Impact of Tertiary Treatment Processes on the Effectiveness of Chloramination for Biological Growth Control in Recirculating Cooling Systems Using Treated Municipal Wastewater

摘要

Adequate biocide addition is the key to control biological growth-related problems in recirculating cooling systems of thermoelectric power plants. The use of monochloramine (MCA) as the primary biocide is as effective as the use of free-chlorine in cooling systems using secondary-treated municipal wastewater (MWW) as the sole makeup water source. However, severe scaling caused by the secondary effluent necessitates incorporation of an additional treatment of secondary effluent (i.e., tertiary treatment) prior to use as makeup water for recirculating cooling systems. In the research reported in this paper, the effectiveness of MCA as a cooling-system biocide was evaluated for three types of tertiary-treated MWW, as follows: (1) acidification, (2) nitrification and sand filtration, and (3) nitrification, sand filtration, and granular activated-carbon adsorption. The impact of these tertiary treatment processes on chloramination was studied in both laboratory and pilot-scale experiments. For the third type of MWW treatment, reducing the total organic carbon (TOC) content in MWW exerted a 37% decrease in biological growth potential but less than a 5% decrease in biological growth rate. A MCA residual of 2C3 mg=L was required to achieve biological-growth control objectives with respect to planktonic and sessile heterotrophic bacteria in cooling systems regardless of the tertiary treatment employed. Legionella analyses indicated that chloramination was effective in controlling proliferation of these species for all tertiary-treated wastewaters. Overall, preformed MCA was effective as a primary biocide in all tertiary-treated MWW evaluated in the research reported in this paper. Nitrification followed by sand filtration was the optimal tertiary treatment among the three investigatedMWW because the biological-control objectives could be achieved with lower doses. The use of granular activated-carbon adsorption revealed no additional benefit compared to nitrification and sand filtration