Frequently Asked Questions On Reverse Osmosis


Today, more and more people in highly-urbanized places are choosing reverse osmosis (known also as R/O) to purify their water. However, there are people still in the dark about water purification, filtration, and the like. The following are the answers to some frequently-asked questions on reverse osmosis.

What is reverse osmosis?

A. Reverse osmosis is a process where water is stripped of minerals and other impurities by forcing it to pass through a semi-porous membrane using pressure. It is the opposite of osmosis, the natural process where water seeps through a semi-porous membrane where liquid is of higher concentration.

What comes out of the reverse osmosis process is fresh, clean water ready for use.

Q. How does industrial reverse osmosis differ from those units used in homes?

A. None, except that the materials used are of industrial strengths for bigger institutions. The big systems usually use spiral wound membranes in high pressure containers. These provide larger surface areas.

Q. Is any pretreatment required?

Ideally, yes. Water should have very low silt (solids) content to keep the membranes from plugging up. This is done by way of pre-filters that remove these solid sediments.

Other TDS (total dissolved solids) like chlorine have to be removed by active carbon filters because it attacks the main R/O membranes. Some, like calcium and magnesium, chokes the membranes, and have to be removed at pretreatment.

Q. How much pressure is required to purify water?

The pressure needed is dependent on the concentration of the sediments and salts on the feedwater (the water to be purified), which is in turn dependent on the pressure from the source.

Q. How pure will the water be?

Purity is determined by two things. One is the reject ratio of the membrane which is pegged at 92% up to 99.5%. The other is the type of TDS (total dissolved solids) present in the feedwater.

However, through length of use, the efficiency (and life span) of the membranes shortens, and leakages occur over time.

Q. How do I clean a system?

Water treated with a cleaning agent is re-circulated on the high pressure side of the system for an hour or so. Then, the membrane is flushed to drain.

Small systems will have to shut down during cleaning, but in larger systems, the individual banks of membranes can be cleaned one by one.

Q. How much maintenance is involved with a system?


A properly set system, with a good pre-treatment in place, usually needs a one-hour cleaning routine once a month. Pre-filters can be checked weekly.

Q. How much does it cost to run a reverse osmosis system?

The cost is dependent on three areas: power, materials, and labor. Check your area on power and labor costs. (Labor is usually low since the system is more or less automated.) Materials are more or less the same everywhere.

Q. What about bugs (bacteria) growing in the water?

Water storage in R/O systems is optionally passed through an UV sterilization system to kill bacteria, usually during cleaning. It is good to have the tanks black or opaque to prevent algae growth.



Q. How long will my reverse osmosis membranes last?

R/O membranes usually last many years. However, they slowly start leaking ions after a time, and have to be replaced once ion levels are not acceptable. Some membranes have been reported in continued use for 20 years.

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