Water Services

The Central Otago District Council's Water Services team has compiled a summary of drinking water treatment for Council supplies and the techniques used.

The level of treatment that is required to meet the New Zealand Drinking Water Quality Assurance Rules is dependent on the quality and consistency of the source water.

All supplies need to be chlorinated to treat bacteria. The requirements to treat protozoa are more complex. This typically requires some form of filtration, and ultraviolet (UV). Membrane filters are one type of filter that do not require UV, but these are significantly more expensive to construct and operate.

Surface water sources require more treatment processes to treat protozoa than groundwater sources. This is because surface water has more risks associated with it, and typically has less consistent quality than ground water. The more treatment processes, and the higher the level of filtration required, the higher the cost it is to construct and operate.

Source water that is affected by turbidity is more difficult to treat. Turbidity is when there is sediment in the water, which is usually caused by rain causing runoff into the streams and rivers. When rivers and lakes are dirty due to sediment, this impacts on the ability of some treatment processes to work effectively.

For these reasons investigation is required to identify the different source water options that are available. Testing of the best option is then needed over at least a 12-month period to identify how consistent the quality and quantity of the water is. This is to ensure that the treatment plant built can treat the source water under all conditions.

There are different requirements in the New Zealand Drinking Water Quality Assurance Rules for water supplies that serve more than 500 people, 100 to 500 people and less than 100 people.

Alexandra and Clyde (Lake Dunstan Water Supply Area)

The new Lake Dunstan Water Supply services the townships of Clyde and Alexandra. This is a membrane treatment plant with chlorination which provides treatment barriers for bacteria and protozoa.

The source water is from bores adjacent to Lake Dunstan. These draw on lake water, which is considered to be a surface water source.

Naseby

The Naseby treatment plant consists of flocculation, sand filtration, UV and chlorination which provide treatment barriers for bacteria and protozoa under normal operating conditions.

The source water for Naseby is from the Hawkdun Irrigation Race, which is a surface water source. This source is vulnerable to turbidity following rain. When this occurs, the plant is shut down, treated water is supplied from reservoirs and a conserve water notice is put in place.

If the reservoir level drops to a point where untreated water is needed to refill it, then a boil water notice must be put in place until the source water clears. This typically occurs when reservoirs have about six hours storage available.

The population in Naseby varies considerably between normal population and peak population. The Naseby plant has to run continuously to produce enough treated water during peak periods. This does not provide sufficient capacity to accommodate typical maintenance outages during peak periods, or changes in the source water quality.

Investigation is underway to identify the options for increasing capacity, and providing resilience to weather events that affect the source water quality.

Roxburgh

The Roxburgh Water Supply services the townships of Roxburgh and Lake Roxburgh Village, and some properties between Roxburgh and Lake Roxburgh Village.

The Roxburgh treatment plant consists of cartridge filtration, UV and chlorination, which provides treatment barriers for bacteria and protozoa.

The source water for Roxburgh is from bores adjacent to Clutha Mata-au which draw on river water. This is considered a surface water source. There is sediment drawn through the bore which has required a larger micron filter to be used. While providing protection, the filter does not fully meet the requirements for compliance with the New Zealand Drinking Water Quality Assurance Rules. The UV still provides a barrier to protozoa.

Council is currently reviewing alternative bore locations to address the sediment issue.

Omakau and Ophir

The Omakau Water Supply services the townships of Omakau and Ophir. The treatment plant consists of sand filtration and chlorination. This provides treatment for bacteria, but not protozoa.

The source water for Omakau is from an infiltration gallery in the Manuherekia Riverbed. This is a surface water source and is vulnerable to turbidity following rain. When this occurs, the plant is shut down and a boil water notice is required. It is also vulnerable to low river flow during summer when water restrictions may be required.

Council is investigating options to provide a resilient and compliant water supply for Omakau. This has involved considering other source water options. A business case has been developed and Council will consider the options within the next two months. Significant investment, in the range of $20-25 million, will be required to build the new plant.

Ranfurly

The Ranfurly treatment plant consists of sand filtration and chlorination. This provides treatment for bacteria, but not protozoa.

The source water for Ranfurly is from two locations on the East Eweburn River. This is a surface water source and is vulnerable to turbidity following rain. When this occurs, the plant is shut down, treated water is supplied from reservoirs. and a conserve water notice is put in place.

At normal water demand, Ranfurly has several days storage in the reservoir.

Council is investigating options to provide a resilient and compliant water supply for Ranfurly. There are turbidity and quantity issues with the existing source water which has required other sources to be investigated.

A business case is being developed and Council will consider the options within the next three months. Significant investment, in the range of $20-25 million will be required to build a new Ranfurly plant with a new water source.

Patearoa

The Patearoa water supply feeds a small urban area and multiple rural properties. The Patearoa treatment plant consists of chlorination. This provides treatment for bacteria, but not protozoa.

The source water for Patearoa is from the Sowburn River. This is a surface water source and is vulnerable to turbidity following rain. When this occurs, the plant is shut down, treated water is supplied from reservoirs, and a conserve water notice is put in place.

If the reservoir level drops to a point where untreated water is needed to refill it, then a boil water notice must be put in place until the source water clears.

Council is investigating options to provide a resilient and compliant water supply for Patearoa. There are turbidity issues with the existing source water which has required other sources to be investigated. A business case is being developed and Council will consider the options within the next four months.

Pisa Village

The Pisa Village Water Supply services the Northern area of Pisa (shaded orange on the map below).

The Pisa treatment plant consists of chlorination and UV which provides treatment barriers for bacteria and protozoa.

The source water is from the Pisa aquifer and extracted with bores. Because the bores are less than 30m deep, they are not given secure aquifer status and require treatment for protozoa. The aquifer water has more consistent quality than a surface water source.

Cromwell

The Cromwell treatment plant supplies water to Bannockburn, Cromwell, Lowburn, Ripponvale and the southern part of Pisa Moorings (shaded green on the map attached). The treatment consists of chlorination. This provides treatment for bacteria, but not protozoa.

The source water for Cromwell is drawn through bores from the Cromwell Terrace Aquifer. The bores are on the Clutha Mata-au arm of the lake, upstream of the Kawarau River confluence. As the bores are less than 30m deep they are not given secure aquifer status and require treatment for protozoa. The aquifer water has more consistent quality than a surface water source.

Investigation has been underway for a new water treatment plant for Cromwell for the past three years. This has included 24 tests for protozoa over a 15-month period which showed no positives for protozoa.

Further investigation work is being undertaken because additional bores are required to meet the high growth needs for Cromwell. This work will be completed in November and will provide sufficient information to enable the borefield and treatment design to be finalised. The borefield and treatment plant upgrades are expected to be tendered in April 2024, with the construction contract commencing in July.

The new pipeline between the borefield and the reservoir is currently being tendered and will be awarded in October. This will take 10 months to construct, with construction due to be complete in August.