Impacts of advanced municipal wastewater treatment processes on monochloramine effectiveness in recirculating cooling systems

Chloramination is commonly used to prevent biological growth in drinking water distribution. Recent studies also demonstrated the feasibility of using pre-formed monochloramine in power plant recirculating cooling systems using secondary treated municipal wastewater (MWW) as makeup. Despite its weak oxidizing ability, monochloramine was effective in controlling biological growth in the cooling system and was more economical compared to other oxidizing disinfectants (i.e., free chlorine and chlorine dioxide). However, due to severe scaling observed in the systems using secondary effluent, it is necessary to employ advanced wastewater treatment processes to make municipal wastewater suitable as makeup up water. In the present study, nitrification, sand filtration, and granular activated carbon adsorption of secondary effluent were evaluated for their impact on the operation of recirculating cooling using municipal wastewater as the only makeup. The impact of these treatment processes on chloramination was determined based on the biocidal effectiveness against heterotrophic bacteria and Legionella, percentage of active monochloramine residual in the recirculating water, and total disinfectant requirement to achieve a desired residual.Laboratory- and pilot-scale studies indicated that a monochloramine residual of 2-3 mg/L as Cl2 is required to control biological growth in the cooling system using secondary effluent treated with nitrification and sand filtration (MWW_NF). Removal of ammonia and organic matter through nitrification significantly reduced the formation of organic chloramines and helped to maintain active monochloramine residual at 85% of total chlorine residual. Further removal of organic carbon by GAC showed no measurable improvement. Furthermore, monochloramine residual around 2 mg/L was unable to control the planktonic biological growth in GAC-treated MWW_NF (MWW_NFG). It is important to note that monochloramine was successful in controlling growth of Legionella in all three wastewaters during 2 months of pilot-scale testing. MWW_NF was also the most suitable makeup in terms of biocide requirement to maintain a desired residual. Use of nitrification and sand filtration of secondary treated wastewater is essential to effective use of monochloramine for biofouling control when using treated municipal wastewater as makeup in power plant recirculating cooling systems.