RO systems.

[DavidCampen]

I plan on assembling my on RO water production system. I want one that is capable of producing upwards of 200 gallons per day. I will use a booster pump to increase my water inlet pressure to around 100 psi. I am also hoping to be able to reduce the waste to product ratio from the usual 3 or 4 to 1 to close to 1:1.

What type of RO systems do other people have? What sort of waste to product ratio do you run at?

[raybark]

David,

I build and sell RO systems (and resell prefabricated ones), giving me a little knowledge in the area, but it doesn't make me an "expert" by any means.

Membranes are typically rated at 65 psi, 77°F input water temperature. Output drops with lower pressures and temperatures.

In standard (read that as "inexpensive and easy to get") household membranes, 100 gpd is pretty much the upper limit, although I have built systems with special-order 150 gpd membranes. I don't recommend using them though, as they tend to have shorter lifespans, and are only marginally less expensive that two 100 gpd membrances. I can get membranes with capacities as high as 2400 gpd, but they require custom-made housings.

Boosting the pressure to 100 psi increases the pure water generation capacity about 50%.

Your goal of reducing the pure:waste ratio of the unit to 1:1 - while admirable - is impractical or costly, depending upon your mindset. The standard 1:4 ratio is not a technological shortcoming, it is an engineering design specification, as the membrane needs that to efficiently flush away the ions rejected by the membrane. If you restrict that flow further, the "cleaning" efficiency decreases, the RO output follows that, and you'll be replacing the membranes far more frequently. There are ways to improve the overall efficiency though, by managing the flush water post-generation.

First, understand that the 1:4 ratio occurs when there is no water in the pressurized bladder tank typically supplied with systems. As the pressure in that tank increases, it slows the production of RO while the flush water continues to flow at it's controlled rate - hence the amount of waste-per-pure actually increases. The closer to full the tank gets, the less efficient the ratio becomes. It is possible to add a "permeate pump" to a system using a pressurized bladder tank, and that uses the waste water flow to lower the pressure on the output side of the membrane so it "appears" the tank is always empty. Not only does that prevent the huge increase in waste water volume, it accelerates the recovery time to refill the tank. If you are producing into an open-air tank (no pressure increase), it's a constant 1:4.

Most folks find a use for the flush water, so it is not a "waste". I use it to fill a small artificial pond next to my greenhouse. Birds and deer use it for drinking year-round, and I get all sorts of frogs in the summer months. Others water their terrestrial gardens with it, or let it spill on the greenhouse floor for humidification.

It is also possible to take the entire system to a "zero-waste" mode by pumping the flush water into your household hot water system, and there are retrofit kits available to do so.

[DavidCampen]

I will use an Aquatec 8852 pump to boost the water pressure to 80-100 psig and a Dow Filmtec brand home drinking water RO membrane. There is a Filmtec Home RO membrance now that is rated at 100 gpd at 50 psig. At first I will use a single of these membranes and if I need true 200 gpd production rate I would add a second of these in series, the limiting factor on production rate then would be the capacity of the 8852 booster pump.

The Aquatec permeate powered pumps are a really neat idea but don't seem to be sized for a 200 gpd system so I will dump product water to an open air tank and then use a second 8852 pump to pump the product up to a pneumatic tank.

I don't have anything convenient to use the brine water so for me it truly is waste. I agree that it would be impractical for most home users to reduce the standard 1:4 product:waste (permeate:brine) ratio but I will instrument my systems with pressure gauges and flow meters so that I can monitor the condition of the membrane. There are various methods available to reduce the waste ratio such as using softened water, increasing the flow rate along the membrane, periodic high velocity flushing, using scale inhibiting polymers, adjusting the pH of the feed water and periodic scale removal by chemical cleaning. I plan to use citric acid for periodic membrane cleaning, citric acid is inexpensive and non toxic.

[raybark]

From a practical perspective, I believe you would want to add a second booster pump if you add a second membrane.

Something you'll want to look into about booster pumps: most are installed on the inlet of the membrane housing, and are accompanied by a 40 psi pressure switch that shuts them off when the tank is full. The trouble with that is that the automatic shutoff valves on RO systems are designed to shut off flow to the system when the outlet pressure reaches 2/3 of the inlet pressure. If the inlet pressure is under 60 psi, then the shutoff valve kills the water supply before the pump pressure switch shuts that off. It seems plausible that if the booster pump was before the entire RO system, you'd be fine, but you had better make sure it can handle the flow rate of the permeate and brine streams, combined.

If you're going to collect the RO in an open-air tank, you don't need to fill the small bladder tank at all, and using another booster pump to do so is just impractical. For one, the typical bladder tank is too small for watering purposes - 3.2 gallons with about a 3 gallon usable capacity. I recently put together a system for a woman using a 20-gallon bladder tank (14 gallons of usable capacity), which roughly doubled the cost of the system, but she had a small collection and did not have space for an open-air tank and pump. My own setup includes a shallow-well pump fed by the RO tank. It has it's own bladder tank, and offers far more pressure and flow rate than a booster pump ever could, allowing me to water quickly and pressurize overhead misting.

I really think you should apply some creativity to the brine issue. All of the instrumentation, custom design and fabrication of membrane housings, and the like are costly, and the use of citric acid, besides being a pollutant, may not be as effective as you would like. To be honest, it seems to me that if the maintenance cost is truly less of a concern than the generation of brine, it would be a lot simpler to just gang several membrane housings with their brine outlets plugged.

[DavidCampen]

Originally Posted by raybark

From a practical perspective, I believe you would want to add a second booster pump if you add a second membrane.

Yes, a full 200 gpd system would require a second Aquatec 8852, with a single pump and a single Filmtec "100 gpd" membrane I should be able to get about 125 gpd depending on my product to waste ratio.

Originally Posted by raybark

Something you'll want to look into about booster pumps: most are installed on the inlet of the membrane housing, and are accompanied by a 40 psi pressure switch that shuts them off when the tank is full. The trouble with that is that the automatic shutoff valves on RO systems are designed to shut off flow to the system when the outlet pressure reaches 2/3 of the inlet pressure. If the inlet pressure is under 60 psi, then the shutoff valve kills the water supply before the pump pressure switch shuts that off. It seems plausible that if the booster pump was before the entire RO system, you'd be fine, but you had better make sure it can handle the flow rate of the permeate and brine streams, combined.

The Aquatec 8800 series pumps do not have a pressure shut off switch just a pressure bypass. If I was filling a pneumatic tank I would install an pressure switch external to the pump to shutoff the pump and feed water. An ASO will not be used since I will use a solenoid valve to shutoff the feed water. Since I will be dumping to an open tank I will use tank level controllers for these functions instead of a pressure switch.

Originally Posted by raybark

If you're going to collect the RO in an open-air tank, you don't need to fill the small bladder tank at all, and using another booster pump to do so is just impractical.

Actually, the pneumatic tank is not only practical but almost necessary. That is why the engineers that designed the well pump that you use as a delivery pump incorporated a pneumatic tank directly into its design as you mention below.

Originally Posted by raybark

... My own setup includes a shallow-well pump fed by the RO tank. It has it's own bladder tank, and offers far more pressure and flow rate than a booster pump ever could, allowing me to water quickly and pressurize overhead misting.

Originally Posted by raybark

I really think you should apply some creativity to the brine issue. All of the instrumentation, custom design and fabrication of membrane housings, and the like are costly,

Yes the instrumentation will be costly but will provide me with entertainment. The custom design I am doing myself and since I enjoy this sort of work I consider that to be zero cost. The membrane housing(s) will be standard.

Originally Posted by raybark

and the use of citric acid, besides being a pollutant, may not be as effective as you would like.

I wouldn't consider the citric acid to be a significant pollutant. It is non toxic and will be dumped into a sanitary sewer where it will be converted to carbon dioxide at the sewage treatment plant.

[raybark]

Originally Posted by DavidCampen

The Aquatec 8800 series pumps do not have a pressure shut off switch just a pressure bypass. If I was filling a pneumatic tank I would install an pressure switch external to the pump to shutoff the pump and feed water. An ASO will not be used since I will use a solenoid valve to shutoff the feed water. Since I will be dumping to an open tank I will use tank level controllers for these functions instead of a pressure switch.

David, I have installed them with Aquatec pressure switches. They are available in 40, 60, or 80 psi.

Solenoid is a good idea. That's how I did a 4 unit assembly not long ago.

Seems to me that a simple float valve would be sufficient.

Originally Posted by DavidCampen

Actually, the pneumatic tank is not only practical but almost necessary. That is why the engineers that designed the well pump that you use as a delivery pump incorporated a pneumatic tank directly into its design as you mention below.

I agree totally. I thought you were referring to the ones usually supplied with systems. The one on my jet pump is much larger and handles much higher pressure.

Originally Posted by DavidCampen

I wouldn't consider the citric acid to be a significant pollutant. It is non toxic and will be dumped into a sanitary sewer where it will be converted to carbon dioxide at the sewage treatment plant.

Whether you consider it so or not is irrelevant. The authorities might think differently. Of course, in small volumes you might get away with it.

Have fun with the design and assembly. When you start looking at purchasing, let me know and I might be able to get you some good prices on components.