Removal of Total Hardness from Surface water using Chemical Softening Techniques

Abstract

Water hardness, primarily caused by dissolved calcium and magnesium ions, is a common signif icant challenge in surface water sources, leading to scale formation and operational issues in both domestic and industrial systems. Conventional softening methods typically employ calcium hydroxide (Ca(OH)2), sodium hydroxide (NaOH), and sodium carbonate (Na2CO3) to precipitate hardness-causing ions. In this study, the combined use of lime (Ca(OH)2) and Polyaluminium Chloride (PACL) was investigated to en hance hardness removal through a series of controlled jar tests. Surface water samples collected from the Per Aru reservoir exhibited total hardness levels ranging from 120 to 400mg/L, determined by EDTA titration. Six parallel experiments were performed using a constant lime dose sufficient to achieve pH10.5, followed by varying PACL dosages. The experimental procedure involved rapid mixing at 250rpm for 1minute, followed by slow mixing at 40rpm for 10minutes, further mixing at 10rpm for 10minutes, and finally 10minutes of settling. Lime addition effectively raised the pH to promote precipitation of calcium and magnesium, while subsequent PACL dosing improved flocculation and particle settling. Results revealed that hardness removal efficiency was strongly influenced by pH, lime dose, PACL concentration, and the timing of PACL addition. The maximum hardness reduction (66.7%) was achieved with lime at pH10.5 combined with 28.5ppm PACL addition. However, higher PACL doses reduced the final pH from 10.5 to 8.46, thereby suppressing precip itation. Overall, the study demonstrates that optimizing the balance between lime and PACL dosages is essential for achieving hardness and turbidity removal in a cost-effective manner, offering practical guidance for surface water treatment applications.