Maintaining good water quality is a foregone conclusion in fishkeeping, but it cannot be stressed long and hard enough when it comes to keeping corals. Perhaps it is not surprising then that some enthusiasts adopt almost clinical levels of cleanliness in their aquariums, with no detectable impurities in the water.
Where it all begins
Due to difficulties in obtaining unpolluted natural seawater, many marine keepers use commercial sea-salt mixes that are mixed with local tapwater. The quality of this artificial seawater is dependent on the purity of tapwater used.
Although tapwater is safe to drink, it still contains a range of impurities. The most obvious are chlorine and chloramines, but is also likely to contain nitrate, phosphates and other organic components, heavy metals and toxic elements.
Acid rain can further aggravate the situation by lowering pH, often to below 6.0. At such low levels, metallic compounds like aluminium, copper and zinc – all of which are extremely toxic in the coral reef aquarium – can easily become dissolved in the supply. To counter this, many marine keepers rely on the high filtering efficiency of reverse osmosis (RO) units to prepare tapwater for use in their aquariums.
To learn about the latest position regarding RO technology, I approached Paul Oglesby, of Flowtech Aquarium Systems, an expert in reverse osmosis systems who has started to manufacture a new range of RO units based mainly on components from the USA.
The ideal RO unit
I asked Paul what we should be looking for in the ideal form of RO unit for a marine hobbyist.
He replied that as RO technology has been mainly developed in the USA to produce very high standards of drinking water, it is suitable for use with marine aquaria.
“The ideal RO unit at present is a four-pod system comprising a sediment filter, carbon filter, RO membrane and a post de-ionising mixing bed resin filter. The water enters the RO unit through the sediment filter cartridge, sometimes called a particle filter, which removes particulate contamination from the feed water, ie the water from the mains supply.
“A wide range of particulate contaminants such as soils and other sediments, rust, etc, are usually present in the mains supply, which can be harmful and impair the performance of the RO unit at later stages of the filtration process.
“The second stage is a carbon membrane cartridge which protects the highly sensitive main RO membrane as it is easily damaged by contact with chemicals such as chlorine and chloramines. The carbon also removes other pretty volatile organic chemicals.”
He went on to say that all carbon filter materials have specified micron ratings, and care is necessary to ensure the RO unit is fitted with filtration to the correct specification.
Carbon cartridges loaded with granular activated carbon (GAC) or powdered activated carbon (PAC) are not as effective as those using extruded carbon block (ECB).
ECB, he explained, is a superior material and prevents channelling. GAC and PAC types contain carbon in loose media form and water can easily channel and bypass areas as it finds the path of least resistance.
“Extruded carbon allows the water to flow easily and evenly through, and the radial design of the cartridge assists the process,” he said. “ECB membranes also come with a rating of around 10 microns, which is easily capable of filtering out material down to bacteria size.”
Cheaper models donï¿½t usually incorporate ECB cartridges. This is a false economy as the carbon cartridges in cheaper units have a shorter life and there is also a big sacrifice in effectiveness terms.
Moving on, Paul said that the water next flows through the main RO membrane. This is a semi-permeable material rolled onto a spindle and contains millions of tiny holes. These are so small that they are measured in a microscopic unit of measurement called angstroms. The sheets are also coated with a layer of de-ionising resins.
The quality of this membrane varies, and with poorer ones, phosphates may easily pass through.
Another feature of inferior quality RO membranes is the water rejection ratio, which is the amount of waste against the usable water produced. Inferior membranes can have a rejection ratio of 5:1. This is uneconomical and slow, producing less than 25 l. in 24 hours. A good quality membrane should have a ratio nearer to 2:1 and deliver around 200 l. in 24 hours at a standard mains pressure of 45psi.
RO units should have a backflush facility to allow any particulate matter on the membrane to be flushed away, and there should be a check valve on the water feed to protect the membrane from back pressure and to avoid back siphoning of water or air when water is connected to an open reservoir.
“The final stage, the post de-ionising filter cartridge, is optional but highly recommended,” Paul continued. “It’s employed basically to mop up anything that the RO membrane may have missed and ensure product water is really pure.
“Various types of de-ionising filter are available. We offer a wide range of mixing bed cartridges and another with the same range, but the added capability of removing silicates.
“RO membranes typically have poor rejection ratios for silicate and silica (some as low as 80%), and where silicates are present in large quantities in the mains supply, the greater capability of this type of cartridge is worth considering.”
Maintaining an RO unit
Modern designs are reasonably trouble free, said Paul. It’s now no longer necessary to run water continuously through a unit to prevent damage to the membrane, and units can be opened and closed as needs arise. In fact, the membrane can hold a large amount of water when shut down, so it’s advisable to run the unit for 15 minutes or so to flush out any water standing in the system.
Maintenance involves the regular replacement of cartridges. Paul said it was possible with the sediment filter to monitor its life by the colour of the cartridge. When it turns a light coffee, it is too clogged for effective use. This can occur in between three and six months if demand is high, but the average marine aquarium with low demand can have a longer life.
“Most extruded carbon cartridges will process 2000-6000 gal. of water, depending on the quality and specification. There is no indication when a carbon cartridge is exhausted, so the maker’s recommendations should be adhered to.
“As all of the water passing through the unit is processed through the carbon membrane – both product and waste water, calculations must be based upon total throughput.
“The de-ionising filter cartridge is not subject to the same problem as only product water from the RO membrane is processed. It can only process around 1500 gal., but as it’s not worked as hard, it’s effective for a period similar to the other two.
“The RO membrane will have a lifespan of perhaps a year in heavy-use situation, and a continuing drop in volumes of product water after replacing all of the clear pod filter cartridges is a good indication that the RO membrane is reaching the end of its useful life.”
Although the initial investment may seem high, an RO unit is valuable in attaining the quality necessary to maintain a successful marine aquarium. There is no substitute for high water purity, whether the aquarium is a colourful collection of coral fishes or a full-blown reef aquarium system.
Problems that may arise when installing a new RO system are few. If you experience low levels of product water, this is usually due to low mains water pressure. Install a booster pump, which boosts the water pressure usually by around 15psi.
Optimum operation of an RO unit occurs at 45psi, and over 60psi, serious damage to the membrane can easily occur.
Attaching a booster pump to a mains supply already in excess of 45psi could lead to damaging consequences. Careful measurement of existing mains pressure is therefore necessary.
Poor removal of dissolved solids is not normally a problem with a new system, and the fault usually relates to the TDS meter that indicates the level of total dissolved solids. The cheaper in-line mounted TDS meters are factory calibrated and often give misleading results.
To get a true reading, use one of the hand-held TDS meters, which can be manually recalibrated for accurate readings.