Many marine and freshwater fishkeepers use reverse osmosis (RO) to produce high quality water. Dave Wolfenden explains how RO units work and how to use them efficiently and safely.
What is reverse osmosis?
Using Reverse Osmosis water is practically essential for success with marine aquariums and ever more important for many freshwater species like sensitive Caridina shrimp and acidic loving species (at least for most of the country who do not have soft water out of the tap). While you can often buy RO water from your local store, it is often easier, especially for bigger systems or those with multiple tank syndrome, to make your own at home.
Components of an RO unit
An RO unit has several parts to them and are often available as a complete kit. The installation of which, needn’t require the services of a professional plumber. A simple screw-in saddle tap can be fitted to the cold water mains pipe to supply the unit and a drain clamp can be fitted to any plastic domestic drainpipe to run away waste water. There are several components that comprise an RO unit, so let's look at the function of each one:
Membrane
The semi-permeable membrane acts as an ultra-fine filter, straining virtually all unwanted constituents from the mains supply and allowing only water molecules to pass through. This part is expensive and easily damaged, and some high-output units may have multiple membranes. It relies on extremely fine membranes, made from Thin Film Composite (TFC) materials to strain unwanted dissolved chemicals.
Flush valve
Some models incorporate a flush valve which bypasses the flow restrictor. This allows deposits to be washed from the membrane, improving efficiency, output and lifespan. Flush valves can be retrofitted to most systems.
Flow restrictor
This valve allows pressure to build up in the system — essential for reverse osmosis to take place —while allowing water through above a certain pressure, preventing the system leaking or blowing apart.
Product water outlet
'Product water' is 'cleaned' tapwater for the aquarium and an efficient RO unit can remove 90% or more of tapwater contaminants.
Regularly test it with a TDS (total dissolved solids) meter or a GH (general hardness) test kit to check if the unit is working correctly. Product water should have a TDS of 10ppm or less and a GH of zero.
Waste water outlet
The waste water has elevated levels of nitrates, phosphates, heavy metals, pesticides and other nasties and shouldn’t be used in the aquarium. Don’t waste any of this water though, as your garden plants will love it!
Pressure gauge
This allows the pressure of the mains water entering the membrane to be monitored. Insufficient pressure will reduce unit efficiency, or prevent it working altogether.
Not all units have a pressure gauge, but they can be retro fitted.
Prefilters
Placed before the membrane, these are essential. They remove sediments, chlorine and other components of mains water which would rapidly block and/or destroy the membrane.
What does an RO unit remove from our tap water?
Our tapwater is subject to strict quality guidelines, but, even so, contains substances our aquarium’s inhabitants wouldn’t like. Tapwater quality and chemistry varies tremendously from region to region, but we’ll often find harmful nitrates originating from agricultural run-off. UK tap water is legally allowed to contain 50 mg/l nitrate.
Phosphates and silicates may also be present, which also act as algal fertilisers promoting nuisance algae. Other chemicals may also be present, including heavy metals, which won’t be appreciated by sensitive invertebrates.
Reverse osmosis purifies tap water to remove such contaminants, including nitrates, phosphates and heavy metals such as copper, leaving only water. This needs to happen under pressure, as it is working against the osmotic gradient of the water — hence the name 'reverse' osmosis. Normally, water would pass across a membrane from a low to a high solute concentration, but, in the case of reverse osmosis, the opposite happens.
Performance and efficiency of reverse osmosis.
The ratio of product to waste water varies, depending on various factors, but expect to waste around 5 l/1.1 gal of water to produce 1 l/0.2 gal litre of product. Before the water can pass through the membrane, it needs to be pre-filtered to remove sediments and chlorine which would otherwise block and destroy the membrane reducing the efficiency. As such, it is important to regularly replace the pre-filter to ensure the membrane is protected.
The performance of any RO unit depends on several other factors too, notably the temperature and pressure of the mains water. Lower temperatures tend to decrease output and a minimum pressure of around 2.8 bar (40psi) is needed. Lower and it’s unlikely that much product water will be made and, in such cases, a booster pump will be necessary to increase the pressure.
Using reverse osmosis water
Using RO water is a doddle for marine aquarists. Simply use the product water to make up a salt mix while the pure RO can be used for freshwater top-ups to replace evaporation without needing additives. Many marine aquarists, and reefkeepers in particular, would consider an RO unit mandatory, as the levels of nitrate, phosphate and silicates in untreated mains water can lead to persistent problems with nuisance algae. If not already using RO for your marine system, seriously consider it!
Things aren’t as straightforward for freshwater aquarists and it’s important to appreciate that neat RO water can be lethal for fish. As it is nearly pure water, it should have zero kH (carbonate hardness) if has some kH, it's not RO! This means that it lacks buffering capacity, making it prone to drastic pH swings, although this can be an important part of using reverse osmosis water for experienced aquarists maintaining aquariums with very soft. Pure water is too sterile and potentially unstable for aquatic life, but can be a ‘blank canvas’ for chemical fine-tuning to create just the right conditions for the aquarium’s inhabitants.
The lack of minerals also plays havoc with fishes’ osmoregulatory systems, as well as creating a poor environment for plant growth as it lacks essential trace elements required. Whilst normal biological filtration requires carbonates (kH) in the water to process nitrogen wastes which as mentioned early, RO lacks.
So RO water needs to be remineralised and there are two approaches. The first involves adding a certain amount of mains tapwater, preferably to which a water conditioner has been added, to RO water to achieve the desired hardness and pH. The more refined method is using bottled or powdered remineralisation products.
Adding tap water to buffer RO
It may seem totally bizarre to add tap water to RO after we’ve just purified it. This method does, to a certain extent, dilute the contaminants in the tap water and this can work just fine for, say, soft water systems needing only a low proportion of it. However, it certainly makes less sense if, for example, very hard water is desired, necessitating lower proportions of RO water and larger quantities of tap water.
You are also adding, although at more dilute concentrations, the nitrates, phosphates and silicates that can feed algae back into the water as well. While these are important for plant growth, in environments with minimal plant life (blackwater conditions, barren fast flowing river style aquariums or typical rift lake cichlid tanks) they can cause many of the problems we try to avoid by using RO.
Remineralising reverse osmosis
A more elegant method of remineralising RO water — allowing for greater control over water chemistry — is using specific remineralising powders or liquids. The water needs to be prepared in a separate container, never mixed in the aquarium, and, with such products, it’s possible to create the ideal conditions for aquariums, ranging from soft, acidic river systems to extremely hard, alkaline rift lake biotopes.
Just make sure you select the right remineralisation product for the type of environment you are replicating!
TFC RO membranes need TLC
The TFC membranes in RO units make up a large proportion of the overall cost of the system. They are delicate and easily damaged, so look after them. As mentioned earlier, regular replacement of the pre-filters, plus regular flushing of the membrane, will help prolong its life. Another key thing to remember is that once an RO unit has been used for the first time, you can never let the membrane dry out, as this will render it useless and require replacing.
Can you get more pure than reverse osmosis?
Some aquarists opt for an additional deionising (DI) stage through which the product water flows before being used in the aquarium. Such systems are referred to as RO/DI. The DI resin removes additional quantities of impurities left behind following reverse osmosis, resulting in water virtually devoid of any contaminants.
For most aquarium applications, however, a standard RO-only system is perfectly adequate.
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