Purain Filter

To design a world class rainwater filter one needs to understand exactly how rain falls from the sky!

The world’s most studied rain filter design

INTEWA studied world rainfall data and discovered the following facts:

  • +/- 97% of world rainfall can be termed “light events” (measured as less than .6mm in 5 minutes)
  • +/- 3% of world rainfall can be termed “heavy events” (measured as more than .6mm in 5 minutes)

By using this little known knowledge, INTEWA has been able to design the world’s first “hydraulic jump” self-cleaning rainwater filter, offering the highest water collection efficiency of any filter tested. Rain filtering is all about quality AND quantity!

The Hydraulic Jump principal

Water enters the filter and passes over the 0.8mm stainless steel screen, trapping small particles carried from the roof, before dropping vertically down into the tank via a 110mm calming inlet pipe.

When a heavy rain event occurs the velocity of the water entering from the left increases and is directed onto the opposite angular wall of the filter. This causes the water to form a vortex or eddy. This eddy removes dirt and particles from the screen and filter chamber through the overflow outlet.

During self-cleaning, the filter loses only 2-3% of total collectible water.

This is the hydraulic-jump principal patented by INTEWA, which enables all PURAIN filters to self-clean – with the main benefits to this being less clogging, more water saved, dramatically reduced filter maintenance and better water quality.

Light Rainfalls

Low rain­fall events ac­count for over 97 % of total an­nual rain­fall. There­fore, it is par­tic­u­larly im­por­tant that the fil­ter be de­signed to col­lect these many light events.

By contrast, most fil­ters are not de­signed to col­lect light falls, either:

a) due to poor design (they clog easily and end up wasting much of the capture) or,

b) they are designed to flush away the first few hundred litres (first flush diverters) of each rainfall event in order to “clean the roof” (a concept we regard as unnecessary and wasteful unless you live next to a major industrial factory or coal mine with potentially high toxic outputs that may collect on your roof).

By losing or discarding this rainwater, a filter’s inability to collect much of the light events results in ratings as low as 50 or 60% efficient. This means that if 1000 litres falls on your roof these filters would collect only 500 or 600 litres in the tank.

By comparison, the rain­wa­ter col­lection efficiency of the PURAIN self cleaning fil­ter is independently rated at 98%.

Heavy Rainfalls

High rain­fall events con­tribute only about 3 % to total water yield. In the PURAIN filters these high rain­falls lead to an increased water flow entering the filter chamber. This creates an eddy, or vortex, and this phenomenon is termed the “hy­draulic jump”.

This eddy spins with such great force that even sand, dirt and stones, that may have ac­cu­mu­lated in the filter chamber, are flushed away and dis­charged through the over­flow.

So these heavy rainfalls are importantly used by the PU­RAIN Rain­wa­ter fil­ter for self-clean­ing.

This enables PURAIN filters to offer 98% collection efficiency AND be self-cleaning – something that virtually all other rainwater filters are unable to offer.

Technical Documentation