RO/DI stands for Reverse Osmosis and Deionization. The product is a multi-stage water filter, which takes in ordinary tap water and produces highly purified water.
Tap water often contains impurities that can cause problems. These may include phosphates, nitrates, chlorine, and various heavy metals. Excessive phosphate and nitrate levels can cause an algae bloom. Copper is often present in tap water due to leaching from pipes and is highly toxic to invertebrates. An RO/DI filter removes practically all of these impurities.
There are typically four stages in a RO/DI filter:
- Sediment filter
- Carbon block
- Reverse osmosis membrane
- Deionization resin
If there are less than four stages, something was left out. 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, 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 under pressure is forced through it. Molecules 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.
There are three types of RO membrane on the market:
- Cellulose Triacetate (CTA)
- Thin Film Composite (TFC)
- Poly-Vinyl Chloride (PVC)
The difference between the three concerns 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.
Reverse osmosis typically removes 90-98% of all the impurities of significance to the aquarist. If that is good enough for your needs, then you don’t need the DI stage. The use of RO by itself is certainly better than plain tap water and, in many cases, is perfectly adequate.
RO by itself might not be adequate if your tap water contains something that you want to reduce by more than 90-98%.
A DI stage by itself, without the other 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 without additional filtration. If you’re only going to buy either a RO or a DI, it would be best to choose the RO, unless you only need small amounts of purified water.
Duplicating stages can extend their life and improve their efficiency. For example, if you have two DI stages in series, one can be replaced when it’s exhausted without producing any impure water. If you have both a 5-micron sediment filter and a 1-micron filter, 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.
RO/DI capacities are measured in gallons per day (GPD), and typically fall within the 25-100 GPD range. The main difference between these units is the size of the RO membrane. Other differences are (a) the flow restrictor that determines how much waste water is produced, (b) the water gets less contact time in the carbon and DI stages in high-GPD units than low-GPD units, and (c) units larger than 35 GPD typically have welded-together membranes.
As a result of the membrane welding and the reduced carbon contact time, RO membranes larger than 35 GPD produce water that is slightly less pure. This primarily affects the life of the DI resin.
Most aquarists won’t use more than 25 GPD averaged over time. If you have a decent size storage container, 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 you need a lot of water in a hurry.
The advertised GPD values assume ideal conditions, notably optimum water pressure and temperature. The purity of your tap water also affects it. In other words, your mileage will vary.
An RO filter has two outputs: purified water and wastewater. A well-designed unit will have about 4X as much wastewater as purified water. The idea is that the impurities that don’t go through the membrane get flushed out with the wastewater.
There is nothing particularly wrong with the wastewater except for a slightly elevated dissolved solid content. It may actually be cleaner than your tap water because of the sediment and carbon filters. Feel free to water your plants with it.