What is rain water harvesting?
There are different methods of rain water harvesting which are described below in detail.
Rainwater harvesting is a green concept of reusing and conserving rainwater. It simply involves the collection of rainwater running off from surfaces like roofs which has fallen directly on the surface, purifying it to a certain level and use it when is required. People started harvesting rainwater since the time of immemorial.
Water is a crucial natural resource and an inevitable component for the living creatures. Although 71% of the world is covered with water only 3% of the earth’s water is fresh. Among these, 2.5% is unavailable as trapped in polar caps, glaciers, soil, atmosphere.
Rainfall occurs to complete the water cycle. Rainfall depends upon the climate, location, and surrounding environment.
Nature also finds its way of harvesting rainwater. Deep landforms such as a pond, lakes act as the storage tank and recharge for groundwater. People started collecting rainwater in barrel, pots, and pits for laundry, cleaning, and other domestic use. Besides, Rainwater harvesting can be done to recharge aquifers, bore wells which act as a source of water.
Rainwater harvesting is beneficial in the economic and environmental aspects. Some of them are as follows:
Importance of rainwater harvesting for future
- It can be used as a water supply system to reduce municipal water usage and bill.
- It can be a supplement in the case of droughts.
- It can reduce flooding and erosion by reducing run-off.
- It reduces groundwater demand, somehow conserving natural resources.
Rain Water Harvesting Methods
1) Barrel Method
It is the oldest form of rainwater harvesting. In this method, large buckets like Barrel are placed at the downspout to collect water which is later used for domestic and agricultural use.
This is a completely manual system, where storage capacity depends upon available barrels. Also, there is a high chance of water being spilled in this system.
2) Dry System
This system is similar to barrel methods. But the barrels are replaced by the storage tank placed underground near to the building. In this system water is directed by gutters, passed through pipes, and stored in the tank.
This system is applied for building with a simple roof network. Tanks can be provided with a float valve to indicate when tanks are filled. Also, the system could be arranged to spill the first few minutes of rainwater which are assumed to be swept off from the roofs.
3) Wet System
This system is a bit more sophisticated uses for complex roof network. Multiple underground pipes are arranged in which area filled with water when rainfall occurs water is the pipes are pushed into the storage tank.
Storage tanks are provided with a Float valve to prevent spill off of the water. Similarly, valves are arranged in such a way that rainwater during the first couple of minutes is not collected assuming they are full of dust and chemical swept off from the roofs.
4) Green Roof
The green roof is a new concept in which plants are grown covering the roofs. In this method, rainfall is first consumed by the plants then excess water is spilled off is a proper manner.
This method has an upside of preventing damages to the roof due to the external aspect and maintains thermal insulation to some extent.
Determining the size of a storage tank.
Determination of accurate rainfall in the area involves consideration of parameters like the intensity of rainfall, duration of rainfall, the material of roof coverings, and others. The formula to calculate discharge is.
m3/s where c= co-efficient of rainfall
i= intensity of rainfall(mm/hr)
In general, from the simple calculation, we know that, if 10mm of rainfall had occurred over 1 square meter area then the total water collected will be 10 liters. But 100% of water cannot be harvested. 20% of loss in rainwater due to splashing, evaporation, adsorption is considered.
Therefore, the total roof area is multiplied by total annual rainfall in the area and co-efficient 0.8 to calculate the size of the tank
Purification of the water
The purification of collected water depends upon the uses of water. Sedimentation will be sufficient in case of water being used for cleaning, flushing purposes.
Furthermore, sedimentation with coagulation can be done by the addition of chemical compounds like Aluminum Sulphate, Sodium Sulphate, Ferric chloride, Ferric Sulphate. Sand filters can also be used Filtration eliminates about 95-98% of suspended and colloidal impurities.
Similarly, the boiling and Solar water disinfectant (SODIS) method can be applied if the water is to be used for drinking purposes. This method can be applied if the water is biologically contaminated.
The rainwater containing heavy minerals must go through other methods. It incorporates placing water filled in polyethylene terephthalate (PET) bottles under direct sunlight for about 6 hours. Also, Chlorine is added as a disinfectant in the water.
Disadvantages of the Rain water harvesting methods
- Poorly constructed tanks can suffer from algae growth and breeding place of insects in the non-monsoon period,
- Sediments in the bottom of the tanks are to be flushed timely.
- The higher the initial cost to establish a full system.
- Rainfall estimation fluctuates on yearly basis.
- Rainwater harvesting is illegal in some cities.
Having said all these, we should stop now taking water for granted and using it wisely. Therefore rainwater harvesting can be one of the elite methods to reuse water.
I hope this article on Rain “Rain Water Harvesting Methods” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rukshana Shrestha