Chlorine dioxide

Chlorine dioxide is an active oxidizing biocide that is applied more and more due to the fact that it has less damaging effects on the environment and human health than chlorine. It does not form hydrochlorous acids in water; it exists as dissolved chlorine dioxide, a compound that is a more reactive biocide at higher pH ranges. Chlorine dioxide is an explosive gas, and therefore it has to be produced or generated on-site, by means of the following reactions:
Cl2 + 2 NaClO2 -> 2 NaCl + 2 ClO2
2 HCl + 3 NaOCl + NaClO2 -> 2 ClO2 + 4 NaCl + H2O.

Chloroisocyanurates

Chloroisocyanurates are applied more and more due to the fact that they have less damaging effects on the environment and human health than chlorine. They do not form hydrochlorous acids in water; they exist as dissolved chlorine dioxide, a compound that is a more reactive biocide at higher pH ranges. Chlorine dioxide is an explosive gas, and therefore it has to be produced or generated on-site, by means of the following reactions:
Cl2 + 2 NaClO2 -> 2 NaCl + 2 ClO2
2 HCl + 3 NaOCl + NaClO2 -> 2 ClO2 + 4 NaCl + H2O.

Hypochlorite

Hypochlorite is a salt from hypochlorous acid. It is formulated in several different forms. Usually, hypochlorite is applied as sodium hypochlorite (NaOCl) and calcium hypochlorite (Ca(OCl)2). These compounds can be applied as biocides. They function in very much the same way as chlorine, although they are a bit less effective.

Ozone

Ozone is naturally unstable. It can be used as a powerful oxidizing agent when it is generated in a reactor. As a biocide, it acts in much the same way as chlorine; it disturbs the formation of ATP, so that the cell respiration of microorganisms will be made difficult. During oxidation with ozone, bacteria usually die from the loss of life-sustaining cytoplasm. While the oxidation process takes place, ozone splits into oxygen and an ozone atom, which is lost during the reaction with cell fluids of the bacteria:
O3 -> O2 + (O)
A number of factors determine the amount of ozone required during oxidation, these are pH, temperature, organics and solvents, and accumulated reaction products. Ozone is more environmentally friendly than chlorine because it does not add chlorine to the water system. Due to its decomposition to oxygen, it will not harm aquatic life. Usually, 0.5 ppm of ozone is added to a water system, either on a continuous or intermittent basis.

Non-oxidizing agents:

Acrolein

Acrolein is an extremely effective biocide that has an environmental advantage over oxidizing biocides because it can easily be deactivated by sodium sulfite before discharge to a receiving stream. Acrolein has the ability to attack and distort protein groups and enzyme synthesis reactions. It is usually fed to water systems as a gas in amounts of 0.1 to 0.2 ppm in neutral to slightly alkaline water. Acrolein is not used very frequently, as it is extremely flammable and also toxic.

Amines

Amines are effective surfactants that can act as biocides due to their ability to kill microorganisms. They can enhance the biocidal effect of chlorinated phenolics when they are applied in water.

Chlorinated phenolics

Chlorinated phenolics, unlike oxidizing biocides, have no effect on the respiration of microorganisms. However, they do induce growth. The chlorinated phenolics first adsorb to the cell wall of microorganisms by interaction with hydrogen bonds. After adsorption to the cell wall, they will diffuse into the cell where they go into suspension and precipitate proteins. Due to this mechanism, the growth of the microorganisms is inhibited.

Copper salts

Copper salts have been used as biocides for a long time, but their use has been limited in recent years due to concerns about heavy metal contamination. They are applied in amounts of 1 to 2 ppm. When the water that is treated is located in steel tanks, copper salts should not be applied because of their ability to corrode steel. Copper salts should not be used in water that will be applied as drinking water either because they are toxic to humans.

Organo-sulfur compounds

Organo-sulfur compounds act as biocides by inhibiting cell growth. There are a variety of different organo-sulfur compounds that function in different pH ranges. Normally, energy is transferred in bacterial cells when iron reacts from Fe3+ to Fe2+. Organo-sulfur compounds remove the Fe3+ by complexing as an iron salt. The transfer of energy through the cells is then stopped and immediate cell death will follow.

Quaternary ammonium salts

Quaternary ammonium salts are surface-active chemicals that generally consist of one nitrogen atom, surrounded by substitutes containing eight to twenty-five carbon atoms on four sides of the nitrogen atom. These compounds are generally most effective against bacteria in alkaline pH ranges. They are positively charged and will bond to the negatively charged sites on the bacterial cell wall. These electrostatic bonds will cause the bacteria to die from stresses in the cell wall. They also cause the normal flow of life-sustaining compounds through the cell wall to stop, by declining its permeability.
Use of quaternary ammonium salts is limited due to their interaction with oil when this is present and the fact that they can cause foaming.

OXYGEN SCAVENGERS

Oxygen scavenging means preventing oxygen from introducing oxidation reactions. Most of the naturally occurring organics have a slightly negative charge. Due to that, they can absorb oxygen molecules, because these carry a slightly positive charge, to prevent oxidation reactions from taking place in water and other liquids.
Oxygen scavengers include both volatile products, such as hydrazine (N2H4) or other organic products like carbohydrazine, hydroquinone, diethylhydroxyethanol, methylethylketoxime, but also non-volatile salts, such as sodium sulfite (Na2SO3) and other inorganic compounds, or derivatives thereof. The salts often contain catalyzing compounds to increase the rate of reaction with dissolved oxygen, for instance, cobalt chloride.

PH CONDITIONERS

Municipal water is often pH-adjusted to prevent corrosion from pipes and to prevent the dissolution of lead into water supplies. During water treatment, pH adjustments may also be required. The pH is brought up or down through the addition of basics or acids. An example of lowering the pH is the addition of hydrogen chloride, in case of a basic liquid. An example of bringing up the pH is the addition of sodium hydroxide, in case of an acidic liquid.
The pH will be converted to approximately seven to seven and a half, after the addition of certain concentrations of acids or bases. The concentration of the substance and the kind of substance that is added depend on the necessary decrease or increase of the pH.

RESIN CLEANERS

Ion exchange resins need to be regenerated after application, after that, they can be reused. But every time the ion exchangers are used, serious fouling takes place. The contaminants that enter the resins will not be removed through regeneration; therefore, resins need cleaning with certain chemicals.
Chemicals that are used are, for instance, sodium chloride, potassium chloride, citric acid, and chlorine dioxide. Chlorine dioxide cleansing serves the removal of organic contaminants on ion exchange resins. Prior to every cleaning treatment, resins should be regenerated. After that, in the case chlorine dioxide is used, 500 ppm of chlorine dioxide in a solution is passed through the resin bed and oxidizes the contaminants.

SCALE INHIBITORS

Scale is the precipitate that forms on surfaces in contact with water as a result of the precipitation of normally soluble solids that become insoluble as temperature increases. Some examples of scale are calcium carbonate, calcium sulfate, and calcium silicate.
Scale inhibitors are surface-active negatively charged polymers. As minerals exceed their solubilities and begin to merge, the polymers become attached. The structure for crystallization is disrupted, and the formation of scale is prevented. The particles of scale combined with the inhibitor will then be dispersed and remain in suspension. Examples of scale inhibitors are phosphate esters, phosphoric acid, and solutions of low molecular weight polyacrylic acid.