How Office Urinals Are Generating Electricity and Recovering Nutrients from Urine

Pee breaks are one of the most relaxing times in the office when you finally get to have some sane moments to yourself. Which is why most employees take numerous such breaks within a day. But what if your normal pee break had you secretly contributing to the generation of electricity? Two hundred and sixty men walked into an office toilet in the north of the Netherlands and became a very small power station. Their contributions went down a dedicated urinal, out through a pipe and into a shipping container parked beside the building. Water board Friesland had asked its male staff to use that urinal, as reported by Dutch Water Sector. It fed a pilot rig for bio-electrochemical treatment, built by Wetsus, the water technology research centre in Leeuwarden. The rig pulled phosphate out of the stream, captured ammonia and generated electricity. Wetsus said the output was enough to run the water board's electric cars about 50km a day.

How bacteria turn urine into current

Bacteria make their living by breaking down organic matter, which means stripping electrons off it and handing them to something else, normally oxygen. Take the oxygen away, put a carbon electrode in front of them, and they hand those electrons to it instead. Wire that to a second electrode across a membrane and the electrons travel the long way round, through the wire. Current, produced by microbes doing what microbes already do. No turbine, no combustion, no moving parts anywhere in the box. Urine happens to be excellent bacteria food: salty, conductive, and loaded with urea. Writing in Trends in Biotechnology, Pablo Ledezma and co-authors from the University of Queensland and Wetsus made the case that separately collected urine is an unusually promising feedstock for these systems, precisely because it hasn’t yet been diluted into the general wastewater stream.

A fertiliser is worth more than the watts

A fact sheet from Eawag, the Swiss aquatic research institute, attributes 85 to 90% of all nitrogen in domestic wastewater to urine alone. Phosphorus runs 50 to 80 per cent, potassium 80 to 90. Every bit of that arrives inside well under one per cent of the total volume. Nitrogen, phosphorus, potassium: the three numbers printed on every bag of fertiliser. Sewage plants currently spend energy aerating tanks to destroy that nitrogen, while fertiliser plants burn gas to manufacture more of it somewhere else. Catching it at the toilet skips both errands. In a paper published in Water Research, Philipp Kuntke and colleagues ran real undiluted urine through an air-cathode cell and recovered ammonium at 3.29 grams of nitrogen per square metre of membrane per day. The energy balance came out with a surplus of 3.46 kilojoules per gram of nitrogen, meaning the cell made more electricity than the recovery step spent.

A related reactor makes hydrogen

In both devices, bacteria clinging to the anode break down organic matter and hand off electrons the same way. What differs is the cathode. In a fuel cell, those electrons meet oxygen there and current flows out on its own. In the hydrogen version, properly called a microbial electrolysis cell, the cathode is kept oxygen-free and wired for a different job: protons pick up electrons and become hydrogen gas, bubbling off to be collected. That reaction needs a push from outside, a small voltage fed in on top of what the bacteria supply, because the anode’s own electron flow can’t quite force it through alone. Kuntke’s group reported in the International Journal of Hydrogen Energy that five-times-diluted urine in a microbial electrolysis cell produced hydrogen at 48.6 cubic metres per cubic metre of reactor per day, while stripping out ammonium and organic load at high rates. Sadly, the output is humbling. Field trials run by the University of the West of England and documented in Environmental Science: Water Research & Technology had a campus urinal wired with 288 fuel cells averaging 75 milliwatts, and a Glastonbury Festival version with 432 cells averaging 300. Plenty for lights inside a cubicle. Nowhere near a kettle. That Glastonbury unit was handling roughly a thousand users a day, which gives some sense of how much biology it takes to keep a light on. Ammonia drifting back across the membrane cut short the stable operating window in the hydrogen work. Kuntke’s team later moved to a hydrogen gas recycling design, published in ACS Sustainable Chemistry & Engineering, aiming to get ammonia recovery working at a larger scale with less energy. A 2020 review in ChemElectroChem traced the whole field back to three papers published independently in 2012, in Britain, the Netherlands and China, and found most of what followed still stuck at proof of concept