Biocides and cidal agents are functional chemicals used to kill or neutralize biological pests such as microbes, mold, and insects. These chemicals ensure preservation and proper function of systems which would otherwise be subject to bacterial growth or other biological infestation.

The use of biocides is often discouraged due to pollution concerns and the harmful chemicals associated with their application. When possible, synthetic and artificial biocides should not be considered a necessity if the thorough cleaning and controlled removal of growth-supporting materials is sufficient to address a problem.

Biocide Selection

Selecting biocides depends almost exclusively on its intended use or application, which will determine the function of the chemical. With the function defined, an industrial buyer should then consider the form, characteristics, and features of the biocide.

Types

Each biocides can be classified into one of five groups by its function (i.e. the killing action it performs) based on its chemistry.

• Algaecides or antifouling agents are designed to killand prevent the growth of algae.Anti-fouling agents are added to marine hull paints to prevent the growth or algae below the water line.
• Bactericides or antibacterial agents are designed to kill and prevent the growth of bacteria.
• Fungicides and fungicidal agents are designed to killand prevent the growth of fungi.
• Germicides or antimicrobial agents are effective in killing smaller microorganisms such as viruses, bacteria, protozoa,and other germs.Certain germicides are targeted to kill specific microbes or specific species of microbes.
• Herbicides or defoliants are designed to killand prevent the growth of plants or vegetation.
• Insecticides and pesticides are designed to killand prevent the growth of insects and other animal pests (e.g. rodents).
• Viricides are chemical agents that can kill, deactivate, or destroy viruses.

Forms of Biocide Products

Form is an important feature in the selection of biocide products. It includes the physical form of the biocide and the product form.

The physical form of a biocide product is its state or consistency. This affects the means for system integration of the biocide, including its shipment, storage, and application. These forms include:

• Liquid or solution
• Colloid or dispersion
• Powder
• Bulk solid or granules

Product form is the tier at which a biocide product or service is purchased. The first tier of the biocide industry consists of the manufacturers of the active ingredients. Some of the oxidizing biocides are simple solutions of common chemicals like bleach. End users can purchase these products, as well as chlorine gas, directly from the manufacturer. The second tier of the industry is made up of biocide suppliers that sell ready-to-use biocides made by mixing the active ingredients with other additives or carrier fluids. The third tier of the industry is made up of independent consultants who provide a full range of water treatment services to users but do not sell or provide any biocide products.

Characteristics of Biocides

Because of the vast range of biocide chemicals, industrial buyers must understand the characteristics of different biocides in order to select an appropriate and compatible product for their system. These characteristics depend on the microbiology and chemistry of the materials and substances interacting. The criteria include effectiveness, safety, compatibility, and handling.

Effectiveness describes the ability of the biocide to kill bacteria. Properties stemming from this include:

• Minimum concentration - minimum concentration to kill microorganisms in a given solution. Measured in lb/gal or kg/L (mass), in lb-mol/gal or mol/L (molar), or ppm (parts per million).
• Contact time - amount of time the biocide must be in contact with the contaminant for disinfection.
• pH (of system) - the acidity or alkalinity of the process fluid. Each biocide has a pH range in which it is most effective. Outside this range, its biocidal activity may be weakened.
• Reactive chemistry (system) - active chemical elements in the process fluid. Because of their reactive nature, biocides are affected by oxidizing and reducing agents, and other reactive chemicals used in the manufacture of products (e.g. surfactants).

Safety describes how safe the biocide is to use. Properties stemming from this include:

• Toxicity - measure of lethality or poisonous nature of a biocide.
• Neutralization requirements - procedure to counteract or nullify the effects of biocides used in a system.
• Discharge to environment - amount of biocide released to the environment during usage.

Compatibility describes how easily the biocide can be integrated with system fluids. Properties stemming from this include:

• Solubility - how much biocide can be dissolved in the process fluid at a given (usually standard) temperature.
• pH (of biocide) - the acidity or alkalinity of the biocide. Excessive changes in pH can have adverse effects on different parts of the system.
• Total dissolved solids (TDS) - amount of solids contained in the process liquid, which determines how the biocide material functions in the solution.

Handling describes the physical means by which a biocide is stored, transported, and used. Properties stemming from this include:

• Freezing point - the temperature at which a liquid material becomes solid.
• Thermal stability - determines the decay rate of materials with increasing temperature. Biocides with high thermal stability can operate in high temperature environments with little reduction in effectiveness.
• Corrosivity - the amount of chemical disintegration caused by the biocide. Highly corrosive materials require special materials to store and transport.
• Shelf life - the amount of time a biocide can be stored before its functional properties deteriorate.

Common Examples

They are too many variations of biocides to list, but some common examples for microorganism control include chlorine, bromine, amines, and bronopol.

Chlorine is the most widely used biocide in industry today. It is a common disinfectant of domestic water and also removes undesirable tastes and odors. In solution it creates hypochlorous acid, the substance responsible for microorganism control. The amount needed for proper disinfection is dependent on the contact time in the system, pH, water temperature, and the amount of chlorine lost through aeration. When possible, alternatives to chlorine are encouraged due to its toxic nature and the pollutants involved in its production (e.g. mercury).

Bromine has a similar chemistry to chlorine, and performs the same functions. It is more effective than chlorine at higher pH but incorporates all the negative side-effects of chlorine in addition to producing the side product bromate, which is a carcinogen.

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. They function best in water with low levels of suspended solids and TDSs.

Bronopol is a dry product sold as a powder or solid stick.

Image Credit: Konar Organics Limited

It has uses in drilling and product fluids in the oil industry. It has a low toxicity but degrades over time and breaks down quickly in high pH.

Applications

Biocide are used in or added into a variety of different products. They are added into metalworking fluids or hydraulic fluids to prevent bacteria growth and the odors associated with the growth. They are also added into plastics or paints to prevent or reduce the mold or bacterial growth on surfaces. They are used to treat or disinfect water. In addition, biocides are used to make treated lumber, which discourages or delays rot or insect attack.

Across applications, a common rationalization for biocide failure or ineffectiveness is that organisms in the system have become resistant to a biocide's chemistry. Manufacturers often cycle or rotate different biocide products in their facilities for this reason. In truth, biocides mainly fail when there is a change in the protected system. Even small changes can have major effects on the activity of a biocide.

References

ASI Magazine - Biocides 

Image credit:

Phase III, Inc.