REVERSE OSMOSIS WATER FILTRATION THE BACKGROUND STORY

Recently much concern has been raised by fishkeepers with regard to tap water quality. The basic problem is that tap water is designed and legislated to be fit for human consumption not for fish.  In many cases this tighter legislation has resulted in perfectly fish-acceptable water being made fish unfriendly by the addition of chemicals such as chloramine to make it safe for humans.   Most traditional tap water conditioners are unable to cope with these new chemicals and so fishkeepers are turning to RO purification as a solution.

What is “RO” ? 

RO stands for reverse osmosis, it is a relatively new process having only been discovered in 1960 by Dr. Sourirajan at UCLA and finally scaled down to a domestic product in the 1980’s. 

The “reverse” part of the name is due to the fact that this process is the opposite of what normally happens in nature. The osmosis part of the name refers to what happens when two fluids are placed on either side of a semi-permeable membrane (fig 1).  In normal osmosis if pure water is placed on one side of a semi-permeable membrane and contaminated water on the other then water flows through the membrane from the pure side to the impure side.  In this way the water content of both fluids is evened up.

In RO the pure fluid is literally squeezed out of the contaminated fluid by applying pressure (fig 2).  This means that the impure fluid is made even more impure and the water content of both fluids is pushed further away from equal, the opposite of what would happen in nature.  The higher the driving pressure, the larger the force through the membrane and the more water output will be achieved from a given membrane.  This is why RO doesn’t work well in areas of low water pressure.

Reverse osmosis is the finest form of filtration known and it is capable of removing particles from a solution at the molecular level.  In practice this means that an RO can remove salts, sugars, proteins, particles, dyes, bacteria and many other impurities.  In fact the pores in an RO membrane are so fine that no-one has yet seen them even with the most sophisticated microscopes, but it is estimated that they are 1 nanometer in diameter.  To save space in the RO machine the flat sheet of RO membrane is wound around a perforated central tube to form a spiral.   In this way the waste water effectively flows straight past the membrane carrying away the wastes whilst the pure water travels through the membrane to be collected in the central tube.

The heart of an RO machine is the membrane (shown above).

This membrane is semi-permeable, allowing only pure water to pass through it, while rejecting the contaminants that remain. Most reverse osmosis technology uses a process known as crossflow to allow the membrane to continually clean itself.  As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected contaminants away from the membrane surface.  

This cross-flow system is why RO’s produce both pure and waste water, the pure water is that which goes through the membrane and the waste water is that which is used to flush the surface of the membrane.  The ratio of these two flows is set by the flow-restrictor and it determines both the quality and quantity of the water that the RO produces.  If a small flow-restrictor is used then the membrane produces a lot of water but of a lower quality and the membrane fouls more quickly.  This technique is often used in “bare-bones” type units to squeeze the maximum performance out of a small (hence cheap) membrane.  If a larger flow-restrictor is used then less water is made but it is of a higher quality and the membrane is far less prone to fouling.  As standard our RO’s come fitted with a larger than normal  flow-restrictor to optimise quality and longevity.

MEMBRANE MATERIALS:

RO membranes fall into two main types cellulose acetate (CA or CTA) and polyamide polymer (TFC or PA). The CTA membranes tolerate chlorine at levels used for microbial control, while even low levels of chlorine will destroy TFC membranes, which is why their pre-filters always incorporate activated carbon. Despite this one disadvantage TFC membranes are generally regarded as better than CTA membranes.  They produce both more water and purer water than CTA membranes and tolerate a wider range of pH and temperature than CTA membranes.  For this reason we only use TFC membranes in our RO’s.

PRE-TREATMENT:

Almost as important as membrane choice is the pre-filtration of the feedwater.  We use standard 10” pre-filters for or RO’s as these are readily and cheaply available from a wide range of manufactures.  We recommend that you use the pre-filter cartridges that we specify as we have chosen them for good reasons:

Particulate pre-filter: We use a Purtrex cartridge as these are made from 100% melt blown polypropylene fibres with no added binder.  This means that these cartridges do not shed fibres or release binder chemicals into the water and are fully FDA approved for food and drug applications.  This method of construction also means that the cartridge gets finer and finer the deeper in the dirt tries to go.  In this way the whole depth of the filter is used rather than just the surface and the cartridges can last over twice as long as a conventional string wound cartridge.

Carbon pre-filter: We use the EP carbon cartridges which have twice the life of normal carbon cartridges and are capable of removing 2ppm chlorine from over 5,000 gallons of water.  This makes our RO units some of the most economical to run as cartridges need only be changed annually.

Factors affecting RO output and Old Wives tales:

Why don’t I get 35gpd from my 35gpd RO?

20% of this can be explained by the fact that RO membranes are rated in US gallons (which are only 3.75 litres) rather than UK gallons, which are 4.5 litres.  In other words 35 US gallons is only about 28 UK gallons.

The other factor is the test conditions, which are usually given as 25°C feed water at 100psi.  Needless to say the chances of encountering 25°C tap water in the UK are slim to say the least!  In general the colder the feed water the more viscous it is and so the harder it is to push it through an RO membrane.  The UK tap water pressure is usually only about 50 to 60psi rather than 100psi as used in the tests. 

As a rule of thumb if you take the standard test results for output and halve them you will not go too far wrong.

Why is there no oxygen in RO water meaning that it needs to be well aerated before use?

Actually RO water has more oxygen in it than tap water!  The reason for this is that oxygen crosses RO membranes very easily and winds up concentrated in the RO water and depleted in the wastewater. 

The often stated need for aeration has far more to do with CO2 and pH than oxygen.

Can I turn my RO off or does it need to run all the time?

Yes.  All of our RO’s use a grade of TFC membrane that is able to withstand intermittent use.  You are free to turn the RO off for periods of up to two weeks without any problems occurring.

Can I use the wastewater for anything?

Yes.  The wastewater is fine for watering plants and gardens.  Washing the car or for cleaning uses.

Why does the water output drop off over time?

As the pre-filters remove particulates from the tap water it is inevitable that they will become blocked.  As this happens the pressure supplied to the membrane falls and so the water output drops off.  This is why with typical levels of silt in the tap water the particulate pre-filter should be changed at least on an annual basis.  The carbon pre-filter becomes chemically exhausted regardless of the level of silt in the tap water and so it should be changed every 3,000 gallons even if the particulate pre-filter is still clean.

If the pre-filters are blocked the overall ratio of waste water to pure water remains virtually the same but the overall level of each falls.

If on the other hand the amount of waste water is found to increase in the face of falling pure water output it is likely that the membrane is fouled.

Membrane fouling occurs when materials from the feed stream collect on or near the membrane surface and restrict water permeation. Fouling may occur as layers of deposition on the surface of the membrane (cake fouling), a hardened layer on the membrane surface (scale), particle insertion into the pore channel or entrance (pore blockage), or chemical attachment of particles to the membrane (adsorption).

In general this takes about 5 years to occur.  If the tap water is very dirty you may find that the membrane fouls very quickly.  In this case we would recommend extra pre-filtration in the form of our sediment removal kit.

TYPICAL OSMONICS MEMBRANE % REJECTIONS

 Our thanks to our suppliers Osmonics for the use of their figures and pictures www.osmonics.com