The new Ohakune water treatment plant

Ruapehu Bulletin, Volume 9, Issue 415, 3 December 1991, Page 2 (Supplement)

The new Ohakune water treatment plant

THE OHAKUNE Water Supply has for many years come from the Serpentine Stream which arises on the lower slopes of Mount Ruapehu and is a tributary of the Mangawhero River. In past years this area has been developed as farm land but is now incorporated into the Tongariro National Park. The supply has always been regarded as a 'pure mountain stream' and this has been seen as being significant in promoting Ohakune's image as a town with a water supply free from the chemical treatments that occur in the water supplies of large municipalities. Unfortunately, the catchment from which the water comes, although protected, is still subject to pollution from animal life and humans. For a number of years, the Health Department has been concerned about the presence of coliform bacteria in the supply and the potential for an outbreak of disease because the water is not disinfected. The Ohakune people have long had a preference for water sterilisation by means of ultra violet light to avoid the problems commonly associated with unpleasant tastes

with disinfection by means of chlorine injection. In recent years the discovery of giardia cysts in the supply has further increased the need for water treatment. At the time of the formation of the Ruapehu District Council in November 1989, consultants for the former Ohakune Borough Council and the Waimarino District Council, had identified possible pipeline routes and reservoir treatment sites. However little information was available on the exact nature of the contaminants in the supply and the District Council initiated a number of tests which were carried out by the Water Treatment Centre in Trentham. These tests revealed that there were a large number of minute organic particles in the water which makes treatment difficult particularly when using ultra violet light sterilisation. To test the effectiveness of various forms of filtration, pilot plants were established in which water from the supply was passed through various types of filters, and various types of coagulant compounds were added to the water in varying concentrations

to determine the best possible treatment. The filter trials were mainly of two types, firstly fine sand filtration which is proceeded by the addition of chemicals causing coagulation and the second type of filter was a diatomaceous earth filter. A lot of consideration has been given to the best method of calling tenders for the construction of the new treatment works. A number of different filter suppliers and filter types exists and it is very difficult to assess the relative merits of each proprietary product and the best combination of these products. PROBLEM OVERCOME To overcome this dilemma, a somewhat innovative approach has been used: 1. Calling for tenders for a design and build contract on a performance based specification. 2. Calling for tenders on a two stage basis. The performance based specification states the desired quality of treatment and this much be achieved before the plant will be taken over by the Council. There will also be a maintenance period of up to one year during which time substantial monies will be withheld to ensure the continued operation of the plant to the required specification. In the first tender round, tenders were invited from any supplier who wished to put in a bid. From all those who supplied prices, four were selected to enter the second tender round. In the second tender round, the four selected tenderers were invited to present a strong concept design of the plant and the Council paid each the sum of $10,000 to produce this outline design. Each of the four tenderers will be required to present their design concepts to a Working Party of the Council who will

then evaluate the best solution for Ohakune. The existing intake on the Serpentine Stream is within the Tongariro National Park. From the intake the present pipeline runs down through the National Park to Ohakune in the vicinity of the railway yards. The new pipeline will connect into the existing pipeline part way along its length and take a different route out of the National Park to a site on adjacent farm land on Soldiers Road. After the water has been treated it will collect in a 1500m3 timbertank reservoir and this will be at a sufficient height to supply Ohakune by means of gravity. The pipeline from its point of connection with the existing pipeline will run along an old tramtrack to the new reservoir site. This tramtrack has now regenerated into bush and every care will be taken to minimise the damage to this bush within the boundaries of the National Park. The Council acknowledges and respects the integrity of the National Park and its asset to the District and to the nation and is keen to carry out its works in ways that minimise environmental impact. The Council is satisfied that the works will be harmonious with the objectives of the Tongariro National Park and that they will be a great asset to Ohakune's future development for many years to come. It is planned to commence the construction of the plant and pipeline in January 1992 and have the work completed by June 1992.