Tantragee Water Treatment Plant

Why the water needs to be treated

Safe water, available to everyone, is a fundamental requirement for public health. Council's previous treatment system relied on screening the water through 1.5 mm mesh (similar to a tea strainer) and adding chlorine. This lack of control had seen boil water orders issued five times in prior years, and it was not acceptable for a growing provincial city that is heavily reliant on tourism and food processing industries.

The system also did not provide adequate protection from protozoa, such as giardia and cryptosporidium, that are becoming more common in New Zealand. Cryptosporidium can be fatal to those with weakened immune systems.

The Ministry of Health (MoH) has the responsibility to make the determination about what level of safety is required. MoH has given all local authorities a very clear steer on what the minimum standards for wholesome water are with the publication of the New Zealand Drinking Water Standards 2005. At present compliance with these standards is not compulsory but the government is planning to introduce legislation to change that.

Power savings

Harnessing the water from the Roding River before it reaches the Tantragee Water Treatment Plant is proving a bonus move. Now in it's fourth year (2012) average kW production is 240,00kW p.a. which means average savings of $26,000 per year.

The raw (untreated) water is pumped up to the Saddle tank at Marsden Valley and then travels down hill and through a small engine just before it enters the plant. The engine works in reverse and generates power which is then offset against the plant’s power consumption.  

Executive Manager Network Services Alec Louverdis, says the system was funded by a loan from EECA (Energy Efficient Conservation Authority).

“The savings from this simple but effective system are offset against the annual power cost of the Tantragee Plant and are a great use of a resource that was already there” Alec says.

“It fits well with Council’s sustainability goals” he says.

Find out more

Read more about Nelson's water supply and planning for the future

Summary of the investigation of alternatives

For almost a decade prior, investigations were conducted on various methods for water treatment. The following is a breakdown of the alternatives Council considered.

Year: 1992

Report

Alternative Disinfection Systems

Description

Assessment of ozone, ultraviolet and chlorine dioxide treatments

Findings/details

Investigations concluded that none of these treatments would achieve the new Drinking Water Standards

Year: 1994

Report

The Costs and Benefits of Installing a Treatment System to Achieve "A" or "B" grading

Description

A full review of treatment systems and what would be required to meet the new standards

Findings/details

Projected conventional treatment systems would cost $18.1million. Identified proprietary systems that could be cheaper. Recommended pilot plant trials

Year: 1996

Report

Household Treatment

Description

An investigation of treatment at point of use (a tap) or at point of entry (supply as it enters the property).

Findings/details

Capital costs ($21 million for point of use and $46 million for point of entry), as well as very high annual maintenance costs and flow rates not cost-efficient.

Year: 1997

Report

Pilot Trials

Description

A year long investigation of a variety of treatment options; ozonation, dissolved air flotation, direct filtration, conventional treatment using 2 stage filtration and adsorption clarifier.

Year: 1999

Report

Water Treatment for Nelson

Description

The results of the Pilot Trials

Findings/details

Recommendation was for a low chemical alternative costing a projected $16.3 million. A visit to Tauranga's membrane treatment plant was recommended - the report back showed the technology was such that it could accommodate future changes in standards (future proof).

Year: 2000

Report

Study Tour of USA and Canada

Description

Visited five treatment plants using a range of membranes. Discussions with designers, owners and operators

Findings/details

Confirmed that membrane treatment was the right choice for Nelson 

Benefits of the new system

The new water treatment plant delivers other additional benefits:

• Drought security will be improved. The treatment plant will be able to treat an additional one million cubic metres of water in the lower levels of the Maitai Dam, significantly increasing the volume of stored water available for use. This increased drought security means that additional water supplies will not be required until well after 2030
• The water from the bottom of the Maitai Dam contains iron and manganese. These elements are dissolved in the water so can't be removed by filtration. If the water with high levels of iron and manganese needs to be used, say if the rivers are running low, then potassium permanganate is added to remove the metals.
• Security of supply will be improved. If the upper section of the Maitai Pipeline is damaged by rockfall or landslide, water supply can still be maintained by pumping directly from the river to the plant.
• The 3000 m3 reservoir at the treatment plant will be able to backup the city supply in the event of damage to the Maitai Pipeline.
• The treatment plant will enable the existing Maitai Pipeline to be upgraded without the need for a duplicate pipeline.
• The new works include dual pipelines to carry water from the Roding supply to the plant and treated water back to Stoke. The Council had already planned for the return pipeline as it will be required regardless of whether or not the treatment proposal goes ahead. The $2.5 million cost of this pipeline is built into the treatment plant costs and compares well with the $3.6 million budget for this work.
• Treated water is available in the upper end of the Brook Valley,Marsden Valley, Enner Glynn and possibly Ngawhatu Valley. The availability of this high level water supply will facilitate development in these areas. Council land holdings along the pipeline route will also benefit.
• Water from all three sources will be able to be mixed, ensuring the best water is available. Chemical usage will be further reduced.
• Why is chlorine added? Before the new water treatment plant came on line, chlorine had to be added to kill all the bugs. The amount added fluctuated according to how dirty the water was. Now the sophisticated membrane technology removes all the dirt and bugs. However, after the water has been treated, a small amount of chlorine is added to help the water stay virus free on its journey from the plant to town.
• The option is future proof, membrane technology is projected as being the way the world's water treatment systems will operate in the future. The membranes are microfiltration 'straws' which filter out the bugs and dirt. They have a 10 year guarantee but could easily last longer than that - so every year they last beyond 2014 is a bonus for Nelson.
• The water treatment plant is surrounded by high security fencing. Sophisticated alarm systems prevent unauthorized entry into the plant.
• Only two operating staff are needed to run this state-of-the-art treatment plant.
• Nelsonians can still walk, jog, bike up to the saddle and beyond. However, access to plant itself is restricted. As before, the public tracks are subject to Carter Holt Harvey's forestry logging operations.

More information about Nelson's water

Nelson's safe, clean, reliable water supply came on line in early August 2004. The city now has a supply it can rely on and be proud of.

How long do the membranes last

The membranes are microfiltration 'straws' which filter out the bugs and dirt. They have a 10 year guarantee but could easily last longer than that - so every year they last beyond 2014 is a bonus for Nelson.

Why chlorine is added

Before the new water treatment plant came on line, chlorine had to be added to kill all the bugs. The amount added fluctuated according to how dirty the water was. Now the sophisticated membrane technology removes all the dirt and bugs. However, after the water has been treated, a small amount of chlorine is added to help the water stay bug free on its journey from the plant to town.

Can all the water from the Matai Dam be used

The water from the bottom of the Maitai Dam contains iron and manganese. These elements are dissolved in the water so can't be removed by filtration. If the water with high levels of iron and manganese needs to be used, say if the rivers are running low, then potassium permanganate is added to remove the metals.

Security arrangements

The water treatment plant is surrounded by high security fencing. Sophisticated alarm systems prevent unauthorized entry into the plant.

How many staff are needed to run the plant

Only two operating staff are needed to run this state-of-the-art treatment plant.

Access to the paths and walks around the plant

Nelsonians can still walk, jog, bike up to the saddle and beyond. However, access to plant itself is restricted. As before, the public tracks are subject to Carter Holt Harvey's forestry logging operations.

What about the future

The 3,000m3 reservoir at the plant means supply can continue if a landslide or earthquake damages the upstream supply pipework.

And finally, because water that was previously untreatable can now be made available, Nelson can feel more confident of water supplies for future years.