The following is a description of a filter which purifies foul water from organic impurities held in solution as well as from suspended solids. Take any suitable vessel with a perforated false bottom, and cover it with a layer of animal charcoal, on the top of that spread a layer of iron filings, borings, or turnings, the finer the better, mixed with charcoal dust; on the top of the filings place a layer of fine clean siliceous sand, and you will have a perfect filter. Allow the foul water to filter slowly through the above filter, and you will produce a remarkably pure drinking water. Before placing the iron filings in the filter, they must be well washed in a hot solution of soda or potash, to remove oil and other impurities, then rinse them with clean water; the filings should be mixed with an equal measure of fine charcoal. If the water is very foul, it must be allowed to filter very slowly. The deeper the bed of iron filings is the quicker they will act.
In Bailey-Denton's cistern filter, the principal novelty is that it runs intermittently, and thus allows the aeration of the filtering material, and the oxidation of the impurities detached from the water. The oxidation is effected by the perfect aeration of the filtrating material, which may be of any approved kind, through which every drop of water used in the kitchen, bedrooms, and elsewhere must pass as it descends from the service cistern for use. As water is withdrawn from this filter, fresh water comes in automatically by the action of a ball-tap; and this fresh water immediately passes through the aerated material into a lower chamber, forming the supply cistern of filtered water for the whole house. The advantages claimed for the filter are that it secures pure water for the whole house. It is attached by pipe to, but is distinct from, the service cistern; it can be placed in any part of the house, and it cannot get out of order. Any approved filtering material may be used, and being aerated between each passage of water through it, oxidation is made certain.
A slate or iron cistern and filter combined may be made by dividing the ' cistern with a vertical partition perforated at the bottom, and placing in the half of the cistern which receives the water, a bed of filtering material, say 6 in. of gravel at the bottom, 6 in. animal charcoal in granular form in the middle, and 6 in. clean sharp sand at the top, covering all by a perforated distributing slab.
Among the things to consider in determining whether cistern water is safe to drink, are the cleanly or dirty condition of the roof, and the materials it is made of; whether leaves from overhanging trees fall upon the roof and lodge in the gutters; whether birds foul the roof; whether it is made of wood, slate, or tin, or of materials inimical to health - as lead, copper, or covered with deleterious paints.
In order to obtain the best results from filtering cisterns, the roof and gutters should be kept free from leaves and dirt, and it is also advisable to arrange the leader with a switch valve, with the handle convenient for operating within the building, so that the first wash may carry away the dust, dirt, or other foul matter, and thus save only the best water.
Caution should be exercised in locating cisterns that are intended to furnish drinking and potable water, that they be away from the influence of cesspools and privies, as clean water reality absorbs the odours, gases, and germs of foul air.
The materials selected for filter beds should be in accordance with the resources of the locality in which the filter is to be used, for the purpose of renewal.
We recommend such materials only as have proved reliable, leaving out all textile or organic substances, as we deem such unfit for this class of filtration.
Pulverised charcoal mixed with sand, or between layers of sand and gravel, so long used for filtering purposes, has a cleansing or antiseptic power, probably derived from the contact of a large carbon surface. Pulverised coke has been used, and is considered a fair filtrant, but less effective than charcoal. Bone charcoal has also been recommended as being highly antiseptic, besides having a strong absorbent power, due to the variety of its chemical components.
Spongy iron, or pulverised hematite mixed with sawdust and roasted; pulverised magnetic iron ore and clean scale from a blacksmith's anvil, pulverised and mixed with clean, sharp sand, have been much used and experimented with in Europe with great success, in not only making fetid water sweet, but it is also claimed that the iron mixtures destroy bacteria and their germs.
A combination of two extremes, a large carbon surface in charcoal and the pungent oxidising qualities of the spongy iron, or its equivalents, will no doubt become the acme of a filter.
From experiments made with the filters of public waterworks in Europe, for the quantity of water that a filter will yield per square foot of surface, it has been ascertained that, with a filter composed of 10 parts fine sharp sand, 1 part coarse sand, 15 parts spongy iron mixed with one-third its bulk of fine gravel, laid upon a strainer of perforated galvanised iron - a bed of brick laid close - or a stratum of gravel covering a perforated iron pipe, a yield of 1 gal. of clear, pure water for each foot in depth per hour for each square foot of surface: 4 ft. being the greatest depth with a yield of 4 gal. per foot per hour - illustrating the probable fact that the velocity of the water corresponds with the depth of the filtering material for equal purity.