Reverse Osmosis Guide
Posted Nov 09th 2012 | By:
Photo courtesy of Keiths
If you are thinking about upgrading the brewery with an RO system, here is some information that may help with your decision.
What is an RO Unit?
RO units are pretty much all the same in concept (with minor differences). They are simply a series of filters. Most RO units consist of 1 or more of the following filters: sediment filter, carbon block filter, RO Membrane, post carbon filter and perhaps a DI stage.
Sediment filters are pre-filters. They remove particulates from the water before it reaches the RO membrane. Their function is to prevent damage to and prolong the life of other components, including the RO membrane and other downstream filters. They come in a variety of pore sizes rated in microns. The smaller the pore size, the more efficient they are at removing particulates. 5 micron is about the largest pore size you should use. Sediment filters should be replaced every 6-12 months according to the manual. This will vary depending on your source water quality and how much water you produce. If you want to get technical about it, you can install a pressure gauge after the pre-filters and replace them when the pressure starts to drop. Replacing pre-filters regularly is necessary to protect the more expensive RO membrane.
Carbon Block Filter
Carbon block filters are also pre-filters. They remove chlorine from the water. Their primary function is to protect and prolong the life of the RO membrane from damaging chlorine. One thing to note is that carbon blocks are not efficient at removing chloramine. If there is a lot of chloramine in your water, an additional carbon filter designed specifically for chloramine removal may benecessary. Carbon blocks are rated by generation capacity. This can range from 2,000 gallons to 20,000 gallons. They should be replaced when you get close to or exceed the rated capacity.
The RO membrane removes dissolved contaminants from the water. This is not possible with a simple sediment filter. Water is pushed through the membrane, drastically reducing dissolved contaminant concentration. The RO water (permeate) flows to a storage tank. At the same time, waste water (brine) which contains all the stuff that was just removed from your water, is sent to the drain. RO membranes do not remove everything from the water. Depending on the unit and the membrane it should be greater than 90%. For more specifics, look up the rejection rates for the unit you are considering. This will tell you how efficient it is at removing specific contaminants from the water. There are two types of common RO membranes. Thin film composite (TFC) and cellulose triacetate. Thin film composite is more efficient at removing contaminants than cellulose triacetate. Another thing to consider is what volume your RO membrane is rated for. The higher the volume, the less efficient the membrane is at removing contaminants. So a 50 gallon per day RO system will produce better water than a 100 gallon per day system will. It's not a dramatic difference, but something to consider. For this reason, it's best to size your system as small as possible while being able to keep up with demand. RO membranes are the most expensive piece of the RO unit. If pre-filters are replaced regularly, the membrane should last somewhere between 5-10 years from what I've gathered, but time will tell the real story. The RO membrane should be replaced when the dissolved solids concentration starts to rise. You can keep an eye on this by using a TDS meter. TDS meters measure total dissolved solids and report the results in ppm (parts per million). You should be able to find a descent TDS meter for around $30 - $40. My unit is brand new. My source water weighs in at about 325 ppm. My RO water comes out at 25 ppm. When the baseline level of my RO water starts to rise, it's time to replace the membrane.
Carbon Post Filter
Carbon post filters are optional. Some units have them, some do not. Their function is to improve the taste of the water before you drink it / use it. Post filters are generally installed after the storage tank. The RO water can pick up tastes from the storage tank or tank liner. The post filter removes this so that you end up with nice clean tasting water. Post filters should be replaced when the taste of your water gets noticeably worse or every 6-12 months.
DI stands for De-Ionization. What this does is remove the remaining contaminants from the water that the RO membrane was unable to. The result is pure water that is basically the same as distilled water. In many cases this step is unnecessary. When brewing for example, you don't need distilled water. Strait RO water is fine. However, you could mix DI water with your source water to achieve different water profiles. When brewing with RO or DI water, it may also be necessary to adjust your water profile using brewing salts. For more information on brewing with RO / DI water, a good place to start is the "A Brewing Water Chemistry Primer" thread. DI water also comes in handy for rinsing pH meter probes, filling humidifiers, fish aquariums and so on. I have a DI stage, but want to be able to choose whether I runoff RO or DI water. What I did was install a tee on the permeate line with a valve and a valve coming out of the DI side. This way I can runoff either strait permeate water or run it through the DI stage at will. DI stages have charged resin beads in them. The beads attract and latch on to ions in the water. Some DI cartridges come apart so that you can replace the resin beads and some do not. It's cheaper to buy replacement beads than it is to replace the whole cartridge. Like the RO membrane, resin beads should be replaced when the TDS of the water starts to rise. When the beads are working properly, the water should be at zero ppm. They also have color changing resin beads. When the beads start to change color you know it's almost time to replace them.
Well, that's basically all there is to an RO unit. So what's the rest of the story?
What's the best? What's the cheapest?
There are a ton of different RO systems to choose from. But the thing is, they all use the same components and achieve the same level of water quality depending on what type of filters are employed. The biggest difference between RO units is the size of the filters and the filter housings. Standard sizes are 10 and 20 inches. There is a better and cheaper selection of filters in the 10 / 20 inch size. What many companies do, is make RO units that have custom sized filters. This forces you to buy their filters instead of using generic standard size filters. So basically, you are paying more money for a brand name and getting the same water that you would from any other RO unit out there. So what unit is the best? The cheapest unit you can find with standard size filters. Also, it is best to get a unit with clear filter housings. This way you can easily monitor the condition of your pre-filters and/or resin beads. To get the best price, you may end up with a unit that has components you would like to upgrade or swap out later or you may need to buy some components separately. For example, my unit came with a sealed DI stage so I cannot replace my resin beads. When the resin beads are spent, I'm going to get a different filter housing so that I can replace them. If I had known this before hand, I would have gotten a system without DI and purchased one separately. If you plan to use a booster or permeate pump, you can probably get those cheaper separately as well. If you don't mind the extra hassle, it may be cheaper to piece meal than to buy a complete system.
How much waste do they produce? Should I get a booster or Permeate Pump?
This can vary. RO membranes work more efficiently at high water pressure. The higher the pressure of the inlet water, the less waste that will be produced. RO units produce more waste water than they do permeate water. In newer units, this should be somewhere in the neighborhood of 4:1.
In cases of poor water pressure, you can improve the waste ratio by using a booster pump. Booster pumps increase the water pressure before it gets to the RO unit. Booster pumps are more expensive and use electricity. A booster pump should not be necessary unless you have very low water pressure like in the case of old homes or homes using well water. Permeate pumps, on the other hand, are incorporated into the RO unit itself. Permeate pumps utilize energy that is generated by the brine (waste water). They do not use electricity or any other external power source. Permeate pumps reduce waste water and make pressurized tanks fill faster. Permeate pumps come in different sizes based on the gallon per day rating of your RO unit. For units rated for 50 gallons per day or less, you need an ERP 500 permeate pump. For units rated over 50 gallons per day, you need an ERP-1000 permeate pump. Use of a permeate pump is not beneficial unless your system utilizes a pressurized tank.
What size unit to get? How about storage tanks?
This depends greatly on what you plan to use it for and how fast you need to produce water. What I recommend is that you figure out roughly how many gallons per day you will need and get a unit rated for twice that amount. RO units will not likely produce as much water as they are rated for. This is because the rating is based on optimal conditions in relation to water pressure and temperature. If water pressure and or temperature get out of range, it will reduce the amount of water produced. As for storage tanks, you have a couple options. Most RO units either come with a storage tank or offer one separately. These tanks are pressurized and will deliver water to a drinking water faucet or ice maker. Since the tanks are pressurized, they only hold about half of the volume capacity of the tank. So, depending on how much you use and how fast your RO unit produces water, you need to size your tank accordingly. If you are not running an ice maker on RO water and do not mind living without an RO faucet, you can use any kind of storage vessel you want. What I decided to do is use a 50 gallon rubber maid trash can. I can get 45 gallons in it. I installed a bulk head/hose barb and a stainless ball valve. In-between the trash can and the ball valve, I installed a carbon post filter. With the head pressure of the water column in the trash can, I can easily dispense water via gravity for drinking or other uses. When I brew, I can hook up to a march pump for quicker delivery to the HLT. Another downside to using a non pressurized tank is that you will either have to manually turn the flow of water on and off or install some kind of shut off valve like a float valve. At the moment, I'm operating mine manually, but I have to check it periodically so that I don't forget to turn it off and overflow the storage vessel.
Well, that's all there really is to know about RO systems. Everything else you need should be in the user/installation guide that comes with the unit.
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