Monday, December 27, 2010

Limitations of Reverse Osmosis for Home Use

Before you buy a treatment system, you need to know the actual characteristics of your water. The test will identify the bacteria and level of minerals that are present. Proper interpretation of the test results will help determine whether treatment is needed and what type of system or systems to consider. The intended use of the water (drinking only, drinking and cooking, laundry, or all household uses) is essential to determine the what treatment is needed and the type of system to select. There is no single “best” treatment for home use, only treatment types appropriate for certain problems. The water treatment the industry has expanded to marketing treatment systems designed treat (or at least sold to treat ) contaminants that may pose a health hazards. Unfortunately, the industry is inconsistent in the skill and knowledge of the companies and their employees and many of the systems installed are inappropriate, unnecessary or have side effects that create other problems. The free in-home water testing provided by water treatment companies is very limited in scope. The only things that they can test for in the in-home tests are hardness, pH, iron and sulfur. In addition, the sensitivity and accuracy of the tests can be limited. Analysis for organics and bacterial contaminants must be performed in a certified laboratory.
Reverse osmosis systems can be used to reduce the levels of total dissolved solids and suspended matter in drinking water. The principal uses of reverse osmosis in are for the reduction of high levels of nitrate, lead, mercury, arsenic, cadmium, sulfate, sodium and total dissolved solids. Removal effectiveness depends on the contaminant and its concentration, the membrane selected, the water pressure and proper installation. Proper selection of the membrane and pressure is essential when selecting a reverse osmosis system. The membrane must be selected based on complete water analysis otherwise the entire system might be useless. In addition, reverse osmosis systems require regular maintenance and monitoring to continue to function properly over an extended period of time. Reverse osmosis has been shown to remove 83%-92% of nitrates from drinking water in both field and laboratory test. This is probably the most appropriate use of reverse osmosis systems.
I am not a fan of these systems in many applications. They are often sold as (very expensive) accessory item to solve the taste and sodium problem created when a whole house water softener is installed or for feared problems without proper testing. Reverse osmosis systems use a lot of water. They recover only 5 to 15 percent of the water entering the system. The remainder is discharged as waste water. Because waste water carries with it the rejected contaminants, methods to re-cover this water are not practical for household systems. Waste water is typically connected to the house drains and will add to the load on the household septic system. A reverse osmosis system delivering 5 gallons of treated water per day may discharge 40 to 90 gallons of waste water per day to the septic system. This is a significant additional load and could impact the life and functioning of your septic system.
Effectiveness of reverse osmosis system depends on initial levels of contamination, membrane size and type and water pressure. The application of pressure reverses the natural flow of the flow of water in osmosis from high concentration so that water passes from a more concentrated solution to a more dilute solution through a semi-permeable membrane. Reverse osmosis systems incorporate pre and post-filters along with the membrane itself in order for a reverse osmosis system to function properly. It is common to have a whole house filter system utilizing activated carbon installed in series with the reverse osmosis system. In addition, because contaminants are removed by forcing water through a membrane, the membrane requires regular maintenance and cleaning. Reverse osmosis systems are normally used to treat only drinking and cooking water supplies and are often installed under the kitchen sink and requires a permanent connection to an existing water pipe. The filter water is dispensed through the existing sink faucet or a separate tap. Reverse osmosis systems are never not appropriate for treating water supplies that are contaminated by coliform bacteria (neither nuisance nor fecal) because they do not remove bacteria.
Reverse osmosis units on the market range in cost from $300 to $3000 and vary in quality and effectiveness. Homes on well water need to purchase low pressure units which are slightly more expensive than the systems designed for municipal water. The size and membrane type are one of the factors that will determine cost. Replacement membranes cost $100 to $200 and filter cartridges around $50. Reverse osmosis is a proven technology that has been used successfully on a commercial basis most famously for removing salt from seawater. Household reverse osmosis systems typically deliver small amounts (2 to 10 gallons per day) of treated water and waste 7 to 20 times the amount of water treated. Reverse osmosis systems can remove many inorganic contaminants from household drinking water supplies including arsenic, sodium and nitrate. The removal effectiveness depends on the contaminant and its concentration, the membrane selected, the water pressure and proper installation and maintenance.

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