Human waste is no waste at all

Human waste could be used to combat climate change, helping to lock up our carbon emissions and build more fertile soils. This is according to Professor Nanthi Bolan of CRC for Contamination Assessment and Remediation of the Environment (CRC CARE) and University of South Australia, who spoke yesterday at CleanUp 2013 in Melbourne.

Biosolids are the solid waste left over after sewage treatment, and Australia produces around 300,000 tonnes of it a year. Large heaps are stockpiled around Australia’s cities - around 70,000 tonnes, according to Professor Bolan - and are largely unused. But Professor Bolan said that “if applied to agricultural soils or revegetation projects, biosolids - as well as increasing soil fertility and boosting plant growth - can lock up carbon”.

Professor Bolan said this is because “biosolids contain nitrogen and other nutrients for crop growth, organic matter to improve soil structure and non-degradable forms of carbon that will stay in the soil for a long time”. He added that Australian biosolids are generally low in toxic heavy metals, making them particularly suitable for agricultural soils.

Furthermore, said Professor Bolan, the cost of transporting the biosolids and spreading them on the soil would be offset “not only by the boost to fertility and soil organic structure, but also by its ability to increase carbon retention in the soil”. Their use could thus be used to generate carbon credits.

He said the amount of carbon that can be locked up depends on several variables - how the biosolids were stabilised at the sewage works, the type of crop they fertilise, and the levels of iron and aluminium in the soils - which combine with, and consequently lock up, the carbon in biosolids.

“Up to 30% of biosolids can consist of non-degradable carbon - and we need to encourage the use of the right sorts of processes in our sewage works to maximise this,” Professor Bolan said.

“If you used biosolids for tree crops or for landscape revegetation, it would then lock up more of the mobile carbon for a much longer time than an annual crop like wheat or canola.”

A further benefit of biosolids is that they contain not only the major nutrients nitrogen and phosphorus but also micronutrients and trace minerals which are increasingly lacking in the modern Australian diet, and whose lack is linked to a number of lifestyle diseases.

“By recycling our waste through agriculture, we could in fact be helping to improve the nutritional quality of Australian food, conserving scarce nutrients and improving public health,” said Professor Bolan.

“Some people may not like the idea of using human waste in the food system, but in reality we have done this for thousands of years - and modern biosolids are far cleaner and safer than untreated waste.

“At a time when fertilisers are becoming ever more costly and soils degraded, biosolids offer a major source of low-cost nutrients and a source of better soil quality.”