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What Happens When You Flush? The Secrets Of Sewage


This is another one of my most-read Forbes pieces - from way back in 2016!

“Yep, it smells about as bad as I expected.” It’s a weird thing to say about somewhere that’s been on your visit-wishlist for a while, but those were my exact words when I walked into a building affectionately known as the "sludge barn" at Crossness.

The site in Thamesmead, South East London, has a long history with water and waste. In the 1800s, London’s river Thames was being used as a dumping ground for everything (and I mean, everything) produced in the city. 1858 brought with it a surprisingly hot summer, but this was not a cause for joyful celebration. The river, smothered in rotting food, animal carcasses and feces, began to smell so badly that the city literally ground to a halt. The period now known as The Great Stink was only confined to the history books when engineers, led by Joseph Bazalgette, built London’s extensive underground sewer network to transport the waste away from the city. And Crossness had a starring role – it was there that huge steam-powered engines pumped the sewage up from the sewers and released it untreated into the river.

Thankfully, London’s approach to wastewater treatment has evolved since Bazalgette’s time – sewage isn’t just dumped into the waterways! To understand the modern treatment process, and to see how it’s changing, I was invited to have a look around the very impressive Crossness Sewage Treatment Works, which is run by Thames Water. Dr. Nick Mills and Aurelien Perrault work in the Wastewater Innovation team, and alongside managing today’s waste, they’re building the next generation of treatment plant, which will turn sewage into big business.

So let’s start with what happens with the flush of a toilet. Everything that’s left your body, any tissue or paper you’ve used and the water in the bowl, leave your home and enter the city’s sewers. There it joins other wastewater, and in the case of London, rainwater, and is carried to a sewage treatment plant like the one at Crossness. There, it is screened to remove any big objects from the wastewater – usually its stuff that shouldn’t be there anyway, like nappies (diapers), condoms and bottles. Any sand and grit in the water is also filtered out at this stage, but unlike the other stuff, it is cleaned and used in construction projects elsewhere. (PS: I was told some horror stories about other objects that have emerged from the sewers, but I won’t repeat them here!) Oil and grease don’t mix with water, so they can also be skimmed off the surface of the wastewater at this point.

And only now does the proper treatment begin. First off, the filtered wastewater is stored into enormous settlement tanks. There, it is gently mixed, to add oxygen and to encourage small particles of contaminant (e.g. feces) to form larger clumps called "flocs." Once they get big and heavy enough, these flocs drop to the bottom of the tank, where they form a dark, gooey material called sludge. In their mixing, the scrapers push the sludge towards the center of the tank where it is pumped away for further treatment.

The now slightly-cleaner-but-still-brown water is passed onto what’s called "secondary treatment," which makes large-scale use of microbiology. Particular species of bacteria are added, to feast on the dangerous pathogens present in the feces-filled wastewater. Because these bacteria rely on oxygen, air is added at the same time, allowing them to thrive and multiply. Once they’ve broken down all of the pathogens, the bacteria have done their job. The water is moved to another tank where is it filtered and disinfected, and then, it’s ready to be pumped back into our homes.

As impressive as the water-cleaning process is, for me, the sludge is a lot more interesting. As I’ve mentioned in a previous post, cities like Stockholm are using it as a source of vehicle fuel, but that’s not the only option. Let’s go through what happens after the sludge leaves the settlement tanks. At this point, the sludge is mostly liquid – in fact, Aurelien told me that typically, only about 3% of the volume is made from solids. So before any major treatment can occur, they need to dry the mixture. They do this using centrifuges, which spin rapidly, forcing the solids in one direction and the liquid in the other. Once the volume of liquid has been reduced (solids now up to ~16%), the sludge is ready to enter the thermal hydrolysis plant (THP).

THP first boils sludge under high pressure, before rapidly decompressing it. The combination of these two steps sterilizes the sludge, and it also makes it easier to break down. The THP at Crossness is an impressive sight – several towering steel silos, radiating heat. “Traditionally, the sector was all about concrete pipes and huge tanks,” said Nick, “We're becoming much more like a chemical engineering process.”

After the THP treatment, the sludge is hot – somewhere in the region of 160°C – so before it moves on, it is cooled to 40°C. Then it’s ready to meet a new class of microbes – anaerobic bacteria – in the digester. These bacteria can break down the sludge and produce methane as a by-product. This gas is directed to the site’s combined heat, power and cooling plant, which is composed of three 2MW engines. This generates electricity – enough to power the entire site – and heat, which is used to produce steam that’s needed in the THP. Yes, the plant that treats sewage is also powered by sewage. It’s also gotten considerably more efficient, as Nick explained. “We used to use 16 digesters to process the sludge on this site. Now, with THP, we need just six to manage the same volume”

Wandering around the enormous site on a cool, breezy autumn day, there was only the slightest hint of sewage in the air. However, as we entered the warm confines of the sludge barn, the situation changed. It STANK. I knew we were nearing the end of the treatment process, but before we opened the barn door, we headed upstairs to another huge plant room – this was where the nutrient-rich, digested sludge is sent before it’s ready for use elsewhere. The room was dominated by several enormous machines, one of which was being serviced, which let me see the mechanism inside. When I mentioned to Aurelien that it looked familiar, he said “These were first developed for use in the cider business, to separate the pulp from the liquid”. These dewatering machines mechanically dry the sterile sludge, by continuously rotating and pressing it. The liquid is passed through membranes to clean it further, and the solids are dropped into the barn below. “Within the next two years, once we bring our new facility on-line, we'll send 50% of our sludge through this type of process," said Nick, "That's equivalent to the waste produced by 7.5 million people.”

The final stop of my tour was the barn itself. Inside the huge space, a digger was driving in and out of alcoves in the barn, each time returning to a waiting truck to load up the clean, dry sludge. Most of this goes to agricultural land – it’s the perfect fertilizer. But just outside the barn, construction workers were digging the foundations for the next stage of Crossness’s development; an Advanced Energy Recovery facility that will take some of the sludge and extract even more value from it!

Rather than burning it all in an incinerator, Nick and his team are building a pyrolysis plant. Pyrolysis uses thermal decomposition to transform some of the solid material into a gas; generally a mixture of carbon monoxide, methane and hydrogen (pretty much in equal parts). This fuel could then be put back into the site’s CHP plant, to produce electricity. Combining THP, sustainable thermal drying and pyrolysis, the Crossness team believe that they could almost double the sewage-to-electricity conversion rate, and this could see them becoming net exporters of electricity.

A study supported by the UK’s Department of Energy & Climate Change showed that if this combined approach was deployed across the UK, sewage sludge could generate an additional 1,310 GWh of renewable electricity every year. If I learned anything on my visit to Crossness, it was that for water and waste treatment to be truly future-proof, it needs to be self-sustaining – a genuine recycling of energy and materials. And it looks like Crossness is well on its way to achieving that.

--- Read the original story over on Forbes