Safer, cheaper way to treat solid waste

Tissa Fernando (left) and Mike Kurvink with sludge residue. Picture / Paul Estcourt

Auckland-based Flo-Dry Engineering has developed a safer and cheaper way to treat human sewage to target a global market the company says is worth millions of dollars.

Flo-Dry project manager Tissa Fernando said the worldwide demand for the technology used to dry out sewage sludge for disposal or recycling was worth €300 million ($597 million) a year and growing.

"The concept is changing," Fernando said. "There's a lot more pressure on treatment works to get rid of their sludge in a safe and beneficial manner."

A city roughly the the size of Auckland with one million people pumps out 250 tonnes of wet sludge every day, which once dried could be reduced to about 100 tonnes.

Many countries no longer allow the dumping of solid waste at sea, meaning it must be treated for safe use or disposal on land.

Once the sludge has been dried it can be used as a fertiliser or as a fuel with half the heating value of coal.

Flo-Dry already made sludge-drying equipment and had built plants here and in Australia but the international market was dominated by a competitor, Fernando said.

Despite the technological and price competitiveness of Flo-Dry's current offering, many customers simply opted for the better-known brand, he said.

In order to gain a competitive advantage Flo-Dry has spent two years and about $1.8 million - including investment of $534,000 from the Foundation for Research, Science and Technology - on a project to dry sludge in a cheaper and safer way.

Every tonne of sludge dried using traditional technology needs to be mixed with between three and five tonnes of already dried material for the process to work.

Traditional equipment ran at temperatures of up to 400C, putting the material at risk of igniting, Fernando said.

"Sewage material once you start drying it because of the carbon content, it becomes very explosive."

It was a risk that had caused some systems overseas to explode, he added.

Flo-Dry has developed a two-stage thermal drying method that doesn't require the reintroduction of already dried and combustible material and can run at up to 700C before later drying at the much lower temperature of about 90C.

WaterCare project manager Graham Barker said biosolids now being disposed of in landfills by the company were about 23 per cent solid, with the rest water.

Drying it out would increase the solid content to up to 95 per cent.