Many years ago, I wrote a feature for 91av about an innovation in waste-water treatment called urine-separation toilets, which, at the time, looked like becoming a desirable accessory for the eco-conscious. To cut a long story short, the Western habit of flushing urine away using clean water then separating it out again in sewage plants is extremely wasteful. Toilets that collect undiluted urine separately and send it off for processing into fertiliser save large amounts of energy and water. We punningly dubbed it ‼-⳦Բ”.
Urine separation didn’t really get off the ground, and I must admit I had forgotten all about it. That is until I read a review paper in the journal Nature Sustainability called and discovered that urine separation is alive and well – along with other ways to recover useful compounds from sewage.
Traditional waste-water treatment focuses primarily on pollution control. Waste water in the form of toilet effluent (known as yellow water and black water) plus grey water from washing machines, baths, showers and sinks enters the system and is cleaned up to drinking water standards. Organic matter is mainly converted into carbon dioxide and nitrogen and vented to the atmosphere, while non-biodegradable material settles out as solid sludge and is sent to landfill or incinerated. This model has performed well in terms of preventing waterborne diseases and protecting the aquatic environment, the authors say, but it is extremely wasteful.
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In recent years, however, a paradigm shift has taken place. Instead of seeing sewage as a waste stream to be managed, it is being re-envisioned as a resource from which valuable products can be extracted. The technology now exists to largely circularise the waste-water system such that every bit of it is reused: sewage treatment plants are increasingly seen not as waste facilities but as chemical factories.
The first step on the journey to sewage circularity is already here, in the form of what the industry calls NEW factories, where NEW stands for nutrients, energy and water. Water is the easy bit – traditional sewage treatment is already very good at turning a mixture of yellow, black and grey water into the kind we can drink. The energy side is also quite mature. Around 11 per cent of Europe’s sewage treatment plants, according to the paper, use microbial digestion to convert organic material from sewage sludge into biogas, a mix of methane and CO2. This can be burned to generate electricity and heat, both of which can be used to sustain the treatment process.
Sewage treatment plants are increasingly being seen not as waste facilities but as chemical factories
Recovery of nutrients, mainly nitrogen and phosphorus, is more challenging, but increasingly doable. Around 25 per cent of all phosphorus applied to agricultural land as fertiliser eventually finds its way into sewage, according to the paper’s authors. If all of this were recovered and recycled, demand for phosphate rocks – a finite resource that could be – would fall by 15 to 20 per cent. The best way to recover phosphate is by precipitating it out of the sludge, a method that also works for recovering nitrogen.
Water, energy and nutrients are the low-hanging fruit, and they aren’t very profitable. But sewage also contains much more valuable commodities, which a few forward-thinking treatment plants are already recovering and selling.
One of those is cellulose, the main component of toilet paper. Around 35 per cent of the solid matter in sewage is cellulose, which usually ends up being buried or burned. But a few plants harvest cellulose from waste, clean it up and sell it to the construction industry. This unlikely market is already worth over $17 billion a year and its value is forecast to more than double by 2032.
The treatment of sewage also generates biodegradable plastic-like materials, notably polyhydroxyalkanoates (PHAs) and extracellular polymeric substances (EPSs), which fetch a high price. PHAs can replace polyethylene or polypropylene, while EPSs have replaced non-biodegradable polymers in fertilisers and seed coatings.
None of this is glamorous, but neither is conventional sewage treatment. As the old saying goes, where there’s muck, there’s brass, and we might as well make it rather than letting it all just add to our pollution problem.
Which brings me back to urine separation. Around 80 per cent of the nitrogen and 40 per cent of the phosphorus in sewage comes from urine, but these are hard to extract as urine is heavily diluted by sewage systems, comprising around 1 per cent of the total volume. There is, the authors say, “a compelling case for urine separation and nutrient recovery”. Don’t say I didn’t tell you so.
Graham’s week
What I’m reading
Inanna, a novel by former 91av editor Emily H. Wilson.
What I’m watching
Adolescence on Netflix.
What I’m working on
I’ve been doing a deep dive into the fascinating world of the small intestine.
Graham Lawton is a staff writer at 91av and author of Mustn’t Grumble: The surprising science of everyday ailments. You can follow him @grahamlawton
