Effect of initial LSI on wastewater softening: simultaneous removal of polyacrylamide residues

Industrial effluent was tested for softening and polyacrylamide (PAC) residue removal and correlated with the Langelier saturation index (LSI). Contrary to previously reported methods, the initial LSI (LSIin) was calculated using the pre-reaction pH, the inherent alkalinities of the original wastewater and the alkalinities added during softening, under the assumption that no physical or chemical reaction had started. During the softening optimization tests, higher LSIin predicted efficient softening, which was further confirmed by lower final LSI (LSIfin). The NaOH dose adjustment was more effective with 100% softening than that achieved by adjusting initial pH. Calcite seeding was more effective at pH 7 with lower LSIin in the range −2 to 2 than that at pH 10 with LSIin in the range 1–4.7. During NaOH dose optimization, LSIin was higher and LSIfin was lower at a higher soda ash to hardness molar ratio of 1.3:1 than that at a lower ratio of 1:1. Compact accelerated precipitation softening process removed 60% of the calcium and 41% of the PAC residues because calcium carbonate precipitates worked as a coagulation aid. Direct correlations between LSIin and softening efficiency in all processes (R2 = 0.95–0.98) confirmed the importance of LSI and explained the effects of calcite addition.