Effect of the removal of DOMs on the performance of a coagulation-UF membrane system for drinking water production

Coagulation with only rapid mixing in a separate tank (ordinary coagulation) and coagulation with no mixing tank (in-line coagulation) were applied prior to ultrafiltration with an inside-out type hollow fiber membrane. In result the filterability at the former conditions was superior in both the crossflow and dead-end modes. Thus the relative importance of the removal rate of DOMs was investigated. Precoating the surface of the membranes with metal hydroxide particles of coagulants was also examined. This method of utilizing coagulants resulted in a smaller consumption of coagulant in the coagulation-UF system.

In the crossflow mode the filterability of a membrane is enhanced by coagulation pretreat- ment, and of the two conditions using a PACl dosage of 4.1 ppm as Al2O3 that at ordinary coagulation conditions was superior to that for in-line coagulation conditions. The removal of DOMs appears to be responsible for the difference in filterability. In the dead-end mode, like the crossflow mode, membrane filterability was improved by the use of coagulation pretreatment, and that at ordinary coagulation w as better than that for i n-line coagulation. In this case the characteristics of the flocs as well as the removal of DOMs as a function of coagulation conditions had an effect on filtration. In the precoating experiments filterability using PACl at 4.1 ppm was worse than that of ordinary o r i n-line coagulation c onditions because of the resistance of aluminum hydroxide particles themselves. However, the reduction of the dosage to 2.0 and 1.0 ppm caused a reasonable improvement in filterability. Thus, it appears that the precoating method could lead to decrease in the consumption of coagulant. Meanwhile filterability for 13.0 ppm ferric chloride as FeCl3 was better than that for 4.1 ppm PACl, because the iron hydroxide particles formed a very permeable cake layer. Thus, in the application of the precoating method as a membrane pretreatment, iron hydroxide particles were found to be more suitable than aluminum hydroxide particles.