Mr Harjit Singh Sir,
Its quite difficult to giv one absolute correct answer coz everyone of them has their own advantageous and disadvantageous.
Nevertheless i would like to share some notes here with u that might shed some light,
Rainwater Harvesting - Pumps or Pressure Tanks
by Doug Pushard
If you are building or planning to install a rainwater collection system, water pressure (i.e. water line pressure) is one of the main issues you need to think about. There one several ways to achieve water pressure when you turn on the faucet. One is gravity. Another is installing a pump to ensure water pressure in your water lines. If gravity does not work for you, you will need to install a pump and there are several options to consider, some new and some old.
System components change over time, but system designers or installers might not change at the same pace. Sometimes it is the old adage “if it ain’t broke don’t fix it”, but it also could be just resistance to new ways or lack of information on why and when to change. Is this the case with new pressure sensitive pumps versus pressure tanks
In designing a system, sometimes it is one aspect of the overall rainwater system that makes one option better than another. Is this the case with the brand new inline pump controllers versus pressure sensitive pumps?
Pressure tanks have been around for a long, long time and have typically been installed on wells and whole-house rainwater catchments systems (i.e. households with no other water system). The tanks hold water for immediate use; consequently, there is no lag in turning on the tap and water coming out the spout as there would be with a standard pump.
Pressure-sensitive pumps are newer and sense the pressure in the water line and turn on when there is a pressure drop in the line (i.e. when the faucet is turned on). Both pressure sensitive pumps and pressure tanks work to accomplish the same end - water at the tap when required.
Newest of all, are inline pump controllers. These are a separate device (i.e. not integrated into the pump) that sense line pressure and turn on the pump. Inline controllers can be easily added to existing installations.
So with three different options, which is best? The short answer is - it depends.
Back to your question, which is most cost effective between gravity and the pumps that is mentioned above, i dun hav any straight answers.
Anybody can enligthen us on this?. Perhaps our Pengarah Kanan Senggara & Fasiliti jalan boleh membantu, coz he's an expert in water and he once headed the Water Department. Hope he can share with us the pros and cons on this gravity and pump.
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