THE ROTARY CLUB

of

Lahore Cantt.

Water Tank Project

THE ROTARY WATER TANK PROJECT FOR

THE PEOPLE OF THE THAR DESERT

Basic Tank design

Structure of the tank comprises the following components (Fig 1.):

The catchment area, which is a circular “pucca” construction forty feet in diameter with a six inch slope towards the center. Rainfall trapped in this catchment flows into the tank constructed at the center of the area.

The storage tank: This is a conical structure with a large circular base and a depth of about eight feet, and a smaller circular opening at its apex. The side wall is firmed up from the inside by pucca or semi-pucca masonry. The tank is built fully underground with its top aligned with the surface of the catchment platform.

Tank cap: This is a fixed metal covering over the tank’s upper opening. Along the bottom of the cap are a series of small openings (holes) which allow water from the catchment platform to flow into the tank for storage. The cap has an upper lid, which is opened for lowering a suitable utensil or container to extract water from the tank.

Strengths of the design

The structure is simple enough to construct and has a cost economy of only about Rs. 3,000 per tank.

The tank is postulated to have a working life of up to fifteen years.

Two tanks are reported to be sufficient for one family, given two rainfall showers per year.

Apart from the masonry requirement, the structure uses a minimum of other resources, limited mainly to fabrication and installation of the simple metallic cover, which has no moving parts except the lid.

The tanks installed by Rotary so far, or to be installed in the present phase, involve estimated construction of 5,000 tanks at a cost of Rs 1.5 million. This target will hopefully be met as required funds have been mobilized. Beneficiaries of tanks already installed consider this to be a heavenly facility. User satisfaction is therefore high, which already represents high achievement on the part of Rotary donors.

The conical shape of the tank leads to a smaller upper opening, which is more economical in terms of cost of the lid and is presumably more convenient in handling.

Weaknesses

The conical shape of the tank would seem to reduce the stability of the walls, which would have an increased propensity to cave in and damage the functionality of the storage area. The life of the tank will thereby be reduced. The weakness of the walls will be directly proportional to the angle of the walls in relation to the base. The more acute the angle the weaker will be the civil structure. This weakness will be further enhanced by weights on the ground surface as may come about by men and or animals (e.g. cows, buffaloes, camels) moving around the close vicinity of the lid.

The large bottom area created by virtue of the conical shape of the tank will leave any remaining water at the bottom of the tank spread out over the larger area in a thinner layer and will thus not be accessible for drawing out. In a time of greater scarcity this would represent a disadvantage.

During the prolonged dry seasons, the catchment area will lie unused and could collect dust, grime, rubbish and refuse of animals, humans and reptiles. If rains should occur in this state of un-cleanliness, much harmful material, including soluble components, will enter the storage tanks along with rainwater. Apart from that, such items as urine of animals or children etc, resulting from carelessness of a tanks’ guardians, will automatically flow into the storage tank, either during the dry season or otherwise, at any time. This could happen unless an efficient cut off stopper system  is in place to obstruct unwanted in-flows into the tank at times when rainwater is not under collection.

Because of the permanent slope of all points of the catchment area towards the tank’s intake points, the catchment area will not open to cleaning because any cleaning fluid used could also flow into the tank along with the pollutants, again, as said above, unless a stopper facility is provided. Even then, it will not be easy to throw or drain out the washings, which will be trapped as a dirty pool towards the center of the catchment area.

Stored water will be short of aeration, which will lead to germination of anaerobic bacteria and mosses ( “kai” ) etc. inside the tank.

The lowering of unclean household utensils into the tank, for water retrieval, may also lead to injection of additional impurities and contaminations into the stored water

There are chances of social disorder if a family short of water should sneak into the storage tank of another, at any time of day or night, and steal stored water from a tank, which remains unguarded and exposed in the open.

Tank shape options

The optimum tank shape would tend to be towards a straight-standing internal wall orientation, whether circular, square or rectangular. The shape will increase in stability the more obtuse is the angle of the wall in relation to the base, and will decrease in stability to the extent of its being acute. We could usefully consider a redesign of the tank’s interior wall at an angle of between 90 and 110 degrees. (Fig 2.)

For purposes of enabling collection of even the last available amount of stored water, a smaller central depression at the base could be built in, with the entire base sloped towards it. (Fig 3.)

Tanks could be pre-fabricated in fiberglass, along with cover, and installed very quickly at destination. Manufacture of the first tank will be more costly, but once the dies are made, manufacture of the rest should be cheaper.

Tank placement and catchment area shape

Placement of the tank at the center of the catchment area would seem to be a cause of some weaknesses stated above, such as inability to cut off unwanted flows into the tank and its increased exposure to possible pilferage of stored water, as well as security of the storage tank. An alternative is to think of the tank and the catchment area as two entirely independent structures, with no necessity of an ordained mutual orientation. The cachment area can be of any shape or size according to configuration of available space. It could be circular, rectangular, square or triangular. Its total available catchment area is the important consideration. The entire area would slope towards a point (Fig 4) from where the water will be carried to a storage tank in the vicinity, even if the tank is within the courtyard of the user. The channel for carriage could be an open surface drain (more susceptible to pollution and damage) or an underground PVC pipe, which could be the option of choice. The inlet (and outlet) point of the pipe would be fitted with a tap to stop at will any unwanted flows into the tank, a facility which can be of high practical value for purposes of retaining the quality of water already stored.

Hillside catchments

As a corollary to the above, we can postulate that, in case of habitations alongside of hills, the catchments may be created on a hillside, in which case storage tanks will not even need to be placed underground, but would have taps towards the base for drawing water.

Tank inlets and water drawal

In using the design regime as above, inlet points of a tank would be a single hole for entry of a pipe or as reception point of a drain. A slide-down door will also seal off the inlet when it is not in use, to prevent entry of insects or vermin. The same opening may also double for insertion of plastic pump pipes to draw out water. This latter option will eliminate the practice of lowering of household utensils or other containers, with ropes or strings into the tank.

Water quality

Water standing stagnant over long periods is susceptible to contamination and growth of bacteria. The following precautions are indicated.

Use of the plastic pump pipes, pumped in reverse, daily to pump air into the standing water for aeration.

Addition of a chlorinating agent for disinfection, whose nature and dosage can be decided in consultation with PCSIR or water board chemists. The possibility of adding any other prophylactics, such as iodides, can also be assessed.

Regular cleaning of catchment areas will not be possible because of water scarcity. However a regime can be established whereby users are trained to use the first few minutes of a downpour for quickly cleaning the surface with hard stick brooms.

It is also necessary for civic minded organizations and individuals to chalk out a program for repeated exhortation or education of users regarding the merits, i.e. the NECESSITY, for boiling water for about 10 to 15 minutes before drinking it.

The design proposed here still leaves one important element un-addressed. Before entering the storage tank, water from the catchment platform preferably needs to flow through a filtration box, through which water should percolate before falling into a receptacle of the flow channel for carriage into the permanent storage tank. The filtration tank could be a simple contraption comprising a wooden frame with a wire gauze base, filled with fine gravel. This is to filter out solid impurities including organic and inorganic matter, bird droppings, bodies of dead insects etc. Fig. 4 shows placement of a filtration box at a point just where the water leaves the catchment area. In actual practice the box needs to be located not here but at a place just before the point where water falls into the storage tank. This is to ensure that trash collected during the water’s travel through the drain also is filtered out. A panel of persons competent in this area needs to be constituted to propose  cheap and simple solutions, which could also include putting in a satchel of bleaching powder into the filter box for chlorinating the water. Important engineering considerations will also pertain to needed slopes in the arrangement of each individual tank project, which may be somewhat specific for different locations, depending on local topography.

 An important existing resource is ground water. This water is reported generally to be brackish and unpalatable. It can, however, still be used for household cleaning, washing of clothes, bathing and multiple other purposes, in this case also for cleaning the catchment areas. Brackish water may collect because of specific underground geological characteristics, which may or may not be extensive. Hence wells over wider areas need to be drilled to prospect for sweet water sources. Furthermore, the available brackish water resource needs to be analyzed to arrive at chemical means for economically removing the main impurities that cause offence, so as to soften the water and make it more usable at least for non-drinking purposes.