Influence of velocity gradient in a hydraulic flocculator on NOM removal by aerated spiral-wound ultrafiltration membranes (ASWUF)

A hydraulic coagulation-flocculation processes combined with aerated spiral-wound ultrafiltration membranes (ASWUF) was designed with the objective of improving natural organic matter (NOM) removal by ASWUF in the treatment of water for human consumption. The pilot-scale experimental system had capacity for treating 0.9 m3/h. Dosage of Cl3Fe as coagulant and hydraulic retention time (HRT) were calculated to generate microflocculation and different velocity gradients (G = 27, 47, 87 and 104 s−1) were applied in the hydraulic flocculator. Operating alone, the ASWUF system achieved an NOM removal performance of 39% without problems of membrane clogging, although there was a significant correlation between effluent and influent quality. Application of microflocculation achieved considerable improvement in NOM removal, but values of G ≤ 87 s−1 resulted in rapid clogging of the membrane due to flocs disintegration in the aerated membrane tank. Particle analysis revealed that the reduction of the velocity gradient had the effect of inclining the particle size distribution towards larger sizes, affecting both NOM removal capacity and membrane clogging. For G = 104 s−1 an NOM removal yield of 90% was reached, while transmembrane pressure (TMP) was stabilised as a result of the control of membrane clogging.