In general terms, the object of filtration may be said to be the separation of the solid from the liquid constituents of a fluid mass by means of a straining medium. Either the solid portion, or the liquid portion, or both, may be the valuable ingredient. As different processes and apparatus are employed according to the character of the fluid to be filtered, it will be convenient to divide the subject into several heads.
Water is undoubtedly the most important fluid submitted to filtration. In this case, the operation is destined to perform 3 distinct functions, at least where the water is required for domestic use; these are (1) to remove suspended impurities, (2) to remove a portion of the impurities in solution, and (3) to destroy and remove low organic bodies.
The first step is efficiently performed by nature, in the case of well and spring water, by subsidence and a long period of filtration through the earth; in the case of river water supplied by the various companies, it is carried out in immense settling ponds and filter beds of sand and gravel. This suffices for water destined for many purposes. The second and third steps are essential for all drinking water, and are the aim of every domestic filter. The construction of water filters may now be discussed according to the nature of the filtering medium.
The usual plan adopted by the water companies is to build a series of tunnels with bricks without mortar; these are covered with a layer of fine gravel 2 ft. thick, then a stratum of fine gravel and coarse sand, and lastly a layer of 2 ft. of fine sand. The water is first pumped into a reservoir, and after a time, for the subsidence of the coarser impurities, the water flows through the filter beds, which are slightly lower. For the benefit of those desirous of filtering water on a large scale with sand filtering beds, it may be stated that there should be 1 1/2 yd. of filtering area for each 1000 gal. per day. For effective work, the descent of the water should not exceed 6 in. per hour.
This simple means of arresting solid impurities and an appreciable portion of the matters in solution, may be applied on a domestic scale, in the following manner.
Procure an ordinary wooden pail and bore a number of 1/4-in. holes all over the bottom. Next prepare a fine muslin bag, a little larger than the bottom of the pail, and about 1 in. in height. The bag is now filled with clean, well washed sand and placed in the pail. Water is next poured in, and the edges of the bag are pressed against the sides of the pail. Such a filter was tested by mixing a dry sienna colour in a gallon of water, and, passing through, the colour was so fine as to be an impalpable powder, rendering the water a deep chocolate colour. On pouring this mixture on to the filter pad and collecting the water, it was found free of all colouring matter. This was a very satisfactory test for such a simple appliance, and the latter cannot be too strongly recommended in cases where a more complicated arrangement cannot be substituted. The finest and cleanest sand is desirable, such as that to be purchased at glass manufactories.
This filter, however, at its best, is but a good strainer, and will only arrest the suspended particles. In a modern filter more perfect work is required, and another effect produced, in order that water containing objectionable matter in solution should be rendered fit for drinking purposes. Many persons when they see a water quite clear imagine that it must be in a good state for drinking. They should remember, however, that many substances, which entirely dissolve in water do not diminish its clearness. Hence a clear, bright water may, despite its clearness, be charged with a poison or substances more or less injurious to health; such, for instance, as soluble animal matter.
To make a perfect filter, which Should have the double action of arresting the finest suspended matter and removing the matters held in solution. and the whole to cost but little and capable of being made by any housewife, has long been an object of much attention, and, after many experiments and testing various substances in many combinations, the following plan is suggested as giving very perfect results, and costing only about 8s.
Purchase a common galvanised iron pail, which costs 2s. Take it to a tin-shop and have a hole cut in the centre of the bottom about \ in. diameter, and direct the workman to solder around it a piece of tin about 3/4 in. deep, to form a spout to direct the flow of water downward in a uniform direction. Obtain about 2 qt. of small stones, and, after a good washing, place about 2 in. of these at bottom of pail to form a drain.
On this lay a partition of horse-hair cloth or Canton flannel cut to size of pail. On this spread a layer of animal charcoal, sold by wholesale chemists as boneblack at about 5d. a lb. Select this about the size of gunpowder grains, and not in powder. This layer should be 3 or 4 in. A second partition having been placed, add 3 in. of sand, as clean and as fine as possible. Those within reach of glassmakers should purchase the sand there, as it is only with that quality of sand that the best results can be obtained. On this place another partition, and add more fine stones or shingle - say for 2 or 3 in. This serves as a weight to keep the upper partition in place, and completes the filter.
By allowing the filtration to proceed in an upward instead of a downward direction much better results are obtained. For a large supply, as in the case of a paper factory, for instance, a good plan is as shown in Fig. 150. Between the factory supply tank and the river providing the water a longitudinal basin is dug in the ground 30 yd. long, 8 ft. wide, and 8 ft.. deep, the sides and bottom being cemented with great care. At the left extremity, which is towards the reservoir, is a well cemented wall, resting upon iron supports, and bearing a trough connected with the reservoir. At the right extremity, which is towards the river, is a second wall, resting upon the ground, and closing in the basin. Stretching across the basin from side to side a number of iron rails are built into the walls, bearing wooden laths upon which is laid the filter bed. This is composed of 4 layers, the lowest of large stones, the second of coke, the third of flints, and the fourth and uppermost of river sand. The water is allowed to flow gently from the reservoir and enter at the left, that portion of the basin underneath the filter bed.