High Efficiency RO/DI Systems

[Myka]

Originally Posted by asmodeus

No Mindy I don't have one I still have the original one from 2002 and just add filter to it as i go. i know that mine doesn't give off alot of waste thou. DOW membrane and i still don't have a flush n mine. my water here in the East end its like 850 by the time it gets to me. so I go through 2 membrane in a year,

Ew! That sucks. My tap water is usually between 120-160 ppm, so my membranes last a couple years(ish). I'm using a Filmtec 150 gpd membrane. I have a manual flush that I use at the beginning, end, and a few times during the day as I'm filling my reservoir. Takes all day.

[Skimmerking]

that is why maybe you are getting a crappy ratio the 150 gal per day alot of the water is wasted, I found a huge difference in the 50 compared to the 100 saved alot of water and the membrane lasted longer now im getting around 7=8 months out of a membrane on a 50 gal

[Myka]

For me, it's not about saving money so much as saving water. It does annoy me that I get dinged for the septic the same as the water usage though. The savings really isn't that significant, but it mostly pays for the cost of fouling up a membrane quicker. I imagine this is where the auto flush kits come in...hoping they flush the membrane often enough to keep it from fouling.

I think I'm going to try putting a 75 gpd restrictor on my 150 gpd system and add a booster pump. See what happens. I have a year on this membrane already and it is still giving me 2 ppm out of the membrane. Lots of life left, but I think still worth the fun. Hehe

Hmmm...from Reef Central:

Originally Posted by Buckeye FS

We feel it is misleading to tell people they can cut down on waste water by adding a second membrane. Here's why.

First - remember that what folks call "waste water" really would be better thought of as "flush water" in that this water serves the important purpose of internally flushing the surface of the semipermeable membrane to keep the membrane from fouling/scaling.

When you configure a system with two membranes in series (the waste from the first membrane going to the "in" port on the second membrane), for this discussion let's say it's two 75 gpd membranes, the system behaves like you have a single long (75 gpd x 2) 150 gpd membrane.

Now - if you use a proper flow restrictor, that is, one for a 150 gpd membrane, you'll have about a 4:1 waste to product ratio. Sounds familiar, right?

If however you don't change the flow restrictor - meaning you keep using the same restrictor you were using when you just had one 75 gpd membrane, then you'll see a waste to product ratio much lower than 4:1. But remember that the recommendation for a ~4:1 ratio comes from the membrane manufacturer. They are telling you that you need about a 4:1 ratio to keep the membrane flushed and keep the membrane from fouling or building up scale. Run the system with a lower ratio and you will foul/scale the membrane(s) quicker than would have otherwise been the case.

Instead of adding a second membrane to lower that ratio, you could have just changed out your flow restrictor ($4) instead. A much less expensive approach to get you to the same endpoint in terms of saving on waste water.

Now, to confuse things just a bit. Filmtec specs call for the 4 to 1 ratio on the basis of assumptions about the water that will be supplied to the membrane. If you have very soft water you MAY be able to get a decent service life from the membrane running at a ratio lower than 4 to 1 (e.g., 3 to 1). Remember that the waste water from the first membrane is about 25% harder than your tap water.

Bottom line: If what you are after is reduced waste water, experiment with a different flow restrictor for $4 instead of messing around with a second membrane plumbed in series.

As a side note, you can also lower the ratio by increasing the pressure delivered to the membrane (with a booster pump), because flow restrictors are sized assuming you are providing factory spec conditions (50 psi and 77 degrees for Filmtec membranes). Increase the pressure and you'll drive more water through the membrane and viola - less waste water. But as I mentioned above, if you do this (just like over-restricting a membrane) - the lower the waste to product ratio, the shorter the lifespan on the membrane.

Makes sense?

Russ

http://www.reefcentral.com/forums/sh...cient+membrane

[phi delt reefer]

Myka - run the waste water line outside your down spot and just have it run off your lawn/driveway.

Or do they measure your sewage usage by just doubling your water consumption (thats how they do it here in London)

[Myka]

Originally Posted by phi delt reefer

Myka - run the waste water line outside your down spot and just have it run off your lawn/driveway.

Or do they measure your sewage usage by just doubling your water consumption (thats how they do it here in London)

Ya, the septic usage is the same number as the water usage.

As much as it chaps my ass to get charged for something I'm not really using (all that waste water is doing is flushing the membrane anyway) it bothers me more to be wasting fresh drinking water that I don't necessarily need to be wasting.

[mike31154]

Check your home's plumbing for a Pressure Reducing Valve (PRV). Most homes not on a well, that is on city water, will have one to protect the house plumbing from over pressure. These valves are generally set to keep the pressure in your house from exceeding 60 psi. I have a Watts series 25AUB and those are the specs. It is adjustable however and I've tweaked mine to give me just above 80 psi. I also have two outdoor faucets plumbed in before the PRV so I get full city water pressure for the garden hose. I've measured that recently at about 110 psi. This drops off a bit with a hose running full blast.

In any case, I've found the 80 plus psi works well with my 75 gpd system particularly in the winter months when the input water temperature is down to 8 Celsius. I consider this tweak my 'free' booster pump. It keeps production at a good pace, but I haven't seen much difference in good to waste ratio, although to be honest, I've never taken the time to accurately measure my good to waste ratio, I'm pretty much guessing. I collect all the waste water in a couple of 55 gal plastic drums and use it creatively around the home & garden. I've pretty much given up on any of the magic bullet 1 to 1 solutions and simply don't consider the waste water as such, since I use it for other purposes.

As far as the membrane & wear etc., my system has been running for just over two years on the original membrane. The only thing I've changed so far is pre filters. Just finished a run of RODI water this evening and at the end I recorded 86 psi input @ 8 degrees C. My second pressure guage before the membrane showed 63 psi, so I'm losing 23 psi through the 3 pre filter stages. Probably time to change the 5 micron poly filter. When it's new, the pressure loss is around 20 psi. So it's safe to say that my membrane is working at close to 10 psi above the pressure (50 psi) that most manufacturers use when rating their membranes. Here's a little tidbit of info I saved to a text file on my hard drive, don't recall where I got it, but I'm pretty sure it's good data.

"Membranes are typically rated at 77 degrees and 50 psi.
For every pound of pressure lower than 50(psi) subtract 2% of the output. For every degree(F) below 77, subtract 1.4%.
Example: Let's take 40 psi water at the 77 degree temperature with a 50 GPD membrane.
50 GPD - 20% (40 psi) = 40 GPD.
Now let's take the 50 psi, but reduce the temperature to about 40 degrees:
50 GPD - 51.8% (since it's 37 degrees colder x 1.4) = about 26 GPD"

With the substantially colder water than manufacturer's rating for nominal production, I'm thinking my extra 10 psi helps a little in keeping the flow going somewhat better than if I was using the standard 50 psi house pressure available by default through a PRV. On the positive side, the colder water allows my 75 gpd membrane to get the input TDS of around 210, down to 0 after the membrane, so my DI media is hardly working at all and I haven't touched that either. If I recall correctly, the warmest input water I get is around 15 degrees C and during that time, the best RO reading I get is 1. I used to run a long length of input tubing through a bucket of water with a heater in it to get the temp up, but gave up on that idea some time ago, since I saw no significant benefit in production.

Sorry, long rant, but bottom line is, I choose to save the waste water for other uses rather than pursuing a 1 to 1 ratio using dual membranes, booster pumps etc. The quote you provided from BFS regarding the flow restrictor sounds interesting though, I might give that a go. Change the flow restrictor and maybe try hooking the input directly to the 110 psi city water pressure... Should probably confirm that all the components can handle the extra pressure though, lest I blow the whole system up!

[wingedfish]

In BC where most (if not all) water is gravity fed, the growing areas have high pressures for the farmers and the houses have pressure regulators. On the prairies, all municipal water is fed by pumps and it is not cost efficient to supply high pressure water and so no need for pressure regulators. Stuck with what the city gives you.

Also, if you figure the price to run that booster pump in electricity and initial cost, you may find the cost of the water might be cheaper.

Rain collection into a barrel filtered and then pumped into the ro might be worth it, but a quarter of a 55G drum at prairie rain rates makes for few and far between water changes.

Cost of water and low efficient RO systems is at this point, a necessary evil of the hobby. 1:1 sounds great, but I'm skeptical.