Roof Rain Water Harvesting System (RRWHS)

RRWHS can be installed in any building having a sufficient roof area and space to store rainwater or recharge as groundwater. Schools can be an ideal setting for installing rainwater harvesting systems, because it not only stores water for students’ and staff’s usage but also initiates dialogue and learning about water conservation and environment within students. Through practical implementation and involvement in the process of maintenance, or record keeping of rainfall measurement, students learn about the process. Government offices, institutions, industries, apartments buildings, bungalows are other places to install rainwater harvesting systems. In large open areas groundwater recharge wells or pits can be made after studying the local topography, rainfall and aquifer conditions.

Types of rainwater harvesting:

Storage:

One way of harvesting rainwater is to store it in a tank after filtering it. Depending on water usage and rainfall availability, a major portion of a year’s water needs can be sufficed by this stored rainwater.

Recharge:

Sometimes there is not adequate space to make storage tanks for rainwater, in such cases it can be used to recharge groundwater. After conducting geological studies to understand the aquifer conditions, one can design recharge pits, wells or through tube-wells.

Combination:

RRWHS can also be a combination of storage and recharge. A storage tank may collect filtered rainwater and once it gets filled, the overflow from the tank can be recharged in aquifer.

Components of a rooftop rainwater harvesting system:

• • Roof: The roof of any building is an important catchment area, which is a surface on which rainwater falls and it can be directed to storage or for shallow aquifer recharge. The roof can be sloping or flat, and it is important to check that the finishing materials or paint used on the roof is non-toxic and does not contain mercury. Before the first rains, it is important to clean the roof thoroughly to avoid dirt and other impurities from mixing with rainwater.

• • Pipes: Rainwater can be brought down from the roof through pipes. The size of pipes can be decided as per rainfall intensity and roof area. For aesthetic purposes, sometimes people use rain chains instead of pipes. These chains direct the water flow from roof to floor, reduce splashing and create a good visual as water flowers down.

• • First rain separator: The first rain of the monsoon season is not stored because the dirt and organic matter mixes with water and such water can create contamination in storage tank. So the first rain separator removes this water, and subsequent rainwater can be collected.

• • Filter system: A filter is needed to prevent stones, leaves, dust and other particles from entering storage tanks or recharge pits. It can be (i) ready filter cartridge available from several manufacturers - to be mounted a few feet above ground on the pipeline bringing water from the roof or (ii) a set of filter chambers usually constructed underground with input from the roof water pipe and output to the storage of borewell.

• Storage: The capacity of a storage tank can be designed based on space availability, needs and usage. If the water is to be used for drinking purposes, the tank can be smaller than if the intended use is for non-drinking purposes.

• Recharge and excess outlet: Once the storage tank is filled, the overflow can be directed to recharge groundwater.

Steps to design a rainwater harvesting structure:

• Calculating supply (rainwater availability):

A = Average annual rainfall of your region.

B = Roof area of your building

C = Runoff coefficient (Runoff coefficient depends on roof surface material - generally taken as 0.7 to 0.8 for concrete roofs)

Available annual rainfall = A * B * C

• Calculating demand of building users (for RWH storage):

X = 135 litres of water used per capita per day (standard for calculation)

Y = Number of residents/users

Z = 30 days in a month (average)

Monthly water demand of unit = X * Y * Z

• For storage type:

  • The size of the tank can be designed as per space availability, water demand and economic reasons. It may happen that all rainwater from the roof cannot be stored in tanks for use. During high intensity rainfall events, the tank may get filled sooner and the usage rate will remain the same, so the excess water can be diverted to recharge groundwater.

  • Existing pipeline systems may have to be modified to take the roof rainwater to the tank through a filter system.

  • Estimate the costing based on pipeline modifications, filter systems, storage structure and initiate execution.

• For recharge or storage + recharge option:

  • The overflow from the RWH storage tank can be directed to recharge wells or recharge borewells. So accurate calculations for tank capacity and overflow management may not be necessary.

  • Existing pipeline systems may have to be modified to take the roof rainwater to the tank through a filter system, and overflow pipelines to be directed towards recharge structure.

  • Estimate the costing based on pipeline modifications, filter systems, recharge structure and initiate execution.

• Maintenance:

  • Check the terrace areas for any major spillage of oil, chemicals or garbage and clean it particularly at the beginning of the monsoon season and then every few months.

  • Using the first rain separator (FRS), water from initial two rain showers should be drained off. Then close the cap of the FRS for subsequent rains, so the water flows from roof through the filter to the tank or recharge as per design.

  • Lid of the storage tank should be kept closed all the time, and water stored inside should not be exposed to sunlight.

  • If rainwater is being stored and used, then regular water quality tests will have to be done to ensure drinking water standards, and perform treatment if necessary.

  • Storage tank should be cleaned once a year especially before the monsoon season.

  • Filter media aggregates or filter cartridges should be cleaned as per their maintenance routine.

  • Stored rainwater can be disinfected by chlorination or boiling water before use.

Rainwater harvesting is a useful method of decentralised water conservation. It is the technique of collection, filtration and storage of rainwater at surface or in subsurface aquifers, before it is lost as surface run-off, such that the collected water can be harvested in the time of need.

Abbreviations:

TDS Total dissolved solids

RO Reverse Osmosis

RWH Rainwater harvesting

RRWHS Rooftop rainwater harvesting system

FRS First rain separator