Coagulation-flocculation of surface water using natural Algerian calcium sulphate: a bio-inspired approach to flocculant efficiency

Surface water contamination, particularly due to turbidity, colour, phosphate ions, and ferrous ions, presents significant challenges to water treatment processes. This study evaluates the effectiveness of Algerian natural calcium sulphate (CS, CaSO₄·xH₂O) as a flocculant in the coagulation-flocculation-sedimentation process aimed at reducing these parameters when they exceed permissible limits. The treatment parameters were optimised by adjusting the doses of the coagulant (ferric chloride) and the flocculant (CS, calcium sulphate), as well as manipulating pH, rapid mixing speed, slow mixing speed, agitation time, and settling time. The optimal dose of ferric chloride was determined to be 25 mg/L, while the optimal calcium sulphate dose was 25 mg/L at a pH of 8. Under these conditions, the treatment process achieved the following removal efficiencies: 52% for turbidity, 100% for colour, 52% for phosphate, and 86% for ferrous ion. The study further determined that the ideal rapid mixing speed is 120 rpm for 20 minutes, sufficient to neutralise colloidal particles and initiate flocs formation. For stable flocs development without disruption, a slow mixing phase of 30 rpm for 30 minutes was identified as optimal. A settling time of 30 minutes, following the slow mixing phase, provided the best correlation between treatment efficiencies for turbidity, colour, phosphate, and ferrous ions removal. These findings highlight the potential of calcium sulphate as an effective, sustainable, low-cost and environmentally benign flocculant for treating surface water contaminated by agricultural and industrial effluents.