materials which are especially resistant to biodegradation. Through their processes, these wastes are transformed into usable products such as biofertilisers, and renewably-sourced gas and electricity. It is an impressive zero waste to landfill policy and is one of the reasons that Bristol is aspiring to be the first UK Gold Sustainable Food city.
So a VERY big thank you to Bertie Traill our chemical engineer tour guide. He gave us heaps of helpful information and answered loads of questions. It was seriously educational to see this circular economy in action… although you’d have to be of a robust constitution to work there amongst all that poo and rotting food. Click here to see photos of our visit.