Ever wonder what can be done to the rain that falls on your roof? Can this water be put to any use at a household level? What does it take to harvest this?
As we saw in one of the earlier posts, the different components of a Rainwater Harvesting system are Catchment, Conveyance, Storage and Filtration.
Let us dive into storage and understand the nuances in determining the type and size of storage.
The rooftop runoff will be conveyed to the ground level through a network of pipes and directed to the storage structure which can be an underground masonry tank or a HDPE (High Density Polyethylene) plastic tank kept above the ground or a HDPE plastic tank below ground level.
While the masonry tank is more robust, the plastic tank is easier to set up and is fairly durable as well. Also, in the case of the plastic tanks, it is easier to relocate them if required whereas the masonry tank is wedded to its location.
The size of the storage is an important parameter while planning the rainwater harvesting system and is the major cost component as well. The storage capacity can be a function of multiple parameters. Some of them are:
- Space available for creating a storage system
- The rooftop area
- The average rainfall quantity and rainfall distribution data for the place
- The number of people living in the house, the garden area. In other words, the daily water demand of the house.
- The availability of other sources of water
- State regulation
Let us consider different scenarios and understand which parameter can be used for determining appropriate storage capacity.
- A family lives in an area where there is severe water shortage during summers when the city utility cannot supply water due to insufficient sources. In such a scenario, it is advisable to build storage big enough to hold all the roof top runoff during the monsoon season and store it for use during the summer. In such a case, the storage should be almost equal to the annual roof top harvesting potential.
To substantiate this, let us assume that the roof area of the house is 200 sq m. The annual rainfall for the place is 1000 mm. To be able to store the entire rooftop generated over the year, the storage capacity required is:
Annual runoff generated = Roof Area * Annual Rainfall * Runoff coefficient
= 180,000 litres or 47,520 gallons
Hence a storage capacity of 47, 520 gallons is required to sail through the summer months with adequate water supply. If the household water demand is significantly lesser during the summer months, then it can limit the storage capacity required. For instance, for the household in question, if the daily water demand is 300 gallons, then the total water demand for the three summer months would be 27,600 gallons. Hence, a 47,520 gallon storage tank might be overkill if the intent is to use rainwater only during the summer months.
- A 6 member household with a daily combined water demand of 600 gallons lives in a place which receives well distributed rainfall – almost over 70 days spread over 6 months. The state also allows usage of rainwater for domestic consumption. In such a scenario, the storage capacity can be for the peak rainfall possible over a 24 hour time period. Since, the water from the tank will be constantly used for household purpose, the tank will keep getting empty to receive runoff from the next shower. Assuming a peak possible rainfall of 150 mm over a 24 hour time period and the available roof catchment of 200 sq m, the storage capacity will be:
Runoff generated = Roof Area * Peak Rainfall * Runoff coefficient
We can observe that there is a significant reduction in storage capacity compared to the first scenario.
- The state of Colorado has a regulation which allows a household to collect upto 110 gallons of rainwater. Here, the storage is driven by the state legislation.
- In dense urban settlements, there might not be enough open space surrounding the house building. This can be a limiting factor for increased storage capacities to do justice to the runoff generated.
Illustration: Construction and final Reinforced Cement Concrete sump tank of 27000 gallons (in an educational institution)
Illustration: Downpipes connected to a 1300 gallon HDPE plastic tank (in a factory)