An overlooked crystallization effect during the O3 participated coagulation improves the performance of dual-membrane process

Till now, the transformation of amorphous flocs formed during the hydrolysis stage of coagulation has not been fully addressed. In this work, by using a conventional coagulant (alum), the transformation process was explored by observing the interactions between O 3 and the amorphous alum flocs formed during flocculation at neutral pH. It was found that O 3 promoted the transition of the in-situ formed alum flocs from tetrahedral Al (Al(O) 4 ) to octahedral Al (Al(O) 6 ) structures, through an accelerated dehydroxylation process, and therefore facilitated the crystallization process to form boehmite . During this process, the surface functional groups of amorphous Al flocs ( Al–H 2 O and Al–OH) were observed to take part in the O 3 catalytic oxidation with the generation of hydroxyl radicals ( OH). This synergistic effect significantly favored the performance of the subsequent dual-membrane (UF-NF) process in terms of both fouling alleviation (1.78 and 1.60 times higher flux of UF and NF , respectively) and final effluent quality improvement, including the reduction in disinfection byproduct (DBP) formation potential (48.1% and 22.4% lower generation of total trihalomethanes and total haloacetic acid, respectively) and the corresponding calculated toxicity. The results obtained here provide a greater understanding of the transformation of amorphous Al flocs formed during flocculation with O 3 , and valuable information regarding its practical application in membrane filtration process with greater treatment performance.