Reverse Osmosis

This small article was written by the proprietor of Water Works in the U.S. It gives an insight into the workings of a reverse osmosis/ deionising unit (ro/di). One of these units is probably overkill when keeping discus, but please read as the explanation is good.


RO/DI stands for Reverse Osmosis and deionization. The product is a multi-stage water filter, which takes ordinary water in and produces highly purified water.


Why do I need it?


Water often contains impurities that can cause problems when added to an aquarium. These may include phosphates, nitrates, chlorine, and various heavy metals. Phosphates and nitrates can cause algae blooms. Copper is often present in tap water due to leaching from pipes and is highly toxic to invertebrates. A RO/DI filter removes practically all of these impurities.


How does it work?


There are typically four stages in an RO/DI filter: sediment filter, carbon block, reverse osmosis membrane, and deionization resin. If there are less than four stages, something was left out (typically the DI stage). If there are more, something was duplicated. The sediment filter, typically a foam block, removes particles from the water. Its purpose is to prevent clogging of the carbon block and RO membrane. Good sediment filters will remove particles down to one micron or smaller. The carbon, typically a block of powdered activated carbon, filters out smaller particles (often down to 1/2 micron or smaller), adsorbs some dissolved compounds, and deactivates chlorine. The latter is the most important part: free chlorine in the water will destroy the RO membrane. The RO membrane is a semi-permeable thin film. Water is forced through it under pressure. Molecules which are larger/heavier than water (which is very small/light) penetrate the membrane less easily and tend to be left behind. The DI resin exchanges the remaining ions, removing them from the solution.


What are CTA, TFC, and PVC?


There are three types of RO membranes on the market: Cellulose Triacetate (CTA), Thin Film Composite (TFC), and Poly-Vinyl Chloride (PVC). Almost all of the membranes sold for aquarium use in the USA are TFC. PVC membranes are currently available only outside the US. The notable difference between these types is how they are affected by chlorine: CTA membranes require chlorine in the water to prevent them from rotting. TFC membranes are damaged by chlorine and must be protected from it. PVC membranes are impervious to both chlorine and bacteria. This FAQ assumes you're buying a TFC membrane.


Do I need a DI stage?


You can save some money by purchasing a three-stage filter lacking the DI stage. Reverse osmosis typically removes 90-98% of all the impurities of note. If that is good enough for your purposes, then the DI stage is not necessary. RO filtration by itself is certainly better than plain tap water and in many cases is perfectly adequate. RO filtration by itself is not adequate if your tap water contains undesirable elements that need to be reduced by more than 90-98%. For example, if there is 10 PPM of phosphates in your tap water, reducing it by 90% takes it to 1 PPM, which is still too high. To save money up front, a DI stage can be easily added to the system at a later date.


Can I use just DI?


A DI stage by itself (without the other RO filter stages) will produce water that is pretty much free of dissolved solids. However, DI resin is fairly expensive and will last only about 1/20th as long when used by itself. If you're only going to buy RO or DI, go for the RO unless only small amounts of purified water are needed. Do I need a ½ micron sediment filter? Opinions vary on whether filtering the input water finer than 5 microns has any value. Finer filtration presumably helps prevent clogging of the RO membrane, but there is a cost in terms of dollars and pressure loss. The pressure loss is particularly a problem if you have low water pressure. Each sediment and carbon stage should be finer than the one before it. For example, a 5-micron sediment filter in front of a 1-micron carbon block will work fine, but using a 1-micron sediment filter in front of a 5-micron carbon block is not advisable.


Where's the value in a 7-stage filter?


Duplicating stages can extend their life or improve their efficiency. For example, if there are two DI stages in series, one can be replaced when it's exhausted without producing any impure water. If both a 5- micron sediment filter and a 1-micron filter are used, they will take longer to clog up. If there are two carbon stages, there will be less chlorine attacking the TFC membrane. Whether the extra stages are worth the extra money is largely a matter of circumstance and opinion (they're more useful if you use a lot of water).


Do I care about GPD?


RO/DI capacities are measured in gallons per day (GPD), typically in the 25 -100 GPD range. The main difference between them is the size or permeability of the RO membrane. Other differences are:

(a) The flow restrictor that determines how much waste water is produced, which must match the membrane, and
(b) The water gets less contact time in the carbon and DI stages in high-GPD units than low-GPD units.


As the GPD rating increases, the purity of the water produced by the RO membrane declines. Membranes above 35 GPD are typically constructed by welding two smaller membranes, meaning there's a seam. 100 GPD membranes are typically more permeable, with a lower rejection rate. The DI stage will make up the difference by removing the remaining impurities but that affects the life of the DI resin. Most aquarists won't use more than 25 GPD averaged over time. If a decent size storage container is used, that size should be adequate. A higher GPD rating comes in handy, however, when filling a large tank for the first time or in emergencies when a lot of water is necessary in a hurry. The advertised GPD values assume ideal conditions, notably optimum water pressure (65 PSI) and temperature (70°F). The purity of your tap water also affects it. In other words, your mileage will vary.


What if I have chloramine in my water?


Some water agencies add chloramine (a mix of ammonia and chlorine) to disinfect drinking water. That's fine, except some carbon blocks are inadequate to neutralize chloramine, so it damages your TFC membrane. It can also pass right through an RO membrane and DI resin, yielding ammonia in the resultant "pure" water. This is particularly a problem with high-GPD units. To find out if you have chloramine in the water, check with your local water company. Chloramine use is particularly common in large municipalities. If chloramine is present in your water supply, this should be discussed with the vendor prior to purchasing a system. The vendor may recommend a second carbon stage, a "catalytic" type of carbon filter, or a lower-GPD unit. At the time of this writing, the single best solution is not yet clear and a combination may be required. In any case, don't trust a vendor who isn't familiar with the problem.


Why are there multiple outputs?


An RO filter has two outputs: purified water and wastewater. A well- designed unit will have about four times as much wastewater as purified water. The idea is that the impurities that don't go through the membrane are flushed out with the wastewater. There is nothing wrong with the wastewater except for a slightly elevated dissolved solid content. It may be cleaner than your tap water because of the sediment and carbon filters. Feel free to water your plants with it. An RO/DI filter may have separate outputs for the RO stage and the DI stage. The RO output can be used for drinking or other purposes that don't need highly purified water.


How do I hook it up?


The input line is connected to a cold water line. It can either be hooked up with a saddle valve that pierces your existing copper pipe, attached to your sink faucet with a special adapter (which is good for those in rental properties or apartments), or to a hose bib. The wastewater goes down the drain. If you have a PVC drainpipe, the waste line can be connected to it. You just need a drill and a saddle. The purified outputs go where you want them. A common approach is to feed the DI output through a float valve to a reservoir. When using a float valve system, an auto-shutoff valve is required to shut off the incoming water to the RO unit. Otherwise, water will continue to pass through the wastewater outlet, consuming water un-necessarily. Check with the vendor at the time of purchase to discuss this option. To make sure you have the correct adapters, installation problems should be discussed when purchasing the system.


How do I know when the filter needs servicing?


Sediment and carbon stages: If you have city water (with chlorine) the sediment and carbon stages should be replaced regularly. The rule of thumb is every six months. Alternately, a swimming pool chlorine test can be used: the carbon is OK if the test reads as zero. This is less critical if you have well water. If you have a pressure gauge you can tell when the sediment & carbon filters are clogged: the pressure will start to drop.

  1. RO membrane: There are two ways the RO membrane can fail. It can develop holes, allowing impurities through, or it can get clogged up. If the input pressure is OK but you're not getting the expected output, the membrane is probably clogged. If the TDS meter shows RO output above 10% of your tap water, it's developing holes. A RO membrane typically lasts 3-5 years.

  2. DI resin: The TDS reading on your DI output should read 0 or 1. The DI resin is exhausted when the reading starts to climb. Some DI resins change colour as they are exhausted. Note that the colour will probably change well before the DI resin really needs to be replaced - use the TDS reading to decide when to replace the resin.


Do I need a pressure gauge?


The gauge that comes with some RO units measures the pressure on the input side of the membrane (or on the waste side, before the flow restrictor, which will give the same reading). This allows you to tell if there is adequate line pressure and if the sediment & carbon stages are getting clogged. Optimum input pressure is in the 60-80 PSI range. Below about 40 PSI the unit will operate less efficiently. The units are typically not rated to operate above 80-90 PSI.


Do I care about temperature?


The GPD ratings are for room temperature (~70 ° F). Colder water travels more slowly through the membrane, which reduces the output. If a high- GPD unit is connected to a cold water line, that can be a problem. You want approximately 25' or 30' feet of tubing from the connection at the cold water running to the RO/DI unit. Fill a 5-gallon bucket with water, and coil the excess tubing in the bucket so it is submerged. Immerse a small aquarium heater set it to 78 degrees F. As the RO/DI unit kicks on, water in the tubing will be warmed up to 78 as well, since it processes rather slowly, and the membrane will be able to produce maximum output in the dead of winter.


Do I need a flush kit?


A flush kit allows periodic flushing of some water across the RO membrane, thereby removing some of the gunk that sticks to it. Regular flushing will extend the life of the membrane.


Do I need a booster pump?


The RO membrane works best when the input pressure is in the 60-80 PSI range. Lower pressure reduces the output and increases the ratio of waste to purified water. Below 40 PSI it will be greatly reduced. Below 30 PSI it won't work well at all. If your input pressure is less than about 40 PSI, it's advisable to consider getting a booster pump. Make sure there is a pressure cut off switch for the booster pump (connected to the RO output). Otherwise, it will run continuously.


Can I drink the purified water?


The RO output water is excellent for drinking. Most vendors offer a drinking water kit that includes a pressure tank, a small faucet which can be attached to your sink, and a post-filter for the drinking water. The post-filter supposedly improves the taste. A DI stage is not recommended for drinking water.


Should I buy a premium RO membrane?


Some of the name brands offer premium RO membranes that are claimed to remove a higher percentage of impurities. Assuming it's true, these will extend the life of the DI resin. For instance, if a "normal" RO membrane removes an average of 95% and a premium membrane removes 98%, the DI resin should last over twice as long. Whether that's worth the extra up-front cost, you can decide for yourself.