Reverse osmosis home filtration systems provide large volumes of pure, clean, color- and odor-free water for people, pets and plants. A “hard wired” RO hyperfiltration unit is a convenience easily within reach of the average DIYer.
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Components of a Reverse Osmosis System
A domestic DIY-ready RO system typically consists of several components, often sold as a kit along with an installation instruction manual:
- Filter Array—four to six filters mounted on a hangable metal housing. Units with more filters deliver slightly cleaner water. All the filters in the array are pre-connected by the manufacturer, so hookup is a snap. An automatic shutoff valve is usually part of the array.
- Holding tank—a 3 to 6 gallon capacity pressurized vessel that stores filtered water ready to flow to a sink- or counter-mounted faucet. Until direct flow systems hit the market recently, the RO process has been too slow to instantly provide a gallon or two of filtered water, hence the need for a tank. Tankless direct flow units are pricier.
- Faucet mounted in a convenient location, usually on the kitchen sink.
- 1/4″ plastic hoses to connect the filter array to feed water and to the faucet, and for waste water discharge
- Feed water valve: either self-piercing saddle type, identical to an ice maker supply setup that taps into a water pipe; or a ball valve installed in-line in the riser tube of a sink’s cold water supply.
How RO System Filters Purify Water
Water flows through the filters in the array and is successively cleaned in “stages” as follows:
- Stage 1 Prefilter, 1 – 5 micron—removes sediment, suspended rust and sand.
- Stage 2 Prefilter, granular activated carbon (GAC) 1 to 5 micron—removes most chlorine, organic chemicals, taste, color and odor.
- Stage 3 Prefilter, either a second GAC or an activated carbon block, 1 to 5 micron—further removes chemical entities Stage 2 filter missed.
- Stage 4 Filter, osmotic membrane—the workhorse filter that gives the system its name. Removes 92% to 98% of all remaining chemicals and dissolved solids in tap water.
- Stage 5 Postfilter, deionization (DI)—removes remaining dissolved solids. Premium systems have 2 of these when ultra pure water is needed for aquariums, hydroponics and laboratories.
Selecting a Reverse Osmosis System: How Large?
The EPA estimates that the average adult consumes 2.0 L (about 1/2 gallon) of drinking water per day. Choose an RO system with a filtration capacity sufficient to meet typical family needs and “surges” like parties that require extra water for coffee, drink mixes and the like. A unit that generates 3 GPH (gallons per hour) has about the same capacity as one rated at 75 GPD (gallons per day), and is large enough for most households.
RO System Pre-Installation Considerations
- Many RO systems require a minimum water pressure of 40 psi. Booster pumps are available if pressure is a problem.
- Consider a whole-house filter, ahead of the RO unit, if incoming municipal or well water is unusually turbid or rusty.
- Choose a spot for the filter array (approximately 18” H x 18” W x 8” D) that’s easy to access, since the unit needs to be serviced twice a year. If the undersink area is too small to stand or hang the array, consider a basement, utility room, etc.
- Select a location for the holding tank (approximately 18” H x 12” W x 12” D). It can be spotted anywhere up to 30 feet away from the filter unit.
- If there’s no available kitchen sink-top hole to install the added separate purified water faucet, replace the kitchen faucet with a pullout spray head model to free up the sprayer hole. Alternatively, drill a new dedicated hole in the countertop or sink. Careful: porcelain, marble, granite and some composites may shatter or crack unless a specialty drill bit and proper technique are used.
- Supplies needed: common hand tools and perhaps an electric drill; Teflon thread paste or tape; extra 1/4″ plastic tubing for longer runs and cable ties to dress up the job; a basin wrench to reach up to faucet nuts under the sink; flashlight; wall or cabinet anchor screw hardware.
Step-by-Step: How to Install the RO System
- First install the faucet (often the most difficult part of the project) on or near the sink. A basin wrench often comes in handy here.
- Run 1/4″ tubing from the faucet to where the filter array will be spotted.
- Mount the filter array where desired. Place a drip pan under it to catch inevitable small leaks.
- Place the storage tank in desired location.
- Connect the feed water valve to a cold (
not hot!) water line and run tubing to the filter array.
- Run a water discharge line from the filter array to a floor drain or utility sink; or into a sink drainpipe above the trap via a saddle usually supplied in RO “kits.”
- Connect the storage tank to the filter array.
- Check all hoses and fittings per the instruction manual. With the faucet open and the valve on the storage tank closed, open the feed water valve. Recheck fittings and eliminate leaks.
- When water flows from the faucet, close it, open the storage tank valve, and let the system “charge” for several hours. When clean water has filled the tank the system usually shuts off automatically. Charging is complete when water stops flowing from the discharge tube.
- Purge the system: open the faucet and let the water run down the drain until only a dribble emerges. This step rids the system of any residual debris.
- Close the faucet and let the system recharge. Enjoy clean water!
How To Maintain the Reverse Osmosis System
Except for the osmotic membrane, which lasts two to three years, change out filters approximately every 6 months or 6,000 gallons. The stage 1 paper prefilter usually fouls faster than the others. To save money, obtain an extra filter and clean the dirty one instead of replacing it with a new one.
Cost Per Gallon of RO Water
An average 75 GPD system and replacement filters to last 5 years run about $300 to $400. Altogether, including equipment amortization and water (RO systems “waste” 2 to 4 gallons for each gallon of purified water), it costs 5 – 15 cents per drinkable gallon.
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