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Water purifiers or water filters (NOT bio filters) - How they work.

The vast majority of us depend on tap water as the source of the pond water in which we keep our koi. 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 mains water available in the EU (yet the sales of bottled water have never been stronger!). At that same time, we are told that the quality of our tap water is the underlying (and often overlooked) cause of many chronic health problems in our koi.

The fact is, both points of view are equally valid, as koi and other captive ornamental fish species are far more demanding when considering the purity of tap water than we are. Firstly, it is standard practice to add certain additives to water to make it safe or better for human consumption (chlorine, fluoride) where as our fish demand water with as few additives as possible. Our water companies have a legal responsibility to provide us with water that is safe to drink. They do not however have the same duty of care for our fishs requirements, and it is our responsibility to take what is suitable for humans, and with a purifier, make it more suitable for our koi.

While our drinking water quality has been improved over years, by tighter standards and policing, we have also seen the number, diversity and complexity of water purifiers also increase. This is because we now have the means of providing our koi with what they require, via our tap water. Technology and understanding of how water purifiers operate has increased, and so too has their throughput and performance, while their cost has moved in the opposite direction, making them more widely available.

We have also demanded better control over the water in which we keep our koi. It is arguably possible to trace the order in which we as koi keepers have challenged and overcome our ponds water quality limitations. These started in the pond, through the use of ever more efficient mechanical and biological filtration to service and process the water as it is recirculated in our pond. And as each water quality challenge within our pond has been overcome, we have traced our remaining and unsolved water quality problems and challenges back to the tap in the quest to provide our koi with a higher overall quality of water. In other words, by using water purifiers, we are trying to eradicate the one remaining uncontrollable risk to the health of our koi - the overall quality of our source water.

As we start to examine and investigate how water purifiers work to produce a better quality of source water, we will have to start thinking at the molecular level, which Im afraid will involve some chemistry. But if I can understand it then so can you!

It is also worth saying that when examining the market for water purifiers, you may be bombarded by various claims and counter-claims. The purpose of this article is not to verify these claims, but to explain how water purifiers work. Furthermore, there is a great deal of commercially sensitive information that manufacturers wish to keep confidential. So rather than explain how individual makes of purifier function, we will look at the chemistry that purifiers exploit in order to produce purer water. Different purifiers will utilise this chemistry in different ways, and it is these different modes of action upon which the various commercial claims are based for each unit on the market.

Time for a little chemistry.

Water and the solutes dissolved within it are made up of ions.

Ions are atoms that are electrically charged and are commonly the building blocks for other molecules. The charge may be positive or negative, depending on the type of ion, with every ion having a charge which cannot change. In electrolysis, which involves the use of two oppositely charged electrodes (negative and positive) to separate out ions in a solution, the positive electrode is called the anode and the negative electrode is called the cathode. Consequently, negatively charged ions are attracted to the positively charged anode (and are therefore called anions), where as the positively charged cations are attracted to the negative cathode.

For example, if salt (sodium chloride) is put into water, it dissolves and then dissociates into two separate ions - a positive sodium ion (or cation, Na+), and a negatively charged chloride ion (or anion, Cl-). So in solution, sodium chloride no longer exists and its ions are free to move and combine with other ions of an equal and opposite charge.

Consequently, all ions can be split into two groups; the positively charged cations such as calcium, magnesium, sodium, iron (all metals) and the negatively charged anions such as bicarbonate, carbonate, chloride, sulphate, nitrate etc.

Fortunately, the vast majority of impurities and inconsistencies between our tap water and that of pure water are due to an excess of specific ions. And as these ions will either have a negative or positive charge, with a little applied chemistry, we can target and remove these offending ions using water purifiers.

How this chemistry is applied to a water purifier.

There are several similarities between how both a pond filter and a tap water purifier function. Both systems are a function of the quality and type of media that is used, as well as sequence in which the media are placed, together with the throughput or flow rate of water. Just as it is key for our mechanical filtration to precede our biological chambers to prevent them from blocking up and thus affecting their performance, so too is a purifiers first media mechanical in its function. On the face of it, tap water is crystal clear and free from suspended particulate matter. But because we want our subsequent purifier media to the act long term at the molecular level, we must ensure that any microscopic debris is prevented from impacting on the purifying media later on. A typical first cartridge will trap particles down to 20 microns. These usually take the appearance of at tightly-bound spool of white wool. But take a look at it after only a month or so and it will have a muddy red or brown appearance, largely due to the interception of iron. These pre-filters will have to be replaced periodically.

Carbon filters

Carbon filters are the next to process the water. Some of the more compact purifier systems use activated carbon filters as the first filter, carrying out the role of a mechanical pre-filter as well. There are many different grades of carbon, ranging from granulated activated carbon (GAC) to ultra compact coconut shell carbon. If this filter is used to filter out sediments, then iron and even calcium will plug up the microscopic areas of adsorption, shortening its effective life considerably. The carbon filter removes a host of contaminants such as chlorine, pesticides, herbicides and other organic materials that could not be removed by ion exchange in the later cartridges.

What is the difference between absorption and adsorption?

A sponge absorbs water into the inside of its porous structure. Ion exchange resins are not porous and so we describe the action by which they attract and retain ions on to their surface as adsorption.

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 its 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.


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