Rainwater Harvesting: When it Rains it Doesn’t need to Pour
It is painful to watch rainwater drain away while your taps run dry.
Only 50% of Nairobi’s four and a half million residents have direct access to piped water and even then the water is rationed. Rural areas and urban slums have it worse with most being unable to connect to piped water infrastructure and depend on uncovered wells, rivers, open springs and water vendors.
Rain water harvesting can boost the availability of water in many households. It simply involves the collection of water from surfaces on which rain falls, and subsequently storing this water for later use. This water can be used to supplement the main supply as well as provide water when there is a drought.
The basic rainwater harvesting system is really a plumbing job where rainwater is captured from roofs, diverted through gutters and drains, and stored in tanks of various sizes for later use.
You’ll first need to appropriately size the rainwater storage and estimate the amount of rainfall that is able to be collected by the area of capture or roof area. The rainfall capturing area must be large enough to maintain adequate flow, with the water storage tank size being large enough to contain the captured water.
Type of roofing material
The quality of water collected is highly dependent on the roof material.
Clay tiles are considered the best material for harvesting rain water because they are made of clay which is a natural filter. However, other materials such as corrugated iron sheets (mabati) and concrete tiles will also collect rainwater whose quality is considered relatively safe for domestic uses such as laundry, toilet flushing, bathing and other general cleaning.
Water collected with the intention of drinking purposes will need some level of treatment e.g. chlorination in order to ensure they meet regulatory standards for drinking water.
Roofs made from grass/reed, shingles and stone coated tiles are unsuitable catchment surfaces since there is high likelihood of water collected to be contaminated by these surfaces.
The storage tanks can be made of Ferro-cement, steel or plastic. The type of materials will depend on whether the tank will be placed above ground, on the ground or below ground (underground).
It is important to ensure that the top of the storage tank remains permanently covered and sealed to prevent the growth of algae or bacteria and infiltration of mosquitoes and dust.
Overflow pipes must be installed in the top of the tank to allow the safe disposal of excess rainwater and to prevent flooding. The overflow water should be drained away to a pit, plant, or stormwater drain.
Devices & Techniques for Better Water Quality
Coarse filtration screens (made of stainless steel or synthetic mesh) can be mounted across the top inlet of the storage tank with the downpipe above the screen. This prevents leaves and debris from entering the system and can improve water quality considerably.
Since contaminants—debris, dirt, and dust—collect on roofs during dry periods, it is important to employ a system for discarding the first portion of each rainfall (also know as the first flush). This can be a simple arrangement such as manually removing the downpipe away from the tank inlet for first flush and replacing once the first flush water has been diverted.
More complex mechanical systems such as semi-automatic first flush systems that do not rely on individuals exist but are more expensive to implement.