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The pay dirt of El Dorado
The high surface of char holds nutrients and bacteria.

Does the Amazon have lessons for managing waste organics? Ron Wainberg and Adriana Downie investigate.

The origin of the ‘Terra Preta’ story has little to do with modern waste management; it goes back to the fable of El Dorado and the search for gold and fortune in South America. The fable tells of Conquistador Francisco de Orellana’s return to Spain in the mid-16th century with stories of great cities and wealth in the Amazon basin. Subsequent expeditions found no evidence to support his claims and they were eventually dismissed as fantasy. Over time, scientists also gave the story no credibility as the soils in the Amazon were noted to be so poor it was doubtful they could have supported the level of agriculture that El Dorado would have needed in order to be viable.

However, some 400 years later, archaeologists noted that areas of dense forest in an otherwise savannah environment were particularly good sites for digs. The curious thing about these areas was that the soil was rich and dark compared to the typical poor yellow soil found elsewhere. It was known by the locals as Terra Preta (Black Earth) and was often mined by local entrepreneurs and sold as a particularly good growing medium.

The origin and nature of this dark soil was a mystery. It was seen to be so fertile it would grow nearly anything; a stark contrast to the yellow soil nearby that was so unsuitable for agriculture. Eventually, some soil scientists performed detailed investigations and their analysis gave a surprising result: the two soils were found to be exactly the same, except the Terra Preta contained large amounts of charcoal. Today, it is present at about 30 per cent in the rich soil.

An analysis of field data covering the extent of Terra Preta and the distribution of archaeological finds suggests the ‘dark earth’ areas could have supported a population of some three million people. Does this give credibility to El Dorado? Even more intriguing from a modern sustainable development perspective, it seems the Amazon’s indigenous people had developed the idea of turning their agricultural waste into charcoal, which was then ground up and added back to the soil. This finding has exciting possibilities in waste management, energy generation and soil sustainability.

Self-supporting char goes far
Charcoal production is easy and has been around for centuries. Wood is heated to a moderate temperature (say about 500ºC) in the absence of air, or in an oxygen-lean environment. Most of the volatile compounds are driven off, leaving carbon and ash. Similarly, heating any biomass under these conditions will produce charcoal, or char.

The off-gas (syngas) from the process has an energy value and can be used internally to fuel the process, with a well-designed system easily self-supporting and any surplus recovered for external use. Using this pathway to produce renewable energy has significant advantages as it not only displaces fossil fuel use, but it is actually carbon negative, considering that a portion of the carbon is returned and stored in the soil in the form of char over the long-term.

Charcoal has many applications, as a fuel, in metallurgical applications and as a filter or purification medium (activated carbon has a large number of pores and a very high specific surface area). However, grinding up the charcoal and blending it with soil is something new for the modern world. Perhaps the 21st century could learn something from the Amazon Indians.

Exactly why charcoal seems to be so beneficial to the soil is not fully understood and is the subject of ongoing research by organisations such as the NSW Department of Primary Industries, BEST Energies, Oil Mallee Company of Australia, the University of Notre Dame and the WA Department of Agriculture and Food. Conventional wisdom has been that the charcoal is inert and at best could help break up a clay matrix. It is quite stable in the environment, with plenty of evidence of charcoal residues from old bushfires persisting over long periods.

Carbon and compost
While carbon is an essential component of a productive soil, it is not a common agricultural practice to restore depleted levels. The compost industry is looking at ways of addressing this through the addition of compost. However, the tyranny of distance means the bulk of the compost is produced a long way from the soils that need it. Also, not everything can be composted. For example, woody material, or contaminated biomass are not suitable feedstocks for composting. They can, however, be charred.

One tonne of wet biomass will produce about 0.25 tonnes of char, which completely changes the transport economics as the product char weighs less than the original feed and is much denser. Additionally, a large part of the mineral content is retained – in fact, these minerals are now more concentrated.

Charring organic waste has huge potential in Australia where soil carbon levels are among the lowest in the world. If it can be demonstrated here, there is opportunity for not only the management of solid waste and recovery of resources, but also for the production of renewable energy and increasing the sustainability of agricultural soils. All this must, of course, be balanced against the cost of producing the char, so there is still a lot of work to do. But if the benefits are successfully demonstrated, a technique used by the ancient Amazon Indians may indeed have lessons for waste management and resource recovery in modern Australia.

Ron Wainberg is from Hyder Consulting (ron.wainberg@hyderconsulting.com) and Adriana Downie is from Best Energies (adriana@bestenergies.com.au).



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