A methodology for calcium sulfate (gypsum) scale control in nanofiltration of saline waters is presented. The methodology involves the use of both theoretically and experimentally determined parameters. Pitzer's thermodynamic equations for electrolytes are used to determine the gypsum scaling potential of the feed water based on its ionic composition, whereas the extent of concentration polarization at the membrane surface is determined from the film model. A proportionality factor that relates the kinetic difference between the saturation predicted by the gypsum solubility model and the actual crystallization is determined using data from glassware crystallization experiments. The last step involves an experimentally developed parabolic equation relating antiscalant (polyacrylic acid) dose to the normalized concentration factors of the saline solution. These parameters are combined into a single model for predicting the required antiscalant dose to control calcium sulfate scale in nanofiltration membranes for any given saline solution. The model is tested by nanofiltration membrane experiments utilizing model solutions simulating saline agricultural drainage waters.
A recent project focused on the recycling challenges posed by the potential presence of the pathogen Cryptosporidium in microfiltration (MF) residuals. After reviewing the various methods available for treating MF backwash streams, clarification was selected because it (1) is successful in treating spent filter backwash water from conventional plants, (2) is economical, and (3) is familiar to water plant operators and US regulators. Successful results from bench‐scale treatability tests guided the pilot‐ and full‐scale studies. Microbial performance indicators for the pilot test included inactivated Cryptosporidium oocysts and aerobic endospores. At a sedimentation overflow rate of 0.25 to 0.5 gpm/sq ft (0.6 to 1.2 m3/m2/h) and with the appropriate coagulant type and dosage, coagulation/clarification of MF backwash streams produced a recycle quality similar to the original raw water. Unlike spent filter backwash water from conventional water treatment plants, polymer alone was not sufficient to treat MF backwash because the feedwater had not been previously coagulated with a metal salt coagulant.
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