Nutrient recovery reactor turns human excrement into high-quality fertilizer

Slough's phosphorus-rich excreta is raw material for facility that converts waste into pellets for farm and garden use

Just a few yards from the choked M4 motorway, beyond the massive settling tanks and a steaming, 500-tonne mountain of black sewage sludge at Slough treatment works, a modern alchemy is taking place that could potentially keep the world in food for a few more years.

The plant is taking the tiny quantities of phosphorus contained in the poo of the Berkshire town's 140,000 people and turning it into high-quality fertiliser fit to grow organic garden vegetables.

At one end of the novel process in Europe's first "nutrient recovery reactor", the human waste is dark and "earthy" smelling. At the other end, bright white, odourless phosphorus-rich pellets drop into sacks. The sewage workers euphemistically say they are "harvesting pearls". Thames Water, which owns the facility, says it is making "Viagra for plants".

According to the water company, Slough's excreta has a "unique vintage", and contains more phosphorus than any other area in south-east England, possibly because of the quantity of meat eaten in the town or because it boasts several large food processing and pharmacueutical works. The company expects to make £200,000 a year from the combination of selling 150 tonnes of its fertiliser to farmers and gardeners, and not having to spend as much money on chemicals to unblock pipes.

"We reckon using this technology Britain could save 20% of the 138,000 tonnes of phosphorus fertiliser that it imports a year," says Piers Clark, Thames Water's commercial director. "Phosphorus is a fast-depleting, non-renewable resource which we will run out of. Without it, all life on the planet will take a nosedive."

It is the key ingredient in fertiliser and essential for farming, says Peter Melchett, policy director of organic trade body the Soil Association. "Without fertilisation from phosphorus, wheat yields will fall by more than half. This technology could offer a solution to securing global food supplies over the coming decades."

"Night soil", or raw human excrement, was traditionally valued highly and spread over fields but because it contains dangerous pathogens and contaminants it is now banned. Instead, treated sewage sludge that still contains some heavy metals is given free to some farmers to use sparingly. The advantage of Slough's renewable phosphorus fertliser, says Clark, is that it is clean of contamination and quantities can be tailored for different crops.

Mineable reserves of phosphorus in countries like Morocco, the US and China are set to be completely depleted in 100 years according to some experts while others say "peak phosphorus" will occur by 2035, after which it is expected to become increasingly scarce and expensive. Its price has risen 500% since 2007.

"The UK is heavily reliant on phosphate rock imports for food production. It can be a pollutant when its concentrations are too high because it leads to the explosive growth of algae which saps oxygen. Using it this way we can see it as valuable reseouce again", said Rosanna Kleeman, a researcher studying Slough's sewage.

"Slough has taken some stick over the years from the likes of Sir John Betjeman and Ricky Gervais, who immortalised its trading estate in The Office. But we are rebranding it as an eco-warrior [town] at the forefront of the effort to save the planet," said Clark.

A case, says one Thames worker, for Betjeman's famous line to be amended from "Come, friendly bombs, and fall on Slough!", to "Come, friendly bums..." © Guardian News and Media 2013

[Image credit: Pouring of chemical fertilizer on farmer hand over green background via]