Best Practices for Nitrite Closed Loop Programs

Posted by Doug Frassa on Jul 29, 2015 8:00:00 AM

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This article is Part 3 of 3 of a series on nitrite closed loop treatment programs.

In Part 1, we introduced nitrite programs for closed loops, and in Part 2, we discussed the nitrogen cycle and how biological problems will show themselves with nitrite based programs by consuming any available nitrite in the system. In Part 3, we will discuss the consequences and ways to correct program deficiencies with nitrite treated systems.

CONSEQUENCES

The use of Sodium Nitrite in a closed system as a corrosion inhibitor at levels of 800 - 1500 ppm of nitrite offers an excellent source of nutrition for the three processes described earlier. Most closed recirculating water systems will have areas of anaerobic and aerobic conditions. Therefore it is possible to have more than one process operating within the same system. At the present time there are no field tests available to determine either qualitatively or quantitatively if these processes are occurring.  

Therefore, a system experiencing a drop in nitrite levels should be investigated with particular attention given to the following:

  1. Corresponding increase in nitrate levels
  2. Corresponding increase in ammonia levels
  3. Microbiological activity increase
  4. Corresponding loss of alkalinity
  5. Addition of make-up water (decrease in conductivity)

If it is suspected that one or more of the microbiological processes described here is operating within the system, it is imperative that the condition be addressed immediately. The loss of inhibitor levels can lead to excessive corrosion within the system. In addition, low levels of ammonia in the presence of low levels of oxygen have been shown to cause severe corrosion cracking. Copper corrosion is accelerated by the presence of ammonia. 

CORRECTIVE ACTIONS

  • Clean the system – If cleaning and purging the system is an option, it highly recommend that the water circuit, including all dead legs and isolated piping, be purged and sanitized with peroxide or chlorine dioxide. The system should then be retreated with a nitrite-free corrosion inhibitor and occasional doses of an appropriate biocide.
  • Filtration - Side stream filtration is highly recommended for most closed systems and is considered a best practice. Besides removing corrosion by products and debris that will cause under deposit corrosion and growth sites for microbes, filtration helps to remove to the dead microbiological matter which will eliminate a source of nutrients for these processes.
  • Biocide selection – Glutaraldehyde and Isothiazoline biocides are typically recommended but have been shown to be marginally effective in control these types of microorganisms. For this reason, the occasional addition of these biocides may not arrest the problem.  
Here is a summary of various biocidal options that may typically be used for  treatment in a closed system.
  • Glutaraldehyde is readily biodegradable and can add nutrients for biological activity. It reacts with ammonia and the rate of killing reaction is slowed.
  • Isothiazoline presents handling and skin sensitizing concerns. Copper versions will increase bulk water copper concentration over time.
  • Use of biodispersants and the microbicide TTPC has been shown to be extremely effective in mitigating the results of denitrification and ammonification.
  • Oxidizing biocides are not recommended for closed systems for routine biological control. These would include sodium hypochlorite, stabilized bromine/chlorine and chlorine dioxide. As referenced above, some oxidizers may be used to clean up a system for a one time treatment.

It is worth mentioning in this part of the article that when a water treater is presented with ongoing biological problems with a nitrite program that are not easily resolved, one of the common ways to combat this issue is after a proper cleanup, to convert to an alternative chemistry such as molybdate. Molybdate based programs, for example, do not pose the risks of biological problems as a nitrite program does but may be a more costly program in many cases.  

While the use of sodium nitrite in closed water systems has merit as an inexpensive and very effective corrosion inhibitor, very careful consideration should be given to the proper application of this program to avoid the potential downsides discussed in this series.

Learn more about Sodium Nitrite in Closed Loop Systems by downloading your copy of our guide today.

Download the Sodium Nitrite Guide

 

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