FILTRATION OF WATER.

New Zealand Herald, Volume LXIV, Issue 19720, 20 August 1927, Page 12

FILTRATION OF WATER.

SECOND PLANT FINISHED. CLEAR SUPPLY AT LAST. THE WAITAKERE STATION. MAINLY AUTOMATIC OPERATION. The second filtration plant., that at Titirangi, will be brought into operation to-day, and as soon as deposit in reservoirs arid pipes is carried away, the "crystal clear" water Aucklanders h.'ive longed for will flow from every tap. Unfortunately people are not inclined to grow enthusiastic over water at the present .time of deluges, nor will they really appreciate the wonderful success of the filtration system until the sun shines and they can hold up a glass of water and watch it sparkle. It is difficult to believe the filtered water is the same supply until one watches one of the filter-beds being air-scoured and washed. The sight is rather horrifying until one realises that the dark brown solution represents the dirt extracted from 500,000 gallons and that the consumption of that quantity would entail hearty teamwork at a temperance convention. The Old and the New. Yesterday one had the privilege of inspecting the Waitakere plant under the guidance of Mr. W. F. H. Dufour, the expert sent from England by the contractors, the Candy Filter Company, Limited, to install it. If one has memories of oldfashioned filters that covered acres and when clogged were dug out by gangs of workmen, the neat two-storeyed concrete building beneath the bushy ridge that is crowned with beautiful kauris is a surprise and a sense of wonder grows when one finds it full of highly ingenious mechanical contrivances that are largely automatic in their operation. Successful operation involves constant employment of the test-tube, but the operation of the plant requires little human aid beyond the turning of valve controls. In a word, given alumina solution, lime solution and efficient attendance, the apparatus does the rest. Use o! Alumina. Alumina is the agent used to coagulate the particles of matter suspended in the water. Alumina added at a rate, varying according to the need, of one to one-arid-three-quarter grains a gallon, collects the particles into "flock" which have sufficient body to be caught in the sand-beds. It comes from England in blocks and readily dissolves, but punips of zinc-free gun-metal and copper pipes have to be used to elevate the solution from the concrete tanks because of its effect upon zinc. This solution then is pumped to an automatic proportioning apparatus which maintains the flow in exact ratio to the flow of unfiltered water over the weir, a float controlling the arm which drops or raises the cylinder with its little weir for the solution. The solution is carried j I by a pipe over the inflowing water and j 'it drips out through holes. Before reach- j , iug the filter-beds, however, the water i has to take a journey through a concrete channel with baffles so placed that it is violently agitated, and so the solution is thoroughly mixed. Addition of Lime, Lime solution is added if the water contains no "hardness," but at present it is not required before filtration. It is added to the filtered water, however, because the effect of alumina upon the carbonates leaves it slightly acid, which the limo corrects. The lime is slaked and the solution has to be kept strongly agitated to prevent the lime settling. From tanks it is raised by elevating-gear to the pipes The water containing the alumina solution is admitted to the six filter-beds, concrete boxes a few feet deep each with a bed 420 square feet in extent The filters are layers of coarse gravel, fine crave], coarse sand and fine sand, the gravel being j on the bottom. The whole is 3ft Gin. thick. The gravel and sand came from | Xgaruawahi*, where a plant was installed to grade it. Clear Water Bubttes XJp. Beneath (he sand there is a reticulation svstem consisting of earthen pipes with 1680 holes and brass nozzles. Through these the water drains off. leaving behind it not merely suspended mat- j ter but-bacteria to the extent of 95 per J cent. The clear water bubbles up in wells in the valve gallery and away to the city. The flow of each' filter is automatically controlled. Every 24 hours under present conditions each filter has to be cleaned. The inlet valve is closed and the water already held sinks down until a floating control operates some precise gear that closes the outlet and opens the wash-out valve, rings a bell and shows a light, the signal for the operator to turn on the compressed air, itself filtered to catch dust and oil, which bubbles up through holes of the reticulating pipes and the sand and bursts through the jdilly-liko coating of dirt on the surface. The process resembles thermal activity in a mud pool at Rotorua. The dark chocolate mass is kept "in the hoi!" for a few minutes and then the upwash valve is operated. This drives up water in place of air and the body of dirty liquid rising quickly is soon pouring away to waste. Guided by the change of colour shown in a tube at the control, the attendant turns off before the water is quite clean, a perfectly clean filter being an inefficient one. A little mud in the sand is an aid to catch mud. In 20 minutes the job is done and a turn of a handle puts the filter to work again. 5,000,000 Gallons a Day. Such is mechanical filtration. Allowing for one of the filters being out of operation all the time the plant has a maximum capacity of 5,000,000 gallons for 24 hours. At present it is doing 3,750.000 gallons. It strikes one as a wonderful example of concentrated efficiency. The results afc Waitakere are practically perfect and only small adjustments have to be made to "maintain them, granting always, of course, that no mistakes are made in add ing a sufficiency of the alumina solution It is estimated the cost of filtration will be about Jkl a 1000 gallons, and loss than Id including interest charges For the benefit received the cost is a bagatelle and the pity is that filter-plants were not installed years ago