(54d) Can Monochloramine Be Used to Control Biofouling in Recirculating Power Plant Cooling System Using Treated Municipal Wastewater?

Biofouling, which involves biological growth in both suspended and sessile form, reduces both heat exchanger and cooling tower efficiency in a thermoelectric power plant. This problem is further aggravated when treated municipal effluent, which contains high concentration of organic matter and nutrients compared to freshwater, is used in a power plant recirculating cooling system as an alternative to freshwater. Although secondary treatment and nitrification remove large fraction of biodegradable components, significant biocide addition is still required to suppress biological activity in cooling systems employing treated municipal wastewater.

The key objective of this study was to evaluate the effectiveness of chlorine-based biocides (sodium hypochlorite, pre-formed monochloramine, and chlorine dioxide) and determine their optimal chemical treatment regimen when using secondary and tertiary treated municipal wastewater as power plant cooling makeup. These three biocides are widely used in water and wastewater disinfection but their effectiveness for biofouling control in highly concentrated (4-6 cycles of concentration, as in recirculating cooling systems) and aerated wastewater has not yet been established. Optimal chemical regimens for achieving successful biofouling control were determined by monitoring biocide usage and heterotrophic planktonic and sessile bacterial populations in both laboratory- and pilot-scale studies. In addition, the cost and degree of environmental hazard associated with each biocide when used under optimal conditions were determined using Life Cycle Costing and Life Cycle Assessment tools.

Laboratory experiments indicated that a pre-formed monochloramine (MCA) dose of 2 mg/L was required to achieve industry biofouling control criteria (below 10⁴ CFU/mL for planktonic bacteria and below 10⁴ CFU/cm² for sessile bacteria) at 40 ºC, while sodium hypochlorite and chlorine dioxide required slightly lower initial doses to achieve the same biocidal efficiency. However, pilot-scale studies revealed that less pre-formed monochloramine is needed to maintain sufficient residual and suppress biological growth. Although laboratory studies demonstrated the ability to form MCA in treated municipal wastewater through the reaction between added free chlorine and inorganic ammonia present in the treated wastewater, the addition of free chlorine in pilot-scale studies to achieve a total chlorine residual of 3-4 mg/L resulted in only 23% conversion to MCA. It was observed that 91% of inorganic ammonia in the incoming secondary effluent was stripped out in the cooling tower, leading to unstable in-situ MCA formation. A cost analysis for these biocides was performed based on the results of pilot scale experiments to provide relevant practical information for wastewater reuse.  This analysis confirmed that the addition of pre-formed MCA for biocidal control in power plant recirculating cooling systems using treated municipal wastewater is effective and deployable at modest cost. Results of Life Cycle Assessment also suggested that pre-formed MCA has the least environmental and human health impact for this application.