How Does Reverse Osmosis Work? What Is Not Removed By Reverse Osmosis?

RO Definition. Principle Of Reverse Osmosis.

Osmosis is defined by WordNet 3.0 as “the diffusion of molecules through a semipermeable membrane from a place of higher concentration to a place of lower concentration until the concentration on both sides is equal”.

One of the best-known natural occurrences of osmosis is the case of plant roots absorbing nutrients from water in the soil surrounding them – the fluid in the root has less nutrients than the surrounding water, and the walls of the plant root are semi-permeable, allowing nutrients pass through them from the outside.

Reverse Osmosis Diagram

Reverse Osmosis, in contrast, is defined as “a technique for purifying water, in which pressure is applied to force liquid through a semipermeable membrane in the opposite direction to that in normal osmosis” (Collins English Dictionary, 12th edition, 2014).

In this case, water containing inorganic dissolved solids - known as solutes - is forced through a filter known as a semipermeable membrane or a thin film composite (TFC) that is fine enough to hold back the solutes.

In other words, reverse osmosis (RO) is a form of extreme filtering of water, using a microscopically fine ‘semi-permeable membrane’ filter, with pores (holes) small enough to prevent substances at molecular level from passing through it.

Only molecules similar in size to, or smaller than, water molecules will be able to pass through the filter, resulting in almost pure water emerging from the RO system. In addition, the water may be passed through several membrane filters in order to increase the amount of impurities that are removed from the water.

How Does Reverse Osmosis Work?​

RO works under pressure, so that untreated water is pushed through the pores of the membrane filter with sufficient force to ensure that there is a flow of water through the minute holes in the membrane.

If household water pressure is less than 30 to 50 psi (2 to 3.4 bar) this may be insufficient to operate an RO system, and a booster pump will need to be incorporated into the design.

RO systems generally include a pre-filter, usually a 1-micron or 5-micron sediment filter, to remove larger particles of impurities from the water being purified, so that the membrane filter does not clog up.

One or more further filters such as a granular activated carbon (GAC) filter impregnated with metallic silver nano-particles will generally be included to remove from the water any organic contaminants that may cause odors and odd tastes, as well as chlorine and other chemicals.

The reverse osmosis stage itself uses a membrane filter with pore sizes in the order of 0.0001 micron to remove impurities including lead, copper, chromium, calcium, iron, magnesium, manganese, sodium and a number of other totally dissolved solids (TDS).

Since a filter that has an absolute pore size of 1 micron or less will be needed to remove parasitic cysts, the RO membrane filter will remove parasitic cysts such as cryptosporidia and giardia, reducing the chances of gastrointestinal infection.

Good quality RO systems will include a post-filter for final ‘polishing’ of the water to remove any remaining traces of odors and tastes. When filtering water from a well or borehole water source, it is highly recommended that a UV filter is included as the last stage of the water purification process.

Most RO systems have an automatic flushing system to clear the membrane filter before it clogs up and the flow of water stops. The membrane filter will need to be replaced about every 24 months.

Advantages Of Reverse Osmosis

  • Since the RO filter removes lime-scale causing minerals calcium and magnesium, up to 99% according to some claims, the RO system can be considered a form of water softener.
  • Since it will also remove sodium, without adding any to the water as would be the case with an ion-exchange water softener, it will be suitable for people on a low sodium diet.
  • In fact, the water produced by the RO system will be extremely soft, and so some systems include an automatic function of adding back beneficial minerals to the water so that it will provide some nutrients when consumed.

Disadvantages Of Reverse Osmosis

  • One of the major drawbacks of RO is the amount of water that is wasted in backwashing the membrane filter in order to clear it of the accumulation of minerals and organic and inorganic contaminants that it removes from the water system. Some RO systems generate 3 to 5 gallons of waste water for each one gallon of filtered water they produce, but the most efficient systems have reduced this to a 1:1 ratio – that is, one gallon of water is wasted for each gallon that is produced.
  • A further drawback is that most domestic RO systems are ‘under-counter’ models which deliver purified water to only one dedicated faucet, usually in the kitchen, which means that all other water in the household is not treated to the same degree. If the RO system is doubling as a water softener, this will not be helpful, since the hot water lines, where most lime-scale buildup occurs, will not be protected.

There are a few commercial RO systems, such as the iSpring RCS5T 500GPD RO system that will supply sufficient purified water for a household of 5 people with relatively hard water (up to 350ppm / 20gpg).

iSpring RCS5T 500 GPD High Flow

This is a tankless system, and, since the output flow of the RO system is only in the order of one third of a gallon per minute, it would be advisable to add a 500 gallon (1,890 liters) storage tank (see on Amazon) and booster pump to the system.

Note also that commercial systems do not deliver water instantaneously, another reason why a storage tank with its own booster pump would be beneficial.