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Biocides

Abstract

Cooling water provides an ideal environment for biological growth, including Legionella. Because Legionella bacteria can be a health hazard, treating your cooling water properly with the most effective biocide (bacterial disinfectant) is an extremely important task.

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Cooling Tower Biocides

Cooling water provides an ideal environment for 
biological growth, including Legionella.  Because Legionella bacteria can be a health hazard, treating your cooling water properly with the most effective biocide (bacterial disinfectant) is an extremely important task.

There are two main classes of biocide:

  • Oxidizing
  • Non-oxidizing.

The difference is found in how the chemical attacks and kills bacteria.

Oxidizing Biocide

Oxidizing biocide kills bacteria through the electrochemical process of oxidation, where the biocide gains an electron from the bacteria, and this electron loss essentially kills the bacteria. Oxidizing biocides are typically made up of chlorine or bromine, and these halogens are the oxidizing agents disrupting the biological cell. Oxidizers are generally most effective in systems that have high water usage.

Non-Oxidizing Biocide

There is a wide variety of chemistry related to non-oxidizing biocide.  Non-oxidizers are typically used on a prescriptive basis and introduced when there are special challenges with the water relative to the particular biological growth, the quality of the water, the pH, discharge limits/toxicity, metallurgy and chemical safety. This biocide class includes DBNPA, Isothiazoline, Glutaraldehyde and Quaternary Ammonium.

Biocides are obviously only effective if they come in contact with the bacteria within the system. Ensuring proper contact time and adequate dosage in the system is vital to keep bacteria under control in your tower.

Oxidizing biocides typically take a couple of hours of contact time at a specific dosage, while non-oxidizing biocides should remain in the system much longer. Other factors that inhibit biocide effectiveness include dead legs (areas of stagnation) in the piping, off-line pumps and filters and areas of low flow.

Reverse Osmosis Biocides
  - Non-oxidizing, non-ionic biocides can be used either on line or as part of a cleaning program to control biofouling in reverse osmosis (RO) membranes. When used as an on-line treatment, the biocide should be dosed prior to the RO system to control bio-growth in the membranes.

 

Application frequency of any selected biocide will normally depend on biological loading and biofilm growth rate. The program should be used to control differential pressures in the RO plant to reduce cleaning frequency. Application rates and frequency of application (biocide program costs vs. effectiveness)  vs. reduction in cleaning frequency, down time, membrane life (operating costs without biocide program) should be balanced to determine the most cost effective biocide dosing frequency.

Dosing biocide further back in the pre-treatment will help control bio-growth but will also greatly increase demand and application costs. The main goal of an effective biocide program is to control biofouling in the membranes to an acceptable and cost effective level compared to cleaning program costs (and other associated costs).

The two ROC products widely used are 2,2-dibromo-3-nitrilopropionamide (DBNPA) and isothiazolone. These products are fully compatible with polyamide (PA) and cellulose acetate (CA) membranes. We also offer two high performance alternatives, chlorine dioxide and a hydrogen operoxide solution synergised with colloidal silver.

DBNPA

DBNPA is fast acting and readily decomposes to harmless by-products on discharge. On-line, it is dosed just prior to the RO system to control biofouling in the membranes. Dose rate is typically 100 ppm for 1 hour. Frequency of application depends on degree of biological contamination in the feed water and rate of biofilm growth in the membranes. Typically frequency of application can vary from every other day to once a month. The half-life of DBNPA is reduced with the increase of pH. In high pH feed waters (> 8.5) the dose rate and contact time should be doubled.

DBNPA biocide should not be dosed with stainless steel injection quills as corrosion of the injection assembly will occur.

Isothiazolone

Isothiazolone has a longer contact time than DBNPA and Isothiazolone is more effective than DBNPA in waters with high organic loading.
 

Dosage Rate Description Product
Based on Parsian Farab Instruction  Cooling tower biocides based on Quaternary Amunium  Hypotreat 1000
Based on Parsian Farab Instruction Cooling tower biocides based on Quaternary Amunium Hypotreat 2000
Based on Parsian Farab Instruction R.O biocides based on Izorhiazoline Hypotreat 3000
Based on Parsian Farab Instruction Cooling tower biocides based on Izorhiazoline Hypotreat 4000
Based on Parsian Farab Instruction R.O biocides based on DBNPA Hypotreat 5000
Based on Parsian Farab Instruction Cooling tower biocides based on Izorhiazoline Hypotreat 6000

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