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The Mangawhai Wastewater Treatment plant recycles water to irrigate farmland.
Kaipara District Council is upgrading the Mangawhai Wastewater Treatment Plant to meet increased demand from development and an increased population over summer.
A new balance tank will be installed to cater for peak demand periods. The tank stores high volumes of waste-water temporarily before allowing it to flow slowly through the system so that it is not overwhelmed.
The plant typically processes 600 cubic metres of water a day but processes double that amount during the summer peak period.
Works manager Eros Foschieri said the plant currently services 2000 households and has capacity for another 1000, even prior to any upgrade.
In addition to the balance tank, the plant’s pump network will need to be upgraded to accommodate new developments, including Mangawhai Central.
Kaipara Mayor Dr Jason Smith said the upgrades would be funded by the recently announced 3.97 per cent rates increase, but would eventually be covered by developer contributions, ensuring costs would not ultimately be borne by ratepayers.
“Infrastructure has to be built by Council before development can start,” Dr Smith said.
“We have always been planning for this. It’s not just Mangawhai Central or the summer peak – there will always be growth.”
He said additional connections to the plant would spread liability across more households, reducing maintenance costs for ratepayers in the long run.
When the plant was first built 10 years ago, it was only servicing 1200 households.
According to the Council’s current modelling, the plant’s load is expected to increase by 100 connections each year.
The balance tank, which will be constructed in 2021, will eventually be converted into a fully operating treatment tank to meet increasing demand over the decade.
Mr Foschieri says it is expected that the plant will then reach capacity in 2028, at which point a further treatment plant will need to be built.
He says by 2043, it is expected the plant will handle 6000 connections, three times what it does today.
Circular system from toilet to table
The Mangawhai Wastewater Treatment Plant last year recycled 240 million litres of water, which was used to irrigate farmland.
The water is currently pumped 4km from the plant to irrigate 65ha of land.
The Council is investigating supplying water for irrigation to the Mangawhai Golf Course, local parks and roadside gardens in new developments in 2021.
Dr Smith said the plant was a gold-standard water recycling facility, the envy of drought-stricken districts.
“In 20 years if the climate hasn’t improved, this facility could be used to keep Mangawhai green,” he said.
“Instead of worrying about how to get rid of the water, we expect to be fielding enquiries from farmers who are looking for good water for irrigation.”
Although there are no plans to bottle it up as Mangawhai-branded drinking water, engineers say it is possible.
Wastewater treatment engineer Andrew Springer said it would only need to be filtered through a membrane to remove bacteria and go through reverse osmosis to remove salt to be drinkable.
KDC general manager of infrastructure services Jim Sephton says that if it wasn’t for the treatment plant, all 600 cubic metres of waste would be heading into the harbour each day.
“The fact that we can swim in the sea without getting sick is huge.”
Engineers care about pet bug, Cass
A dedicated team of engineers and operators spends each day ensuring their “pet bug” of bacteria is strong and healthy enough to do its job.
The treatment plant uses a cyclic activated sludge system (CASS) that employs bacteria to break down ammonia and organic compounds in waste matter. The bacteria effectively consumes human faeces.
Operator David Allan takes a sample twice a day of the sludge that emerges from the bacteria to check on its health.
“If the bug is breaking down nitrogen well, more sludge will settle at the bottom of the tank.”
Thirty-five cubic metres of sludge is produced every day. It is then compressed to remove any final traces of water and sent to landfill.
The clean water rises to the top of the tank and is siphoned off for use in irrigation.
Occasionally, some of the bacteria is drained to ensure it doesn’t grow too big.
“A treatment plant like this would double its biomass in a week if left unchecked,” David says.
If the bug becomes ill, then it might be fed molasses to bring it back to health.
David says they must keep it lean by monitoring its fat intake. If Cass gets too much fat, it will grow too quickly and die.
For this reason, households are meant to tip oil from their fry pan into a bottle and then compost it, not pour it down the sink.
A screen is used to block out plastics, rags, sanitary products and anything else that can’t be broken down by the bacteria.
The plant is apparently one of the best smelling waste facilities, thanks to a sophisticated system that draws odour from the surface of the water.
The air is pumped into a storage tank and filtered through tree bark. This removes smelly methane and hydrogen sulphide.
