A pressure pump would be needed to supply water to the RO. Booster pumps will only ‘boost’ mains water pressure, the pressure from the water butt will need to be at a reasonable water pressure before the pump of 15 PSI or over. Even then the booster might still not be able to boost over 45 PSI so the efficacy of the membrane would be poor.

 

A water butt filled with rain water will contain a very high TDS and also large dissolved solids. The solids (leaves from gutters, sand etc) may eventually cause the booster pump valves to get stuck open or shut or if you place the booster after the pre-filter the filter may get blocked quite quickly. The high TDS will also shorten the working life of the membrane/filters.

 

If the water butt freezes or the water is almost at 0⁰c the membranes efficacy will be very poor or the RO unit may freeze and become damaged.

 

In short whilst this can be done its really not the best way to produce water and can be uneconomical long term.