Introduction to Biocides

Bacterial growth in a cooling water system is a major concern that can cause setbacks in an operation. It can result in the formation of biological fouling, or biofilm, on surfaces in contact with water. Biofilm acts as a thermal insulator that decreases heat transfer efficiency in the production equipment. In turn, corrosion rate increases due to formation of anaerobic areas underneath, which creates galvanic couple corrosion and forms metabolic byproducts (e.g. hydrogen sulfide) that attack base metals. Severe cases have resulted in cooling system failures due to biomass plugging.

Waterborne disease is another safety concern because of the possibility of bacterial infection upon human contact. These could all be controlled by maintaining a biologically clean system through periodical treatment of the cooling system with a biocide.

A biocide is a substance that can deter, control, or render harmless any organism through chemical or biological means. It works effectively when a critical dosage is reached and maintained within a certain time frame. Effective critical dosage point and time required varies substantially depending on the type of biocide and the overall condition of the system. Biocides have two major classes: oxidizing and non-oxidizing types.

And oxidizing biocide destroys the cellular structure of an organism. Because it attacks through destructive means, it is virtually impossible for organisms to develop immunity against the treatment substance. Its low unit cost, rapid action against the target organism, and low effective dosage makes this chemical cost effective. But this is not to say that oxidative biocide is without its drawbacks. Some substances can decrease system water’s pH  uncontrollably, while others increase the corrosive nature of cooling water. Some, still, produce undesirable byproducts that are potentially harmful for the environment, particularly chlorine. Corrosion and scale control chemicals are rendered inactive upon contact with some oxidizing biocides, and almost all of these treatment substances cannot penetrate and/or remove organic slime layers.

On the other hand, a non-oxidizing biocide interferes with the metabolic activity of an organism. Because of the bacterial variety, immunity to a particular biocide will be more likely with this type. Follow-up doses will become less potent as microbial population shifts to varieties that they are immune with. They can also be costly in the long run because of the high dosage required, long contact periods, and high unit cost. However, this treatment can function despite the process contamination, and doesn’t contribute to system corrosion and scaling.

To solve this conflict, suppliers use both types of treatment alternately in weekly or biweekly fashion. There are also products that offer both of these types, so called all-in-one packages, although these may not be effective in controlling biological fouling because of the inability to alternate biocides or adjust it's dosage, independent of the other inhibitors. The nature of your water system should be studied well in order to determine the best type of treatment possible.

References:

Biocide. (n.d.). Wikipedia, the Free Encyclopedia. Retrieved July 8, 2011, from http://en.wikipedia.org/wiki/Biocide

Keister, T. (2006, May 5). Managing Cooling Water: Preventing Biological Fouling - Archives - Process Cooling. Process Cooling. Retrieved July 8, 2011, from 

http://www.process-cooling.com/CDA/Archives/ce9094fe7160b010VgnVCM1000