Effect Of Al-Fe Based Coagulants On Membrane Fouling In Coagulation-ultrafiltration Hybrid Process

Coagulation before ultrafiltration is an effective method to mitigate membrane fouling and the choice of coagulant plays an important role in the performance.In this study, kaolin-humic stimulated water was used in coagulation-ultrafiltration (C-UF) hybrid process and the effects of Al species in Al-based coagulants and polyaluminum ferric chloride (PAFC) on floc properties and membrane fouling were studied. The main content and results were as follows:Impacts of Al species on floc properties and membrane fouling in C-UF process with different pH conditions were investigated. Polyaluminium chloride (PAC) was prepared with alkali-titration method and Al speciation distribution was tested with Al-Ferron method. Three kinds of coagulants (AC (AlCl3), PACb, PACc) consisted primarily of Ala (monomeric species), Alb (medium polymer species) and Alc (colloidal or solid species) respectively were used as coagulants under pH conditions of 4.0,6.0 and 8.0. Floc properties were evaluated using a laser diffraction particle sizing device and ultrafiltration experiments were conducted by a dead-end batch unit. The results indicated that pH condition and aluminum species had significant influences on size and fractal dimension value of flocs. For the three Al species, relatively low pH condition was favorable to form larger flocs, while it would diminish the floc growth rate. And the loosest flocs were formed at pH 4.0. Under the same pH conditions, Ala tended to form the largest flocs and Alc achieved the loosest floc structure. For each coagulant, membrane fouling under different pH conditions was in the following order: pH 4.0> pH 8.0> pH 6.0. Alb and Alc showed a wider tolerance for changes of pH conditions than Ala. And under the same pH conditions, Alc achieved the lightest membrane fouling.Impacts of n(Al/Fe) in PAFC on floc properties and membrane fouling in C-UF process were investigated. Three kinds of PAFC with different Al/Fe molar ratios of 9:1, 7.5:2.5, and 6:4 respectively were prepared, and they were called PAFCo.i, PAFC0.25, and PAFC0.4 respectively. Properties and membrane fouling of PAFC was evaluated compared with PAC in C-UF process, and the impact of Al/Fe molar ratio on the performance was studied. The results showed that percent of Fe in PAFC have a major effect on membrane fouling performance, with a relatively low percent (PAFCo.i) reducing membrane fouling comparing with PAC, while a higher percent (PAFC0.25 and PAFC0.4) raised membrane fouling. And floc properties played a key role in affecting the extent of membrane fouling. Flocs formed by PAFC tended to be larger in size and looser comparing with PAC, and they were more obvious when the percent of Fe was relatively high. However, PAFC tended to form more fragile floc and the volume of floc was much larger than that of PAC. These are likely the reason why PAFC0.25 and PAFC0.4 caused more serious membrane fouling than PAC.Effects of the basicity of PAFC on the performance of coagulation-ultrafiltration hybrid process were also studied. Four kinds of PAFC with B value of 0.5,1.0,1.5 and 2.0 respectively were prepared when the Al/Fe molar ratio was 9:1 and 7.5:2.5 respectively. The results indicated that the basicity didn’t have distinct effect on the coagulation performance. However, there is an unquestionable link between basicity of PAFC and floc properties. For PAFC0.25, when B value was 0.5 or 1.5, floc sizes and floc strength were lower and when B value was 1.0 or 2.0, floc sizes and floc strength was higher. For PAFCo.i, size of flocs formed by different PAFCs with different basicity was in the order:B=1.5> B=2.0> B=0.5> B=1.0, and floc tended to be compact when B value was 1.5, while floc tended to be loose when B value was 1.0. Basicity also had great effect on membrane fouling in coagulation-ultrafiltration hybrid process for PAFCo.i. When B value was 1.5 and 2.0, the lightest membrane fouling was acquired and when B value was 1.0, membrane fouling performance was the most serious. The difference of floc size of different PAFCs could be the most important reason and large floc tend to be reduce membrane fouling in C-UF hybrid process.