'Squeezing' sewage may save millions

19:19, Apr 07 2014

Landfill costs and technology development delays are likely to see the Palmerston North City Council invest $253,000 in new equipment to squeeze the moisture out of sewage sludge so it can be used in compost.

The move could help avoid a bill of $3 million to $5m for dumping sludge at the Bonny Glen landfill when the wastewater treatment plant runs out of storage space every seven to 10 years.

Water and waste services manager Rob Green said the dewatering process would help extend the life of a wet storage lagoon and a drying bed at the Totara Rd plant until new options became available.

The estimated project cost for buying and installing the dewatering unit has come in well below the Annual Plan budget of $800,000.

It would add just over $100,000 a year in operating costs.

The unit would be used every day to cope with sludge as it came into the plant, producing material that can be mixed with green waste to make compost. The compost is used to cover the closed Awapuni landfill in Palmerston North.


The organic content means it cannot be sold commercially.

The council already uses a centrifuge system to spin moisture out of sludge produced in the clarifier during the 100 days a year when it is used to remove phosphorus from wastewater. The centrifuge reduces the amount of sludge from 135 cubic metres a day to about 2.5 tonnes of dry solids.

Green said an option would have been to use that unit during the balance of the year to remove water from primary sludge - about 150 cubic metres a day that is still 95 per cent water.

However, he was worried about the potential costs of wear and tear if the centrifuge was in constant use, and said the new belt press would be more effective in dealing with primary sludge.

Capacity to store and dry sludge in two lagoons at the plant would be reached within four to six years if they kept filling at the current rate.

Green was optimistic that by stretching their life out to a decade or more, there would be new technology available to break sludge into its constituent, reusable parts.

One of the technologies offering future hope is wetox, or wet air technology.

A pilot plant to test its commercial possibilities was set up at the wastewater treatment plant last year using a $1.1m grant from the Government's waste-minimisation fund.

However, that experiment is on hold.

Wetox manager Don Smith said further research was needed on devising a heater element for the small unit, as previous components had burnt out.

"The trial was initially promising, but we need to keep it operating for a sustained period before we can demonstrate it to commercial investors.

"The council has been very supportive allowing us to remain there but, overall, it's been a disappointing year."

Deputy mayor Jim Jefferies said buying the new machine was a low-risk option.

But not all of the councillors at yesterday's planning and policy committee meeting were convinced, with the spending recommended on an 11-3 vote.

Councillor Chris Teo-Sherrell said the spending was not essential as there was still capacity in the lagoons for storage, and there was scope to make better use of the centrifuge.

Manawatu Standard