Reverse osmosis system questions

[BuckleyBrew]

I recently purchased a used RO system that includes 3 pre-filters (I believe they are sediment, GAC, and ??), an RO membrane, and a final "polishing" filter. I don't know how long it was in previously use or when the filters were last changed, but for $20, I couldn't pass it up.

When I installed the system, I bypassed the pressure tank using the logic that I'll only be slowly filling my HLT with RO water over multiple hours prior to brewing and I don't need to have RO water on demand at a high flow rate.

I tested the system last night and noted the following:
- The RO water flow rate was extremely slow, like ~1 drop/sec, compared to the wastewater flow rate, which I didn't measure but would estimate was around 0.5 liter/min.
- TDS of the RO water was around 155 ppm; TDS of the source water (municipal) was around 190 ppm. I verified the accuracy of the TDS meter using bottled DI water, which registered 0.0 ppm.

Obviously the RO system isn't doing its job. Questions for RO system users:
1. What steps should I take to improve the rejection %?
2. What is the normal RO/wastewater flowrate I should expect?
3. Could bypassing the pressure tank contribute to the low rejection % and/or low RO flow rate?

Thanks!

 

[TechFanMD]

First, systems come in different rates, ie 50gdp (gallons per day), 75gpd, etc.

Permeate = the water you want
Brine = the stuff that goes down the drain

RO grows more efficient with a higher pressure differential across the membrane. On my RO system that I use for drinking water, I use a pump to boost the incoming pressure. As efficiency is better (ratio of permeate to brine) with higher pressure differential, it also means when your tank is empty OR no tank at all, efficiency is better. As you approach where the systems stops making RO (auto shutoff) the water is lower quality than the water it makes when the tank is empty.

My guess is you need all new filters (your pressure may even be low due to the sediment filter being clogged) and membrane. The plus side is they aren't expensive and if only using for beer, you may not need to buy them again for many years. Even with my household use my membrane can go 2 years, but I do replace the sediment and charcoal filters annually.

So:
1. What steps should I take to improve the rejection %? Replace the filters. At least test it with the sediment filter removed.
2. What is the normal RO/wastewater flowrate I should expect? ~1-3 gallons per hour
3. Could bypassing the pressure tank contribute to the low rejection % and/or low RO flow rate? No

Edit: the flow restrictor (which is on the waste/brine line) is kind of what determines the size of the system (though its more complicated than that)- which is how you how you know which membrane to buy (or just buy a new flow restrictor to match the membrane) if the system and membrane aren't labeled. The trick is that some flow restrictors are built into the membrane housing and it is hard to tell. It can be removed - google that part.

 

[day_trippr]

Almost certainly the membrane is shot, and if that's true the pre-filters are probably well past replacement.
If the system uses industry standard cartridges and the throughput is rated around 50gpd the system could be refreshed for ~$60-70 on up...

Cheers!

 

[TechFanMD]

Feel free to PM me and I’ll help you figure it out as far as what to buy, etc.

Funny thing is I have a quick supply of quality RO and still just use my well water. It makes great beer and I don’t go to the trouble of collecting enough RO to brew.

 

[Bobby_M]

It's really helpful to know what your source water pressure is. If it's a well, you are probably running your well pump around 40-50psi which you can adjust upward 10-15 psi without any problems. If that's the case, replace the membrane with a 50 or 75GPD and no higher. The higher flow membranes absolutely need higher input pressures. All RO systems benefit from a booster pump with pressure switches. It may seem like overkill, but running my RO at 120psi has my incoming TDS (400ppm) all the way down to 9ppm on the RO side and I nearly halved the amount of waste water.

Yes, you might as well replace the sediment and carbon cartridges since you don't know how old those are either.

One final thought is that even with an older membrane, I don't think you'd be getting such horrible rejection percentages. I have a feeling like your auto shutoff valve has a leaking membrane. You can take the four screws out of it to inspect the membrane... The one time I saw my output TDS go up significantly, that was my problem. The incoming raw water was leaking into my RO output.

 

[BuckleyBrew]

Thank you all for your input - this is really helpful.

I'll take a look at the flow restrictor to figure out the rate of the RO system. I'll check out the auto shutoff valve as well

I'm on a municipal water supply (not a well). I'm not sure about the pressure; I'll have to see if I have a gauge I can hook up to test it.

 

[TechFanMD]

Thank you all for your input - this is really helpful.

I'll take a look at the flow restrictor to figure out the rate of the RO system. I'll check out the auto shutoff valve as well

I'm on a municipal water supply (not a well). I'm not sure about the pressure; I'll have to see if I have a gauge I can hook up to test it.

Keep this in mind: If you're not using a tank and just want to catch 8-9 gallons of water in kettles or whatever....you can just remove the auto shutoff. That's there to shut it off when the tank is up to pressure - you wouldn't need it. If you have municipal water, and you're only using this for brewing - don't worry too much about inlet pressure. Get it working and time how long it takes to fill to where you need.

The danger will be that you need to make sure you don't forget that you are filling so you don't have a flood....I'd recommend doing it near a drain or in a sink - you're probably going to forget. Consider - at 50gpd it will take ~4.5 hours to get enough water to brew, at 75gpd it will be ~3hrs. You'll get busy, run an errand, go to dinner and suddenly 3-4 gallons per hour is going on the floor. (you could do shutoff floats but this gets tricky).

 

[BuckleyBrew]

That makes sense re: the auto shutoff. I took it out and still got TDS around 95 ppm in the permeate. I tried bypassing the prefilters to see if there was any pressure loss that was affecting the efficiency of the membrane (nope). So it looks like replacement filters and membrane is the next step.

The flow restrictor says 800cc; from what I’ve read online it sounds like that would be a 75 or 100 gpd membrane. Does that sound right? Anything else to look for or avoid when bring replacement filters/membrane?

 

[day_trippr]

I like Filmtec membranes, I've been running a TW30-1812-100HR (100gpd "high recovery") which yields ~7-8 tds from a 300+ tds source...

Cheers!

 

[Buckeye_Hydro]

Feel free to give us a call when you are in front of your RO system if you'd like to troubleshoot it.
Russ
513-312-2343