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Drinking Water Treatment:Reverse Osmosis

Author:海讯集团    Time:2013/11/9

 Drinking Water Treatment:Reverse Osmosis


Abstact  Homeowners are increasingly concerned about contaminants in their water supply that may affect health or cause taste and odor problems. The reverse osmosis (RO) water treatment method has become popular for household drinking water treatment to resolve these concerns. This guide discusses the principles and process of RO treatment for household drinking water.Contaminants removed from water by reverse osmosis.


Reverse osmosis (RO) systems frequently are used to reduce the levels of total dissolved solids and suspended particles within water. These systems remove a variety of ions and metals as well as certain organic, inorganic and bacterial contaminants. Some contaminants treated effectively by RO are listed in Table I. This table is not an exhaustive list of contaminants that RO may remove, but rather lists those for which RO can be a practical treatment method for treating household drinking water. Most RO systems also include activated carbon (AC) filters and the carbon provides the treatment for some contaminants, as noted in the table. The RO membrane alone may not be an effective method for total removal of these contaminants, but a properly designed system may be effective in reducing these contaminants to safe levels. Contaminant removal by the system may vary depending on operating conditions and equipment. Refer to the equipment section of this guide for further explanation of activated carbon filters combined with RO.


Reverse osmosis can remove microorganisms. However, it is not recommended for that use (i.e., only coliform-free water should be fed to the system) because membrane deterioration can occur due to the bacteria, and contamination may occur through pinhole leaks.


     There are some contaminants not removed from water by RO systems. These include dissolved gases such as hydrogen sulfide, a common nuisance contaminant with characteristic rotten egg odor, which passes through the RO membrane. Some pesticides, solvents and other volatile organic chemicals (VOCs) not listed in the table above are not completely removed by RO. Refer to Extension Circular EC03-703, Drinking Water Treatment: An Overview for a discussion of possible water quality problems and appropriate treatments for these contaminants. Further information can be obtained from the appropriate treatment guide in the Drinking Water Treatment series (listed at the end of this publication).


The RO membrane's efficiency in reducing the amount of contaminant in the water depends on the contaminant concentration, chemical properties of the contaminant, the membrane type and condition, and operating conditions. Refer to the section in this guide on the RO process for explanation of these factors.


No one piece of treatment equipment manages all contaminants. All treatment methods have limitations and often situations require a combination of treatment processes to effectively treat the water. AC filtration and/or sediment filtration is commonly used in conjunction with RO to help remove silt particles or chlorine that may foul the RO membrane and also remove certain pesticides and organic solvents that the RO membrane does not remove. The section in this guide on equipment discusses this


The rejection rate is the percentage of contaminant that is not allowed to move through the membrane. A rejection rate is calculated for each contaminant separately, as well as for Total Dissolved Solids (TDS). For contaminants that cause health concerns, the rejection rate needs to be high enough to reduce the contaminant to a safe level. The quality of the incoming water, or feed water, is crucial here. For example, if the water supply contains nitrate at 40 mg/L, an RO membrane with 85% rejection would reject 40 x 0.85 = 34 mg/L nitrate, leaving 6 mg/L in the treated water. However, if the water supply contains 80 mg/L nitrates, an 85% rejection rate would reduce the nitrate concentration to 12 mg/L in the treated water. This nitrate level, even after RO treatment, is above the maximum contaminant level (MCL) of 10 mg/L nitrate set by the EPA.


Conclusion


Drinking water treatment using RO is one option for the homeowner to treat drinking water problems. RO is an effective method to reduce certain ions and metals, such as nitrate and arsenic. It also can remove certain pesticides, organic and inorganic compounds, though it is not effective for others. It is often used in combination with AC filtration. Selecting an RO system should be based on water analysis and assessment of the individual homeowner's needs and situation. Regular maintenance of the membrane and replacement of any filters/cartridges are critical factors in maintaining effectiveness and reducing bacterial contamination of the system. NSF and the WQA test and certify products and this certification and validation can help guide selection.