Look After Your Membrane

High quality seawater has been said many times to be the number one requisite for success with a marine aquarium of whatever type. It doesn’t matter if the aquarium is fish only or reef, seawater quality counts, though it has to be said it usually counts more with a reef.

The aquarist spends time making sure the seawater quality is high by doing tests for various parameters and carrying out routine seawater changes to reduce pollutants, help replenish trace elements and generally freshen things up.

What about the other end of the operation? When the seawater is made up some aquarists use tap water. Others use reverse osmosis (RO) water, which is tap water that has been passed through an RO filter unit. Using RO water is highly recommended. In this way a large amount of the pollutants that are in tap water – and these vary from area to area – are removed. RO water is generally 95 to 98% pure.

The pollutants are removed by forcing tap water, using mains pressure, through a membrane. This membrane will only permit (nearly) pure water to pass. The rest is allowed to run to waste, unless the aquarist collects it for gardening purposes or whatever. The ratio of purified water to waste water is around 1 to 4 or 5.

The advantage of using an RO filter can be lost. RO filters are pretty simple and require little maintenance but nevertheless occasional attention must be given to ensure efficiency is maintained.

The heart of the RO filter is the membrane. This is where the good is separated from the bad, so it follows that failure here will negate the operation. So protection of the membrane is the order of the day.

There are two types of protection required. The first and probably most important is protection against chlorine. If chlorine has access to the membrane, the membrane will be damaged and will not remain properly functional. Water purity will fall and it is likely that a replacement membrane will be required. (The membrane is the expensive part of the RO filter and it may be worthwhile obtaining a complete new unit.) Protection against chlorine is achieved by using a carbon unit, which is supplied as part of the RO filter. During use there is nothing that needs to be done to the carbon unit. However, when the RO filter is first purchased a note should be made of the manufacturer’s recommendations as to how long the carbon unit will last. This is usually given in gallons and the number will be high. In the home situation the carbon unit will last a long time, so it is worthwhile either logging down the amount of each usage in gallons, or multiplying up the known weekly usage. The aquarist will of course know the amount required for seawater make-ups for routine changes, and also the amount required for evaporation top-ups. The two added together is the weekly usage though it might vary a little. If the manufacturer’s recommendation for gallons is divided by the weekly usage, the answer is the life of the carbon unit in weeks. Once the end of this period is being approached a new carbon unit should be ordered.

The second protection needed is against detritus. There is detritus in tap water though hopefully there isn’t much. Many manufacturers recommend cleaning the detritus filter at intervals of X gallons, and this is all that is normally needed. Looked after, the detritus filter should last the life of the RO unit overall.

Though some more expensive RO filters use more sophisticated methods, an occasional check of the quality of the RO water being produced is worthwhile. A check for nitrate and phosphate is usually sufficient. The test kits should be suitable for fresh water of course.

Maintaining the efficiency of the RO filter helps the aquarist achieve from the start the conditions required for success.


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  1. Another consideration is the size of your membrance. Most RO membranes can be upgraded from 30-35 gallons per day to a membrane that can make 75 to 100 gpd for very little cost.

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