FILTRATION SCHEME

Sun (Auckland), Volume II, Issue 525, 30 November 1928, Page 2

FILTRATION SCHEME

PURIFYING WAIRAU FOR NORTH SHORE WATER ! A scheme for augmenting the North Sh r< v. r s ipph I y impounding the | waters of the Wairau Stream, and ; after treatment, pouring it into Lake Pupuke. W hich would thus bt restored ; to its former level, has been explained • by Mr. W. H. Dufaur. manager of thCandy Filters (New Zealand). Ltd. At a meeting of the North Shore Water Board yesterday afternoon it I ' vas resolved to ascertain the attitude ; of the Health Department on the que«s- ---; tion. Mr. E. Aldridge. Mayor of Devon- | port, was in the chair. Dr. H. Chesson | medical officer of health, and Mr. F. K [ Powell, consulting engineer to the Water Board, w ere present. Mr. Dufaur said that modern purifi, j cation methods had proved themselves, and in England and America especially | old prejudices as itnst the use of any other than a first-class supply were i disappearing. In London, approxii mately 60 per cent, of the supply was I from the River Thames. 25 per cent i from the River Lea, and the balance from wells and springs. LAKE FALLING Quoting from the Auckland Water Com i mission's report. Mr. Dufaur said it was | held that the lake derived its water from I the rainfall upon its surface and upon [ the adjoining catchment area. The level had been falling within recent years, and in the five years from January, 3922 March, 1927, th*- fall had been 6fr 2in. This was in spite of the fact that the Auckland rainfall during the period averaged 53in. or 23 per cent, more than the average sihee 3 855. In 1912. Mr. W. E. Metcalfe, civil engineer, estimated the average yield of (the lake at 2OO,fK»o.of>n gallons, and in i 1912 Air. Powell estimated the quantityavailable in a j ear of average rainfall at 130,000,000 to 150,000,000 gallons yearlr. On the latter igure. on th«- basis of a daily consumption of 50 gallons a head, the lake could supply a population of 8.2*0. The population of Devonport Takapuna, Northcote and Birkenhead in 1926 was 21,254. and the total quantity of water pumped for the year ended March 31. 1927, for those districts was 352,000,000 gallons, or close upon two and a-half times the average quantity that could be yielded by the lake. PURIFYING THE SUPPLY Mr. Dufaur said he understood tin* Health Department had condemned iht> Wairau stream as being polluted and could not be allowed to run into the lake. Though he (the speaker) did not consider it a seriously polluted supply, it would not matter if the supply was as polluted as the River Thames. The suggestion was to tap the Walrau Stream just above the point where the Wairau Road crossed the stream. A small concrete dam would be constructed, together with intake chambers, equipped with screens, and a concrete conduit would carry the water practically in a straight line to the lake, the inlet to the conduit being controlled by penstock valves. A gas chlorinating plant, housed in a small building, would be installed, cap r able of dosing the water at the maximum flow, and in this way every drop of water running into the lake from the stream would be completely sterilised. From the bacteriological point of view the water running into the lake would be purer than the lake water itself. The distance between the intake and a suitable point of delivery into the lake was approximately 2,770 ft, and the greater part of the conduit would be tunnel work, and a certain amount of land would have to be acquired at each end of the conduit. The conduit would have a capacity of about 30,000,000 gallons for 24 hours, and he estimated that in 60 days during the winter months some 900,000,000 gallons could be run into the lake. This, together with the natural lake yield of 150,000,000 gallons, would give a total supply of I, gallons in the year, equal to a consumption of about 3,000,000 gallons a day, which, at the present rate of consumption, would serve at least two and a-half times the present population. Thchemical analysis of the water was satisfactory, and the large body of water of the lake -would itself afford considerable purification. The capacity of the Devonport station should be increased to deal with the w’hole supply, instead of the thre*- plants now operating. This would entail tht addition only of a pumping set, and mains would be laid connecting the reticulation systems now’ supplied by the Birkenhead and Northcote pumps. A floating intake would extend into the lake, from which the low-lift pump would draw the water and deliver it to a gravity filter-plant built next to the pumping station. The high-lift pump would take the filtered water from the filters and deliver it into the mains. The plant w’ould be of the same type as that at Waitakere, but of smaller capacity. The filters gave a clear, sparkling water and effected high bacteriological purification. COSTS COMPARED The cost of the scheme was approximately .£47,723. The cost of water, including cost of purification, both chlorination of the stream and filtration of the lake water, cost of pumping, and interest on capital, worked out at 7Jd a thousand gallons for 2,000,000 gallons a day. compared with 15d and 16.6 d and 9d and 11. for the Rangitopuni scheme. The Wairau would provide an assured supplyIn reply to questions, Dr. Chesson said that so far as the lake basin was concerned, the Health Department would not lift the existing restrictions. He would go into any question the board cared to submit and would not take any personal responsibilities, but would consult the department It was decided to submit a series of questions, so that the attitude of the department toward every phase of the question could be ascertained. Dr. Chesson said the lake water was good water, and that it did not need purification if taken from a distance out, treatment being desirable only in the case of water taken along “the margin of the Jake.