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

by Curtis Grossi | Last Updated: August 6, 2020

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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

Disadvantages Of Reverse Osmosis

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).

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.