Figure 161 illustrates a method of preparing an ordinary house cistern for filtering. The pipe and fittings should be of galvanised iron; black or plain iron is better as long as it lasts, as it rusts fast; in either case it is better to waste the water first drawn, for the water absorbs both the zinc and the iron when standing over night. The zinc is not healthy, and the taste of the iron is unpleasant.
The perforations should equal 3 or 4 times the area of the suction pipe, which in ordinary cisterns may be 1 1/4 in. pipe, while the branches may be 3/4 in. pipe. The holes, if 1/3 in., should number at least 200, distributed along the lower half of the pipes. Smaller holes are preferable; of 1/16-in. holes, 800 will be required.
For the filtering material we recommend a layer of fine gravel or pebbles for the bottom, 3 or 4 in. in depth, or heaped up over the perforated pipes; upon this a layer of sharp, clean sand, 9 in. in depth; upon this a stratum of pulverised charcoal, not dust, but granulated to size of peas or beans, or any of the material above mentioned, 4 in. deep; and upon this a stratum of fine, clean sand 6 to 12 in. in depth.
Such a filter should be cleansed at least twice in a year by pumping out all the water, taking out the mud or settlings, and one-half the depth of the top layer, and replacing with fresh sand.
The double filter cistern, Fig. 162, has much to recommend it, having a large receiving basin which in itself is a filter placed in a position for easy cleaning. The recess at the bottom may be covered with a perforated plate of galvanised sheet iron, upon which may be laid a filter bed of gravel, sand, charcoal, spongy iron, and sand in the proportions as stilted above. This enables the frequent cleaning by removing the top layer of the filter bed without disturbing the water supply. The cover should fit tight enough to keep out insects and vermin.
A double-bottomed basin perforated and filled with clear, sharp sand and charcoal should be attached to the bottom of the pump pipe, as shown.
This enables the small filter to be drawn up and cleaned, without the necessity of emptying the cistern or interrupting the water supply.
The half barrel or keg filter, as illustrated in Fig. 163, is a convenient form of cistern filter where filtered water is required from cisterns already filled.
This is also a convenient form for readily cleaning or changing the filter without the necessity of discharging the water from the cistern.
This filter can be made from an oak: keg or half barrel, such as is used for liquors or beer. Take out one of the heads and cut away the edge, so that it will just drive into the end of the keg, fasten 2 battens of oak across the head with oak pins left long enough to serve for legs for the filter to rest upon.
Bore this head full of holes 1/4 in. diameter. In the other head bore a hole 1 1/4 in. diameter, and bolt an iron flange into which the pump pipe is to be screwed. Let the bolts also fasten upon the inside a raised disc of galvanised sheet iron, perforated with a sharp point or chisel. Proceed to charge the filter by turning the top or Hanged head down, and placing next the perforated plate a layer of fine gravel 3 in. thick, then a layer of sharp, clean sand 3 in. thick, then a layer of pulverised charcoal free from dust, 3in. thick, then a layer of sharp clean sand mixed with spongy iron, pulverised magnetic iron ore, or blacksmiths' scales, followed by a layer of coarse sand, gravel, and broken stone, or hard burnt bricks broken into chips to fill up. Place the perforated bottom in as far as the head was originally; bore and drive a half dozen oak pegs around the chine to fasten the head. Then turn over the filler, screw the pump pipe into the flange, and let it down into the cistern.
Such a filter requires to be taken out and the filtering renewed in 6 to 12 months, depending upon the cleanliness of the water catch. With the precautions mentioned above in regard to the care of the roof, such a filter should do good work for one year.
A country resident thus describes the manner in which he utilised rain-water, falling upon an ordinary tin roof, covered with some sort of metal lie paint, said to contain no lend, and flowing into a large cemented brick cistern, whence it was pumped into the kitchen. The cistern differed from the usual construction in this manner: across the bottom, about 3 ft. nearer one side than the other, was excavated a trough or ditch about 2 ft. wide and 2 ft. deep; along a brick wall from the bottom up to the lop of the cisstern. and having a few openings left through it at the very bottom. The whole cistern, bottom, sides, and canal included, was then cemented as usual, excepting the division wall. Upon each side of the wall, at its base, 6-12 in. of charcoal were laid, and covered with well-washed stones to a further height of 6 in., merely to keep the charcoal from floating. The rain-water running from the roof into the larger division of the cistern, passes through the stone covering, the charcoal, the wall, the charcoal upon the other side, lastly, the stones, and is now ready for the pump placed in this smaller part.
It is much better that the water at first pass into the larger division, as the filtration will be slower, and the cistern not so likely to overflow under a very heavy rainfall. He used this cistern for many years, and was troubled only once, when some toads made their entrance at the top, which was just at the surface of the ground, soon making their presence known by a decided change in the flavour of the water.
If the house chances to be in a dusty situation, several plans will suggest themselves whereby a few gallons at the first of each rain may be prevented from entering the cistern. Should the house be small, and therefore the supply of water from its roof be limited, do not lessen the size of the cistern, but rather increase it, for with one of less capacity some of the supply must occasionally be allowed to go to waste during a wet time, and you will suffer in a drought, whereas a cistern that never overflows is the more to be relied upon in a long season without rain.
Rainfall varies exceedingly in different places, and even in the same situation it is impossible to foretell the amount to be expected during any short period of time, but the most careful observations show that about 4 ft. in depth descends at New York and vicinity every year, or nearly 1 in. a week. If this amount were to be furnished uniformly every week, the size of a cistern need only be sufficient to contain one week's supply, but we often have periods of 4 weeks without receiving the average of one, and we must build accordingly.
The weekly average of 1 in. equals 1 cub. ft. upon every 12 ft. of surface, or 3630 cub. ft. upon an acre, weighing about 113 tons. Upon a roof 40 ft. by 40 ft., 1600 sq. ft., it would be 133 cub. ft., 1037 gal., or about 26 barrels of 40 gal. each. A cistern 8 ft. across and 10 ft. deep would contain 502 cub. ft.; and one of 10 ft. across and 10 ft. deep, 785 cub. ft., or 6120 gal. - about the average fall upon a roof of the above size for 6 weeks; while the smaller cistern would hold 3900 gal., or a little less than 4 weeks' rainfall. The weekly supply of 1037 gal. is equal to 148 gal. per day, or nearly 15 gal. to each individual of a family of 10. This is certainly enough, and more than enough, if used as it should be; but where water is plentiful it is wasted, and in our capricious climate, whether we depend upon wells or cisterns, it is wise to waste no water at all, at least during the warm summer months, and lay by not for a wet but a dry day.
In Fig. 164, abcd show the excavation that must be made for the cistern, and supposing the diagram to exhibit, as it does, a section of the cistern, the receptacle for the water will be, when finished, taking the relative proportions of the different parts into consideration, just about 9 ft. wide and 4 1/2 ft. deep. Of course, the excavation must be made greater in breadth and depth than the dimensions given, to allow for the surrounding walls and the bottom. The walls may be of brick, cemented within, and backed with concrete or puddled clay without, or of monolithic concrete; but the bottom, in any case, should be made of concrete. The trench efgh running across the bottom of the cistern is 2 ft. broad and 2 ft. deep. In the middle of this opening is built up a 9in. brick wall, or a party-wall of concrete, ik. Along the bottom of the wall openings l are left at intervals. The party-wall divides the entire space into the larger outer cistern m, and the smaller inner cistern n. Supposing the breadth from e to f to be 2 ft., and the wall 9 in., spaces of 7 1/2 in. will be left on each side of the wall. These are filled to 3/4 the height, or for 18 in., with lumps of charcoal, smooth pebbles, 1-3 in. in diameter being laid along the top of the charcoal till the trench is filled up.
The cistern is so constructed that the water from the roof enters m; it passes downwards through the stones and charcoal, as shown by the arrow at /, passes through the opening, and forces its way upwards in the direction of the arrow at e into the cistern n, in which it rises to the level of the water in m, to be drawn thence for use by a small pump.