Queenstown Lakes District Council strategic project manager Martin O'Malley checks drying sludge. Photo by Christina McDonald.
Ever wondered what happens to sewage after you flush it away?
For most people, it is a case of out of sight, out of mind,
but the process to "desludge" sewage-treatment ponds in
Queenstown has been ongoing since March at a cost of more
than $1 million.
The task is nearing completion and Queenstown Lakes District
Council strategic project manager Martin O'Malley said the
three ponds should not need to be desludged for another 10-15
Mr O'Malley said the purpose of the Shotover oxidation ponds
was to treat the waste through a "purely natural process" by
encouraging three layers: aerobic, anoxic and anaerobic.
Since they were built in 1973, the ponds have accumulated up
to 30% of sludge, which was reducing their volume, and liquid
waste was spending less time in the treatment ponds before
filtering into the maturation pond.
After passing through the maturation pond, liquid waste is
discharged into the Shotover River, which is a legal and
The council contracted Conhur to desludge the ponds at a cost
of $1.1 million, and this began in March.
Sludge taken from the ponds is sun-dried, to reduce the
moisture, on adjoining land near the airport runway. At this
point, it has very little odour and carries essentially the
same risk of disease as regular compost.
The dried waste is then deposited in landfill at a cost of
between $76 and $159 a tonne, less moisture equating to less
While primarily human waste comes out of the pipes into the
pond, occasionally there have been tennis balls, tea towels
and even a hamster, Mr O'Malley said.
He said the liquid discharged into the Shotover River should
be green in colour, which might concern some people. However,
the colour showed there were algae present, evidence of the
natural treatment process.
More than 8500 tonnes of wet sludge has been taken from the
ponds and between 2300 and 2800 tonnes of dried sludge will
be going into landfill.
"At the moment, there's not an accepted use other than
putting it into landfill," Mr O'Malley said.
Although the dried waste was only as toxic as general
compost, Mr O'Malley said people were reluctant to use it
because of where it came from.
The Lake Hawea oxidation pond is next in line for desludging,
with contractors scheduled to start on the $150,000 task in
the new year.