# Expected RO reduction in PH



## Phil Onion (10 Jan 2022)

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?


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## Maf 2500 (11 Jan 2022)

RO water, if from a good unit that is working properly, will have near zero ppm hardness and alkalinity and neutral pH. This effectively gives you a blank canvas which you can remineralise how you choose.

I don't use RO so will leave more detailed responses to people with hands on knowledge.

What I will say is that RO units used to filter hard water create a lot of waste water, often several times the volume of RO water, so if you are on a water meter you might want to look at using rainwater instead for a more cost effective option that has the benefit of being more environmentally friendly.


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## Hanuman (11 Jan 2022)

Personally I would not think in terms of PH.
RO filter/membranes will NOT remove all DS from water. In fact this should be turned differently. RO filters can filter so much and their filtering capacity is a function of the source water and the quality of the RO membrane. Your RO filter can very well output Xppm or XXppm even with a brand new membrane if your source water's ppm fluctuates and is high.
In order to have close to/or 0 ppm you would need to post filter your RO water with a deionizing resin unit.

Edit: obviously if your source water is very soft and your TDS shows a ppm value sub 100 then your RO water will output a ppm close to 0.


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## Maf 2500 (11 Jan 2022)

Normally the RO units available in the UK are DI also. At least the good ones. Not sure of the specific model mentioned in the OP.


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## Hanuman (11 Jan 2022)

Here in Thailand I have yet to see DI. Most people only use a 5 or 6 stage filter (sediment+carbon+resin+RO+post carbon/UV or whatnot). Water here is usually rather soft but during the dry season Na sips inland (specially Bangkok and coast cities) and TDS sometimes can go as high as 1200ppm for a couple of weeks.


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## Wookii (11 Jan 2022)

Phil Onion said:


> 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.
> 
> ...



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.


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## bazz (11 Jan 2022)

I have a the standard/cheapo 150 USG/day (570l) 4 stage (sediment/carbon block/activated carbon/membrane) from 'Osmotics'. I have just measured the TDS as I'm in the process of drawing off water at the moment and the TDS of my tap is 344 while the filtered water is 10. With JBL GH and KH test kits both register 0. I ought to have a booster pump as my water pressure is low (50 PSI) and I'm only producing about 200l/day (will sort this while I'm thinking about it). As for the PH, the same as @Wookii describes above. I have an extra module for DI (which removes the last 10 TDS) but not deem it necessary to use with freshwater. For the record I remineralise to 5GH and 1KH.
Hope this helps!


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## dw1305 (11 Jan 2022)

Hi all,


Hanuman said:


> Personally I would not think in terms of PH.





Phil Onion said:


> 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


As @Hanuman says you can just ignore pH, it just isn't a <"very useful measurement"> as you <"approach pure H2O">.


bazz said:


> I have just measured the TDS as I'm in the process of drawing off water at the moment and the TDS of my tap is 344 while the filtered water is 10.


<"That is the one">, you just need a <"low range conductivity meter">.


Phil Onion said:


> 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.........Would an RO get the water down to a PH < 7 and a Hardness < 150?


I live in a <"hard water area"> and <"keep soft water fish">, and I can do this <"because I use rain-water"> in the tanks, and <"I recommend it for every-one">.

cheers Darrel


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## Phil Onion (11 Jan 2022)

Thanks to everyone. I was looking at a Vyair RO-100M 4-Stage with DI and this will be a low tech tank.


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## dw1305 (11 Jan 2022)

Hi all, 


Phil Onion said:


> I was looking at a Vyair RO-100M 4-Stage with DI


That will produce water that should be less than 5 microS (3 ppm TDS). 

cheers Darrel


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## Conort2 (11 Jan 2022)

I’ve got a Vyair unit and I’m very happy with it, I have the one with the added booster pump.  

As @dw1305 mentions rainwater is also a great alternative. When I had my smaller tank I just used rainwater and had great results with it. We get enough rain in the uk so if you have space for a water butt or two this can be a great cheaper alternative.

Cheers


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## Phil Onion (11 Jan 2022)

I'm trying to work out if I need a pump. I thought my tap water pressure was good, but not so sure. The pressure is about 10 l/min, which I believe is 1 bar. Tha Vyair states it needs 3 bar to operate. Is any UK tap kicking out 3 bar.


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## Wookii (11 Jan 2022)

Phil Onion said:


> I'm trying to work out if I need a pump. I thought my tap water pressure was good, but not so sure. The pressure is about 10 l/min, which I believe is 1 bar. Tha Vyair states it needs 3 bar to operate. Is any UK tap kicking out 3 bar.



I also have 1 bar mains pressure - I needed a pump. Without it your production rate ( RO vs waste) will be way down.


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## Hanuman (12 Jan 2022)

Wookii said:


> I also have 1 bar mains pressure - I needed a pump. Without it your production rate ( RO vs waste) will be way down.


Not even sure it would work at all with 1 bar. I think the minimum for a standard small RO membrane 40/50 psi (2/3 bars) is required for you to see water coming out.

Bangkok water pressure is also ridiculous. All houses are equiped with a water pump just to make water go to the 1st floor else you get "nada". Even with that, pressure is not enough for an RO filtering unit and so, again, all RO units also need a booster pump.
The house pump:




The RO unit which I am upgrading this weekend with 2 additional 20 inch canisters (sediment + carbon). The booster pump goes up to 80psi (5.5 bar) which to me is the bear minimum to run a small RO unit like mine with decent flow.


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## Phil Onion (12 Jan 2022)

Once again, thanks for your help. Have ordered a Vyair RO-100M 4-Stage with DI and 6 x 25l plastic bottles. Should do me ok and if not, I'll bottle it and sell it to tourists as Swindon Spring!


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## sparkyweasel (13 Jan 2022)

Phil Onion said:


> I'll bottle it and sell it to tourists as Swindon Spring!


And this time next year you'll be millionaires.


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## KirstyF (13 Jan 2022)

sparkyweasel said:


> And this time next year you'll be millionaires.



Nah, You’ve not seen Swindon buddy! 😂

Said with all the well intentioned fondness of someone who lived there for 12yrs. 😊


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