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Note on Mechanical Filtration and Gravitation Filtration (Paper & Discussion)

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Note on Mechanical Filtration and Gravitation Filtration (Paper & Discussion) 69 Victorian Institute of Engineers. A FEW NOTES ON MECHANICAL FILTRATION IN CONNECTION WITH WATER SUPPLIES. Read by MR. GEORGE SWINBURNE, 7M August, 1901. In this short paper I make no pretence at originality, but with a view to initiating discussion, will endeavour to give the result of a few enquiries on this important matter in which I have taken a little interest. The system adopted in older countries of filtering through sand beds is not adaptable to Australian conditions in the great majority of cases; yet there is no doubt that something must be done in the near future to better the condition of water supply in the smaller towns of Australia. In the large centres of population, where watershed areas are preserved, a good state of things _ mày exist, but in the country districts, the only available supplies are from doubtful springs, lather stagnant rivers which have already been polluted further up, or dams open to dust storms and all kinds of impurities, which are only filled when it rains in the immediate neighbourhood. It is rarely that the people themselves take much interest in the question, except to grumble when things are very bad. To engineers and sanitary authorities it is one of the most important questions in our modern life. Pure water means good health, but half the population of this continent is content to put up with Typhoid and the ills attending a supply of bad water, rather than help or pay to have the supply placed on the best possible footing. In my opinion this state of things exists on account of the supplies being in the hands of the local authorities or trusts, and the members not realising the importance of the matter. With a municipally-owned concern there are not always the same watch- dog regulations as in the case of a privately owned supply. A private company would have been compelled to put down filters, or use some means to improve the quality of the water which is now put up with by the inhabitants of many towns. Where a supply is owned by a company any excuse that the undertaking was not paying would be treated as being beside the question ; the order would be—" You must give us the best possible water and use every known means to achieve that result regardless of cost." This would be quite right, but why should many Water 70 MECHANICAL FILTRATION Trusts he content to give a poor supply instead of aiming at the best and considering more the health of the town ? They should not be lacking in what would be expected from a privately owned scheme. In the scarcity of perennial brooks and clear pebbly streams it is desirable that something should be done to secure the waters we have from many preventable sources of pollution, and to adopt severer regulations to compel authorities to use available methods of purification, and I trust that our discussion of the subject may tend to supplement the efforts of the Water Supply Department and Public Health Boards in pressing forward the improvements that necessity demands. I am not a chemist or bacteriologist, but, from an engineering point of view, I can see that what has been done in America can be done with advantage here. Mechanical filtration aims to purify large volumes df water with a small body of filtering material, relying on frequent washings to keep the material clean. The washing of the sand or quartz is accomplished by a simple reversal of the current, sometimes accompanied by agitating the purifying material by revolving arms. Instead of scraping the sand at intervals of some weeks, as is the case with ordinary open sand filter beds the filtrant in the mechanical filter is.washed in the filter itself at short intervals. There are, in America, several firms manufacturing mechanical filters, and wherever impure water is reported the filters are vigorously pushed. They are now working in connection with about aoo water supply schemes. Most of the filters are designed to remove suspended matter and colour in cases where these are disagreeable, but are generally supplemented by the use of some chemical, such as alum, which acts as a coagulant and precipitant, and also assists in the efficiency of filtration ; in fact it has been said that there is no mechanical filter made, which, without the use of some coagulant, will more than take out the logs," or remove more than the visible floating or suspended particles of matter in the water. I may say that there are two kinds of mechanical filters made which are classed as Pressure Filters and Gravity Filters. • Being Chairman of the Broken Hill Water Supply, I have taken an interest in the working of the mechanical filters put down in connection with that undertaking, under the supervision of Mr. Thos. Houghton, M.I.C.E. of Sydney, and the resident engineer, Mr. E. A. Whitehead, member of the British Association of Waterworks Engineers. They are known as the " Warren Gravity Filters." Broken Hill is one of the most difficult places in Aus- tralia to supply with water. A town of 30,000 people, and the only means of supply is to catch the water from a watershed about 200 square miles when it rains. The creeks only run after a downpour, and then only for a short time after the rain stops. MECHANICAL FILTRATION. 71 For the last five years the average rainfall per annum has been 5i inches, and as many as 62 months have elapsed without any catchment of water in the reservoir, so that you will see the difficulty the company has in supplying pure water when all that is available is what has been stored under a broiling sun sometimes for many months without any fresh in-take. The mechanical filters at Broken Hill are the only filters, as far as I know, that are used in connection with water supply in Australia, and are estimated to treat 1 million gallons of water per 24 hours. The water to be filtered is pumped from the catchment reservoir through roi miles- of main composed of 14", 16" and 18" pipes, and is received by a cast-iron tank, measuring 33'6" x 37'6' x 8'4" maximum depth. This tank is divided by three timber partitions into four channels, each opening into the next at alternate ends, thus forming a continuous water-way having a length equal to four times that of the tank. As the water enters the tank there is added to it a small quantity of sulphate of alumina, of such strength that it is equivalent to from r grain to 2 grains of alum per gallon of incoming water. The sulphate of alumina is dissolved in filtered water, heated by steam in an open wooden vat 4'6" diameter. From this vat it is run into two lower vats, each 6'o" diameter, where it is diluted and graded to the required strength to suit the quality of water being treated. The strength is determined by means of a urinometer indicating specific gravities from i.o to 1.6. From the lower vats the solution is pumped to a lead-lined storage tank situated above the top level of the receiving tank, and from thence gravitates through a brass pipe to the alum feed box, in which the level of the solution is maintained constant by means of a vulcanite ball-valve Working in the feed box, and driven by a six-bladed propeller placed in the mouth of the rising main, is a vulcanite radial pump—a sort of reversed. Barker's Mill— which picks up the solution and delivers it through a lead down pipe to the in-coming water. As this pump is driven by the in-coming water, it follows that the quantity of solution delivered by it should he always pro- portionate to the quantity of water flowing into the receiving tank. The chief point in the mechanical filtration of public supplies of water is the use of a coagulant or precipitant, the nature of which depends largely upon the composition of the impurities in the water to be treated. As already indicated, the coagulant used at Broken Hill is sulphate of alumina (alum) of 17i% purity, the action of which is described as follows :—When the solution of sulphate of alumina is added to the water it is broken up into sulphuric acid and 72 MECHANICAL FILTRATION. hydrate of alumina, the acid combines with the lime or other salts in the water forming sulphates, while the hydrate of alumina pre- cipitates as a gelatinous mass, carrying down with it much of the solids in suspension and a certain portion of the organic matter. It further assists filtration by partially filling the small in terstices of the upper surfaces of the filter-beds, and thus more quickly bringing them into a state of efficiency. The precipitating action of the alum takes place in the receiving tank, which it is found necessary to clean out at least once in every four weeks. From the receiving tank the water passes to the filters. These are four in number, each 12'6" diameter and g'II" total depth, built up of $" steel plates. The filtrant used is quartz, free from iron and other impurities or colouring matter, and crushed to pass through a sieve having 16o holes per square inch ; any quartz which passes through a sieve having 1600 holes per square inch is rejected. Each filter bed has a depth of 2'2", and is supported upon perforated copper plates fixed on suitable arranged timbers at a height of 18" above the cemented floor of the filter, the space under the copper bottom forming a clear water chamber.
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