RO unit and TDS


Purchased 13 months ago complete with resin pod .noticed my marine tank NO3 
reading had increased over past 2 / 3 months to 10ppm from 0.25 no changes in 
my routines . After careful checking took readings from my unit and was 
surprised to 5ppm NO3. And the 80ltr tank of RO 10ppm.use this amount weekly
TDS reading into unit around 270 and after 6/7 ppm.
Decided to replace all cartridges including membrane and resin ,have produced 
a little over 25ltr and the TDS reading is now 3ppm. When 0 after 2ltr
Had replaces pre and carbon after 6 months.
Before the tap water reaches unit it has already passed through my Purity on 
Tap Hilux pre filter and chlornomore cartridges.


Dear Mr E

A couple of pointers if I may.

Testing RO water with traditional reagent based NO3 kits is doomed to fail in the main, this is because the reagents used are only capable of working within certain Ph tolerances as they often include weak acids. Ro water has no buffer system as its so pure, as such the reagents used effectively poison the sample as they will push the Ph of the sample outside the resolution of the kit. To work the sample must be able to resist the Ph change of the reagent , obviously if the sample has no buffer the Ph will swing wildly with the addition of any acid or alkaline solution even if it is very weak.

To gain a better idea of what is going on it maybe better to mix in a small sample of Nitrate free aquarium salt in a very clean glass 35g of salt per 995gr of water and test again, be mindful that washing up detergent is high in nitrate and Po4 so make sure the mixing vessel is spotless and well rinsed in RO before you make up a stock salt mix to test. This should provide a better idea of whats going on if the Nitrate kit is known accurate.

The rejection rate of your unit is pretty good, generally we are looking at a reject rate of between 95 and 98% depending on water temperature and correct pressure being applied to the membrane, for a tap water TDS of 270 this is around 5-13ppm TDS pre Di resin.

Also be mindful that the majority of that TDS will more than likely be dissolved CO2 which can make it through the RO membrane and gas off naturally over time.Also check that your collection vessel is clean as that will also taint the water with TDS as the RO water will literally suck TDS from the environment due to its purity as by its nature it is an excellent solvent.

With a fresh Di pod I would expect the effluent to read at 0ppm TDS even with relatively high TDS going in, this will exhaust faster the higher the TDS in but at 7ppm in I would expect it to last for at least 200l of product water before you start to see the TDS creep up. You may want to calibrate your TDS meter if you were seeing TDS at the outlet of fresh Di resin.

Another tip will be to install a tap and T piece pre Di resin and point the water to waste , the reason for this is that the initial 100-200ml of product water will contain high TDS after the unit has been turned off then on again from standing for a while, this is due to TDS migrating across the membrane due to low pressure on the membrane when the system is off. By allowing the first 100 or so ml to go to waste before directing the water to your Di resin when you run the unit between on/off cycles  it will extend the life of your Di resin by flushing that initial high TDS water to waste whilst pressure increases on the membrane again.

To further enhance the production rate of the unit and decrease TDS further you may also consider adding a pressure gauge to monitor the mains pressure to the unit, ideally you will be looking for over 50 PSI , if the pressure on the membrane is under 40 PSI you may want to consider a booster pump, this will then increase efficacy of the unit further.


Hope this helps Tony