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

► Gypsum scaling in a discharge pipe formed despite the use of an antiscalant (PC-191). ► The scaling was enhanced by a decrease in the reject brine pH. ► Batch experiments show that the antiscalant effectiveness decreases with pH decrease. ► Experiments are described with a single rate law and a pH-dependent adsorption model. ► Utilizing the laboratory data, the scale in the pipe is quantitatively described.