Don't just choose a Ro that will fill your requirements now, think of the future, if you take on more wfp work you might well need more than 250Ltrs a day. Make sure that you either get a ro that will give you 300-500ltrs a day or at least one that can be upgraded. You'd be surprised at just how much water you can get through, especially if you turn the flow rate up at the brush!!
Also keep in mind that the output of any unpumped ro system will vary depending on your mains tap water pressure, if you have pressure like I have, eg 35psi, you wont get as much pure water produced as the description on your ro says it'll produce.
A merlin will produce as much water as you need without the need for a static tank even if you upgrade and get a 1000ltr tank in your vehicle and even if your tap pressure is not much you can get a booster pump and still produce enough watre.
As for the turning pressure up on your poles get microbore hose and change your fittings on your poles in now get through 350 ltrs a day instead of 700 and still manage to get a good block of 70 flats out of 35o ltrs.
Ok so you can produce water with a static ro while your working but a merlin £250 pounds wont need to change any parts for 12-24 months and the parts are not expensive in relation to what you can actually earn.
Some will say that a merlin produces alot of waste water ok it probablly does more than other ro's but same again to what you can earn is alot more than the waste and you can put your water bills through your accounts.
As to your question yes you can fit a merlin to an inside or outside tap hose into the drain and away you go.
you will however need resin to polish the water down to 000 but again if you use microbore hose and samller outlets on your poles then your resin costs will be minimal.
