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Long term deterioration of filtrate quality after an increase of flow rate through a filter

Congress: 2008
Author(s): Dąbrowski W., Piątkiewicz D.
Cracow University of Technology 24 Warszawska Street, 31-155 Kraków, Poland tel. +48-12- 6282551, fax. +48-12-6282042
AbstractLong term deterioration of filtrate quality after an increase of flow rate through a filter Dąbrowski W., Piątkiewicz D. Cracow University of Technology 24 Warszawska Street, 31-155 Kraków, Poland tel. +48-12-6282551, fax. +48-12-6282042 email: wdabrow@usk.pk.edu.pl Oral Congress Sub-themes 1. Development of water resources and infrastructure - Data, monitoring and information technology 2. Water availability, use and management - Water quality management: surface and ground water keywords : water treatment, depth filtration, filtrate quality deterioration Abstract Deterioration of filtrate quality resulting from sudden temporal changes of flow- rates has been intensively observed and investigated at least through the last forty years. As long as this time ago Baumann and Cleasby concluded that for the deterioration of filtrate quality the speed of the change is more important than its value. After forty years from that time this conclusion is still accepted. However, it refers to a sudden answer of a depth filtration system to a temporal, or to a permanent, change in flow rate. Experiments carried out on recirculation of filtrate in a closed loop, just after a filter run, proved that an increase in flow rate results in changing of a sediment structure, realizing some water from it. This can be easily explained by creating higher shear stresses on the boundary between recirculating filtrate and sediment trapped previously in the filter media. Such a process occurs continuously during depth filtration of coagulated suspensions as the shear stresses rise due to filter media clogging. However, the changes of sediment structure are more rapid if caused by a rise in filtration velocity. As such changes take some time in an order of magnitude let say two hours, so an extended deterioration of filtrate quality may likely occur. To verify this hypotheses a filtration column has been equipped with capillary sampling devices. Turbidity of suspension flowing through and of filtrate flowing out of the column were measured. The results clearly indicate possibility of occurring long term deterioration of filtrate after an increase of flow rate. This deterioration is not so visible as the one occurring immediately after the rise in flow rate but still is of some practical concern.
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