Secondary-treated municipal wastewater (MWW) is a promising alternative to freshwater for use in thermoelectric power plant cooling systems, especially in arid regions. One of the key challenges for successful use of MWW in cooling systems is potentially severe mineral deposition (scaling) on pipe surfaces. In order to develop effective scaling mitigation methods and advance the wastewater reuse, it is necessary to understand the scaling behavior of MWW with respect to mineral precipitation potential and chemical composition of solid precipitates under normal operating conditions. In this study, chemical equilibrium calculations conducted using MINEQL+ were compared with experimental results to evaluate the feasibility of using chemical equilibrium modeling for predictionof the scaling behavior of MWW in cooling systems. Equilibrium calculations for synthetic MWW indicated that dolomite and hydroxyapatite are the main mineral scales formed for a recirculating cooling system with MWW as makeup water operated at four cycles of concentration. However, solids characterization with X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) indicated that magnesiun calcium carbonate (with a saturation inclusion ratio of Mg at about 7%) and amorphous calcium phosphate were the predominant minerals precipitated from synthetic MWW in laboratory tests. Water quality monitoring revealed that MINEQL+ calculations over-estimated the precipitation potential of the synthetic MWW, which could be partially ascribed to the antagonistic effect of several co-existing species on precipitation. Additional analysis revealed that the incorporation of Mg into the lattice of calcium carbonate slowed the kinetics of precipitation but had negligible impact on equilibrium concentrations. Meanwhile, soluble orthophosphate could inhibit the precipitation and encourage the formation of more soluble amorphous calcium carbonate rather than stable calcite. Overall, it was found that chemical equilibrium modeling can be used for qualitative assessment of mineral precipitation potential but has limited utility in quantifying the scaling process.

Feasibility study on scaling prediction with MINEQL+ when using secondary-treated municipal wastewater in cooling systems

243rd National Meeting (2012)

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