A new approach to control and monitor coagulation/sand ballasted sedimentation as pre-treatment of water feeding ultrafiltration membranes

The technological and economic efficiency of membrane water purification installations depends largely on the quality of the medium supplied to the devices carrying out the process. For typical technological solutions of desalination and demineralization systems, consisting of reverse osmotic membranes followed by nanofiltration membranes and electrodeionization units, pre-treatment of the feed water is required to meet the requirements of osmotic membrane manufacturers. The preliminary treatment of water aims to limit the phenomenon of membrane fouling, which is mainly caused by suspended solids, colloidal substances, and natural organic matter present in the water. The paper presents the results of research conducted in the technical coagulation/flocculation with sand ballasted sedimentation system followed by ultrafiltration units, used as preliminary water purification before osmotic membranes. Standard monitoring measurements were unable to detect situations where the process parameters deviated from the values determined during jar tests. Additionally, the obtained results of the research showed that coagulation/flocculation process can be conducted according to the criterion of minimizing the pressure loss during the filtration cycle and the strategy of maximizing pre-treatment effectiveness preceding ultrafiltration membranes. The condition for ensuring such parameters is the use of appropriate tools allowing for effective operational monitoring of the process. The results of the research on the application of a simple vacuum filtration test showed that it is a particularly good tool for monitoring and controlling the operational parameters of coagulation. During the research, high correlation was shown between the filtration time values of samples and the quality of the raw water feeding the treatment system, which affected the subsequent course of ultrafiltration. Similar dependencies were observed in clarified water supplied to UF membranes.