Initial rain has been diverted to remove debris and impurities, and subsequent rainfall running off the roof will be headed to the filter. But how is the filter designed and how does it work? Filter design is generally influenced by the:
- end use intent of the harvested water
- size of the roof top
- rainfall intensity
For harvested rainwater designated for non-potable purposes like gardening or washing, a simple filter capable of removing physical impurities is adequate. An example of this is to fold a piece of fine cloth in 8 folds and use it as a sieve over the mouth of the rain barrel. This is effective in removing silt and debris to a large extent.
Illustration: Cloth Filter
Filter construction
For harvested water fated for domestic consumption (presuming the catchment water quality can be confirmed), a simple masonry structure with filter media inside will do the trick. If a masonry structure is too difficult to construct, a barrel would work as a filter structure.
The size of the filter should be the equivalent of a minimum of 2 mm of rain falling on the rooftop. For instance, if the rooftop area is 100 square meters, then the runoff generated for a 2 mm rain shower would be:
Runoff for 2 mm shower over 100 sq m roof area = 100*2*0.9 = 180 liters
Hence the volume of the filter should be 0.18 cubic meters. Generally, a cube shaped structure is preferable.
Illustration: Masonry Filter construction and charcoal inside
Fill the filter structure with aggregates/ pebbles at the bottom, then a layer of washed wood flake charcoal, then another layer of aggregates/ pebbles. Each layer can be separated by a mesh.
The gravel/ aggregates/ pebbles aid in blocking physical impurities in the rain water while the charcoal is effective in eliminating any odor due to gases dissolved in the water.
This filter design is relevant for above ground structures. In cases where the down pipes are already below ground level, a sub-surface filter is required. This type of filter is constructed below ground level and though the filter media is the same, the flow of water is different than with the surface filter.
Wall mounted filters
In cases where space is at a premium and the construction of such a filter isn’t possible or doesn’t meet the aesthetic expectations of the client, a compact wall mounted filter can be used. This filter is basically a cylindrical pipe with a couple of perforated stainless steel meshes inside.
The outer mesh has a fairly large pore size while the inner mesh has fine pores of size 50 microns. The inlet to the filter is through the side and the water swirls in a centrifugal motion around the insides of the mesh. The mesh screens out the dirt. Pure water is collected on the sides and comes out through an outlet pipe which is perpendicular to the flow of water. The heavier silt laden water drops down through an outlet which is in line with the water flow direction.
Illustration: Wall mounted filter
These filters are generally adequate for filtering the harvested water for a house. Yet it’s absolutely necessary to utilize secondary filtration inside the home to treat the water for any bacteriological contamination.
For larger roof areas like apartments/ institutions/ commercial buildings, advanced filtration systems might be required. These systems are a combination of a pressure sand filter or activated charcoal filter combined with a chlorine dosing unit or a UV lamp (to eliminate bacteria).
With a little ingenuity and elbow grease, water filters can be created in numerous shapes and sizes to meet the needs of any household or business.
References:
- http://www.theprepperjournal.com/2013/07/11/100-year-old-way-to-filter-rainwater-in-a-barrel/
- Image source: Biome Environmental Solutions