The efficiency of poly-aluminium chloride (PACl) and of aluminium sulphate (alum), two commonly applied coagulant agents, was studied comparatively in this work, during full-scale experiments in a drinking water treatment plant. The removal of suspended solids (turbidity) and the residual aluminium concentrations were carefully monitored and they were used for the evaluation of effectiveness for each coagulant, as well as for the determination of optimal operative conditions. Two alternative treatment processes were examined: (a) the conventional coagulation-flocculation-sedimentation process, followed by gravity filtration through sand filter beds, and (b) the direct filtration process, i.e., coagulation-flocculation and sand filtration, but without the intermediate sedimentation step. PACl proved to be a more efficient coagulant than alum, as lower dosages of PACl, about 1.35mg Al/L in this case, resulted to the production of treated water with low turbidity and residual aluminium content. In addition, the direct filtration process through dual sand-anthracite filter beds was found to be equally sufficient, as the conventional one, i.e., when applying the sedimentation step; in this case, 0.70mg Al/L of PACl resulted in low turbidity water (around 0.1 NTU) and residual aluminium content (lower than 150g/L). In addition, the operation time of filters was extended to more than 24 hours.