Reverse Osmosis Process - What does it achieve?
Reverse osmosis is a process in which the incoming (dirty) water is forced through a special cellophane-like RO membrane under elevated pressure. The hole size in an RO membrane is less than 0.0001 microns (1 Angstrohms).
Water (H 2 O) happens to be nature's smallest liquid molecule at room temperature and it passes through the minutest holes in the RO membrane, while all other larger molecules are stopped. The dirty water left behind is drained off at a constant rate by a capillary waste flow controller. In small RO systems, the ratio of dirty to RO water is set between 3-5 to one.
There are two types of RO membranes: Thin Film Composite (TFC) and Chlorine Tolerant Acetate (CTA). TFC membrane generally lasts longer than CTA but cannot take chlorine. If source water is chlorinated, TFC follows a carbon stage which reduces chlorine. Small RO systems range between 3- and 5-stages of filtration.
Reference on reverse osmosis filter - North Dakota State University, June 1992
Reverse Osmosis (RO) removes substantial amounts of most foreign substance in water other than oxygen. Mechanical filters and activated carbon filters almost always accompany RO membrane. First, the mechanical filter (1) removes dirt, sediment, rusts, and such that would otherwise clog the membrane. The RO (2) filter is next. Pressurized water storage container (3) holds RO water. Activated carbon filter (4) further reduces foreign matters that pass though the membrane. Separate water tap (5) dispenses the processed water. Reverse Osmosis uses large amounts of water. Typically, about 75% or more gets discarded together with the contaminants.
Source water quality requirements - not that strict for drinking water RO filter
- Water temp: 85F max
- TDS: 2,000 ppm max
- Iron tolerance: 0.5 ppm max
- Hydrogen sulfide: must be removed
- Silica tolerance: less than 125 ppm, antiscalant should be considered for over 75 ppm
- Turbidity: must be removed
- Hardness: over 10 gpg should be softened
- pH range: 3-11
Water outside the above limits will shorten the natural life of a membrane, large and small. Hence, source water is pretreated for large production RO systems where membranes are expensive and numerous. In contrast, it is seldom necessary to pretreat for the smaller drinking/cooking RO models listed below with a production rate of, say, 100 GPD or less. The reason is that waste water is drained at 2-4 times the volume of RO water so that the membrane chamber is kept relatively clean, and a membrane is not costly. This kind of water wastage is not practical in large GPD RO systems.
Municipal water should come in the allowable quality range. A simple source water analysis is recommended for individual well water of unknown quality. GPD (gallons per day) rating of a membrane is defined at 77 degrees Fahrenheit, 25 degrees Celcius, and at 60 psi differencial pressure between the source and permeate water.
At lower source water temperatures, RO production decreases and the GPD rating has to be divided by 1.19 at 68F (20C), by 1.47 at 59F (15C), by 1.89 at 50F (10C), by 2.58 at 41F (5C). Pressure effect is less noticeable down to 45 psi or so, but production will drop precipitously below 40 psi. You need a booster pump then. Top
What does an RO system reduce?
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