Central Otago District Council (CODC) made drinking water quality a priority in its 2018 - 2028 long-term plan, says mayor Tim Cadogan, but the costs involved mean it will be a long and costly process.

Residents of Alexandra and Clyde have recently enjoyed the benefits of the first phase of those plans, with upgrades to the Lake Dunstan water supply and delivery pipelines that have cost more than $10M.

These resulted in a noticeable difference in water taste and an absence of the limescale that has long been a problem in Alexandra and surrounds.

Water has become a muddy topic after an outbreak of cryptosporidiosis in Queenstown, led the drinking water regulator, Taumata Arowai, to serve a compliance order on the Queensland Lakes District Council, saying the Two Mile water treatment plant was non-compliant with the Water services Act 2021. Specifically, it lacked an adequate barrier for protozoa, a parasite that causes illness in humans.

Not all water treatment plants in Central Otago have protozoa barriers, including Omakau and Ophir, Patearoa, Cromwell and Ranfurly, which have varying types of source water. 

The council was either investigating or putting together a business plan for greater resilience at those treatment plants, CODC communications officer Mary-Jo Tohill said.

The following is a summary of drinking water treatment for CODC supplies and the techniques used, provided by CODC water services, followed by specific details about the water treatment plants in Roxburgh, Alexandra, Cromwell, and Pisa.

Tomorrow, we will look at the water treatment plants in the Maniototo.

All supplies need to be chlorinated to treat bacteria, CODC water services said. 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.

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.

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.

Cromwell

The Cromwell treatment plant supplies water to Bannockburn, Cromwell, Lowburn, Ripponvale and the southern part of Pisa Moorings.

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.

Pisa Village

The Pisa Village Water Supply services the northern area of Pisa.

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.