Water purifiers or water filters - How they work. Thanks to Ben Helm for this article
The vast majority of us depend on tap water as the source of the pond water. This is the same tap water that we use to drink, cook, wash and bathe in, and our water companies claim that it is amongst the safest and highest quality yet the sales of bottled water have never been stronger!
What is the difference between absorption and adsorption?
A sponge absorbs water into the inside of it's porous structure. Ion exchange resins used for purifying water are not porous and so we describe the action by which they attract and retain ions on to their surface as adsorption. see article here for introduction to purifying water
There are several new generation adsorptive media that seek to replace or improve upon the purifying performance of activated carbon. Some are natural media, while others boast patented technology that enables them to adsorb most heavy metals and dissolved gases.
Next: Dealing with the dissolved ions.
After these first two media have worked on the raw tap water, there should only be a significant quantity of inorganic compounds remaining as ions which can then be removed using ion exchange technology.
What is ion exchange?
Ion exchange is a reversible chemical process in which the specific ion (such as sodium, Na+) are released from the insoluble solid medium (which is the ion exchange resin) and exchanged for none-desirable or target cations such as heavy metals. There are two types of ion exchange that can be caused to occur within a water purifier; that which removes target cations and that which removes target anions.
Ion exchange was first discovered in 1845 by an Englishman called Thompson who passed an ammonia-rich solution of manure through some ordinary garden soil, only to discover that the ammonia content of the liquid manure was greatly reduced. It was later shown that the soil contained fine particles of a natural material called zeolite which would even later be shown to have ion exchange properties. We still of course use zeolite today to remove unwanted ammonia from pond water. The water industry has not looked back since, but developed better and more efficient media to do the job of water purification.
How cation exchange works.
Cation exchange resins are usually made from an inert compound called polystyrene-divinylbenzene which is heated in its manufacturing process with concentrated sulphuric acid, causing a sulphonic group (SO3-) to be permanently fixed on to the structural chemistry of the resin beads. Because these sulphonic groups have a negative charge, they can be charged with positively charged ions (cations) typically sodium (Na+), potassium (K+) or even hydrogen (H+). When tap water containing dissolved cations (such as heavy metals) pass by the resin, then these are exchanged for, and trade places with the loosely held sodium ions on the resin. There will come a time when no more cations can be removed by a fully reacted resin which is then described as being 'exhausted', and which must then be replaced. The better a resin is protected by pre-filtration from fouling contaminants such as iron and chlorine (which can actually cause the resin polymer beads to disintegrate), the longer it's active life will be. Cation exchange resins will remove most metallic, positively charged ions such as barium, cadmium, copper, iron, manganese, zinc, calcium and magnesium.
Consequently, if the flow rate has been sufficiently slow and there has been sufficient active areas for cation exchange on the resin, then the levels of contaminant cations would have been reduced, and retained within the resin. All this leaves is the negatively charged contaminants or anions which must then be removed before the water can be used to fill the pond.
How anion exchange works.
Anion exchange units use a different resin that works in the opposite way to a cation exchange resin. It is charged with either chloride (Cl-) or hydroxyl (OH-) ions, which will then be released into the pond water in exchange for the less desirable contaminant anions. Anion exchange removes nitrates, sulphates and other negatively charged ions.
Mixed bed ion exchange.
As the term suggests, these ion exchange media contain both anionic and cationic exchange media, combined in one cartridge. To ensure that there is efficient purification, mixed bed ion exchange resins are usually used in a series of multiple cartridges, preceded as ever, by at least a carbon filter and at best an additional fine micron mechanical pre-filter.
In summary, water purification uses a series of complementary filtration processes that involve both mechanical and chemical means to produce 'purified' water. Our tap water can deliver quite unpredictable levels of ions and other 'contaminants' such as herbicides and pesticides, as well as chlorine and chloramine that would otherwise accumulate in our pond, causing koi chronic health problems. Different purifiers boast different qualitative and quantitative performance figures; a function of the different types and configurations of media used in these purifiers. The team of different media work to target and remove these contaminants, whether they are present in our tap water or not. It never ceases to amaze me that by using the innate 'electrical' features of the dissolved contaminants themselves, the manufactured media or resins can effectively remove them from tap water, with no power or electricity required to power them.
Water purifiers water filters adsorption, absorption ion exchange