Biological Control and Biocides
Cooling water towers can be ideal breeding grounds for biological growth, including algae, bacteria, sulfate-reducing bacteria, protozoa, and fungi. If not properly controlled, these organisms will form a layer of bio-slime that acts as a natural adhesion surface for scale formation, resulting in increased corrosion rates, restricted water flow, and reduced system efficiency.
ChemTreat engineers begin their biological control program with a thorough survey of the cooling water towers to determine the exact type of biological growth control necessary. This information allows the engineer to select a customized treatment program to produce the best kill rate for specific organisms. Other considerations include:
- The volume of the water being treated as measured through a molybdate-tracer program or the system piping.
- The recirculating water pH to ensure the water is compatible with the selected biocides.
Once the treatment program is initiated, our engineers can continually monitor the biological activity, as biocides may need to be alternated to provide the best treatment. In most instances, weekly or bi-weekly alternation provides the best results.
We have a complete portfolio of biocides for controlling both macro/mollusk and microbiological growth:
Enzyme poisons are effective against sulfate-reducing bacteria. These compounds permeate cell walls and chemically react with enzyme functional groups to prevent them from performing their normal metabolic activities. Methylene-bisthiocyanate (MBT), heavy metals (such as tin), dithiocarbamates, and chloromethylisothiazolin are all enzyme poisons.
- MBT complexes ferric ions in the cell cytochrome and prevents the oxidation-reduction reaction necessary for cell respiration and energy production. Microorganisms do not develop resistance to MBT, which is also extremely effective against sulfate reducers. MBT hydrolyzes and loses half-life at pH values above 8.0. It is deactivated by high ferric iron concentrations.
- Heavy metals such as bis (tributyltin) oxide permeate cell walls and attack thiol and other functional groups essential to enzyme functions.
- Dithiocarbamates chelate metallic ions essential to organism metabolism in the cell. Dithiocarbamates also complex copper ions.
Oxidizing and Nonoxidizing Biocides
These compounds oxidize protein groups within the cell, resulting in the loss of normal enzyme activity necessary for respiration and cell metabolism. They also destroy cell walls.
Chlorine is the most widely used oxidizing biocide; however, some anaerobic bacteria, such as sulfate reducers, can develop resistance to chlorine. Other organisms can encapsulate and resist chlorine by moving to other locations in the system. Bromine donors are more effective at higher pH levels. All chlorine and bromine donors are oxidizing biocides, including stabilized isocyanurates and slow-release hydantoins.
In smaller systems, nonoxidizing biocide use has remained an industry standard. However, because a single biocide is usually not as effective as chlorine, two or more different products are normally needed to achieve adequate microbial control. Contrary to popular belief, cooling water microorganisms do not become immune to nonoxidizing biocides. This genetic property exists in viruses and insects, but not in algae, bacteria, or fungi. While waterborne bacteria do have limited adaptive defense mechanisms against enzyme poisoning and surface-active agents, they do not reproduce generations of new organisms immune to these biocides. Since all organisms are not equally affected by the three classes of biocide kill mechanisms mentioned, some organisms cannot be controlled for extended periods by any one biocide. When alternating biocides, it is essential to select products with different kill mechanisms.
Alternating Oxidizing and Nonoxidizing Biocides
Alternating chlorination with nonoxidizing biocide treatment provides the ultimate microbial control with minimal treatment costs. Alternating these biocides provides the following benefits:
- Unsurpassed bulk water cleanliness
- Reduced nonoxidizing biocide dosage and frequency
- Nonoxidizing biocide alternation is not needed
- Heat exchanger cleanliness is greatly improved
- Low overall treatment costs
- Each program’s limits are overcome by the other program
While these advantages have been obvious for a number of years, the problems of installing gaseous chlorine feed systems in all but the largest cooling water systems have discouraged this option. Organic, stabilized chlorine donors have solved this problem. Available in concentrated tablets with 90 percent available chlorine, these products can be manually slug fed or continuously fed using inexpensive basket or tube feeders. Storage and handling are also easy, as the products are available in plastic pails or fiber drums.
Cell Wall and Cytoplasm Toxicants
Cell wall and cytoplasm toxicants include polymeric amines, quaternary ammonium compounds, and chlorophenols. These materials are surface-active compounds that orient and absorb on cell walls and reduce cell permeability, thereby disrupting the normal flow of nutrients into the cell and normal waste discharges out of the cell.
Since their action is primarily mechanical in nature, relatively high concentrations are required. High suspended solid levels in the water absorb much of the biocide, requiring prohibitively high dosages and deactivating some of these products. The mechanical process also requires more time for system cleanup.
- Quaternary ammonium compounds attack phospholipids in the cytoplasmic membrane, causing lysis of the cytoplasm. Cationic materials also form electrostatic bonds with negatively-charged sites in the cell, blocking normal activity. Organisms acclimatize to the biocides by increasing the phospholipid content of the membrane and developing thicker cell walls.
- Chlorophenols absorb on cell walls through hydrogen bonding, penetrating the membrane and forming colloidal suspensions with the cytoplasm and precipitate protein. Though highly effective, these biocides are seldom used today because of their persistence in the environment and their manufacturing hazards.
- Glutaraldehydes react in a similar manner as chlorophenols, reacting with and precipitating protein groups in the cell nucleus. Kill mechanisms are rapid at high pH, but take more time in low-pH waters. Microbial adaptation is minimal, but ammonia or other primary amines, if present, will react with glutaraldehyde and reduce its effectiveness.
- Dibromo-nitrilopropionamide (DBNPA) functions as a cytoplasm toxicant, reacting with proteins in the cell nucleus. Like MBT, DBNPA hydrolyzes at pH values above 8.0, and the biocide’s half-life in the cooling water is greatly reduced. DBNPA also functions as an oxidizer by selectively brominating metabolic sites and destroying cell walls.
- Chloromethylisothiazolin functions by inhibiting respiration and food transport through the cell wall. As with all surface-active biocides, considerable retention time is required for isothiazolones to perform effectively.