Introduction:

Biocides are a group of chemicals that are designed to control and limit microbial growth in water treatment systems. Controlling bacterial growth can solve a variety of related issues such as:

• Health hazards especially the one related to legionella bacteria.
• Affecting the efficiency of treatment chemicals.
• The development of biofilms.
• Loss in the efficiency of heat transfer.
• Flow restrictions.
• Under deposit corrosion.
• Microbiologically induced corrosion (MIC).

Therefore, it is crucial to implement the proper biocidal treatment regime and conduct regular analysis to ensure that the microbial population is kept under control in the treated system.

Biocides are used in a wide range of applications like drinking water, the pulp and paper industry, process water, open recirculating water systems, closed cooling circuits, swimming pools, and recreational waters.

Keeping the system free of microbial contamination, frequent cleaning is applied along with, dosing the proper type and dosage rate of a biocide, either continuously or through a schedule of shock dosage are the usual methods used to suppress microbial development.

Biofouling

Fouling is the accumulation of unpleasant material on solid surfaces. The fouling materials can consist of either living organisms or non-living substances like inorganic or organic compounds.

Thus, biofouling problems can be classified into two groups in terms of composition, slime which is characterized by being adhesive to the surface and sludge which is accumulated in low flow areas.

Slime: is the type of fouling that contains a greater amount of microorganisms compared to inorganic substances. Slime attaches to the surface of heat exchanger tubes, or pipes, due to the sticky nature of the organic compounds in it, which is produced by microorganisms despite the flow of water in the system.

Sludge:  is the fouling type that contains a greater amount of inorganic substances compared to microorganisms. Sludge is normally accumulated whenever having a low flow rate of water or stagnancy conditions.

Microorganisms Classification

Below are the main types of microorganisms that are present in water systems with the subcategory of each type:

• Algae:
• Blue-green algae
• Green algae
• Diatom
• Fungi :
• Phycomycetes
• Mycomycetes
• Bacteria :
• Zoogloea sp.
• Sphaerotilus sp.
• Iron bacteria
• Sulfur bacteria
• Nitrification bacteria
• Sulfate reducing bacteria

However, the microorganisms that are responsible for biofoulings can be classified into two types according to their location in water, either Planktonic or Sessile organisms:

Sessile bacteria are anaerobic microorganisms permanently attached to large masses of system debris or foulant such as biofilms. Their secretions and degradation products are responsible for the development of biofouling.

Planktonic bacteria are aerobic microorganisms passively floating in the water bulk of a system.

Types of Biological Control Chemicals

Many chemicals can be used to control biological growth, these chemicals can be classified based on the mechanism of killing the microorganism or based on the application as well.

When selecting a biocide for a system many factors should be considered:

• Dosage method maintenance dose biocide or a shock dose biocide
• Contact time
• Required concentration.
• System temperature.
• pH of the treated water.
• Compatibility with system conditions and other used chemicals.
• Efficacy against microbes present in the system
• Cost-effectiveness.

REDA technical team offers you support on biocide selection to apply for the most suitable program with minimum operational cost.

Below are the main categorized biocides:

Oxidizing biocides

Oxidizing biocides are the type of biocides that attack microorganisms by oxidizing the cell structure, disrupting nutrients from passing across the cell wall such as chlorine.

Oxidizing biocides may often be used at lower dosages compared to non-oxidizing biocides making them more cost-effective, another advantage of oxidizing biocides is the fast-killing effect against water-borne bacteria, as well as breaking down the biofilms that hold pathogens in water systems. It is extremely effective against planktonic and sessile microorganisms, in including bacteria, fungi, algae, and yeast. It is ideal for use in water hygiene and pipework treatment applications where it quickly eradicates Legionella. Pseudomonas and other microbes.

Non-oxidizing biocides

Non-oxidizing biocides are Organic compounds that act as “poisons “; they disrupt the metabolic or reproductive processes of micro- and macroorganisms and are therefore toxic such as quaternary ammonium compounds.

Using one product of non-oxidizing biocide may lead to the development of a resistant flora in the treated system, which will then cause microbiological fouling due to uncontrolled biological growth. The common remedy for this issue is using two, or more non-oxidizing biocides alternatively in the treated system on regular basis. Non-oxidizing biocides are more efficient when applied in slug dosages, unlike oxidizing biocides.

Algaecides
Algae has a different metabolic and structure from bacteria thus it is generally more difficult to be controlled using common biocides. Algicides are biocides that are used for killing and preventing algae growth.

Algae needs moisture, sunlight, and nutrients to grow, thus, open water systems are the most commonly affected system like cooling towers, swimming pools, and open ponds. Because these systems are open to the ambient, so microbes, sunlight, and nutrients like (nitrogen and phosphorus) are often allowed to enter the water. As a consequence, algae can rapidly grow out of control if not treated properly.

Ideally preventing algae growth by having the correct biological treatment is much more efficient than treating the problem once it’s created. Algae can rapidly grow out of control if not managed early.

Algaecides can be selective against algae. selectivity is depending on the species, dose and timing of application, product formulation, and water chemistry. Algaecides are commonly applied in slug dosages on a regular basis.

Biodispersants

Biodispersants are generally nonionic surfactants, they are typically used in combination with oxidizing and nonoxidizing biocides to enhance effectiveness.

Biodispersants can increase microbial control in cooling systems and are an essential component of comprehensive biocontrol strategies. These chemicals do not kill microorganisms, but they do assist break up biofilms and suspend bacteria in water, allowing biocides to more easily destroy them. Dispersants can inhibit or delay microbe adhesion to heat transfer surfaces. The main objective of a dispersant is to maintain the system surfaces clean from biofilms.

They are normally applied in open cooling systems and are compatible with other treatment chemicals, they also have a wide pH range of effectiveness, and are thermally stable at ordinary cooling water temperatures. Overdosing with a biodispersant can cause foaming issues and so a defoamer can be used to control it. Keeping in mind that the use of antifoam can diminish the efficiency of the biodispersant, thus both should be used with caution and at the optimum dose rate. Slug doses of biodispersants are typically used as a maintenance treatment for the system.