Waste Goes To Hotrot

Palmerston North City Council is the first customer of a compost-making machine developed for New Zealand conditions by LincLab, a Wool Research Organisation subsidiary.

The machine, known as HotRot, is sited at Palmerston North’s sewage treatment plant. The compost will be used initially as landfill cover, which the council has previously had to pay for. Other uses include roadwork restoration and in council gardens and parks.

The device, developed for use by industry and smaller communities, consists of an insulated horizontal tank several metres long, into which waste is fed. An auger, or spiral screwdriver, pushes the waste from one end to the other over 10-20 days, depending on material being composted.

“It’s not aimed at big-volume green-waste – only composting,” LincLab general manager Peter Robinson says.

“Many overseas technologies are not suited to New Zealand conditions. Some cities in the United States and Europe have the population to produce hundreds of thousands of tonnes a year. The technology doesn’t scale down easily for New Zealand.”

HotRot is aimed at multi-waste composting for communities from a few hundred to about 300,000 people.

The unit – developed with funding from the Foundation for Research, Science and Technology – deals with smelly or other obnoxious or difficult-to-handle rotting wastes. These might include sewage grit and screenings, industrial and sewage sludge, restaurant and household food wastes. Green-waste, sawdust, paper, straw and chipped wood can also be processed as amendments.

Dr Robinson says the device could solve the problem of what New Zealanders, and many similar-sized communities overseas, do with a lot of their rubbish.

“Small-town dumps have closed or are closing, and councils now are moving to regional landfills. Fifty to 60 percent of stuff going to landfills is compostable. Twenty-nine out of 73 councils have integrated a ‘zero waste to landfill’ or some other waste-reduction target in to their strategic planning.”

He says the typical backyard compost bin is inefficient and often does not result in true composting because it lacks oxygen and does not get hot enough.

“Composting breaks down organic matter by the actions of micro-organisms. This process requires plenty of oxygen to allow the micro-organisms to grow and to avoid the production of unpleasant odours. Temperature rises naturally in the mix because of what those microbes are doing. High temperatures – up to 80C – speed up the breaking-down process and ensure the resulting compost doesn’t harbour microbes that could pose a health risk,” he says.

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