Fixing the holes where the rain gets in
16 December 2008
Fixing the holes where the rain gets in
When it rains heavily, stormwater gets into Wellington’s sewerage system – sometimes causing it to overflow into the harbour and Cook Strait – something which absolutely is not supposed to happen.
The overflows pollute the harbour and coastline and can make swimming and other recreational activities unsafe.
The rogue incursions of stormwater fill the sewage network beyond its designed capacity. Not only do overflows occur from the network throughout the city, but the huge volumes also overwhelm the city’s sewage treatment plants. It means untreated or partially-treated sewage must be allowed to ‘bypass’ the plants and enter the sea
The Moa Point sewage treatment plant, for example, is designed to deal with a maximum inflow of 3000 litres of wastewater per second. However during heavy rain the flow, swollen by stormwater, can reach 4000 litres/sec. This extra 1000 litres/sec can’t be treated and has to be pumped into Cook Strait.
It’s estimated that about half of this stormwater gets in through faults in the Council’s sewer mains – and the other half via holes, cracks and faulty joints in private drains.
The Council has spent more than $130 million upgrading the sewerage network since 1993. However work is now required on the private network if the overflow problem is to be overcome.
So the City Council’s Strategy and Policy Committee has given the go-ahead for a study on the possible ways of fixing the holes and cracks.
And depending on the recommendations in the report, property owners with leaky drains may face hefty repair bills.
Cr Celia Wade-Brown, the Council’s Environment Portfolio Chair, says the ‘inflow and infiltration’ (I&I) problem is a particularly challenging issue for the Council and the city as a whole.
“This is one of those problems that will be difficult for the Council to ‘sell’. Most people don’t even realise there is a problem – because few of us hang around Moa Point or the Karori sewage treatment plant when it’s pouring with rain. So the sewage overflows usually go unnoticed by the public.
“Most people wouldn’t imagine that the sewer pipe running off their property and safely buried in the ground could be cracked or holed – and that it could contribute to big problems at the beach or at the treatment plants.
“Even further than that is the challenge of getting property owners to realise that the private drain is their responsibility.”
There are an estimated 50,000 private sewer drains across the city. Property owners faced with renewing defective drains might be faced with costs ranging from $1000 to $20,000-plus - with a likely average cost from $4000 to $7000.
Council engineers and consultants have been asked to spend the next three months investigating and costing possible ways of dealing with inflow and infiltration.
The possible options are wide-ranging and expensive. They could include:
Increasing the capacity of the Moa Point and Western (Karori) treatment plants so they can deal with high volumes without the need for overflows.
substantial spending to increase the capacity of the infrastructure – this would likely involve building very large underground storage tanks at various locations around the city to hold large volumes of wastewater during and after heavy rains.
Requiring private property owners to upgrade or replace private sewers – especially those running from older properties.
A combination of several approaches, including different ways of funding work.
It is known that a small number of private sewer drain replacements are done each year – generally in conjunction with other building works, says Cr Wade-Brown. “Therefore it’s likely that a negligible amount of maintenance is occurring on most sewers on most properties.”
The Council’s Infrastructure Planning Manager, Maria Archer, says one means of compelling property owners to fix private sewers could be by including information about the age and presumed condition of the drains on land information memoranda (LIMs).
“The Council could require that private sewers and stormwater drains be inspected and, if necessary, fixed as part of a property sale process – so the condition and possible need for their replacement could become a matter of negotiation between buyer and vendor.”
Maria Archer says that, in conjunction with all of these options, it will be recommended that the Council fund an education campaign to inform property owners of the issues.