The first disinfectant, Carbolic acid or phenol, was used in the operating room by Joseph Lister in the late 19th century. The usage resulted in fewer post-operative infections and led to the discovery of many more disinfectants and thus science of disinfectants was born.

Hard water and presence of metals make all kinds of disinfectants less effective by reacting with them. Some produce harmful byproducts. Organic content can trap or bind to the disinfectants, making them ineffective. All disinfectants are toxic to fish and therefore should not be used without knowing the right amount to use. But when used correctly, disinfectants can prevent parasites, bacteria and viruses from spreading from one tank to another, reduce algae and sterilize the pond before putting the fish into the pond.

Chlorine
All four halogens - Chlorine, Fluorine, Bromine and Iodine,  are disinfectants. Chlorine is cheap and abundant. Bromine is more expensive and milder than Chlorine. Fluorides are toxic. Iodine compounds can cause thyroid disorders. This is why Chlorine is the most popular disinfectant.

Chlorine destroys a wide range of pathogens. It prevents algal growths, removes iron, manganese, hydrogen sulfide, bleaches colors caused by organic compounds like tannins, controls slime growth, leaves pipes clean. But Chlorine, after reacting with organics, produces Trihalomethanes that are known to be carcinogenic.

How does Chlorine work?
Chlorine gas hydrolyzes in water to form hypochlorous acid.

Cl₂ +H₂O --> HOCl + H⁺ + Cl^-

Hypochlorous acid a weak acid so it breaks down to form hydrogen and hypochlorite ions

HOCl --> H⁺ + OCl^-

HOCl has much higher germicidal effect than OCl^{-. The breakdown is highest above pH 8.5 and zero below pH 6.5. This means germicidal effects is much higher at lower pH (6.5 or below) because HOCl is the dominant germicide in that range. Note that chlorine produces H+ ions that bring down the pH of the solution.}

Chlorine is gas and can not be stored in homes. So, commercially available bleaches contain compounds that produce hypochlorite ions when dissolved in water - Sodium hypochlorite and Calcium hypochlorite.

Sodium hypochlorite has 12.5% available chlorine. It is made by dissolving chlorine gas in Sodium hydroxide. When dissolved in water, Sodium hypochlorite breaks down to form hypochlorite ions.

NaOCl + H₂O --> HOCl + Na⁺ + OH^-

The difference is instead of hydrogen ion ( H⁺ ) as in chlorine gas treatment,  we get a hydroxyl ion (OH^-) that will increase the pH of water. If Sodium hypochlorite is contaminated with Sodium hydroxide (because it is made with Sodium hydroxide), it can further increase the pH, reducing the germicidal effect. Sodium hypochlorite (12.5%) will degrade to 10% in 30 days under best conditions. Increased temperature, exposure to light, contact with metals will increase the degradation. While using Sodium hypochlorite one must remember that it is corrosive (pH 12).

Calcium hypochlorite is made by mixing chlorine gas with a  solution of like or Calcium oxide and Sodium hydroxide. About 65% of Calcium hypochlorite is Chlorine. When mixed with water, Calcium hypochlorite produces hypochlorite ions

Ca(OCl)₂ + 2H₂O --> 2HOCl + Ca⁺⁺ + 2OH^-

Just as Sodium hypochlorite, Calcium hypochlorite also gives hydroxyl ions that increase the pH of water.  Calcium hypochlorite should be stored away from heat because spontaneous combustion fires have been reported. Calcium hypochlorite loses its activity by 3-5% per year.

Hypochlorite ions damage cell membranes of most pathogens. Since Hypochlorite ions are more effective at lower pH, Chlorine gas is more effective than both Sodium and Calcium hypochlorite if no pH adjustments are made. For example, virus inactivation requires 50% more contact time at pH 7.0 than at pH 6.0. Six times higher contact time is needed if pH is 8.0 (Culp 1974) Similarly, contact time should be increased 2-3 times to achieve comparable inactivation levels when water temperature is lowered by 10*C (clarke 1962)

Benzalkonium Chloride (Alkylbenzyldimethylammonnium Chloride)
Quaternary Ammonium Compounds (Quats) were first introduced in 1935. Quats are effective disinfectants and surfactants. They are ammonium compounds containing a positive charge that helps in attracting negatively charged compounds present on cell membranes of pathogens. Soluble in ethanol or acetone, not in water. When shaken they produce foam.

Quaternary ammonium compounds are the main active ingredients of common antiseptic solutions like Dettol, Lysol, floor cleaners, hand wipes, contact lens solutions, eye drops and pool Algaecide.  They are active against fungi, some viruses and gram positive bacteria. Hard water and presence of soap or any other surfactant (clay) can reduce the biocidal activity of Quats. Quats are known to disrupt the cell membranes of pathogens. Since the disruption is selective, not many byproducts are formed (unlike with chlorine) and very little amount is sufficient.

Quats are toxic to fish, kill aquatic invertebrates and are moderately toxic to birds. Bacteria like Pseudomonas species are resistant to Quats. But soaking nets and other cleaning materials in Quats can kill protozoan parasites and gram positive bacteria.

Potassium Permanganate
It is a strong oxidant but poor disinfectant. Potassium permanganate directly oxidizes cell walls of pathogens.  During this process, it is reduced to Manganese dioxide and precipitates. Reactions are exothermic.

MnO₄^- + 4H⁺ --> MnO₂ + 2H₂O

MnO₄ ^- + 8H⁺ --> Mn²⁺ + 4H₂O

Permanganate oxidizes Iron and Manganese present in water which will precipitate as hydroxides. Manganese dioxide formed also attaches to all charged micro organism making them clump and fall to the bottom of the pond.

3Fe²⁺ + KMnO₄ + 7H₂O --> Fe(OH)₃ + MnO₂ + K⁺ + 5H⁺

3Mn²⁺ + 2KMnO₄ + 2H₂O --> 5MnO₂ + 2K⁺ + 4H⁺

About 1.49mg/l as CaCO₃ per mg/l of alkalinity is used for Iron and 1.21mg/L for Manganese oxidized. This means alkalinity is reduced when Potassium permanganate is added.

Alkaline conditions enhance the effectiveness to oxidize organic matter but opposite is true for disinfecting power. It also inactivates Legionella more rapidly at pH 6.0 than at 8.0. It is also more effective at higher temperatures. If lots of organics are present, disinfection is reduced. Algae and phenols are degraded.

Even in ideal conditions, Potassium permanganate is not a good germicide. Dosages as high as 20mg/L and contact times of 24 hours are necessary to inactivate bacteria like Vibrio cholerae, Salmonella typhii and Bacillus flexner but not completely. However, E. coli were destroyed with 1-2mg/L with 10 min contact time in Lake Mead Experiment. About 50mg/L potassium permanganate with contact time 2 hours was necessary for inactivation of Polio virus. Since protozoa are more resistant than viruses, dosages and contact times may be higher. Potassium permanganate is toxic to skin and mucous membranes. While handling, one must be very careful.

Repeated treatments or high dosages of Potassium permanganate can deplete Chloride ions from fish cells, gill tissues causing them to swell. Experiments conducted with Channel Catfish with 2.19mg/L Potassium permanganate (12 hour exposure) showed ten-fold increase in blood levels of Cortisol and reduced levels of Chloride. Loss of chloride alters electrolyte balance stresses both kidney and lung/gills.  Toxicity of Potassium permanganate to fish is greatest at lower temperatures, in hard water or in alkaline conditions. So, lower doses must be added in these conditions.

Chloramines

Chloramines are formed by reaction of Ammonia with Hypochlorous Acid or Aqueous Chlorine (HOCl). They are more stable than Chlorine and are effective in controlling bacterial growth.  They form fewer Trihalomethanes.

Cl₂ + H₂O  --> HOCl + H⁺ + Cl^-

HOCl --> OCl^- + H⁺

Both HOCL and OCl^- are powerful oxidants. Between pH 7 to 8.5, HOCl reacts with ammonia to form Chloramines

NH₃ + HOCl  --> NH₂Cl (monochloramine) + H₂O

NH₂Cl + HOCl  --> NHCl₂ (dichloramine) + H₂O

NHCl₂ + HOCl  --> NCL₃ (trichloramine) + H₂O

Monochloramines have the highest germicidal effect of all Chloramines. They are able to penetrate biofilms. Since they do not react with organic compounds no worries about Trihalomethanes but germicidal effect is 200 times less than that of Chlorine. Over a period of day or two, Monochloramine will degrade to Dichloramine. Dichloramines are unstable in the presence of HOCl.

If ammonia present in water, Chloramines will be partially nitrified. The nitrites produced will  reduce free Chlorine, accelerate decomposition of Chloramines. Nitrifying bacteria are more resistant to Monochloramines than free Chlorine. Chlorine oxidizes nitrite, kills nitrifying bacteria and removes excess ammonia. Chloramines if present in water, can damage gill tissues, enter red blood cells and cause acute blood disorder.

Ultraviolet radiation

UV can disinfect bacteria and viruses with proper dosage. Optimum UV range for germicidal effects is 245 to 285nm. By using either low pressure lamps with 253.7nm wavelength or medium pressure lamps from 180 – 1370nm or lamps emitting wavelengths in a high intensity pulse manner one can target the bacteria and virus.

What is UV dosage?

D = I x t

Where: D = UV Dose, milliWatts×seconds/cm^2, I = Intensity, mW/cm^2, t = Exposure time, seconds

When germs are exposed to UV radiation, a constant fraction of them get destroyed during each progressive increment in time. This means a high intensity UV energy over a short period will kill equally well as a lower intensity UV energy at a proportionally longer time. Effectiveness is dependent on dose and water quality and NOT the density of microorganisms. Since the waves are electromagnetic, pH, temperature, alkalinity and total inorganic carbon do not impact effectiveness. Hardness will cause scales on UV lamp. The presence of oxidants like Ozone and Hydrogen peroxide increases effectiveness.  Dissolved and suspended matter (iron, sulfites, nitrites, phenols absorb UV light) will shield germs from UV.

UV demand is different for each water. Percent transmittance is measured as absorption energy per unit depth at 254nm)

Percent Transmittance = 100 x 10^-A

Excellent water: 0.022 absorbence  units/cm means 95% transmittance

Good water: 0.071 means 85% transmittance

Fair water: 0.125 absorbence  units/cm or 75% transmittance.

The UV lamps typically use a quartz tube filled with argon and small quantities of mercury. UV radiation is emitted from electron flow through ionized mercury vapor to produce UV energy. (Like fluorescent lamp but the latter is coated with phosphorus which converts the UV light to visible light).

Over time, UV radiation makes the UV lamp opaque (solarization). Each time UV lamp is cycled off and on, the electrodes wear off. So, frequent switching on and off ages the lamp prematurely. Average life expectancy of a UV lamp is about 8800 hours. UV lamps should be discarded when the last 30% is remaining.

UV dosage required to kill

• Bacteria 2,500 – 26,400 μWs/cm²
• Yeast 6,600 – 17,600 μWs/cm²
• Algae 11,000 – 330,000 μWs/cm²
• Viruses 2,500 – 40,000 μWs/cm²

Bibliography
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