Humans have been using their own waste matter as fertiliser for thousands of years, but with modern treatment processes biosolids have never been safer, according to one expert.
Professor Mike McLaughlin, a science fellow with the CSIRO and professor at the University of Adelaide, has studied soils and the use of biosolids for a number of years, and said the treatment process makes sewage safer even than other organic fertilisers.
“Potentially soils can carry disease – soils are full of bacteria, too. But if [biosolids] are treated properly, there shouldn’t be any additional risk. It’s probably safer than using cow manure, for example – cow manure isn’t treated,” he said. “People think fresh is good, but sometimes it’s not.”
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Professor McLaughlin said the Chinese were using human waste as fertiliser 2000 years ago, and pointed out that the colloquial name for waste treatment facilities, sewage farms, originates from late 19th or early 20th century England, when the waste was sent to actual farms for recycling and reuse.
The treatment process for the sewage usually involves three phases – sedimentation, biological treatment to break down organics, and stabilisation to make sure odours and volatiles don’t attract flies.
The biological treatment includes putting organisms into the waste to process the organic matter, which breaks down the natural nasties that live in sewage. But problems start to arise when contaminants that can’t be broken down – such as pesticides or metals – are introduced into sewage systems.
“It’s not actually poo coming out the other end. It’s dead bugs, basically, and things that can’t be degraded. In that sense, the human poo has been totally recycled into microbial bodies,” Professor McLaughlin said.
“Everything we put down the toilet, and everything the industry puts down the drain, will go through the waste water treatment plant, and if it can’t be degraded there, it’s either going to come out in the water or the solids.
“If the sewage treatment plant is working correctly, the organisms, as I say, are not the big issue. The big issue is more the trace contaminants, to make sure they don’t build up to a level that will, over the long term, affect agricultural production.”
Professor McLaughlin said biosolids are used in a number of regions, including in NSW, WA, and SA, and said it’s important not to waste the waste.
“To flush that stuff into the ocean would be a real shame because the world’s short of nutrients, and nutrients are becoming more expensive and food’s becoming more expensive, so it’s a very wasteful process to throw that away. So it’s really important that we try to recycle it,” he said.
In the ACT, biosolids are burned before they are recycled, which Professor McLaughlin said was a more certain way to kill any bad organisms, and reduced weight, but it was expensive and left a less nutrient-rich product.
“It does then save on transport costs because obviously you’ve removed all the water. But unfortunately you’ve removed a lot of the organic matter and nutrients, so from that point of view it’s not good because you’ve removed a lot of goodness from the material.”