Posts Tagged ‘rooftop water collection’

Excerpted from Domestic Sanitation and Plumbing by A. Herring Shaw

Collection and storage of the rainfall from impervious surfaces such as roofs, yards, and courts is occasionally adopted in country districts for domestic services other than drinking water, and in some instances it also is used for the latter purpose.

Roof surfaces are generally chosen for the collection areas, and the water from there can be stored in a tank in the high part of the house. The various fittings throughout the premises are then supplied by gravitation.

A large portion of the organic matter which accumulates on roof surfaces is conveyed by the rain-water into the storage tank, especially at the commencement of a heavy downpour. This is an objectionable feature, for the stored water gives off an unpleasant odor, especially when heated for use in baths, lavatories, and sinks. Such water is totally unfit for drinking or potable purposes.

Fig. 28.—Rain-water Separator. (Apparently these devices are no longer being made. This Roberts Rainwater Separator was available during the latter 1800’s. Might make for an interesting venture for the mechanically inclined to develop a new device to place on the market.ds)

In many instances the rain-water is conveyed by the ordinary fall pipes to an underground tank constructed in the vicinity of the building.

It is advisable to adopt an arrangement which will either reduce the amount of organic matter conveyed by the rainwater from the roofs, or eliminate it entirely before the water passes into the distribution tank. Two methods are available for this purpose. One consists in fixing a device which will automatically divert the first portion of the rainfall, and discharge it into the foul-water drain. The roof-washings are thus prevented from mixing with the purer portion of the roof-water.

Fig. 28 shows a vertical section through Roberts’ rain-water separator. This appliance consists of a series of chambers and small balancing tanks enclosed by a movable drum which turns about a central pivot, M, attached to a stationary part of the apparatus. The water enters through the screening chamber A, and passes through E into the foul-water chamber N, whence it passes by a special pipe to waste. In the compartment C a small adjustable sluice, B, allows a stream of water to pass into the chamber F, which, when full, overflows into the canting tank J. When a certain quantity of water has accumulated in J, the centre of gravity of the drum is altered, and a movement takes place which shifts the outlet of E from the chamber N to the chamber 0. From the latter the water passes to the storage tank. When the flow from the roof ceases, the tank J is emptied by the siphon L, and the drum regains its normal position with the outlet of E over the foul-water chamber N.

Fio. 29.—Scheme for Collection of Rain-water.

Fig. 29 shows the application of the rain-water separator to a rain-water storage system. The water is led to an underground storage tank from the pure-water outlet, and is pumped as required into a distribution tank fixed in the high part of the house. Care should be taken to make the storage tank watertight, and provision for ventilation of the enclosed space should be made. The top of the access shaft should be raised about 12 ins. above the surrounding earth and be banked with concrete.

If the rain-water is required for drinking and potable purposes, it must be filtered previous to delivery into the distribution tank. To affect this, one or more sand-filters are required in addition to a large storage tank for the raw water and a smaller one for the filtered water.

Fig. 30 shows a plan and sections of a scheme for the collection, storage, and filtration of rain-water for supplying a large country house with potable water. The rain-water is collected from the various roof-surfaces and conveyed through the main channels M M to the large storage tank, K. At E a horizontal type of rain-water separator is showD, but is not always used. The bottom of the tank K slopes towards the scour-pipe, where a sump is provided to facilitate the proper cleansing of the tank. The overflow is connected to the scour-pipe at the far side of the valve N. A pipe conveys the raw water through the valves D, C, on to the pair of filters H, J. The scour-pipe P from these filters is connected to the scour-pipe from the large storage tank. The outlets from the filters are guarded by the valves E and F, and enter a Y junction which conveys the water to the filtered water-storage tank G. This tank is provided with an emptying pipe, 0, which joins the scour-pipe.

Great care is essential during the construction of the tanks and filters, to guard against the possibility of pollution of the stored water. The tank bottom can be of Portland cement concrete 6 ins. thick, and the walls of brick 14 to 18 ins. thickness, with a layer of sheet bitumen behind the inner 4£ ins. of brickwork. The bituminous sheeting must be continued over the bottom of the tank, and 3 ins. of brickwork should be placed above the sheeting. All brickwork must be set in cement mortar. Blue bricks should bo used for the interior, and hard-burnt red bricks for the outer walls.

The respective capacities of tanks and filters will be governed by local conditions and requirements, assuming (a) that the total requirements of the house per diem = 500 gallons; and (b) the collection area, i.e. the roof surface, is 140 ft. x 100 ft.; and (c) the mean annual rainfall = 50 ins.

The portion of the rainfall that can be stored will depend upon the amount which is rejected by the separator (if one be used) and the quantity lost by evaporation. The latter quantity is fairly considerable during warm and windy weather, and especially if the rainfall be intermittent and also not heavy in character.

It is usual to allow from 6 to 8 ins. depth for evaporation, and assuming a reasonable amount—\ the total rainfall—to be rejected by the separator—the total quantity available for storage is as follows:—

Requirements per annum = 500 x 365 = 182,500 galls.; leaving a surplus of 266,666 – 182,500 = 84,166 galls.

The amount to be stored varies in different districts as from 60 to 150 days. Assuming that 100 days’ supply has to be stored, then the capacity of the storage tank –

Assuming the depth of the tank to be 10 ft. and the length 25 ft., the breadth –

The two filters should each have a surface area of 9 sq. ft. This will allow them to be worked under a slight head, and will give a highly efficient filtrate. Two pipes connect the filters to the scour-pipe, and provide for running the water from the filters to waste for a period of twenty-four hours after cleansing operations.

The duplication of the filters permits of cleansing being carried out without risk of a shortage of filtered water occurring.

The storage capacity of the filtered water-tank should be equal to three days’ requirements. The capacity of the tank would then be—

which would be obtained by the following measurements: 8 ft. x 6 ft. x 5,ft.

From this tank the water requires to be lifted by means of a pump into a storage tank fixed in the high part of the house.

It should be borne in mind that rain-water collected in country districts is soft, well aerated, and usually slightly acid. These features are responsible for the solvent action which rain-water has upon lead, iron, zinc, and copper. This action may be neutralized by allowing the water to come in contact with limestone or chalk; the C02 dissolved in the water combines with the lime carbonate, and the water is rendered alkaline. If care be taken regarding the use of the limestone, the solvent character of the water can be changed, but the water is liable to become “hard “—a feature which is considered objectionable. A hardness of several degrees is, however, preferable to a solvent property.

In rare instances special rain-water collection surfaces are provided by enclosing and carefully under-draining grass land. Great care is essential to prevent the subsoil water in the surrounding earth from entering the collection channels of the prepared area.