Mining Our Manure Mountains
A mountain of waste that grows by a million tons every three years from the sewage plants of Australia’s major cities and towns is the target of an unusual mining venture.
Scientists in the CRC for Contamination Assessment and Remediation of the Environment (CRC CARE) are working on ways to turn one of the nation’s major urban organic wastes into a safe source of fertility for the continent’s depleted soils.
In a 21st century quest to turn “˜manure into gold’ they are using one of Australia’s most sophisticated scientific instruments, the Australian Synchrotron, to find new ways to lock up the toxic heavy metals which accumulate in human waste, so the beneficial nutrients it contains can be used to enrich farmland.
Associate Professor Enzo Lombi and Dr Erica Donner of CRC CARE and the University of South Australia are working to discover new ways to ensure metals like cadmium and lead in the biosolids from urban sewage don’t re-enter the food chain.
At the same time they want to make available the vital nutrients phosphorus, nitrogen and potassium as well as micro-nutrients like copper and zinc, and organic carbon, to enrich our soils and grow better crops and pastures in a time of rising global food insecurity.
The hope is that urban waste can supply nutrients to help maintain the fertility of Australian soils in the event of growing global nutrient scarcities and soaring fertilizer prices.
“Australia’s sewage works produce a total of over 300,000 tons of biosolids every year, derived from the settling process in primary treatment and the waste bacteria from secondary treatment,” Prof Lombi explains.
“This contains several thousand tons of valuable elemental nutrients. Some is used to improve soils now, but you can’t apply too much in a small area or you risk contaminating the soil with the heavy metals it contains. Most biosolids material is simply stored in huge dumps, where it poses a long-term management issue.”
Municipal wastewater contains almost every metal known to man ““ from iron to gold and silver. However most of these are in quantities too small to extract economically. The presence of toxic metals like lead and cadmium is a major obstacle to the widespread re-use of biosolids as soil improvers.
The team are using the synchrotron to study the chemistry of the bonds that bind these toxic metals to particles within the waste and to pioneer novel ways to bind them more permanently, so preventing their mobility into the environment. This offers a much lower cost solution than attempting to remove the unwanted metals.
Dr Donner says the synchrotron is being used to investigate how particular forms of the target metals bond to other particles and minerals and whether the bonds are durable enough to stop them from being available in the environment over a long time.
“Also we want to optimize how these metals are fixed or released,” Dr Donner says.
“For example, you may want the cadmium and lead to be locked up tight, but the nutrients available to improve plant growth in deficient soils.”
The synchrotron is also being used to create “˜maps’ of the metals in biosolids, so that their interactions with various solid particles can be studied in detail.
Apart from tackling one of society’s major waste headaches ““ what to do with the mounting piles of biosolids accumulating in our cities ““ their research also offers an important approach to dealing with the looming global problem of food insecurity.
One of the drivers of food insecurity is the fact that mined nutrients are finite, whereas world demand for food is rising strongly as populations and economies grow. The hunt is now on, says Prof Lombi, for alternate sources of strategic minerals like phosphorus and potash.
“If we can recycle nutrients from the world’s urban waste streams ““ as indeed our ancestors did for hundreds of generations ““ we can make the food system more sustainable and help protect both farmers and consumers against the impact of global nutrient shortages,” he says.
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