The effect of pH on the formation of a gypsum scale in the presence of a phosphonate antiscalant Original Research Article

A massive gypsum scale in a discharge pipe of a desalination plant occurred despite the presence of an antiscalant (PermaTreat, PC-191). The scaling occurred following a sharp drop in the reject-brine pH when an acidic effluent (pH < 2) was introduced. Batch experiments revealed that the antiscalant is effective in postponing nucleation and hindering the crystal growth kinetics of gypsum in the supersaturated brine (Ωgypsum > 2) at neutral pH. However, as pH decreases both nucleation and crystal growth accelerate.RateSBET=k1⋅(Ω1/2−1)10+k2⋅(Ω1/2−1)2,