Water treatment and purification

by George J. Reclos

Water is the most essential part of every aquarium since it is the media that our fishes and plants live in and totally rely upon for their feeding and survival. However, unless you live by the lake or the sea that your fishes and plants live in, you have to use water that is not meant for them. Thus, you need to treat the water and make it as suitable as possible for the aquatic life you keep. What we need to know is what kind of water we start with in order to evaluate what we need to remove or add to it before using it in our tanks. Therefore, water treatment can be defined as any procedure or method used to alter the chemical composition or natural "behavior" of a water supply. Water supplies are classified as either surface water or groundwater. The majority of public or municipal water comes from surface water such as rivers, lakes, and reservoirs. The majority of private water supplies consist of groundwater pumped from wells. The quality of the latter may differ significantly from place to place, so you need an analysis of your particular water to evaluate which of these techniques may help you to improve its quality. If you use water from your well, you may go directly to the "On site - treatment" part.


Most municipal water found in a city or community today has been treated extensively. Specific water treatment methods and steps taken by municipalities to meet local, state, national, or international standards vary, but are categorized below.

Screen prefiltration
A coarse screen, usually 50 to 100 mesh (305 to 140 microns), at the intake point of a surface water supply, removes large particulate matter to protect downstream equipment from clogging, fouling, or being damaged.

Clarification is generally a multi-step process to reduce turbidity and suspended matter. Steps include the addition of chemical coagulants or pH-adjustment chemicals that react to form floc. The floc settles by gravity into settling tanks or is removed as the water percolates through a gravity filter. The clarification process effectively removes particles larger than 25 microns. The clarification process is not 100% efficient; therefore, water treated through clarification may still contain some suspended materials.


The addition of lime (CaO; Calcium oxide) and sodium carbonate (soda ash; Na2CO3) reduces the level of calcium and magnesium and is referred to as "lime softening." The purpose of lime softening is to precipitate calcium and magnesium hydroxides (hardness) and then clarify the water. The process is inexpensive but only marginally effective, usually producing water of 50 to 120 ppm (3 to 7 degrees GH) hardness. Warning: Lime treatment may result in water with very low GH (as low as 3) but a very high pH (over 8.0). Furthermore, water treated this way will be practically depleted of carbonates which means a KH of almost 0 (almost no buffering capacity). In this aspect it should be carefully used in systems with low KH values and a CO2 injection (pH may drop dramatically). On the plus side, it will not affect any well buffered tank even one established in the acidic range since the low buffering capacity of the added water will let the pH stay almost intact. There is also a lesson to be learned here by most hobbyists who believe that soft (or extremely soft) water also means acidic water. Those two coincide most of the time but it is not necessarily so. In many USA States aquarists have had nasty surprises because of lime treated water.

Disinfection is one of the most important steps to municipal water treatment. Usually, chlorine gas is fed into the supply after the water has been clarified and/or softened. The chlorine kills bacteria. In order to maintain the "kill potential", an excess of chlorine is fed into the supply to maintain a residual. The chlorine level must be constantly monitored to assure that no harmful levels of chloramines or chlorinated hydrocarbons develop. Needless to say the harmful levels are assumed for consumption by humans and not fishkeeping. The harmful level for fishes is much lower.

pH adjustment
Municipal waters may be adjusted to a pH of approximately 7.5 to 8.0 to prevent corrosion of water pipes, particularly to avoid dissolution of lead into the water supply. In the case of excessive alkalinity, the pH may be reduced by the addition of CO2.


After the water is delivered from the utility or the well, there are many on-site options for further treatment to meet specific end-use requirements.

Chemical addition

  • pH adjustment. Certain chemicals, membranes, ion exchange resins, and other materials are sensitive to specific pH conditions. An example of this is to prevent acid corrosion in boiler feed water by adjusting the pH so it is in between 8.3 to 9.0.
  • Dispersants. Dispersants are added when scaling may be expected due to concentration of specific ions in the stream. Dispersants disrupt the scale formation, preventing growth of precipitate crystals.
  • Sequestering (chelating) agents. Sequestering agents are used to prevent the negative effects of hardness, preventing the deposition of Ca, Mg, Fe, Mn, and Al. This has an additional advantage (or disadvantage) for the fish hobbyist. The metal ions will not precipitate from the water column, making them available to the life forms we keep, which are able to use the chelated complex and "extract" the ion for their own use. This is of particular interest to aqua gardeners, since chelated Calcium, Magnesium, Iron, and Manganese can be utilized by their plants. It should be noted that Ferric hydroxide ((FeO(OH)) could also be used in the purification of water. Ferric sodium edetate (C10H12FeN2-NaO8) is a chelated form of iron (III), which is also used as an iron source for humans.
  • Oxidizing agents. Oxidizing agents have two distinct functions: as a biocide, or to neutralize reducing agents. Among them, some of the most widely known oxidizing agents in the aquarium hobby are ozone, potassium permanganate, chloramines, and chlorine. Some further information concerning the chemistry of the last two - which may help hobbyists understand how they act and why their removal from our water system is highly recommended - are given at the end of this article.
  • Potassium permanganate. Potassium permanganate (KMnO4) is a strong oxidizing agent used in many bleaching applications. It will oxidize most organic compounds, and is often used to oxidize ferrous iron to ferric for precipitation and filtration. These two properties are of particular interest to the aquarium hobbyist as it is used as a "disinfection" agent (mainly by dipping objects like nets) while it may lead to iron deficiencies if present in planted tanks. May dye some material. If living tissue (plants, fishes) is to be treated, a very short dip is recommended. Rinsing in purified water afterwards is also highly recommended to remove any traces of potassium permanganate. It should be noted the even dilute solutions of potassium permanganate are mildly irritating while concentrated ones are caustic. Use with extreme care and avoid contact with skin.
  • Reducing agents. Reducing agents, like sodium metabisulfite (Na2S2O5), are added to neutralize oxidizing agents such as chlorine or ozone. In membrane and ion exchange systems, they prevent the degradation of certain membranes or resins, which are sensitive to oxidizing agents. It should be noted that sodium metabisulfite is the active ingredient of most water conditioners used in the aquarium hobby. Of course, one of the disadvantages is that it remains (and usually accumulates) in the aquarium water if used in excess.

Tank-type pressure filters

A typical filter consists of a tank to house the filter media and a valve or controller to direct the filter through its various cyclestypically service, backwash, and rinse. This is not applicable for the filters commonly used in our tanks which require a manual service at specific time intervals or when the flow at the exit of the filter is reduced. In some advanced sump filters this technique can be applicable.

Easily the most critical aspect of pressure filter performance is the relationship of flow rates to filter media surface area. This relationship is the primary cause of failure or trouble in filter systems. If problems develop, the most common reason is that many filters are inaccurately "sized" for the job. In the aquarium hobby, this is most often the reason for inadequate filtration. In this case, the filter size (or better still, the capacity of the filtering media) is too low, or alternatively the filter media is too much for a specific filter which blocks it and reduces its flow rate to very low levels. Some examples of pressure filters and their applications are:

· Sand filters. Sand or other filtration media are used to remove turbidity. However, the location of the fine media on top of the coarse media causes the sand filter to clog quite quickly and the coarseness of sand allows many smaller impurities to pass through. In the aquarium hobby, this filter is mainly used as a biologic filter and not as a primary mechanical one. Sand traps are also used in the filtration systems of large (usually public) aquariums and they are very efficient. This type of filter when correctly assembled allows for backwash cleaning.

· Neutralizing filters. Neutralizing filters usually consist of a calcium carbonate calcite medium (crushed limestone or marble) to neutralize low pH water. The same technique (although not named like that) is usually applied in the filtration system of many tanks when the desired pH of the water differs greatly from the one that the tap water has. We use crashed coral as a filter media to raise the pH (for African cichlids) or peat to decrease it (South American tanks).

· Oxidizing filters. Oxidizing filters use a medium treated with oxides of manganese as a source of oxygen to oxidize and precipitate iron, manganese, hydrogen sulfide, and others. An analog in our tanks is the ozonizer, which adds a strong oxidizing agent in the water column.

· Activated carbon filters. Activated carbon (AC) is similar to ion exchange resin in density and porosity. It absorbs low molecular weight organics and reduces chlorine or other halogens from water, but does not remove any salts. These filters must be changed periodically to avoid bacterial growth, but are not easily reactivated in the field. Accumulated solids require frequent backwashing of the filter unless installed after reverse osmosis or ultrafiltration.

When considering Activated Carbon for taste, odor, and color removal applications, it is necessary that the influent be relatively free of iron for maximum service life. It is also necessary that the influent be free from oil and other suspended matter, as these constituents may reduce the overall efficiency of the activated carbon.

Activated carbon is a very mature technology that is designed to help remove taste and odor from water through adsorption of the compounds that cause problems. There are a variety of different types of carbon that are used industry-wide. They include wood, lignite, coal, and coconut as the most common sources for activated carbon.

Activated carbon operates through adsorption. Adsorption is a surface phenomenon and is therefore directly related to the surface area of the media. In the case of activated carbon, the surface area is related to the pore structure of the raw materials. The cost of the media is also related to the raw materials, so there are other factors that must be taken into consideration besides the total surface area.

Adsorption takes place due to intramolecular attraction between the carbon surface and the substance that is being adsorbed. The force of the attraction can be altered by increasing the density of the carbon or by reducing the distance between the carbon surface and the substance being adsorbed (typically by reducing the median pore size). As the fluid (often water) passes over and through the carbon, the attractive forces between the compounds that are the most attracted to the carbon are adsorbed onto the surface. The compounds that are the most highly attracted are typically organic compounds (which can cause taste, odor and appearance problems), volatile organic compounds (VOCs), and halocarbons such as trihalomethane (THM) compounds and other process wastes.

Once all of the surface area of the carbon has been exhausted through adsorption, the carbon can be regenerated in a number of different manners. The most common is offsite furnace re-activation which involves heating the carbon up to drive off the organic materials that are adsorbed. You can read more about the use of activated carbon here.

· Dual- or multi-media filters. Dual-media filters remove suspended solids to as low as 20 microns in size, but no dissolved solids. The top layer is coarse anthracite followed by fine sand. Usual materials that are commonly used in the aquarium hobby are the sponge pads that come in different pore sizes. In this case, the larger pore sponge should be the first material the water should pass through. Sometimes special aquarium floss (which has the smallest porosity) is added as the last layer. Clogging of these pads will reduce the filtration capacity of the system very quickly.

Pre-coat filters
Usually with a media of diatomaceous earth, pre-coat filters remove very small particulate matter, including some bacteria. They are practical only for limited volume applications, but are common for swimming pools, beverage plants, and small installations. Not so common in aquariums, but they have gained some popularity lately. Its use is to polish the water and it is usually used at regular intervals or when needed and not on a permanent basis.

Cartridge filters
Cartridge filters can now be described in two general ways: as depth filters or surface filters.

· Depth cartridge filters. In a depth cartridge filter, the water flows through the thick wall of the filter where the particles are trapped throughout the complex openings in the media. The filter may be constructed of cotton, cellulose, synthetic yarns, or "blown" microfibers, such as polypropylene. The best depth filters have lower density on the outside and progressively higher density toward the inside wall. The effect of this "graded density" is to trap coarser particles toward the outside of the wall and the finer particles toward the inner wall. Depth cartridge filters are usually disposable, cost-effective, and are in the particle range of 1 to 100 microns. Generally, they are not an absolute method of purification since a small amount of particles within the micron range may pass into the filtrate. This is of particular use for large sumps that support large aquariums.

· Surface filtrationpleated cartridge filters. Pleated cartridge filters typically act as absolute particle filters, using a flat sheet media (either a membrane or specially treated non-woven material) to trap particles. The media is pleated to increase usable surface area. Pleated membrane filters serve well as sub-micron particle or bacteria filters in the 0.1 to 1.0 micron range. Newer cartridges also perform in the ultrafiltration range: 0.005 to 0.15 micron.

· Ultrafiltration cartridge filters. Point-of-use ultrafiltration cartridges are used to remove pyrogens and other macromolecular compounds from ultrapure water. They are built in a spiral-wound configuration. This allows a crossflow mode of operation to help keep the surface clean.

Ion exchange systems
An ion exchange system consists of a tank containing small beads of synthetic resin. The beads are treated to selectively adsorb either cations or anions and exchange certain ions based on their relative activity compared to the resin. This process of ion exchange will continue until all available exchange sites are filled, at which point the resin is exhausted and must be regenerated by suitable chemicals.

· Water softening. The ion exchange water softener is one of the most common tools of water treatment. Its function is to remove scale-forming calcium and magnesium ions from hard water. In many cases, soluble iron (ferrous) can also be removed with softeners. A standard water softener has four major components: a resin tank, resin, a brine tank, and a valve or controller. However, water softening is disadvantageous when high quality water is required since sodium ions will be present after the ion exchange process.

· Demineralization/deionization. Ion exchange deionizers (Dl) use synthetic resins similar to those used in water softeners. Typically used on water that has already been prefiltered, DI uses a two-stage process to remove virtually all ionic material remaining in water. Two types of synthetic resins are used: one to remove positively charged ions (cations) and another to remove negatively charged ions (anions). Resins have limited capacities and must be regenerated upon exhaustion. This kind of resin will result in a higher level of purity than the previous one.

· Two-bed and mixed-bed deionizers. The two basic configurations of deionizers are two-bed and mixed-bed. Two-bed deionizers have separate tanks of cation and anion resins. In mixed-bed deionizers, the anion and cation resins are blended into a single tank or vessel. Generally, mixed-bed systems will produce higher quality water with a lower total capacity than two-bed systems. This is the type of resin I use for my discus tank.

Deionization can produce extremely high-quality water in terms of dissolved ions or minerals, but they do not generally remove organics and can become a breeding ground for bacteria.

This system has some disadvantages. It requires the assistance of another system to produce absolutely pure water. Small fragments of the ion exchange resin are washed out of the system during operation, and stagnant water in the cartridges can actually encourage the growth of bacteria. It does not remove all of the dissolved organics from the feed water, and these can foul the ion exchange resin. It needs to be combined with other purification technologies to achieve a high level of purity.

Organic scavenging
Organic scavengers, or traps, are ion exchange resins that contain strong-base anion resin regenerated with sodium chloride brine. Since most naturally occurring organics have a slightly negative charge, the anion resin absorbs them. After the resin is loaded, the organics can be displaced by high concentrations of Cl during regeneration. It is used to remove trace organic contamination to levels as low as 5 ppb when pretreated with reverse osmosis.

Distillation is the collection of condensed steam produced by boiling water. Most contaminants do not vaporize and, therefore, do not pass to the condensate (also called distillate).

With a properly designed still, removal of both organic and inorganic contaminants, including biological impurities and pyrogens, is attained. Distillation involves a phase change which, when properly carried out, removes all impurities down to the range of 10 parts per trillion, producing water of extremely high purity.

Careful temperature monitoring is required to ensure purity and to avoid contamination of the purified water. Organics with a boiling point near that of water are very difficult to remove due to carry over into the vapor. In these situations, a double distillation system is often required for complete pyrogen removal.

Electrodialysis (ED) and electrodialysis reversal (EDR) employ specially prepared membranes that are semipermeable to ions based on their charge, and they employ electrical current to reduce the ionic content of water. Two flat sheet membranes, one that preferentially permeates cations and the other anions, are stacked alternately with flow channels between them. Cathode and anode electrodes are placed on each side of the alternating stack of membranes to draw most ions through the membranes. This leaves much lower concentrations of ions in the water of the alternate channels. Recent developments have improved the efficiency of ED by reversing the polarity of the electrodes periodically. This is called EDR and has reduced the scaling and fouling problems common to ED.

Crossflow filtration systems
Crossflow (also called tangential flow) filtration is the pressurized flow of the feed water, or influent, across a membrane, with a portion of the feed permeating the membrane and the balance of the feed sweeping tangentially along the membrane to exit the system without being filtered. The filtered stream is called the "permeate,” because it has permeated the membrane. The second stream is called the "concentrate" or "reject,” because it carries off the concentrated contaminants rejected by the membrane. Because the feed and concentrate flow parallel to the membrane instead of perpendicular to it, the process is called "crossflow" or "tangential flow." Depending on the size of the pores engineered into the membrane, crossflow filters are effective in the range of reverse osmosis, nanofiltration, ultrafiltration, and more recently microfiltration.

Crossflow membrane filtration allows continuous removal of contaminants, which under normal filtration would "blind" (cover up) or plug the membrane pores very rapidly.

· Reverse osmosis.

Nature dislikes imbalance, and reverse osmosis takes advantage of this natural need. Osmosis is the movement of water across a semipermeable membrane from the side that is less concentrated, and more pure to the salty, more concentrated side. This continues until either the concentration is equal, or the pressure on the concentrated side becomes strong enough to stop the flow.

In reverse osmosis, the process happens in reverse, due to the pressure of a high-pressure pump. The pump applies a strong pressure to the concentrated side, forcing water molecules over to the pure side, and leaving the salt, minerals, and other impurities to be removed as waste. While the percentage of feed water to pure water is large, the process is very efficient in removing contaminants. The following chart reflects expectations:

Because this process removes contaminants so efficiently, it is very cost effective for pre-purifying tap water, which is then purified again before use in other technologies. It removes a high percentage of bacteria and pyrogens, so it is often combined with the ion exchange to prolong the life of "polishing" cartridges in deionization systems. It also provides high quality pre-purified water, which is suitable as is for many routine laboratory purposes.

Reverse osmosis (RO) was the first cross flow membrane separation process to be widely commercialized. RO removes virtually all organic compounds, and 90 to 99% of all ions. A large selection of reverse osmosis membranes is available to meet varying rejection requirements.

RO can meet most water standards with a single-pass system and the highest standards with a double-pass system. RO rejects 99.9+% of viruses, bacteria, and pyrogens. Pressure, on the order of 200 to 1,000 psig (13.8 to 68.9 bar), is the driving force of the RO purification process. It is much more energy efficient compared to heat-driven purification (distillation) and more efficient than the strong chemicals required for ion exchange. No energy-intensive phase change is required. Reverse osmosis is now widely used in the aquarium hobby especially in marine systems (an absolute must for reef tanks) and freshwater systems requiring a precise chemical composition or extremely pure/ soft water (discus breeding tanks, for instance). The downscale models which are used in fish keeping are able to work under much lower pressure in the order of 4-8 bar.

Contaminant % Removal efficiency (one pass)
Suspended solids 100
Bacteria 99.5
Viruses 99.5
Pyrogens 99.5
Organic molecules with a molecular weight greater than 250 Dalton 97-99.5
Monovalent inorganics (Na+,K+,Cl-,F-,I-,NO3-,NO2-,NH4+ etc.) 94-96
Divalent inorganics (Ca++,Mg++,Fe++,Cu++,S--,SO4--, CO3-- etc.) 96-98
Trivalent inorganics (Fe+++, Al+++, PO4--- etc.) 98-99

· Nanofiltration. Nanofiltration (NF) equipment removes organic compounds in the 300 to 1,000 molecular weight range, rejecting selected salts (typically divalent), and passing more water at lower pressure operations than RO systems. NF economically softens water without the pollution of salt-regenerated systems and provides unique organic desalting capabilities.

· Ultrafiltration. Ultrafiltration (UF) is a similar process to RO and NF, but is defined as a crossflow process that does not reject ions. UF rejects contaminants in the range of 1000 Dalton (10 angstrom) to 0.1 micron particles. Because of the larger pore size in the membrane, UF requires a much lower operating pressure: 10 to 100 psig (0.7 to 6.9 bar). UF removes organics, bacteria, and pyrogens while allowing most ions and small organics, such as glucose, to permeate the porous structure. Often used instead of a microporous filter, this system is particularly useful with particulates, microorganisms, and also pyrogens. This makes it the companion filter of choice whenever pharmaceutical applications are involved, since a microporous filter does not remove pyrogens well. It uses a membrane similar in design to a reverse osmosis system, except that its pores are slightly larger. Very often, this filter is used to remove pyrogens from water that has already been purified of other contaminants, so a large amount of water passes through it. Because of this, it will eventually plug if not carefully maintained. In a properly designed system, the ultrafilter is regularly washed clean of contaminants, and should be easily accessible to this kind of care.

· Microfiltration. Microfiltration (MF) membranes are absolute filters typically rated in the 0.1 to 2 micron range. Traditionally available in polymer or metal membrane discs or pleated cartridge filters, microfiltration is now also available in crossflow configurations. Operating pressures of 1 to 25 psig (0.07 to 1.7 bar) are typical.

Crossflow microfiltration substantially reduces the frequency of filter media replacement required in normal flow MF, because of the continuous self-cleaning feature. Typically, crossflow MF systems have a higher capital cost than MF cartridge filter systems. However, operating costs are substantially lower.

Microporous filtration works best with applications involving particulates and microorganisms, and is a useful addition to any system that also uses activated carbon filtration or deionization. It removes the carbon particulates from the carbon system and the resin fragments and bacteria from the deionization one. It uses a membrane (or hollow fiber) with an absolute pore size of 0.2 micron that prevents passage any larger contaminant. Since NCCLS considers water to be particulate-free when it has been passed through a 0.2 micron filter, microporous filters are an important part of a many water purification systems.

Microporous filters have uniformed pore sizes and will remove 100% of contamination that is larger then the rated pore size

Ultraviolet oxidation
Ultraviolet oxidation is another method that works well as an addition to other systems. It does a good job eliminating bacteria, and an excellent job cleaving and ionizing certain organics so that they are easily removed in the polishing loop by the deionization and organic scavenging cartridges. It works by the use of ultraviolet radiation at the biocidal wavelength of 254 nanometers, which is murder on bacteria. For more information, you can also see the relevant article.

Some elements about the chemistry of Chloramines and Chlorine.

What are chloramines?
Chloramines are biocides. Like Cl2, they are oxidants, and kill bacteria by penetrating their cell walls and disrupting their metabolism. Chloramines are, however, much slower to "react". They remain as a reside ("unused") in water and are consumed as they come in contact with bacteria or break down over time as a matter of course. As with chlorine, municipalities aim for I to 2 mg/l (ppm) chloramine residual in the potable water supply. Several disinfectants, ranked by their "biocidal efficiency" and "stability,” are noted below. (Biocidal efficiency was considered as the effectiveness of the disinfectant against a-number of viruses and bacteria in the pH range of 6 to 9. Stability reflects a lack of reactivity with constituents other than microorganisms and is a measure of persistence in the treated system). It should be noted that the name "chloramine" used to describe these compounds refers to their general structure. It should not be confused with the active ingredient that carries the common name "chloramine" used in medicine, which refers usually to Chloramine-T (used as an antibacterial agent in humans and a topical antiseptic in animals). Additionally, dichloramine is also the common name of the compound correctly named Dichloramine T with similar properties. Monochloramine and dichloramine are used in the table below to describe the real compounds, which are present in tap water and may cause severe burns in fish gills.

Biocidal Efficiency (best to worst)

ozone > chlorine dioxide > free chlorine > chloramines

Stability (best to worst)

chloramines > chlorine dioxide > free chlorine > ozone

Their chemistry
Chloramines are typically generated on-site by the addition of ammonia (NH3) to water containing free Cl2 (HOCl or OCl depending upon the pH of the water). The optimum reaction pH is on the alkaline side; pH 8.4 (i.e., NH3 (aq) + HOCI « NH2Cl + H2O) Three forms of chloramine can result as well as undesirable but unavoidable interference reactions (#4 below).



Molecular Weight

Predominant pH

Biocidal Activity

1) NH2Cl





2) NHCI2



4 to7


3) NCl3



1 to 3


4) RNHCl

Organic Chloramine




Organic chloramines cannot be distinguished from the other forms of chloramines with standard methods of chloramine analysis.

Chloramines are not highly disassociated in aquatic solutions (in other words, only minimally ionic). That fact, and their low molecular weight, makes them difficult to remove via RO (discussed in more detail below). The monochloramine form is the best biocide, and as is noted, is the dominant entity at pH 7 and greater. Since slightly alkaline waters are less corrosive, municipalities in many cases maintain the monochloramine form and reduce corrosion potential at the same time. Note that at an alkaline pH, chlorine exists as the hypochlorite ion (OCl), which has a higher oxidative potential than hypochlorous acid (HOCl), but is 80 to 100 times less effective as a disinfectant.

Due to the complexity of chlorine chemistry, the activity and forms of chlorine is not well understood. Its reactions are not always "predictable" and are somewhat unique. This memo is intended to provide some simple insight of the most common forms of chlorine. Please note that chlorine is a very powerful irritant and may even cause fatal pulmonary edema in humans (at a very high concentration, of course). It will irritate the fish gills and cause fatal edema very quickly if not removed or neutralized. It can be removed if the water is "aged" for 24 hours with agitation (by running an air pump) or by the use of reducing agents like sodium metabisulfite (Na2S2O5) - the basic active ingredient of most tap water conditioners available to hobbyists today.

The most basic form of chlorine is Chlorine Gas (Cl2). This is usually the cheapest form of chlorination, yet somewhat complicated. Therefore, it is usually used only in large installations (municipal water supplies, etc.). When Cl2 is added to water the following reaction occurs:

Cl2 + H2O <----> HOCl (Hypochlorous Acid) + H+ + Cl- (Hydrochloric Acid).

K = 3 x 10-5 (pH = 4.5 at equilibrium)

Thus, the water pH swings to the acidic side.

At a higher pH, the hypochlorite ion (-OCl) is formed. The hypochlorite ion has a higher oxidation potential than hypochlorous acid, yet hypochlorous acid is a better disinfectant. The fact that hypochlorous acid has no charge allows it to penetrate microbial cell walls easier. Therefore, the lower the pH, the better the disinfecting power of a chlorine solution due to hypochlorous acid formation. The hypochlorite ion is overall more reactive, being harder on membranes and other materials of construction.

Hypochlorites (household bleach), perhaps the most common form of small-scale chlorination, tend to form high pH solutions due to the formation of NaOH.

NaOCl + H2O = NaOH + HOCl

The dissociation constant regulates the acid form on the basis of this equation.

HOCI <------> H+ ClO  - K = 3 X l0-8

At pH 7.53, concentrations of HOCI and -OCI are equal. At pH = 10, the predominant form is hypochlorite ion (OCl-).

Chlorine Dioxide (also known as Chlorine peroxide; ClO2) is a form of chlorine that is finding new uses as a disinfection agent. It has unique properties that may prove to be valuable for the crossflow filtration industry. It has been found that ClO2, while being a strong disinfectant, does not necessarily attack other components it contacts. ClO2 does not form trihalomethanes (THM's) and will not form chlorinated compounds with organic substances. Several CIO2 based disinfectants are being marketed now where the ClO2 is produced on a demand-type basis. The triggering action for ClO2 production in these disinfectants is thought to be sugar-like substances, which are an integral part of bacterial cell walls. ClO2 formation from chlorite ion (via chlorous acid) is accelerated in the presence of bacteria or organics consisting of sugar-like compounds. These demand-release disinfectants, due to these phenomena, have a very low toxicity to mammalian cells. Another advantage of ClO2 is that it is also an effective disinfectant at high pH for most microbes. However, it is most effective at the lower pH range. It should be noted that chlorine dioxide will also irritate the fish gills and cause fatal edema very quickly if not removed or neutralized. It can be removed if the water is "aged" for 24 hours with agitation (by running an air pump), or by the use of reducing agents like sodium metabisulfite (Na2S2O5). As previously shown, chlorine dioxide not only exerts a very strong oxidizing action but is also relatively stable in aquatic solutions. 

Chloramines, formed when hypochlorous acid and ammonia are present together in solution, produce long lasting residuals and do not form trihalomethanes, yet their disinfection capability is limited. Additionally, chloramines can cause tastes and odors in finished water (e.g., swimming pool odor is attributed to chloramines).

Why remove them?
In summary, chlorine in most forms is a more effective disinfectant in the acidic range than the basic. However, the hypochlorite ion, which is most prevalent in high pH solutions, has the highest oxidation potential and is more corrosive to other components it comes in contact with. As a result, sanitation with chlorine in an acidic solution (pH <7) is more effective and more desirable from a compatibility standpoint.

Like any other molecule, chloramines contribute to the overall total dissolved solids content of the water and like chlorine, are selectively reactive, thus may have deleterious effects on downstream processes. In equilibrium with chloramines are trace amounts of ammonia and/or hypochlorite ions. Their (NH3 and HOCl) presence must also be recognized when one is designing an ultrapure treatment system to remove chloramines.

From the aquarist's point of view, chloramines, chlorine gas, hypochlorite ions and chlorine dioxide are all elements with oxidizing potential, which will cause damage to fish and should therefore be removed from the water. It should be noted that most water companies add an increased amount of chlorine gas during the summer months to compensate for losses due to the higher water temperature, the faster metabolism and multiplication of the microorganisms, and the lower solubility of the gas in the water. Therefore, a quantity of sodium metabisulfite known to neutralize chlorine and chloramine compounds in a specific water volume in the winter may prove to be too small in the summer months. In contrast, aging of the water during the summer months will allow chlorine gas and chlorine dioxide to escape faster. Do not rely on chlorine smell since the threshold for smelling chlorine is 0.20-0.40 mg/l while even 0.10 mg/l (very common in tap water, especially in summer months) will be acutely fatal in aquaria, especially newly established ones. The maximum tolerable limit for continuous exposure to chlorine is 0.003 mg/l while any detectable level is undesirable.

Chlrorine as well as chloramine will readily react with organic matter present in the water column and become neutralized. However, in newly established aquaria the organic matter present in the water is not enough to neutralize these chemicals thus a large proportion of these compounds will have to be removed with other means. Otherwise, those chemicals will exert their action on the gills and mucosa of the fish. Aging the water and aeration will work well for chlorine and chlorine dioxide but will not eliminate chloramines. Carbon filtration will readily remove chloramine from the water. Usual chemical neutralizers (such as sodium thiosulfate) will break the chlorine - ammonia bond of the chloramine (and neutralize chlorine at the same time) thus introducing a quantity of ammonia in the water which has to be removed too. Moreover, some of these "neutralizing agents" will give a false negative ammonia reading in most tests. Zeolite (an ion exchange resin) can be then used to remove the ammonia from the water but the aquarist should be very careful when using it. Usually, a good quality zeolite (like Clinoptilite) will remove approximately 2 mg ammonia per gram of zeolite exchanging it with sodium ions. Large particle sizes of zeolite are less efficient but smaller particles are easily clogged. Zeolite will be less efficient in hard water since magnesium and calcium ions will bind to it, thus reducing its efficacy by 50%, while in marine water (sp. gravity 1.027) its efficacy is reduced by 95%. It should be noted that zeolite has a specific capacity for ammonia and needs to be regenerated (with a 20% NaCl solution for  30 minutes). 

Acknowledgements: I would like to thank Dr. Nikolaos Graekas for critically reviewing this manuscript and Mrs. Carli De Busk for making it a bit more suitable for the average hobbyist.

Some elements for this article were retrieved from the "Pure Water Handbook" published by Osmonics®, Inc. of Minnetonka, Minnesota USA. Osmonics Inc. is a company specialized in water purification - at any scale.

Useful information about the properties of the various chemicals referred to in this article were retrieved from "The Merck Index", Twelfth Edition, 2001, by Merck & Co., Inc., Rahway, N.J., USA.

Another source of useful information was "Fish Disease - Diagnosis and Treatment", by Dr. Edward J. Noga, 1995 Edition, published by Mosby - Year Book Inc., St. Louis, Missuri, USA

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