The 4040 R/O's are not cheap, I've been using a 200 gallon a day unit for the past 2 and a bit years now, takes just over 24 hours or so to fill up my 1000l IBC tank, I've had to change the 2 membranes once (my own fault, I didn't flush them for months on end
)
The replacement membranes are about 9 months old...(I think
) and are still reducing my TDS reading down to 0.001-2.
A litre of resin lasts for a couple of months.
Don't bother mixing pure water with tap water, you will be undoing all the work that has already been done, and I am not sure whether in reality the TDS will actually be diluted in quite the way you might expect....I feel a little experimet coming on!
And if you did have water with a reading of 0.050 ppm you will eat through resin pretty quickly.
Work out how much water you need per day, add 25% to that and upgrade to an R/O that will produce you the volume you will need.
Leave your tap running over the weekend and always start the week with a full holding tank, you shouldn't have any worry about running out then.
Ian