The cost of the unit you linked to is less that want I'd currently expect to pay for a decent membrane alone, let alone a decent RO unit.
With those, the pre-filters are a special size, so you can only buy Their replacements. This is expensive, as they can charge what they like for these replacements... £8-99 is OK for a full set (which is what they are asking), but I bet that applies only to the set when you buy them with the unit. That assumes it is for the pre-filters only. If you get a membrane in that set, I wouldn't hold my breath on that membrane being any good...
How much water do you need to produce, and in what time frame? My 50GPD unit will actually produce something more like 30gpd in winter, of 40g in summer. This is well maintained and on a tap water supply with better than average pressure. On a typical tap water supply, you'd get more like 30gpd in summer, 20gpd in winter...
A good 50GPD unit IMO would be one like
this. That would need three
ball valves and two pressure gauges (that Osmotics don't appear to be carrying in stock ATM. May need to wait a week or two to get them) to get it to all you need. A
TDS meter would then be required to be able to tell what the unit is doing by way of RO water quality and hence enabling you to tell the condition of the membrane. That concludes the essentials, I'd recommend a full replacement set of filters while you are at it to prevent having to pay postage again later. That would be a
50GPD membrane, a
50GPD flow restrictor (flow restrictors need replacing with the membrane) and a three stage unit
pre-filter set
Personally, I consider extra pre-filtration or any post-filtration to be a waste of time. All you need is a sediment pre-filter, a carbon pre-filter and a membrane. DI resin, carbon post-filters and a second carbon pre-filer just bump up your running costs for no extra benefit. Actually, I feel DI is counter-productive, as all that that removed is Organic Carbon after the membrane, something a lot of people are now adding in the form of Vodka, Sugar or Vinegar to reduce Nitrates and Phosphates.
/cgi.ebay.co.uk/3-STAGE-AQUARIUM-REVERSE-OSMOSIS-SYSTEM-50-GPD_W0QQitemZ250541272303QQcmdZViewItemQQptZUK_Pet_Supplies_Fish?hash=item3a556c6cef
