Estimating the size and design of the required systems

Estimating the size of the required systems

The actual size of the system will depend on various factors. Primarily the size will be influenced by the cost of the system, the amount of rainwater to be collected, expectations and needs of tank owners and the level of external support.

The key to the solution here lies in matching the supply and demand to the satisfaction of the user(s) at lowest cost possible. The amount of rain and its distribution over the year, the size of the catchment area and the projected supply ultimately determine the size of the tank. Usually the worst scenario will present itself during the longest dry spell. The size of the catchment area and tank should be enough to supply sufficient water for the users during this period.

But in the urban housing nowadays people no bother about storage of rain water storage, so developers has installed ground water recharge system on the site before taking building user certificate.

General design features

Rooftop water harvesting systems can provide good quality potable water if the design features outlined below are taken into account.

Design considerations for rooftop catchment system:

1. The substances that go into the making of the roof should be non-toxic in nature.
2. Roof surfaces should be smooth, hard and dense since they are easier to clean and are less likely to be damaged and release materials/fibbers into the water.
3. No overhanging trees should be left near the roof.
4. The nesting of birds on the roof should be prevented.
5. All out lets ends should be fitted with a wire mesh screen to keep out leaves, etc.
6. A first-flush rainfall capacity, such as a detachable down pipe section, should be installed.
7. There should be no possibility of contaminated wastewater flowing into the system (especially for tanks installed at ground level).
8. Water from other sources, unless it is a reliable source, should not be emptied into the system through pipe connections or the manhole cover.
9. There should be a reliable sanitary extraction device such as a gravity tap or a hand pump to avoid contamination of the water in the tank.
10. There should be no possibility of contaminated wastewater flowing into the tank (especially for tanks installed at ground level).
11. Water from other sources, unless it is a reliable source, should not be emptied into the tank through pipe connections or the manhole cover.

Design of system components

By now, it is clear that a rooftop catchments system has three main components, viz. a roof, an outlet and first flush device and system.

The roof - The roof should be smooth, made of non-toxic substances and sufficiently large to fill the tank with the available rainfall conditions. Existing roofs of houses and public buildings can be used for a rooftop catchments system. In some cases enlarged or additional roofed structures can be built.

Outlet and first-flush device - Outlet is intended to protect the building by collecting the water running off the roof and direct it, via a down pipe, to the storage tank. Outlet should have a uniform slope of 0.5 per cent large enough to collect the heavy runoff from high-intensity rain.

Recharge Bore Well System: water from surface sources is conveyed or stored in-situ at places above the aquifer areas and made to percolate and recharge the ground water. This includes Surface spreading Techniques and Subsurface Techniques.

With all roof catchments tanks, the first rainwater running off the roof should be discarded. This helps keep the water potable because this first flush of rainwater contains large quantities of leaves and bird droppings. The importance of such first flush devices became clear from a study undertaken in Malaysia. The study showed how the faucal coli form count in runoff water was reduced from 4 to 60 per litter to zero, as the first five litters of run-off washed a roof measuring 15 M2 .

PVC a positive choice of today and tomorrow:

The use of PVC for production of pipes is one of the most dramatic success stories in Industry, where a new material, technology and entrepreneurship has so significantly fulfilled the basic need of assured availability of clean and hygienic water. Traditional material like steel, cast iron, stoneware or asbestos suffer from effect of corrosion, health hazards, cost, transportation, breakage and a short as well as uncertain life span while PVC pipe do not suffer from these problems and have gained overwhelming preference world over in a short time span. They are the positive choice of today and tomorrow.

Why PVC pipes?

To make the system cheaper, it is advisable to adopt PVC as material to channel the water from the roof top.

Main benefits of PVC:

A. Corrosion resistance

1. No surface Corrosion
2. protective coating required
3. Immune to galvanic or electrolytic erosion
4. PVC pipes are not attacked by low or high concentration of acids, oxidizing agents, alkalies, Oils fats and halogens.

B. Low thermal conductivity means it maintains uniform temperatures in transforming fluids.
C. Energy saving, Environment friendly recyclable and Flexible.
D. Biological resistance.

1. Fit for purity water applications.
2. Stability of PVC pipes against rodent attack.

E. Abrasion resistance- Due to toughness and inner bore smoothness its ideal for abrasive resistant application.
F. Always available in wide variety of custom colors.
G. Can be installed at very low cost further more it is maintenance free (no painting or no coating required) and up to certain extent they are fire resistance.
H.The Co-efficient of thermal expansion of rigid PVC pipes is 5-6 x 10-2 per 1 degree Celsius. This means that in a 100 feet run, the length of pipe will alter by 1 inch per 10 degree Celsius change in temperature. Compared to PVC the co-efficient of thermal expansion for CI/GI pipes is 1-2 x 10-2 per one degree Celsius.

Utilization of dried up wells

Lowering of groundwater levels in the last 2 to 3 years, has led to the drying up ofmany of the open wells as well as some shallow bore wells. Instead of discardingthem, these dried up wells can be utilized as recharge wells by connecting them tothe water outlet pipes from the roof tops. This cost effective process not only savesthe precious rainwater going as waste, but also helps in restoring the local groundwater availability.

Conclusion

In short water harvesting is a process of collecting and concentrating runoff water from a runoff area into a run-on area, to be either:

1. Directly applied to a cropping area and stored in the soil profile for direct Uptake by crops and trees (i.e. runoff farming) or
2. Stored in a on-farm water reservoir for future productive uses
3. Domestic consumption, livestock watering, aquaculture, irrigation
4. Stored into the aquifer (i.e. recharge enhancement).