Life inside the mind-boggling site where all your sewage goes - and the bizarre things that turn up in it

There's possibly no experience in the wide tapestry of human life that leaves less of an impression on its participants than flushing the toilet. You flush and you move on with your day; no further reflection needed.

But after that parting moment between human and excrement, whatever you've flushed has a very long journey to make. Along the way it can cause all manner of problems, especially if you've flushed something that won't break down before it completes its trip to a treatment works.

Today I've made that journey with it, and find myself at Dwr Cymru Welsh Water's Cardiff wastewater treatment works in Tremorfa; sewage to the left of me, sewage to the right - here I am, stuck in the middle with poo. Here, frankly mind-boggling quantities of sewage arrive from as far out as Pontypool and Pontypridd as well as Cardiff city centre and its suburbs; when at full capacity (6,060 litres per second), it's enough to fill an Olympic swimming pool in seven minutes or 210 times in a day.

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On what looks like more or less any other industrial site you'd find off Rover Way or indeed anywhere else in the UK, some fascinating processes are at work that convert sewage into much cleaner water and use its byproducts for things most people probably never knew about.

We start the tour on top of a 31m shaft containing inlet pumps which bring sewage flows up to ground level so the plant's screening can begin. It’s here, at the very start of the process for wastewater coming in to the Cardiff works, that problems will rear their heads. For the latest analysis of the biggest stories, sign up to the Wales Matters newsletter here

The site, next to the water, as seen from above with the vats of sewage, balloons for biogas, and car parks visible.
The site as seen from above -Credit:Richard Swingler Photography
Gavin, stood on a platform at  the treatment works, points upwards while speaking
Gavin walks us through the complex processes that happen here -Credit:John Myers

Sharon Ellwood, head of environmental services, is on the tour with us - and explains: “You’ll see the consequence of blockages and wet wipes in the network, the bit you don’t see is what happens when it still gets to our treatment works and blocks the pumps and everything here.”

If detritus does get through the system and ends up here, it’s no small feat to remove it. It takes three cranes, a specialist confined space team and multiple contractors - in short, it’s expensive and a pain in the backside.

Gavin Stephens, operations supervisor, has been working for Welsh Water for more than 20 years. He explains the worst offenders when it comes to blockages: “It’s those type of things that don’t necessarily break down. If you put a toilet tissue in a bottle and shook it, it would naturally start to break down into next to nothing.

“Do exactly the same with a wet wipe, or even sanitary products to a point, you shake that as much as you want - that won't break down. Even the toilet cleaning products these days - attachments you can clean around with and flush down the toilet at the end of it all.”

Wet wipes are a problem, Sharon adds, because of the plastic - specifically microplastics - they leave behind (something being investigated at a UK level). Fats, oils and greases are also a huge part of the challenge - and there’s some weirder stuff too (whether that’s from the usual sources in people’s houses, or illegal dumping).

"Only flush the three P's," Gavin tells me, "Poo, pee and paper. It’s not uncommon for us to have bricks, timber, even an alternator from a car. How on earth that got into the sewer network… ”

Sharon adds: “We found a snake, still alive, which we believe escaped; it was returned to its owner."

"False teeth,” Gavin contributes. “Sanitary products,” says Sharon, who tactfully adds: “And other products that might be used in the bedroom.”

Sharon, on site, wearing an orange high vis
Sharon's job is to try and make Wales' water better quality and more environmentally-friendly -Credit:John Myers
A complicated apparatus comprised of dozens of pipes and tanks
This tangle of tubes carries out the complex work of turning the byproducts of all this into biogas - more on that later -Credit:John Myers

There was a cow - “A Cow,” I ask? Yes, A Cow. Jewellery, presumably flushed in a romantic rage, a goldfish, a family of otters, a coconut… name it and there’s a chance it might have ended up in the sewers. How a cow ended up in the there is still a mystery, but the cow, found alive and well, wasn’t telling.

All the water coming through the plant - and anything in it - goes through two sets of screens - first a wider mesh, then a finer one to catch all the smaller bits. We’re looking at them through narrow gaps in the machinery but it’s in the next room where it’s visible up close.

“Do you ever get used to the smell?” I ask Gavin as we walk across the site (it's not particularly pleasant). Not only do you get used to it, he explains - you get attuned to it. A sniff can tell you whether or not things are going as expected or if there’s something wrong with the treatment process. It’s quite remarkable. I suggest that makes him a sommelier of sewage; he seems to quite like that.

Around the back, in a room that feels like a massive garage, we see the outcome and it’s basically the polar opposite of the noisy, gushing process of filtering the sewage outside. A giant pipe juts out horizontally in the room, and in it a giant, congested cylinder of dry, claggy waste sits completely still. Soon, it will unceremoniously plop into a skip; the end of a long journey for some very unpleasant-looking waste.

This dry mass is what happens when the sewage is filtered and filtered again, squeezed dry and compacted. This means two things; firstly, what we see looks impossibly dry compared to where it was just a few minutes ago, and secondly that we can see basically everything that the filters have caught within a couple of square metres here: condoms, sanitary products, tinfoil, rogue pieces of plastic and masses and masses of wipes (many of which are described as flushable).

Looking at these frankly underwhelming piles of dry rubbish it’s hard to comprehend the scale of the problems they cause. But, Sharon explains, the problems are very big indeed. “Your sewer pipe on your property can block, that’s the first point. That sends all of that flow back into your home and you don’t want that. Or you can flush and it goes, but it affects your neighbour’s property, or your neighbour’s neighbours’. That’s the consequences to people, but a blocked pipe further in the network can lead to a storm overflow discharge overflowing when it shouldn’t, so you’ve got the environmental consequences.”

A compacted cylinder of waste emerges from a metal tube
Once it's screened, dried and compacted, this is what the stuff you shouldn't flush looks like -Credit:John Myers
A close-up of the pile of compacted waste
You can pick out some fairly unpleasant stuff in here -Credit:John Myers

These storm overflows are one of the most pressing concerns for Wales' water network. In a great many locations across Wales, sewage is discharged into our waters, often in periods of heavy weather - but the way we gather the data means we don't actually know exactly how much. You can read more about that here.

There's at least a partial solution in the works; with a £13m scheme planned to create a wetland in New Inn, near Pontypool, which will serve as a natural filtering system for water from a nearby storm overflow. It will include a screening process to remove wet wipes and anything else that shouldn't be in the sewers, and aerated reed beds and two wetland ponds will treat the sewage itself as it passes through the wetland before it is released into the Afon Lwyd, which feeds into the Usk. It's a partial solution and it's a while off, with work set to be complete by March 2025. But Dwr Cymru says removing storm overflows from the system entirely would cost between £9bn and £14bn and require digging up almost every street in Wales.

Even beyond the problems caused for overflows, unflushable waste in the system has a knock-on effect, as Sharon explains: “Then there’s the cost to clear blockages - 20 to 25% of which is wipes, rag and fat. That combination together is what you usually see in fatbergs. The cost keeps the bills the way they are. The hidden costs that you don’t see are the costs when you get here - the pumps block, operators have to deal with that, that all costs money and is keeping bills inflated.”

After another special device is used to remove grit and any remaining debris and the wastewater moves onto the next stage and the de-watered liquids are then returned to the beginning of the treatment process to be re-processed. As we move along we see a pair of contractors repairing one of the screens, sections of it caked in the same stuff we saw in the skip.

Before long we’re staring across a grid of 16 vast vats of bubbling brown water. This is the sewage after it’s been filtered, and what we’re seeing and hearing is the work of bacteria that live off the stuff. A long time and a lot of effort was spent on cultivating and harvesting the bacteria. It’s a digestive system of itself and it now produces itself but it has to be taken care of - no shocks, no sudden temperature changes - and if treated properly, it does its thing.

A view of the large, square vat of bacteria
In this vat the bacteria work away at the water - and on the other side of the walkway is the finished product -Credit:John Myers
Bacteria scum on the surface of brown water
Bacteria are constantly fizzing away -Credit:John Myers

Gavin points to one side of the wall we’re standing on. “If you imagine this was put into a bottle,” he says while staring out over a sea of fizzing opaque watter, “it would look like a muddy puddle.” He turns to the other side - greyish, but clearer - “This side is fully treated wastewater.”

It doesn't exactly look like tap water but it's cleaner - and crucially it's regulated by a NRW permit with spot checks to make sure it meets the required standard. Ultimately, that’s it for the wastewater, most of which now goes out to sea (some of it to wetlands).

But that’s not it for the treatment plant. Firstly, the processed sludge is turned into a bio-gas, held in a giant sphere on site and used to run the plant; the energy produced equates to 22 gigawatt hours of energy per year (that’s enough to power 5,800 homes).

The sludge is passed through a hydrolysis plant that heats it up to 165 degrees as part of a process known as advanced anaerobic digestion. Gases released from the digesters are passed through combined heat and power engines which help to power the site.

But possibly the more interesting bit is found in a shed full of what looks like - and, more or less does the job of - manure. It’s known as cake (and while it doesn’t smell like cake, it does have a cleaner, more chemically odour than where we’ve been so far) and it looks like playdough.

A lump of "cake" - a brown, doughy substance - in a gloved hand
This doughy lump might not look much but it's got a very important job -Credit:John Myers
The cake piled up at the end of a large barn/shed
This cake will be shipped out to farms across Wales -Credit:John Myers

This cake, a rich fertiliser, is shipped out to farms across Wales - where Welsh Water has already done the work of sampling soil and selecting which fields on each farm are suitable for use of the cake. This is massively important because it needs to be ensured the nutrients in the cake don’t run off anywhere they shouldn’t.

“Nine times out of 10 it’s of mutual benefit to both Welsh Water and the farmers,” Gavin explains. “They get a land condition assessment and support from the agricultural team, which we provide, and we know we have the comfort that a farmer followed our conditions and hasn’t just gone willy-nilly and spread it wherever. They get the benefits, we get the benefits.” The alternative is incineration, so this is the environmentally-friendly option.

And that was it - just as the sewage found its way out to sea after a brief time in the treatment works, so too did we leave. Our journey home was probably nicer.