Scaling Control for Heat Exchangers in Recirculating Cooling Systems Using Treated Municipal Wastewater

Abstract

Treated municipal wastewater (MWW) is recognized as a viable alternative cooling water source for power generation. One of the key challenges for the successful use of the effluent from wastewater treatment facilities for cooling is the potential for significant condenser fouling when this water is concentrated as much as 4–6 times in recirculating cooling systems. In this study, two types of treated municipal wastewaters, namely secondary-treated MWW with pH adjustment (MWW_pH) and secondary-treated MWW subjected to nitrification and sand filtration (MWW_NF), were evaluated as the sole source of makeup water for recirculating cooling systems in both laboratory and pilot-scale studies. The tests revealed that synthetic MWW_pH had significant crystalline fouling potential on the condenser surfaces when the pH was adjusted at 7.8 and that hydroxyapatite was the main component of the mineral scales formed on condenser surfaces. Addition of antiscalants, e.g., polymaleic acid (PMA), was shown to be effective in fouling mitigation by inhibiting the transformation of amorphous calcium phosphate to hydroxyapatite. In the case of synthetic MWW_NF, significant crystalline fouling was observed at pH 7.2 while bulk precipitation reduced the driving force for crystalline fouling when pH was adjusted to 7.8. Pilot-scale studies with actual MWW revealed that the addition of PMA alone is not sufficient to control scale buildup and that controlling pH at 7.8 is needed to ensure proper operation of the cooling tower, as predicted by laboratory studies with synthetic wastewater. Alternatively, nitrification and sand filtration would accomplish the same goal without any chemical addition. Overall, this study demonstrated that it is possible to manage crystalline fouling on hot condenser tube surfaces associated with the use of treated municipal wastewater just by pH control and antiscalant addition and that significant additional treatment of municipal wastewater prior to use in recirculating cooling systems may not be necessary.