You should be getting more than a year out of your membranes. The use-able lifespan of the membranes is most affected by your pretreatment - what you do to the feedwater before it reaches the membrane.
In your case:
- Contact your water utility and see what they use as a disinfectant. It will either be a form of chlorine, or it will be chloramine (chlorine + ammonia). The answer to this question will have a big effect on the appropriate size of your backwashing carbon tank, and on the type of carbon to use. At 5 years of age you're near or at the time to replace the media in this tank. Test your concentrate for chlorine as OurTown mentioned. Chloramine will show up as Total Chlorine. Chlorine will show up as Free Chlorine. Although often not used in the carwash world, a lockout switch on the carbon tank is a great idea and is widely/typically/commonly used in other business types.
- At 6 gpg hardness, as mentioned above, you need a softener. Just like hard water deposits scale on your plumbing fixtures, it will deposit scale inside your RO membranes. The scale will plug the membrane (slowing down the production of purified water), and it will create small holes in the membrane and cause higher TDS in the permeate (like you're seeing now). What is the TDS of your feedwater? Depending upon the operation of your wash, you may or may not want a twin tank alternating softener rather than a single tank softener. We can talk in detail about this if you'd like. If you're going to get a softener do yourself a favor and use one with a Clack brand valve. You'll thank me when it comes time for maintenance on the valve!
The life you get out of your membranes will also be affected by the way you have your system set - typically on a commercial RO you have at least two flow gauges - one for the permeate (RO water) and one for the concentrate (flush water or "waste" water). The concentrate flow can be adjusted - and changing this flow will affect your "pump out" pressure and your permeate flow. You may also have a gauge and needle valve for a recycle function. If you do... because you do not have a softener, you should not be recycling any of your concentrate.
To suggest a replacement 4040 membrane (this means a membrane that is 4" in diameter and 40 inches long, nominal) for you, we would look at these settings on your RO system, have you adjust if needed, and look at your feedwater temperature, and then look at the lowest energy (pressure) membrane that will do the job for you.
Russ
