Soon-to-flow Waikato river tap water will be as good as that now drunk by Aucklanders, write DAVID SINCLAIR and VIRGINIA HOPE*.

Many people still have reminders of the 1994 Auckland drought and water crisis - a brick in their toilet cisterns or the toilet flush gizmo distributed by councils. For most, however, it is only an unpleasant distant memory.

The crisis seriously affected people and businesses and posed threats to public health and safety. Yet few people, businesses or councils have done much since to reduce demand, and consumption continues to rise.

Another drought, or the failure of one of the existing main water treatment plants, would be disastrous.

Drinking water, both the quantity and quality, is something that happens in the background for most of us until something goes wrong. But ensuring adequate quantities of good-quality water needs long-term planning and development by councils and suppliers and ongoing vigilance by suppliers and public health services.

Councils and water suppliers had been considering options for additional sources for more than 20 years, during which the population doubled. The drought made water supply security urgent.

Some 90 options were considered, including further dams in the Hunua and Waitakere ranges and at Riverhead, bores along the banks of the Waikato, and even taking water from Rangitoto Island. Additional dams, of course, would still be affected by drought, merely postponing the need for severe water conservation measures if 1994 were repeated.

After much assessment, consultation, costing and consideration by councils and Watercare, the Waikato river option was chosen. The resource consent process allowed many issues to be worked through in a public forum.

The Waikato river is used as a drinking water source by 22 communities, the largest being Hamilton, which has an A grading for its water supply and treatment plant and meets the 2000 drinking water standard. Ngaruawahia and Huntly, downstream of Hamilton, also have A gradings.

Like many other rivers used for drinking water, the Waikato is affected by bacteria and waste from farming, industry, stormwater and treated effluent from sewage treatment plants.

The river also contains geothermal chemicals, including low levels of arsenic (which is effectively removed by standard water treatment, such as the new plant), and boron (of lesser health concern but still below the drinking water standard and World Health Organisation guidelines in the raw and treated water).

By international comparison the Waikato river is of reasonable quality since it has a small population and water use. In proportion to the river's flow, there is comparatively small contamination from industry, farming and urban sources. Quality has improved substantially in the past 20 years through controls on discharges.

Bacterial counts increase from upstream of Hamilton but decrease lower down the river, so by the time the river reaches the new treatment plant, counts are comparable to those in some protected catchments used for drinking water in Auckland.

Waikato river water does have higher levels of bacteria, protozoa, geothermal chemicals and other contaminants than deep-bore water or the existing lakes, so it requires a higher grade of treatment and better monitoring and management.

Treatment at the new plant near Tuakau will include standard clarification, as used in big treatment plants, followed by membrane microfiltration and carbon filters. The water is chlorinated to kill any viruses or bacteria that remain or may get into the pipes through leaks. All water must go through each of these stages, and cannot bypass them. Membranes and carbon filters are additional to the technologies used in most treatment plants in New Zealand.

Water treatment plants and pipelines need approval from a district health board's medical officer of health. This involves assessing the technology, the plant operations and management plans, the quality of the source water and testing the treated water to see if it meets the drinking water standard.

Watercare conducted detailed testing after negotiation with the public health service. The process of approval has taken about 18 months.

The Auckland regional public health service (part of the Auckland District Health Board) took its own samples of treated and untreated water at the plant for analysis by independent laboratories for a range of the bacteria, viruses and chemicals specified in the drinking water standards.

Tests done by Environment Waikato and Watercare have also been reviewed. Results show the plant comfortably meets the standard, so the product will be safe for use by the public.

We have concluded that the new Waikato water treatment plant is able to provide safe, high-quality water to Aucklanders and respond to demands on the supply. When it starts flowing from taps in two weeks, the treated water will be at least as good as the water now supplied to Aucklanders.

* Drs David Sinclair and Virginia Hope are Auckland medical officers of health.