Installing an In-line Water Filter: Pure Water From a Reverse-Osmosis System

Tap water can be disagreeable stuff to drink brackish, sulfurous or reeking of chlorine. And in some cases, what comes down the pipes can be bad for your health. Convenient in the short term, bottled water becomes an expensive hassle before very long. But there are several popular and effective alternatives. One of them is reverse-osmosis, or RO, filtration.

How It Works
Reverse osmosis works by forcing water through three filters. The first is a 5-micron carbon filter (a human hair has a diameter of about 50 microns). The cheaper systems don't have this first filter, but you need it. It removes sediment and chlorine; either of these can destroy the membrane filter, which is next in line. This filter performs the actual RO that purifies the water. Impurities collect on its input side and are washed down the drainpipe, along with a lot of water. The third filter, activated carbon, eliminates questionable tastes and odors. An RO unit will remove 90 to 98 percent of all sediments, impurities, inorganic minerals, bacteria, chemicals, odors and tastes from your drinking water.

No single filtration system catches everything, but RO, with its sequence of membrane and carbon filters, comes close. If the filter is rated at "absolute 1 micron," it will eliminate pathogens like Giardia (5 to 6 microns) and Cryptosporidium (4 to 6 microns). For truly ghastly water, you may want to look into more aggressive filters (ultraviolet, for example), or you may just want to buy your drinking water if groundwater in your area is unhealthy and unpalatable.

Caveats
If your water supply contains 0.1 parts per million (ppm) or more of iron or is excessively hard (10 grains or more of total dissolved solids per gallon), it's beyond what an RO system can handle by itself. You'll need to install a full-house treatment system.

For every gallon of filtered water an RO system produces, 4 to 10 gallons go to flush impurities down the drain. This is because the filters have to be "washed" constantly or they become clogged.

Water pressure is also an issue. An RO system requires at least 40 psi to work properly, and several manufacturers recommend a minimum of 45 psi. How can you find out what your water pressure is? Your local water company may be able to tell you; just ask for the static water pressure. If the answer is a range of psi, use the lower number. Or you can measure your own water pressure with a pressure gauge. If your pressure is low, you'll need a booster pump. Some systems have a booster pump built in, or you can install one on your own.

If your kitchen sink is cast iron and doesn't have a ready-made hole for the faucet that dispenses the filtered-water, you may want to install the faucet in the countertop next to the sink. (We'll discuss this a little further on.)

A Typical Setup
As noted above, the better RO systems have three filters: one for chlorine and solids, the membrane filter and an activated carbon filter. Typically, the filters are replaceable cartridges that fit into an inert plastic housing; you unscrew the housing to change the filter. Usually these housings are mounted to a manifold plate that holds two or three cartridges. But some systems feature integral modules that are both housing and cartridge.

Since RO filtration works slowly (producing 10 to 50 gallons a day, depending on system size and water pressure), most systems collect the filtered drinking water in a small tank included in the system. You draw from that supply as you need it. Obviously, when you use up the water in the tank, it will take some time to filter a new tankful. If you need a larger reserve of filtered water, you could install two systems in parallel.

All RO systems provide 3/8-inch-OD water tubing for the interconnections. High-quality systems use very simple connections and color-coded tubes to make it easy to identify lines carrying drain water, the unfiltered water supply, and the filtered water.

Plastic containers can give the water stored in them a slight plastic taste or smell. The RO system stores drinking water in a plastic reservoir, so a small carbon filter is installed just upstream of the dispenser (often piggybacked on top of the manifold). It acts as a final defense against odd tastes or odors.

Almost all RO filtration systems deliver water to you through a small faucet usually a single-handled water dispenser that mounts on the sink bowl or nearby. The best systems also have two indicator lights in the dispenser base that connect to an electronic sensor in the manifold. The lights tell you either that the system is OK or that it's time to change the filters.

An RO system usually attaches to both the water supply and the drain by means of a pair of tap-on valves, one small, one large. The small one taps into the water pressure line and the large one taps into the drain.

Step by Step
1. Install the system. Location, location, location. Where you mount the system determines how difficult it will be to install and maintain. Though many manufacturers call their systems "undersink" models, my preferred location is not under the kitchen sink. You need convenient access to the cartridges so you can change them easily whenever they require it. Now, think of all the junk under your kitchen sink. A better place for your system is in the basement, immediately beneath the sink; if that won't do, put it elsewhere in the basement. No basement? A utility room is okay, if it's near the sink. If there is no good alternative, put it under the sink after all but pledge to maintain it enthusiastically.

You'll need a wall to secure the cartridge block. It usually needs to be mounted so that the cartridges hang vertically. Also consider the location of your water supply: you'll have to bring in a water source to feed the unit. The holding tank weighs about 40 pounds, so set it on something secure (the floor will do). Lastly, if circumstances permit you to install the filter someplace other than under the kitchen sink, you'll need a utility sink, floor drain or some other place for the flexible drain line to discharge impurities removed from the system. Install the cartridge block somewhere above the drain not necessarily directly above it.

To simplify running the tubing, many systems use grabber fittings. You don't need tools for these; you simply push the tube into the fitting. To release the tube, pull back on the fitting's collar. These work well, but the tube must be inserted properly or it'll blow out of the fitting under pressure. Cut the tubing end straight, not at an angle, and be sure that the tubing is free of debris and cuts. Push it into the fitting all the way. As the tube is pushed into the fitting it will pass a group of stainless steel grabber rings and an O-ring. Don't stop pushing until you feel the pressure of the O-ring against the tube; then push it beyond, making sure it has seated.

2. Install the drain tube. Insert the tube that carries wastewater into the outflow port of the cartridge block. Take the other end of the tube to a drain. If the drain in question is a basement utility sink, cut the drain tube so that there's a 1-inch air gap between its end and the sink's overflow rim. If wastewater should back up in the sink, it must not touch the tube.

If you have to set up your filter under the kitchen sink, you can put the tube directly into the drainpipe, immediately above the P-trap if the system's instructions say you can (most do). If they do, it means that the faucet that dispenses your filtered water has a built-in air gap. Don't buy a system without such a faucet design if you intend to use the kitchen sink drain. If the dispenser has an integral air gap, you can use the tap-on valve supplied with the system to make the connection. Manufacturers' tap-on valve drain designs and installation instructions may vary, but basically you clamp the tap-on valve tightly onto the drainpipe. Then drill a 7/32-inch hole though the clamp's porthole and into the drain itself. To attach the drain tube, first slip the compression nut over the tube, then slide the plastic ferrule on, with its tapered edge toward the valve. Then slip the brass insert into the tube end to make it rigid. Push the tube into the clamp and tighten the nut.

3. Install the water supply tube. Insert one end of the tube into the cartridge block. The other end taps into the cold-water line that runs to your sink. The system you purchase should supply a main water valve so that you can turn off the water running into the filter system when you have to do maintenance.

To get the cold water through a valve and into the filter's water line, the manufacturer typically supplies a tap-on valve like those used for tapping into a water line for an ice maker. The valve fits around the pipe and clamps down on it. As you screw the handle down, it punctures the pipe.

The sad secret is that this works only for copper and CPVC. The puny little cutter in the tap-on valve stem will not penetrate galvanized steel pipe. If you have galvanized supply pipes, you'll have to shut off the water and drill a small hole (say, 1/8 inch), then install the tap-on valve. PEX pipe will be too flexible you'll have to get a plumber to install a special fitting in the line.

4. Install the filtered-water tube. The first tube you need for this step has one ordinary end and a T-fitting at the other end. (The T-fitting is another grabber fitting supplied by the manufacturer.) Attach the plain end to the cartridge block. Connect a second tube of the same color (whatever it is) to one crossbar of the T, and attach the other end of that tube to the water reservoir. The tube attached to the other side of the T-fitting goes to a little in-line carbon filter. Yet another tube will take the purified water from the other end of the carbon filter to the water dispenser. The carbon filter will most likely be connected to the dispenser by compression fittings, like those you installed on the tap-on valve: nut, plastic ferrule, and brass insert.

5. Install the dispenser. If the installation has a difficult part, this is it. No problem as long as you have a stainless-steel sink with an extra hole for mounting the faucet for your filtered water, but many sinks don't. If your sink has a sprayer, you can remove it and put the RO dispenser in its hole. If you have a three-hole faucet, you can replace that with a single-hole faucet, but then after installing the RO dispenser you'll have one hole too many. You can always stick a soap dispenser in the extra hole and be done with it.

If you want to keep your kitchen sink faucets as they are and you want the filtered water dispenser mounted to the sink, the only option is to punch or drill a hole in your stainless-steel sink, using a carbide-tipped or bimetal hole saw. Take great care not to disfigure your sink.

A cast-iron sink presents a major problem. The first (and not especially practical) solution is to buy a painfully expensive porcelain-cutting hole saw. Second, use a carbide-tipped or bimetal hole saw to slowly and carefully cut an opening into the sink, adding water to the cutter as you drill. This is difficult, but it works though it's very easy to scratch the sink. Put several thicknesses of tape around the hole to keep from doing so. Or you could replace the sink: it will be easier than trying to cut cast iron.

All in all, the best solution to the problem of a holeless sink is fast and easy: Install the faucet on the countertop next to the sink. If you make this choice, be sure the RO kit you buy comes with a dispenser that extends far enough into the sink that water coming out of it won't splatter on the counter.

Different manufacturers specify different-sized faucet-mounting holes some as small as 1/2 inch but 1 1/4 inch is typical. You will need a 2-inch-diameter flat surface for the faucet to sit on, and the depth of the sink edge and counter must not exceed 1 1/4 inch. Check under your sink for strengthening webbing that adds to its depth. Once you've got a hole, apply a bead of silicone caulk around its edge, stand the faucet body in the hole and, from underneath the sink, tighten down its holding nut.

6. Hook up the power. Once the faucet is installed, you'll make one last connection the telephone cable that connects the printed circuit board on the manifold with the small green and yellow lights on the faucet. As long as the green light is on, the system is working well. A yellow light means the system isn't working: replace all the cartridges. If the cable supplied by the manufacturer isn't long enough, you can add another with an ordinary double female connector for telephone cables.

By the way, the electricity for the lights (actually light-emitting diodes) comes from a tiny battery whose voltage is so weak that there's no danger of your getting a shock. You'd get a far larger zap by scooting your feet across a carpet, picking up static electricity and then touching someone.

7. Start the filter. Follow the manufacture's startup instructions. Note: Be sure to install the battery for the circuit board.

8. Keep up on maintenance. Maintenance simply consists of changing the modules. Always wear gloves when you do this chore. First, turn off the water to the system and turn on the faucet for a moment to bleed off any residual pressure. To change any of the three modules, push up and twist counterclockwise: the old cartridge simply unscrews. Push up and turn clockwise to install the new one. A few drops of water may leak out during the change. How often you have to replace modules has to do with the quality of water that's going through the filter. The first and third module will need changing every three to six months, depending on the amount of chlorine in the incoming water. If you let it go too long, chlorine will permeate the carbon module and ruin the membrane module (which will otherwise last up to three years); then the yellow light will come on, and you'll have to replace the membrane. If your water supply has no chlorine in it, change the other two modules every six months. Expect a 10 percent diminishment of water output with each passing year because of impurities clogging up the membrane's pores.

Hire a Pro?
If any or all of this job is something you'd like somebody else to handle, check out our Services channel. We'll help you find find trustworthy, prescreened professionals.

Rex Cauldwell is a third-generation electrician and a master plumber. He has written widely on both subjects, most notably in Fine Homebuilding magazine and the Journal of Light Construction. His book Wiring a House was published by Taunton Press in 1996.

TOOLS AND MATERIALS
Utility knife
Silicone caulk
Cordless drill
Telephone cord coupler
Drill bit set
Telephone cord
Bimetal 1 1/4-inch hole saw
Screwdriver
Adjustable wrench