In laboratory tests a maximum tolerable microbial population limit in systems is determined. When these data are known in many cases the number of bacteria and other microrganisms needs serious reduction. This can be accomplished by addition of biocides; chemical compounds that are toxic to the present microrganisms. Biocides are usually slug fed to a system to bring about rapid effective population reductions from which the microrganisms cannot easily recover. There are various different biocides, some of which have a wide range of effect on many different kinds of bacteria. They can be divided up into oxidising agents and non-oxidising agents.
Chlorine is the most widely used industrial biocide today. It has been used for disinfection of domestic water supplies and for the removal of tastes and odours from water for a long time. The amount of chlorine that needs to be added in a water system is determined by several factors, namely chlorine demand, contact time, pH and temperature of the water, the volume of water and the amount of chlorine that is lost through aeration.
When chlorine gas enters a water supply it will hydrolyse to form hypochlorous and hydrochlorous acid. The latter determines the biocidal activity.
This process takes place according to the following reaction:
Cl2 + H2O -> HOCl + HCl
Hydrochlorous acid is responsible for the oxidation reactions with the cytoplasm of microrganisms, after diffusion through the cell walls. Chlorine than disturbs the production of ATP (adenosine triphosphate), an essential compound for the respiration of microrganisms. The bacteria that are present in the water will die as a consequence of experienced breathing problems, caused by the activity of the chlorine.
The amount of chlorine that needs to be added for the control of bacterial growth is determined by the pH. The higher the pH, the more chlorine is needed to kill the unwanted bacteria in a water system. When the pH values are within a range of 8 to 9, 0.4 ppm of chlorine must be added. When the pH values are within a range of 9 to 10, 0.8 ppm of chlorine must be added.
Chlorine dioxide is an active oxidising biocide, that is applied more an more due to the fact that is has less damaging effects to 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.
Chlorine dioxide in bags
These are organo-chlorine compounds that will hydrolyse into hypochlorous acid and cyanuric acid in water. The cyanuric acid reduces chlorine loss due to photochemical reactions with UV-light, so that more hydrochlorous acid will originate and the biocidal action will be enhanced.
Hypochlorite is 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 is naturally instable. It can be used as a powerful oxidising 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 microrganisms will be made difficult. During oxidation with ozone, bacteria usually die from loss of life-sustaining cytoplasm.
While the oxidation process takes place ozone parts 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 continuous or intermittent basis.
Quaternary ammonium salts
In many cases oxidising agents are not effective biocides. Non-oxidising agents are than applied.
Acrolein is an extremely effective biocide that has an environmental advantage over oxidising biocides, because it can easily be deactivated by sodium sulphite 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 are effective surfactants that can act as biocides due to their ability to kill microrganisms. They can enhance the biocidal effect of chlorinated phenolics when they are applied in water.
Chlorinated phenolics, unlike oxidising biocides, have no effect on respiration of microrganisms. However, they do induce growth. The chlorinated phenolics first adsorb to the cell wall of microrganisms 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 microrganisms is inhibited.
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-sulphur compounds act as biocides by inhibiting cell growth. There are a variety of different organo-sulphur compounds that function in different pH ranges.
Normally energy is transferred in bacterial cells when iron reacts from Fe3+ to Fe2+. Organo-sulphur compounds remove the Fe3+ by complexion as an iron salt. The transfer of energy through the cells is than stopped and immediate cell death will follow.
Quaternary ammonium salts
Quaternary ammonium salts are surface-active chemicals that consist generally of one nitrogen atom, surrounded by substitutes containing eight to twenty-five carbon atoms on four sights 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 of 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.