Harvest rain water

Jul 04, 2005

Water is life and Uganda is well endowed with it. We have two rainy seasons a year giving us an average annual rainfall of about 45 inches. However, it is common to suffer the effects of waterlessness. Crops fail and domestic animals die. Sometimes communities migrate from their villages to distant

By Samson Kibende

Water is life and Uganda is well endowed with it. We have two rainy seasons a year giving us an average annual rainfall of about 45 inches. However, it is common to suffer the effects of waterlessness. Crops fail and domestic animals die. Sometimes communities migrate from their villages to distant places in search of water.

A Ugandan visiting Libya was asked by his host whether Uganda is a desert area. Having learnt that we get so much rain and Uganda is very green, the Libyan wondered why then we were so poor.

Water harvesting is the principle of collecting and using precipitation from a catchment’s surface.
Rainwater harvesting promotes self-sufficiency. It also promotes water conservation.

Rainwater harvesting also conserves energy as the energy input needed to operate a centralised water system is bypassed. Many systems require only a small pump to create water pressure in household pipes.

Rainwater is one of the purest sources of water. Its quality always exceeds that of ground or surface water. It does not come into contact with soil where it can dissolve minerals and salts nor does it come into contact with many of the pollutants often discharged into local surface waters or contaminate ground water supplies. However, rainwater quality is influenced by where it falls. rainfall in areas where industries or crop dusting are prevalent may not have the same purity as rain in other areas.

Amounts gathered
The rule of thumb is 10 litres of water per cm of rain per 100 sqm of catchment area. Calculate the catchment area using the footprint of the building plus the length of the overhangs.

Efficiency is about 75% - 90% depending on system design.
Rainwater can provide clean, safe, and reliable water so long as the collection system is properly constructed and the water is treated appropriately for its intended use. Although rainwater is one of the purest sources of water and it’s quality almost always exceeds that of ground water, once rain contacts a collection surface like a roof it can wash many types of contaminants off that surface and into the tank. Contaminants include mold, bacteria, algae, protozoa, dust, pollen, feathers, leaves and dead insects.

Health concerns regarding rainwater for drinking include micro organisms like bacteria and contaminants such as pesticides, lead, and arsenic. Rainwater must be treated in some manner to kill micro organisms and remove contaminants.

Catchment systems
Rainwater catchment systems can range in size from a barrel to a built system costing millions of shillings. However, rainwater-harvesting systems are simple and can often be assembled with available materials by owners with a basic understanding of plumbing and construction.

Concrete: Reinforced concrete tanks can be built above or below ground. Because of their weight, they are usually poured in place to specifications. However, concrete tanks can also be fashioned from prefabricated components, such as septic tanks and from concrete blocks.

Concrete is durable, but is subject to cracking; below ground tanks should be checked periodically for leaks, especially in clay soils where expansion and contraction may place stress on the tank. An advantage of concrete cistern chambers is their ability to decrease the corrosiveness of rainwater by allowing the dissolution of calcium carbonate from the walls and floors.

I have constructed two 10,000litre above ground tanks at my home as rainwater catchment facilities. Besides fired bricks and mortar, no other reinforcements were necessary. The first tank has served for over six years.

Polyethylene: These tanks are commercially available and can be constructed for above or below ground installations. Polyethylene tanks are gaining popularity due to their relatively low cost and long life — although they are still dearer than the brick ones. Their lightweight makes them easy to transport while their smooth interior makes them easy to clean.

To ensure their long-life, polyethylene tanks which have ultra-violet (UV) inhibitors for outdoor use, or painted with a protective surface to provide protection from the sun should be chosen. Black tanks have the greatest UV resistance, with a life expectancy of 25 years, though tend to absorb heat and thus can affect water quality.

Galvanised corrugated iron: They are the cheapest. They do not last as they get corroded easily and the corrosion could release dangerous substances into the water.

Safety issues
1. A cistern may not be located closer than 50 feet from a source of contamination like a septic tank.

2. A cistern must be located on a grade lower than the roof washer to ensure that it can fill.

3. A rainwater system must have an overflow pipe, which empties into a non-flooding area.

4. Inlets must be made to dissipate pressure of in-flowing stream and minimise the stirring of any settled solids.

5. Overflow from rainwater systems cannot flow into sewage drainage systems.
6. Cisterns should be accessible for cleaning.

(adsbygoogle = window.adsbygoogle || []).push({});