Kaipara District Council could potentially discharge treated wastewater into the ocean off Mangawhai as part of future sewerage infrastructure expansion for the country's fastest growing coastal settlement.
Mangawhai is growing quickly with wastewater treatment under pressure Photo: Northern Advocate Photo: Northern Advocate
A yet-to-be confirmed quantity of Mangawhai's treated wastewater is potentially destined for the ocean as Kaipara District Council (KDC) grapples with an almost at-capacity treated wastewater disposal and major increasing sewerage infrastructure demand over the next decade and beyond.
More than 15,000 people are forecast to live in Mangawhai in 20 years' time, from the 4000 residents at the moment.
KDC infrastructure planning manager Eros Foschieri said ocean outfall was one of three options on the council radar. Discharging into Mangawhai Harbour is also on the table. Discharge onto land is a third option.
Foschieri said the quantity of treated wastewater likely to be discharged into the ocean had not yet been established.
"Further detailed investigations and public consultation needs to occur before we progress any further with investigating these options," Foschieri said.
The scheme's current disposal systems are at capacity, limiting opportunity to take more wastewater into the scheme's treatment plant, according to KDC's website.
The number of Mangawhai wastewater scheme connections has gone up by more than a third from 1860 since 2018. Connections are forecast to at least double during the next decade.
The scheme services 2500 connections, but this is forecast to grow to between 5000 and 7000 connections within the next 10 years.
The situation has forced KDC to look at an immediate option of discharging treated wastewater to the settlement's golf course. This is expected to take care of about 500 connections as a way to address immediate short-term needs. Wastewater would be pumped from Mangawhai's sewage treatment plant to a 500 cubic metre green steel holding tank in the golf club's carpark and spread at night.
KDC also wants to boost the wastewater discharge capacity of its Brown Rd farm about 10 kilometres south of Mangawhai, where it spreads the material across land.
Mangawhai Harbour's outgoing tide, a potential KDC wastewater discharge option Photo: LDR/ Susan Botting
Mayor Dr Jason Smith said converting the farm to a wetland could be an option. KDC is looking at additional property suitable for the new discharge to land option.
Foschieri said there was a lot of work to do to arrive at a decision on which discharge option to go with. He said there was currently no best option, each had its pros and cons.
"The best choice is what the community wants," Foschieri said.
The council is next month setting up an informal community group including representation from mana whenua, technical specialists, business, community group Mangawhai matters, a councillor and KDC communications staff.
Foschieri said the discharge option chosen was important because it in turn influenced how treatment plant expansion proceeded.
Huge cost overruns for Mangawhai wastewater treatment in the past resulted in the government putting commissioners into KDC. Ratepayers were initially told that scheme would cost no more than $10.8m when it was first announced in 2003, but that rose to $37m by 2009 and to more than $60m by 2013.
KDC ratepayers only last year finished paying off their allocation of Mangawhai wastewater treatment plant debt blowout repayment.
Tangowahine-based KDC councillor Victoria del la Varis-Woodcock warned ratepayers across Kaipara would now also potentially have to pay 50 percent of the cost of interest on money borrowed for the new planned future wastewater treatment expansion.
The KDC west coast/central ward councillor said the figure for the ocean outfall cost would undoubtedly increase from its current potentially $37 million - to as much as $70 million. Cost increases had consistently been the case for Mangawhai's wastewater treatment plan with a history she described as a "debacle" and a "woeful saga".
"In the wastewater world, is discharge to ocean the right environmental choice," del la Varis-Woodcock said.
Her comments came amidst a sometimes-heated one and a half hour questioning and debate during a Mangawhai wastewater scheme presentation by Foschieri at KDC's first 2022 meeting. This was a briefing meeting held face-to-face in the Dargaville Town Hall where del la Varis-Woodcock's attended virtually.
She said KDC needed to put the horse before the cart in the face of under-pressure Mangawhai infrastructure. It needed to control the amount of development there rather than spending big money on wastewater treatment expansion.
Del la Varis-Woodcock said Mangawhai developers' required KDC financial contributions paid towards the cost of infrastructure supporting their developments were "woefully inadequate".
She said the likely cost of Mangawhai golf course discharge has already gone from an original $1.5 million to as much as $6.5 million.
The cost of the Brown Rd land had initially been indicated at $361,000 but that had increased to $14 million.
Meanwhile, a new effluent storage or 'balance' tank being built at present has also increased in price. It was initially budgeted at $3.1 million but has increased in cost by 65 percent to $5.1 million.
Councillor David Wills said evidence indicated Mangawhai's growth was potentially outstripping even KDC's highest 2018 forecasting. He questioned what would happen if the 5000 connections forecast for 10 years' time happened earlier.
Wills also questioned what would happen to capacity requirement predictions should the pipeline bringing Whangārei District Council water from Bream Bay to Mangawhai happen. The pipe would bring reticulated water to a community which currently used only water tanks. Residents would use more water, in turn affecting the amount of wastewater produced.
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