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

Upgrades to the Lake Dunstan water supply and delivery pipelines have cost more than $10M and resulted in a noticeable difference in taste, and an absence of the limescale that has long been a problem in Alexandra and surrounds. Naseby and Omakau residents, however, are no strangers to notices instructing them to conserve or even boil water when downpours muddy their source waters or droughts leave the reservoirs worryingly low.  

Ranfurly earlier this year experienced demand for water that exceeded supply, resulting in a conserve water being put in place on February 4, a water restriction on February 7, and a boil water notice for eight days from February 8 to 16. 

The 2018 - 2028 LTP states that treatment upgrades were planned to improve treatment standards for Cromwell, Ranfurly and Patearoa from 2021. Other projects, such as reservoir construction, changes to pipe configuration, valves and backflow prevention would improve resilience. In some cases upgrade projects would also respond to projected growth.

Clean drinking water has become a national concern 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 council water services, followed by specific details about 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.

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.

The CODC 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 the council would consider the options within the next three months. Significant investment, in the range of $20-25 million would be required to build a new Ranfurly plant with a new water source.

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 Manuherikia 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.

The 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 the council would consider the options within the next two months. Significant investment, in the range of $20-25 million, would be required to build the new plant.

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.

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.

The 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 was being developed and the council would consider the options within the next four months.