SWITZERLAND • If the banana trees at Zoo Zurich are particularly lush, it is thanks to a fertiliser with an unusual ingredient: human waste. During spring last year, zoo employees cleared a bamboo grove in Zurich's Masoala Rainforest to plant the trees.
Within a few months, the saplings had reached an impressive height and produced a cornucopia of yellow fruit. "We were really surprised how fast the plants put down roots," said Mr Martin Bauert, curator of the tropical area of Zoo Zurich.
The reason for this fast growth has a name - terra preta - which is Portuguese for "black soil". It is a particularly fertile substrate created from compost, charcoal (biochar) and human faeces.
The company that provides Zoo Zurich with the fertile substrate is Greenport, started in 2015 by four friends. "We wanted to break some taboos with our products," said Mr Tobias Mueller, a former carpenter and inventor.
To obtain the raw material, the start-up team developed a mobile dry toilet, the Greenport.
The human waste drops into a container, which Team Mueller carts to a pyrolysis facility. Pyrolysis is the chemical decomposition of organic materials through the application of heat.
The nutrient-rich matter is exposed to temperatures of up to 800 deg C, destroying toxic germs, viruses and hormones but leaving nutrients, trace elements and water intact.
The process yields charcoal (biochar) with a high storage capacity, and this extracts toxic substances from the soil and returns water or carbon dioxide to it.
The biochar is supplemented with compost and soil organisms to obtain terra preta.
"We take human waste back to where it belongs - nature," Mr Mueller said.
Greenport practises in miniature what science has tried to work out on a much grander scale: How to recover valuable substances from sludge.
In Switzerland alone, sewage treatment generates 200,000 tonnes of sludge each year, from which 6,000 tonnes of phosphorus - a high-grade nutrient for the production of fertilisers - could be recovered.
For the moment, the annual production of terra preta is limited to 200 cubic m, though Mr Mueller's start-up may well lead the way for production on an industrial scale.
One and a half years after Greenport's foundation, initial investments in production have been amortised, and Mr Mueller envisages the creation of a partner-network-system with branches across Switzerland.
"Rented toilets are a market of billions. If we can have a small part of it, we're satisfied," he said.
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