First, let’s take a minute to examine why we need pure water. Since ‘hydroponics’ means growing plants in water without the use of soil, then it makes sense that the quality of the water you employ should be good. Pure water is also an integral part of soil gardening—chemicals and other contaminants are detrimental to the health of your plants whether you are growing hydroponically or traditionally. The water you give your plants should be the best you can make it. I say ‘make it’ because you can control the contaminants in your water—you don’t have to be stuck with that chemical tap flow from the municipality or the sludge in your well.

The nutrient feed formula you give your plants is based on a measure of parts per million, which indicates how many parts of nutrients there are per million parts of water—we’ll refer to this measurement as ‘ppm’ from here on. When your water comes out of the tap it already has a ppm measurement that must be accounted for before adding nutrients. This means that if you already have 300 ppm water you’ll have to decrease the nutrients you administer by that number to avoid overloading your plants. This is a problem, though—because now your plants are getting fewer nutrients because of the non-nutritious crap that is present in your water source. The stuff present in your water is also often not only non-nutritious, but might actually be toxic to your plants.

Calcium and magnesium—minerals found in almost all water—are important nutrients for both plants and the human body. The calcium and magnesium found naturally in water, however, is present in the form of large blocky particles that bind against the roots of your plants and can cause nutrient lockout—basically, the particles crowd up against the roots and don’t allow any of the good stuff to get through. While this sounds dramatic, it’s not even the worst news about your water supply—chlorine or chloramines are also generally added to your water by your local municipality in order to kill bacteria and render the water safe for drinking. Unfortunately, these substances are great at killing plants as well and are the reason plants like algae don’t grow in swimming pools and hot tubs. You’d probably never consider using pool water for your plants, but city water straight from the tap is really just a milder version of the same stuff!

Although some wells are ‘shocked’ with chlorine, generally well water users don’t have to worry as much about chlorine or chloramines—but calcium and magnesium are often found in much higher levels in well water. Additionally, contaminants such as cadmium, arsenic, boron, manganese, hydrogen sulfide (for that just-peeled egg aroma) and iron can be present in levels that are not only toxic to plants but to the human body as well. Some of these substances—though more common to wells—are found in your tap water too, as municipalities don’t have the means to remove them.

Now that I have you good and worried, let’s talk about how an RO system can rid your water of these nasty contaminants so you can have a healthy garden. Water purifiers for your garden come in two types: dechlorinators and reverse osmosis systems. The difference is an extra filtration step in the RO systems. Dechlorinators employ two steps of filtration, the first being a sediment filter that traps large particles and keeps them from causing the next step—the carbon filter—from getting clogged up. Sediment filters can usually be rinsed and reused regularly before their annual replacement. The next step in filtration is the carbon filter, which removes chlorine from the water—if your municipality uses chloramines, you’ll want to use a special KDF carbon filter. Chloramine is a chlorine molecule bonded with an ammonia molecule. This chemical is used in place of chlorine because it lasts longer in the water and won’t bubble out and evaporate like chlorine will. Good for water treatment, but bad for your plants! For simple dechlorinators this is where it ends—chlorine, chloramines and sediment are removed and water is improved. Ppm will not be greatly affected. If your water was under 150 ppm out of the tap, this might be enough filtration for your plants. However, if your ppm if higher than 150, if you have any funky contaminants in your water or if you simply want the purest water for your garden, an RO system is what you need.

This is where the membrane comes in. In RO, a membrane looks like a plastic cylinder with a hole in either end. It’s called a membrane because the process of osmosis is actually the diffusion of water through a membrane, which is a barrier with small openings—or pores—that only lets certain things through. The cell walls of organisms (like humans and plants) are membranes and allow certain substances through but lock others out. Our skin is a membrane that lets some things in but keeps things like bacteria out.

An RO membrane works the same way—it’s composed of a super-thin sheet of polymer punctured with tiny holes. By tiny I mean 1/1,000th of a micron, the perfect size for a water molecule to pass through. The sheet of polymer is then rolled up to create layer after layer. When the water is directed into the membrane, it has to diffuse through all these layers, meaning every single water molecule of purified water has to go through hundreds of these tiny openings. It’s like an exclusive night club—the bouncer only lets the right kind of molecules past the velvet rope. If you have used an RO system, you might have been surprised that the water didn’t come out immediately and with as much volume as it went in. Well, now you know why—it’s been waiting in line to get in to the club! It takes time for the good water to pass through the membrane molecule by molecule and the rest passes out of the membrane as waste water.

People new to the process of RO are often surprised by the waste water factor; that is, the amount of unpurified water that remains for each measure of pure water. Most systems are built to support a ratio of three parts waste water to one part pure water—they are built this way because this ratio best supports the life of the membrane. Bear with me while I extend the nightclub metaphor—some molecules just don’t make it into the club. Calcium, magnesium and various other contaminants won’t get past the bouncer. This also helps explain why so much waste water is necessary. It’d be nice if these contaminants simply left, but they’re stubborn and kind of belligerent and they need to be forced to leave. The bouncer turns them away at the door, but then they hang around causing trouble, so good water molecules have to wash them away to make them go. The contaminants are stubborn and want to hang around the door fighting with the bouncer. The more waste water there is, the easier it is on your membrane (bouncer). Should you choose to use a flow restrictor to reduce your waste water there won’t be as much waste water to help wash those contaminants away and you may decrease the lifespan of your membrane.

Damage to your membrane can happen in one of two ways: it can become either ‘fouled’ or ‘corroded.’ When contaminants build up in the membrane they won’t allow any other molecules through those tiny pores—effectively shutting down the club—or they might actually smash right through the opening, enlarging it and rendering it an ineffective barrier. Basically, they take out the bouncer and then anything can get in. In the former case, the membrane becomes clogged up—this is called fouling or saturating the membrane—and it will cause a dramatic decrease in your flow rate. Where there was a healthy stream there will only be a trickle, because those water molecules can’t get through. Should you neglect to change your carbon prefilter, you will have the opposite problem—as the chlorine that is now getting through your tired carbon filter will enlarge the openings and ‘corrode’ the membrane. Now the flow rate will actually increase, which seems great—until a ppm reading reveals that your rejection rate has decreased and you are no longer getting clean water. Either of these two things might have happened when it’s time to change your membrane, but restricting your waste water might mean that they have happened sooner rather than later because you are concentrating the water and you might be going through membranes a bit more quickly than before. If saving water is of primary importance this could be a good trade, but individual needs vary from person to person.

When considering how to best extend the life of your membrane, it’s good to recognize that this fouling/corroding issue also depends on what’s actually in your water. If your water is very high in chlorine or chloramines, corroding will be the main issue. If your water has iron or very high mineral content, then fouling will occur. In both cases, proper prefiltration can help preserve your membranes. Finding out what’s in the water by getting a water test or checking with your local water treatment center will help you to choose the right prefilter, such as a KDF—or, in the presence of iron, a philox filter. If you have hard water, a water softener is also an excellent way to preserve your membranes. It works by exchanging hardness (minerals) for salt, which doesn’t sound too plant-friendly—but salt is actually the ideal contaminant for RO to remove. It doesn’t get in the club but takes the rejection politely, leaving your membrane intact and still able to perform its job.

It’s a good idea to explore what’s in your water. Start by finding the website of your local municipality or by doing an internet search of your area and its water content. Your local hydro store is also an excellent resource—the employees there know all about what’s in the local water and can guide you to the product that will work best for you. And when you do get your RO system, be sure to change those prefilters to protect your membrane and save money. Once you start using pure water in your garden, the difference will amaze you.