Hi,
Has anyone experience of using RO units with fairly high hard water. I was looking at the Vyair RO units, but they don’t put a figure on PH or hardness reduction. I don’t want to buy one and then find it doesn’t reduce PH or hardness to a point that would allow me to keep South American fish. Yes, I know I could look at African lake species that like hard water, but prefer SA.
My tap water is, according to water company and testing:
PH - 7.5
Hardness - 287 ppm
Alkalinity - 220 ppm
Would an RO get the water down to a PH < 7 and a Hardness < 150?
I have a Vyair 400 RO-400 which is a pumped unit with three pre-filter stages (sediment/carbon block/chloramine in my case) and dual membranes - they are very good.
As
@Hanuman points out, RO units remove a percentage of solids from the water. A good unit will remove 97-98%, but that percentage will depend on other factors such as water temperature, pressure at the membrane, age of the membrane etc.
The TDS of your outcoming water will therefore depend entirely on the TDS of your incoming water. My unit comes out fairly consistently at 6ppm output year round - I assume that is because though the TDS is around 260ppm in winter, and can be up to 360ppm in the summer, the colder water in winter reduces performance offsetting any performance gains from the lower TDS. I would consider it pointless to have to deal with DI resin to remove that final 6ppm.
In terms of pH, pure water has a pH of 7.0, but as soon as it absorbs atmospheric CO2, that pH drops. Water from the tap typically has a higher level of CO2 than atmospheric equilibrium anyway, so the pH of RO is likely to be pretty low on freshly produced RO and will gradually increase as it off-gasses some, but I would expect it to be less than 7. How much impact a 6ppm of remaining solids has on the pH is hard to say (I have to admit I've never tested it - I probably should just out of academic interest).
It is all a bit of a moot point though, for all intents and purposes the KH of the RO water is zero. It has such low carbonate content that as soon as you add it to the tank it will take on a variety of organic acids which will react with any trace of carbonates remaining, and the pH will drop. In a soft water South American dedicated tank you'll probably be adding wood, dried leaves and other items anyway that will leach tannic, fulvic and humic acids further buffering the pH downwards. Finally if you inject CO2, you'll also be dropping the pH on a largely permanent basis anyway (permanent because CO2 rarely fully off-gasses overnight), even with a measurable low KH value.