Providing Pure Wafer

Manawatu Times, Volume 61, Issue 45, 24 February 1936, Page 7

Providing Pure Wafer

Wellington, Christchurch, And Invercargill Schemes

PART PLAYED BY ARTESIANS

The ratepayers of Palmerston North go to the polls on Wednesday to pass or reject loan proposals submitted by their council for the improvement of the city’s water supply. Considerable difference of opinion is evident as to the merits or the proposals put before the ratepayers, and with the object of clarifying the issues involved the ‘‘Times” has obtained reports upon the water supplies of some cities wbere artesians play an important part. WELLINGTON CITY Approximately one-third of Wellington’s summer water supply is obtained from artesians. At Gear island, near Lower Hutt, the council has put down 17 artesian wells and water from these is pumped at high pressure into a gravity supply main of 21-inch diameter. The council is restricted by Parliamentary sanction to draw off from these artesians 3,000,000 gallons per day. When the Wellington City Council commenced its artesian development the Hutt Valley boroughs becarno alarmed. They contended, and evidently with good reasons, that the subterranean water supplies were not unlimited, and that if Wellington city drew an excessive supply, then they might be faced with a definite shortage. Eventually a limit was placed on Wellington’s draw-off by Act of Parliament. Wellington has spent approximately £20,000 on artesian development and obtained therefor a most useful auxiliary supply. The population of Wellington city is 114,000, and the water consumption averages about (30 gallons per head per day, falling to 50 gallons in winter and rising to 80 gallons and over in summer.

Looking to the future, Wellington considers artesian water as being but a “stop,-gap ’ ’ supply which may suffice to carry the city over the next ten or even twenty years; but eventually an enlarged gravity scheme is contemplated. At present the main supply is obtained from the ranges to the north-east of Petone. In the distant, or perhaps not so distant future, a greater gravity scheme will be developed in the Akatarawas. The City Council there holds a water works reserve of 82,000 acres of mountains and bush land.

CHRISTCHURCH SUPPLY The Christchurch city water supply is obtained entirely from artesian, 34 wells being used for this purpose. As Christchurch provides an outstanding example of the utility of artesian w r ehs for city supply purposes, the “Times” obtained a report on the system, and herewith are given pertinent extracts from that report: —

“The water is very clear, pleasing to the taste, and odourless, except in the last particular of that drawn off from dead ends in tho reticulation system, of which there are very few, when there is a slight inoffensive odour. The artesian water has not any bad effects upon the hot water cisterns and household fittings generally. The water, although slightly hard, causes no difficulties whatever in connection with laundering or internal encrustation in the pipe. An analysis of the water taken several years ago is appended herewith for your information, and at tho present time a more de tailed investigation is being carried out by Dr. H. G. Denham, of Canterbury College. The water itself is of such exceptional purity that it is pumped direct into the mains and distributed without any treatment whatever. Its purity is such as to permit of its use freely for industrial purposes. There is an unlimited supply from the artesian areas under the city, and during the bad droughts of the last three or four years we have met all the demands so successfully that there has not been the slightest shortage of water. There has been no internal corrosion in the pipes, but external corrosion has developed somewhat considerably in the steel tubular mains through electrolysis and soil acidity, and in such cases the bitumen wrapping round the pipes has been perforated or damaged and the bolts of the joints have been, in many instances, completely corroded. Where cast iron has been used, there has been practically no internal or external corrosion worth mentioning, and these pipes are as good as the day on which they were laid down some 26 years ago, and are evidently quite fit for another 50 years’ service. It may be stated that the greater quantity of the water is drawn from the 100-feet depth known locally as the first stratum, and the deeper you go the less the quantity of water obtained, but the greater its purity and static head.” The analysis received shows that the water has a very faint smell when heated to 100 degrees Fah., that it is entirely free of organisms, and then gives details of the presence or otherwise of the six important chemical compounds and of the total solids content. The water is rated as of excellent quality.

SUPPLY AT INVERCARGILL This city relies wholly on wells, which are not strictly “artesian.” Invercargill has a population approximately identical in numbers with that of Palmerston North. As their system is supplying a city of our size, the “Times” obtained a report upon it. Extracts are given herewith: — “In this city the water is not strictly ‘artesian,’ but is obtained from wells sunk about 140 feet, deep into water-bearing gravel. When at rest (i.C., when no pumping is in progress) the water stands at about 60 feet below ground level, and during pumping

it falls to about 100 feet, from which depth it is pumped to the surface. The somewhat high content of dissolved iron is objectionable, since upon contact with air the hitherto invisible dissolved iron is changed to visible particles of iron rust or ferric hydrate. This causes the water to be very discoloured and to leave a rusty precipitate on standing. To mitigate this it was found necessary not to deliver the water direct, to consumers, but first to aerate it thoroughly by spraying in air. This forces the oxidation of the iron to take place at once, aud not at a later stage. Retention in reservoirs, besides affording necessary storage, affords, opportunity for some of the iron to settle, out of the water as sludge in the reservoirs, rather than in its subsequent history iii mains or in users’ receptacles. It will therefore be understood that the use of aeration and storage at Invercargill is for special reasons and is essential. The cost of aeration is .merely that of the power required to lift the water the extra height (or to impart the extra pressure, which is the same thing) to operate the spraying nozzles or devices. In Invercargill this is about 10 feet, or less than 5 per cent. The total annual cost of the pumping system is about one-quarter of the probable interest charges alone that would be involved in an upland (gravity) supply. ’ ’

COMMENT From the account of these three systems it will be seen that artesiatis are now playing an important part in the supply of water to New Zealand cities. Christchurch apparently has an almost perfect supply; Invercargill ouo that is not so good but one that has nevertheless been made suitable by treatment. In Wellington, artesians are playing a part similar to that they now perform in Palmerston North. As to what extent • artesians should be used, this depends-upon the supply obtainable and the quality of the. water.