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.