Brine-free softening of tap water using tapioca mono-starch phosphate in a hybrid ultrafiltration system: adsorption kinetics, isotherms and fouling behaviour

ABSTRACT Hardness removal remains critical for industrial water reuse; however, conventional ion exchange (IX) generates chloride-rich brines, while lime–soda softening requires strict pH control and produces large volumes of sludge. This study evaluates a brine-free softening approach using tapioca monostarch phosphate (TMSP) integrated with ultrafiltration (UF) as a polymer-enhanced ultrafiltration (PEUF) system. Batch screening of modified cassava starches identified TMSP as exhibiting the highest hardness affinity due to TMPS functional groups, with an apparent uptake of 7.6 mg as CaCO3 g−1. Adsorption kinetics followed a pseudo-second-order model (R2 = 0.998), indicating chemisorption-dominated Ca2+/Mg2+ binding. Equilibrium data were better described by Freundlich and Temkin isotherms (R2 ≈ 0.91), whereas Langmuir fitting yielded unrealistic parameters. The optimised starch was applied in a continuous UF system under constant transmembrane pressure. At an initial flux of 25 L m−2 h−1 and a 50:50 permeate-to-retentate ratio, the TMSP–UF process achieved 40–48% hardness reduction with stable operation >60% flux decline. Lime–soda pretreatment followed by TMSP polishing reduced total hardness to approximately 48 mg L−1 as CaCO3 at near-neutral pH. These results demonstrate the potential of a starch-derived, brine-free PEUF system as a low-residual option for partial softening in industrial reuse and ZLD.