Phosphate mines will run out across the world, which is bad news for agriculture, as this mineral is essential for the growth of crops. Luckily, there is a solution: recycling phosphate from our waste flows, for example human excreta. In the Netherlands, regional water authorities treating wastewater search for innovative ways to recover phosphate from it, but the waste water sector alone cannot solve the phosphate problem. In her postdoctoral project at Wageningen University and Research, Saskia van der Kooij looks at ways in which the various links in the phosphorus cycle might work together better to successfully recycle phosphorus from human excreta. And that quest begins… with the humble toilet.
“In the Netherlands, developments to recover phosphate from wastewater are mainly geared towards struvite or sewage sludge ash," says Saskia van der Kooij, researcher at the Environmental Policy group, Wageningen University and Research. “While regional water authorities are well underway, there are additional ways to keep phosphate within the cycle. Our project looks at what we already know about the different recycling methods and how we can encourage cross-pollination by linking different strands of expertise. Colleagues from a range of other disciplines are involved, from crop system science and soil quality to environmental technology.”
Struvite from wastewater
The multidisciplinary project is named ‘From toilet to food’ and starts with a literature review to map the expertise that is currently available. “I’m surprised by what we do already know,” Van der Kooij states. “For example, struvite can be produced during the treatment of wastewater, but we do not exactly know what happens in the soil when struvite is used in agriculture. We do know more about other ways to apply recycled phosphate on agricultural soils, however: in the US and UK, for example, sewage sludge can be spread over agricultural fields, and long-term studies have shown how contaminants that may accompany the sludge behave in the soil. We can deploy this kind of knowledge in the development of new methods to return phosphate to the fields and better understand whether possibly accompanying contaminants are harmful on the long run.”
All the knowledge
“We need to combine all the knowledge we can get,” Van der Kooij emphasises. For instance, there is much to learn from crop science. “A hundred years ago, ‘night soil’ collectors ‘’ would drive through the streets of cities to collect buckets of human waste and bring them to farmers. The root systems of the plants back then were adapted to extracting nutrients efficiently from that waste. Today’s crops are fully adapted to synthetic fertilisers, however. If we now want to close the phosphorus cycle with sludge compost or struvite instead of synthetic fertiliser, they might have to switch to crop varieties with different root systems. Farmers could consider intercropping – planting different crops that jointly make the most of the timing of the release of phosphate from struvite.”
A serious game
To close the circle, gaps need to be closed throughout the chain. This is why Van der Kooij and her project team are starting a so-called ‘serious game’ – a ‘game’ with a scientific purpose – in which it can be simulated what happens if all parties involved – from food consumers to toilet users, work together on comprehensive solutions. Such solutions could start at the toilet. But what would toilet users think of that? “Our projects aims to address that question as well. We will certainly have taken a step forward if we can find a way to more easily talk about possible relationships between food and human waste.”