I have a Merlin RO and TDS in is between 69-71, out between 000 and 001 @ 48psi.
Production rate is OK but the waste seems to be very high.
Would it be cheaper in the long run to DI only?
Water rate is £1.30 plus 80 odd pence for waste plus vat.
Say £2 per 1000 litres for water. Waste ratios vary on a Merlin but £6 for 1000 litres of pure would be a fair figure maybe? That's with an RO. The TDS out will climb after a while so there would be a small resin charge to add.
Without RO.
A tap TDS of 70 should give you around 7000 litres of pure per bag of resin. If £70 per 25 litre bag that would be about £10 per 1000 litres - and you might need a twin DI setup to achieve that. Of course you will still have a the £2 per 1000 litres charge too.
Of course things can vary. The waste ratio from the Merlin may vary. Also, as the TDS climbs, there will be increased resin usage after the RO but not very much really. Also, there are ways of getting the waste water charge reduced (others will know more about this than me).
Membranes and pre-filters need to be factored in too.
So without RO maybe £12 per 1000 litres.
With RO maybe £6 per 1000 + a bit of resin + filters.
Too many variables to be too accurate though.
Looks like with RO is cheaper though once the oiriginal kit is paid for.