[simp]

At what point do you replace the membrane?
My system originally was 330ppm in and 13ppm out. Its now coming out at 20ppm befor the DI. Is that acceptable?
I don't want it to get too high and end up using loads of resin, although I do have a 11L DI vessel, so up to now seems to be fine.
I would like a 4040 but don't want to pay out unnessasarily.

[ftp]

I changed mine after two and a half years with readings like yours. Producing 5tds now. I didn't know when to change really.

[matt]

its got to be worth it to use the resin upto 30 - 40 TDS , anymore than that and its time to change it

[simp]

Any one else? At what point do you change it?

[steven ainger]

get a booster pump, that should knock it down again, i use a ro-man 200gpd and my membranes are 2 yrs old now, tds in, 572 at the moment, tds out 6.

[simp]

I,ve added a booster pump and increases the pressure from 55-100psi. It makes no difference, I'm still getting the same reading at the RO 

[ftp]

What 

[mark dew]

You would be better off changing the prefilters 1st and if that doesn't help then get new membranes. 
Assuming you flush the ro regularly, your membranes will last longer now you have a booster pump.
What size ro is it?

[Ian Lancaster]

My 4040 is 18 months old.  Gone from 5ppm to 21 ppm, just ordered a new membrane as the resin in my cylinder is only lasting a couple of weeks.  New membrane about £190 so works out at about £2.60 a week.  Producing up to 2klt a day so I reckon I've had good value out of it.