BOROUGH WATER SUPPLY

Timaru Herald, Volume CXLIII, Issue 20700, 13 April 1937, Page 9

BOROUGH WATER SUPPLY

ENGINEER’S REPORT FILTRATION SYSTEM Extracts from a full report by the Borough Engineer (Mr T. O. Fox) on the water supply of the borough, written in April, 1935, were presented to a meeting of the Council last night. The extracts dealt chiefly with filtration. The extracts were as follows: “In the installation of filters, there are two types from which to choose, in both of which filtration is carried out by passing the water through layers of graded sands. In each case a coagulant, sulphate of alumina, is used in order that the finest particles of sediment may be collected on the surface of the filter. This sulphate also forms a gelatinous film upon the filter, is a very effective agent in securing a high bacterial efficiency and many records exist to show that the removal of bacteria by this means may amount to 90 per cent and even more. For this reason I am of opinion that the addition of a chlorination plant should not be necessary. Furthermore, there are certain objections to the use of chlorine as a germicide in a town supply. Gravity Type. “Of the two types of filter mentioned above, one is known as the gravity type through which, as its name implies, the watfer gravitates to a clear water reservoir, whereas the other, the pressure type, is placed upon the supply mains and is worked under a head equal to the difference in height between the level of water in the supply and of the site of the filters. If the gravity system were used on the borough supply it would be necessary to pump the water in the race to a height of about 15 feet in order to obtain sufficient head for the filter to work, the purified water then entering the reservoirs. These filters would require to have a maximum capacity of 100,000 gallons per hour, the storage in the reservoirs taking care of the maximum draw-off between 8 and 11 a.m. “If pressure filters were placed on the supply mains, these filters would require to have a capacity of 300,000 gallons per hour as they would have to be capable of dealing with the maximum demand in the town. Furthermore, as the town is supplied by means of the 18in main to Wai-iti Road and the 14in main to North Street, it follows that there would have to be two independent installations and in each case the highest part of the filtration plant would have to be not less than 15 feet below the bottom of the reservoirs. Suitable Positions. “The only suitable positions are those where the mains cross the Otipua Creek and these positions would entail increased post of operation because of the supervision required over plants in separate positions. In view of the higher capital cost of a pressure filter over the gravity type and the fact that the former would require to have three times the capacity of the latter together with the increased labour charges in the cost of operation, I consider the pressure filter to be entirely out of the question. The cost of gravity filters in the locality of the reservoirs is estimated at £lO,OOO while the operating costs are estimated at £1375 per annum.

“One other point in connection with the installation of a filtration plant is that such a plant would enable the borough to postpone consideration of the installation of a further storage reservoir for a few years as it would be possible to use water from the Pareora when the latter is slightly discoloured, whereas at the present time it is necessary to turn this to waste. The estimated cost of an additional reservoir to hold 15,000.000 gallons, including the purchase of land and the necessary outlet pipes, etc., is £8700.” Summary of Costs. To summarise the costs mentioned in the above report they are as under: £ Installation of filters, capital cost 10,000 Annual cost of filtrationinterest and Sinking Fund Fund at 6* per cent ... 650 Power 425 Chemical treatment 50 Wages 750 Plant maintenance 150 2,025 Concreting of remainder of race 50,000 Construction of additional reservoir 8,700