WATER FILTRATION.

New Zealand Herald, Volume LXIII, Issue 19265, 2 March 1926, Page 11

WATER FILTRATION.

CITY SCHEME DESCRIBED. TWO SEPARATE PLANTS. HIGH STANDARD REQUIRED. CHLORINATION NOT NECESSARY. Missappreliension exists as to the nature of the water purifying scheme for which the City Council will submit a loan proposal for £70,000. The specifications required a filter that should fulfil a threefold purpose: (a) to rejnove suspended matter and render the water clear, bright and colourless; (b) to provide efficient safeguard against any bacterial or other organic pollution that may occur accidentally or otherwise; (c) to correct the acidity of the water and render it non-corrosive to pipes.

It appears to have been assumed that the chief purpose of the proposed installation is to chlorinate the water, a precaution that requires to be taken only when water is drawn from a source that is polluted, particularly through human agency. The water from the ranges is regularly analysed and obviously, if a safe standard were not maintained, chlorination would immediately be carried out. Chlorination has at times been practised. When a quarry was being worked above one of the dams the ordinary precautions observed against contamination of the water were made doubly safe by this chemical treatment, and on odd occasions when, during the repair of a break in a line, the Western Springs have been drawn upon, chlorination at this point of supply has been absolutely essential. Ho Chemical Additions.

The Waitakere water is not impure in the sense that it carries dangerous bacteria. Tho objection to it is that it contains so many particles of vegetable and other matter that its appearance is often too colourful and its taste too often the reverse of tasteless. Tho Candy filtration process which has been accepted subject to the loan being sanctioned, will remove these objections and it is officially stated that the water will contain nothing that can possibly injure the elements of electric hot-water cisterns about which a correspondent to the Herald lias raised a query, but that the correction of the corrosive tendency in the water by the addition of a small proportion of free lime—a proportion of one to 70,000 —will probably increase the life of the electrical apparatus. It is pointed out that such cities as London, which draws part of its water from the Thames, and New York, are bound to chlorinate as well as filter their water, but the use of electricity for heating water is in no way restricted. In his report upon his observations during a tour abroad the assistant engineer, Mr. J. Tyler, said he had been amazed to find that many of the large cities of the United States were compelled to take water from streams and rivers the turbidity of which in some cases was as high as 3000 parts per 1,000,000, but the effective system of coagulant filters and sterilising was marvellous in operation. In the United States it was the general practice to filter and sterilise all water for human consumption, chlorination taking place prior to tho water passing through the filter beds.

Eleven Million Gallons Daily,

The scheme proposed for is two gravity filtration plants, one of six filters in reinforced concrete tanks measuring 35ft: by 12ft., two miles below the Waitakere dam and the other of seven tanks of similar dimensions at Titirangi for the Nihotupu water, the site being alongside the break-pressure tank at the east end of the Exhibition Drive. The first will filter five million gallons and the second six million gallons daily. Sand, which is Nature's filter and which has been used for filtration down through the ages, remains the medium of the most modern plant. Science has taken this material and with the aid of chemical and mechanical means has devised a method for filtering large quantities of water quickly and for cleansing the filters alternately without cutting off supplies to the reservoirs.

The sand-bed has a depth of 3ffc 6in. of sand. At the bottom there are three layers each of 4in. of gravel and coarse sand and on top there is 2ft 6in. of fine sand. The class of sand required is obtainable in New Zealand. When the water is turned on a coagulent in the form of eulphate of alumina is added by an automatic apparatus in the proportion of one grain to one gallon or one part to 70,000. This tbe suspended particles sufficiently large to be removed from the water as it passes through the sand. By the same means colouring matter is absorbed and caught. Cleansing the Sand-beds.

A filter will operate efficiently for 24 hours but it must then be put out of action for cleansing. This is effected by agitating the sana by compressed air from the bottom and then washing it by filtered water introduced dnder pressure also from the bottom. After being thoroughly agitated and washed in this mantier, the sand-bed is drained and once more the filter is ready for use.

The onlv remaining operation before the filtered" water is discharged into the pipe leading to the break-pressure tanks is the addition, when necessary, of ordinary lime in the proportion of one to 70,000 to correct the corrosive characteristics the water may possess. The Waitakere has only slight acidity but what lime is necessary will be added" to prevent galvanised service pipes from rusting, which causes discoloration and reduces the pre!sure.

The power for driving the motors for the air compressors and the apparatus that supplies the chemicals will be obtained from the Wai teniata Power Board. The total borse-pfiwer will be fro m3O to 40 but this amount of power will not be required continuously. Electric alarm signals will be provided to indicate any stoppage of the motors driving the chemical plant or any failure of the water supply. The filter tanks will be housed in handsome buildings and doubtless houses will be erected for the staffs the strength of which cannot be decided until it is known whether two or three shifts will have to be worked. Terms ol the Specifications. The council's specifications contained the following clause on the standard of purity: "No tender will be considered unless the tenderer is prepared to guarantee the following maximum standard of purity without thp addition of chlorine or other sterilising agent: (a) the total number of bacteria cultivated for 24 hours shall not exceed 20 per cubic centimetre; (b) there shall be no typical sewage organisms present that can be detected in 10 cubic centimetres when cultivated by any standard laboratory process; (c) turbidity and colour shall be removed to the extent that a platinum wire, one millimeter in diameter, shall be visiblu at a depth of four feet in the open at mid-day, not in direct sunlight, to a persoa of normal vision; (d) there shall be no trace in the filtrate of coagulant added to the raw water that can be detected by any ordinary chemical method.'-