Rainwater harvesting basics: Filtering the water before storage and use

Overview

Rainwater is amongst the purest water one can get distilled as it is by the sun. However, in a rainwater harvesting system, the water comes in contact with several surfaces, such as the roof or gutters. Its flow becomes possibly mixed with leaves or dust.

To get water fit for use at the end of the harvesting process, apart from keeping these surfaces clean, we can filter the water before storage.

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Materials required and site specification

Materials Required

  1. PVC drum or ferro cement
  2. Filtering material – coarse jelly, smaller jelly, sand
  3. Wire mesh
  4. Outlet pipe for allowing filtered water to go out
  5. Overflow pipe to allow excess water to flow out

Site Specification

  • Usually placed below vertical down pipe
  • Can also be placed adjacent to sump tank

Note
A drum of 90 litres can filter the water of a roof area up to 100 m².

Filtering material

Placed at the mouth of the rainwater pipe, the filtering material can be very diverse.


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A combination of perforated aluminium box with mesh and sand or perforated plastic bottle with mesh and gravel are efficient.

PVC drums

  • Advantages – light weight, easy to transport, easy installation
  • Disadvantages – cannot be kept in sun, limited capacity
  • Cost - around Rs. 600. If kept in the shade, it can last over 6 years

Ferro cement filters

  • Advantages – size can be made to suit the requirements
  • Disadvantages – heavy, requires skill for construction
  • Cost - around Rs. 1700

Stainless steel filter

  • The stainless steel box filter is ready to use.
  • The water enters the box from the top and passes through the mesh to the down flow pipe. Thus, stone and sand filling is not necessary.
  • This filter is easy to clean
  • A stainless steel filter costs around Rs. 6000

Construction technique

Step 1
Make the filter using PVC or Ferro cement. Provide an outlet at around 2” from the base.

Step 2
Tie nylon or wire mesh to the mouth of the outlet for holding back any suspended impurities.

Step 3
Place the filtering media in 3 layers. Put coarse jelly at the bottom and smaller jelly above it. The topmost layer must be of sand. All layers are usually 15 cm deep.

Step 4
Place a nylon or wire mesh over the sand to hold back bigger suspended impurities like leaves etc., while letting water to percolate down. You can also use a perforated plastic cap.

Caution

  • Always keep the filter clean
  • Remove the mesh after every rain