POWER OF THE SUN

HELPS CUT LANDFILL SLUDGE

Selwyn council had a problem in 2011. Fast-growing communities in Prebbleton, Lincoln and Rolleston were putting pressure on its waste management system, and there was an influx of people as a result of the Christchurch earthquakes.

Deciding there had to be a better solution than moving tonnes of sludge by road from its effluent treatment plant in Eastern Selwyn to a landfill, the council asked MWH New Zealand to look at the issue.

The firm redesigned the original plant to expand it as a waste sludge gravity thickener and aerobic digester, attached a dewatering plant, and solar air drying hall that includes a robotic sludge turner.

Rainer Hoffmann, the firm’s chief process engineer, says: “The solar drier is the first in New Zealand. It is a test to see whether it can work with the South Island’s cooler climatic conditions in winter. These systems are used in Australia, but it is hotter there. But the Germanmade system is used across Europe. Results so far prove the idea works well.

“The solar drier works on natural heat and ventilation. The biomass is still quite active, so there is heat generation, and as it is turned by the sludge manager it brings in oxygen, and in that process you generate more heat, which kills off lots of the pathogens.”

Rainer says there are no odous to manage because the sludge is already quite well stabilised when it is spread in the drying hall, “by the time the sludge goes into the greenhouse it is already at an advanced stage of stabilisation”.

The new system is designed to handle 500 tonnes of dry solids a year. Right now it is handling the waste of 30,000 people, but the installation has been future-proofed to meet the needs of 60,000 people. Expanding the capacity of the plant will essentially mean duplicating much of the equipment that’s been installed.

Rainer says: “We are achieving 90 percent dry solids, and everyone is pleased with how well the syetm is working.”

Currently, all the dry biosolids are being stockpiled at the Kate Valley landfill site near Christchurch. But there are plans to dilute it with ‘green’ waste and spread it on land (subject to resource consent). Rainer says there is a huge potential for this technology in New Zealand.

“It’s relatively low-tech, low energy, and makes use of the sun – it is sustainable, environmentallyfriendly – it gets all the ‘green’ ticks,” he says. “One of the biggest problems in New Zealand at the moment is that there are is a lot of sludge and ponds, and what do we do with the sludge?

“Because the sludge contains so much water, when it comes to transporting it, you are really moving a lot of water (80 percent) and 20 percent solids. With the technology we have used, the council ends up with just 10 percent water.

“There are lots of facilities in the country that could use this technology, maybe 20 to 30, there are small communities and dairy waste too.”

The drying hall for biosolids has provided a cost saving of about $3 million to $4 million for Selwyn council by reducing the sludge destined for the landfill.