Alternative sources of drinking water are generally sought when groundwater cannot be accessed due to hard ground conditions, or is unfit for drinking or when there is no surface water.
Rainwater harvesting is the most relevant alternative for regions having proper rainfall. Rainwater harvesting technology is used for collecting and storing rainwater from rock catchments, the land surface and rooftops with the help of pots, jars or underground check dams.
Most modern technologies used for providing drinking water include the exploitation of groundwater from boreholes and wells, and surface water from lakes, streams and rivers. These techniques, however, account for only around 40% of total precipitation.
Rainwater harvesting ensures independent supply of drinking water at times of water scarcity across regions. Rainwater harvesting systems are easy to install and operate. They are efficient too as rainwater is collected from the surfaces upon which it has fallen directly excluding the runoff from land watersheds into lakes, rivers and streams.
The quality of collected rainwater is generally sufficient for household needs, mainly reducing the requirements for large amount of detergents as pure rainwater is soft.
Rainwater Harvesting System
The principal components of rainwater harvesting include the following:
Rooftop Catchments – In this technology, rainwater is collected in vessels placed at the roof edge or through gutters that drain water to the collection vessel via down-pipes, or diverting water from gutters to containers for settling particulate matters before being drained into the storage tank.
The volume and quality of rainwater collected on the rooftop depends on the area and the roofing material used. Pure rainwater can be collected from the roofs made of slates, tiles, aluminum or asbestos cement sheets and galvanized corrugated iron.
Thatched roofs with bamboo gutters and properly laid slopes can also yield the same amount of water inexpensively. However, roofs constructed with metallic paint may change the taste and color of the collected water.
Land Surface Catchments – Rainwater harvesting using land surface catchments involves collecting the runoff using drain pipes and storing the collected water in tanks. This technique provides a larger surface area for collecting rainwater when compared to rooftop catchment techniques.
Rainwater can be captured by retaining the flows of small streams and creeks in small storage containers. The water stored using this technique can be used for agricultural purposes.
The runoff within the land surface catchments can be increased by reducing soil permeability, increasing the land slope using artificial ground cover and clearing vegetation cover.
Storage Tanks - Storage tanks used for collecting rainwater may be either above or below the ground. Open containers are not preferred for collecting rainwater for drinking purpose. Mortar jars and cylindrical ferrocement tanks are the commonly used rainwater storage facilities.
Mortar jars are large jar-shaped vessels made from wire reinforced mortar. The ferrocement tank includes a lightly reinforced concrete base on which a circular vertical cylinder is erected. The cylinder is in turn covered with light wire mesh layers, forming a tank frame.
It is essential to ensure that storage tanks provide cover for preventing breeding of mosquitoes or algal growth, and an enclosure to reduce contamination from human or animal or other environmental contaminants.
Rainfall Water Containers - Battery tanks that are interconnected and made of polyethylene, ferrocement or pottery can be used as an alternative to storage tanks. These tanks are provided with a number of openings that can be fitted with connecting pipes. They are compact and convenient for cleaning.
Conveyance systems help in transferring the collected rainwater to the storage tanks through the connections made with one or more down-pipes linked to the rooftop gutters. Dirt and debris from the rooftop and gutters will be washed into the down-pipe when it first starts to rain.
Clean water can be collected only after some time and there are several choices for collecting pure water. With the help of manually operated down-pipe flap, it is possible to direct the first flush of water flow via the down-pipe, while the latter rainfall is diverted to the storage tank.
On the other hand, it is necessary to ensure that excess water can overflow and the blockages in the pipe do not contaminate the water supply. With the system designed to have larger drain-pipe than the rainwater tank feed-pipe, excess water can be allowed to bypass the storage tank.
Advantages of Rainwater Harvesting
The key benefits of rainwater harvesting include the following:
- The construction of a rainwater collection system is very simple and can be achieved using inexpensive materials.
- It can provide good quality water in a cost-effective manner.
- The rainwater collection system has low operation and maintenance costs.
- The collected rainwater can be directly used without treatment providing a clean collecting procedure has been followed.
- The rainwater collection system is flexible and adaptable.
- It avoids flooding, erosion and storm water drains.
- It can be used to recharge ground water.
Disadvantages of Rainwater Harvesting
The following are some of the major limitations of rainwater harvesting:
- High initial investment costs for constructing rainwater collection system.
- Regular maintenance, cleaning and repair will be needed for successful collection of rainwater.
- Poorly constructed storage containers or jars may suffer from algal growth and invasion by lizards, insects or rodents.
- Water supply can be contaminated by animal/bird droppings on the catchment areas if they are not cleaned properly.
- Droughts or long periods of no rain conditions can affect the water supply.
The feasibility of rainwater harvesting technology in a specific location is mainly based on the intensity of rainfall. Other variables include catchment surface type and catchment area, which can be altered as per the household requirements.
Rainwater collection techniques can only serve as a supplementary source of water supply as rainfall is unevenly distributed all through the year. In addition, the viability of the rainwater collection systems is highly dependent on the quantity and quality of water available from other sources, budget available, household size and per capita water requirements.
Therefore, it is necessary to balance the total cost of the project with the budget available. Similarly, the cost of environmental degradation related to the development of available alternative sources should also be analyzed and added to the economic analysis.
Sources and Further Reading