jeasko
Fishaholic
Membrane should be replaced when your TDS readings go high it can be anywhere upto 18 months/2 years depending on how much you use the unit, just as a guide the membrane can be upto £40 with the pre filters being around £15 for both so with your £20 starting price your at about £75 even the your not sure that the waste valve is ok or set to your GPD so to air on the side of caution I'd replace that which while only about £5 brings your second hand RO unit to £80 (most likely under but can't be sure), then you might want to add a de ionising unit if not already fitted to bring down TDS further.
Also I would say that the prices I quote are for a standard RO unit from reputable outlets, you can get cheaper but I don't really have experience with the cheaper ones so can't comment on them.
Also I would say that the prices I quote are for a standard RO unit from reputable outlets, you can get cheaper but I don't really have experience with the cheaper ones so can't comment on them.