The application of calcium sulphate dihydrate (CaSO 4 ·2H 2 O) as a coagulant-flocculant alternative to the conventional use of aluminium sulphate in the primary treatment of wastewater was evaluated using a jar test apparatus. Samples from the State Water Commission (CEA) in Queretaro, México, were collected for the experiments. Turbidity and pH were measured before and after applying the calcium sulphate dihydrate (CaSO 4 ·2H 2 O). Turbidity readings obtained for the doses of 4 g·ℓ -1 of

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... nium sulphate varied from 3.91 to 3.87. The corresponding water pH was 3.90, giving the water an acidic character. Use of aluminium sulphate in the clarification of wastewater, thus, has financial and environmental implications due to the need to raise the pH of the treated water to 6.5-8.5, the recommended optimum interval for the physical-chemical-biological removal of pollutants. By contrast, calcium sulphate di-hydrate (CaSO 4 ·2H 2 O) (gypsum) doses of 1, 1.5 and 2 g·ℓ -1 resulted in a pH of between 7.04 and 7.51 repeatedly. These findings suggest that the application of calcium sulphate di-hydrate (CaSO 4 ·2H 2 O) as coagulant-flocculant, followed by the process of sedimentation, may be a suitable alternative for the clarification of wastewater. However, the turbidity reported for the same doses was 74.05, 80.5 and 74.5 NTU, respectively, well above the international standard of 5 NTU. The effect of gypsum on turbidity warrants further research.