A holistic view on microbial control in ammonia cooling water systems

Proactive monitoring and data management

Efficient removal of microbial contaminants is preferred to prepare water for optimised cooling water chemistry performance, which has a direct impact heat exchanger efficiency. The best treatment strategy requires a proactive approach that includes a monitoring plan built around key performance indicators, live transmittance of data and alarms for rapid response and service expertise to make changes to the treatment program based upon the data. Critical data points that are part of a monitoring strategy include application location, oxidation reduction potential (ORP), residual and bioactivity measurements from plating, adenosine triphosphate (ATP) and dip slides. Mining process information with key environmental and microbial data can generate algorithms to further optimise an ammonia plant’s microbial control performance and reduce total costs of operation.

Best practices with microbial chemistry

Chlorine gas, bleach (sodium chlorite), and bromine have long been used for microbiological control of cooling water systems. Unfortunately, these halogen-based oxidants have inherent limitations especially in “dirty” cooling water systems operating under alkaline conditions. Chlorine dioxide (ClO₂) has shown success in the ammonia industry, not only to address the performance challenges described above, but also meet growing safety, regulatory and environmental concerns.

Chlorine dioxide, a broad-spectrum biocide at low concentrations, is highly effective in ammonia process cooling waters under conditions unfavourable to chlorine. Because chlorine dioxide does not dissociate in water; its biocidal activity is not affected by system pH. Due to its low reduction potential, ClO₂ is not consumed by organic contaminants such as ammonia-nitrogen present in the cooling water. Consequently, the dosage required for biocidal control remains fairly constant over a wide range of cooling water conditions (Figure 1).

Figure 1. How system microbial chemistry impact dosage requirements. 

Safety, regulatory and environmental

In addition to its performance advantages, chlorine dioxide helps producers meet several safety, regulatory, and environmental initiatives. It eliminates health and safety risks relating to the handling, storage and integrity of one-ton chlorine gas cylinders. Under OSHA Standard 29CFR 1910.119, Process Safety Management (PSM), the threshold quantity (TQ) of chlorine is 1500 pounds. As a result, even a single ton cylinder on site obligates the user to comply fully with the requirements. The Risk Management Program (RMP) in Section 112(r) of the Clean Air Act requires those affected to go beyond PSM. For chlorine, the threshold quantity (TQ) is 2500 pounds and those affected must comply with the RMP Rule. RMP requirements include the preparation of emergency response programs, public notifications, and further record keeping. Chlorine dioxide avoids both the PSM and RMP requirements and the associated costs with record keeping and man-power. Since chlorine dioxide requires on-site generation, only small quantities are produced for cooling water applications thus keeping the quantities significantly below the 1000 pounds threshold.

Regarding environmental regulations, there are concerns with meeting discharge limits for trihalomethanes (THM). THMs are formed as byproducts when chlorine and bromine are used for water disinfection. THMs represent one group of chemicals generally referred to as disinfection by-products. They result from the reaction of chlorine or bromine with organic matter present in the water being treated. Chlorine dioxide is an excellent oxidizer with high efficacy at low dosage rates. Instead of chlorination it oxidizes and does not contribute to the formation of undesired THM by-products. Chlorine dioxide should be considered as an alternative if THM discharge limits cannot be easily met with the use of chlorine.

Conclusion

A comprehensive view on microbial control in a nitrogen processing facility is critical to help maintain and improve plant efficiency and overall total cost of operations. When combined with process information, key microbial information such as application location, residual, bioactivity and ORP can provide insights to optimise microbial control and improve plant capacity. Chlorine dioxide is a predictable, broad spectrum oxidizing biocide that provides effective microbiological and biofilm control in industrial cooling systems. When performance safety, regulatory, and environmental considerations are taken into account, the case for chlorine dioxide as the best available solution for microbiological control along with proactive monitoring and control is very compelling.

Author: Melissa Callejo, Senior Marketing Manager, Nalco Water

Read the article online at: https://www.worldfertilizer.com/special-reports/13032019/a-holistic-view-on-microbial-control-in-ammonia-cooling-water-systems/