IWRA Proceedings

AbstractUsually, rapid filters are operated under constant flow rate, eventually constant head loss. In practice, filters cooperate one with each other and the filtration rate in time. Variable Declining Rate Filters operate with orifices installed at outflows instead of flow rate controllers. Appropriate relations between laminar head loss of flow through filter media and turbulent head loss of flow through orifices let to keep declining filtration velocity between limited values. Lower hydraulic load at the end of filter runs under Variable Declining Rate control system results in rarer backwashes of the filters and give financial profits. Smooth change of declining filtration velocity during the run and low value at the end of filtration cause that quality of total outflow from Variable Declining Rate Filtration plants should not be poorer than from traditionally operated filter plants. Experimental research has confirmed that theory. Poorer filtrate quality do not follow economic profits of Variable Declining Rate Filters. Results, even revealed that quality of filtrate produced under Variable Declining Rate Filtration was better for higher ratio of maximum to average rate filtration. Similar conclusions were received from numerical simulation realized based on dynamic model connecting equations describing hydraulic work of declining rate filters and filtration kinetic equations compiled from theory of limited trajectory. The computer simulation considered particle size distribution analysis, for Variable Declining Rate operation led to improved removal of smaller particles, but also a decrease in removal efficiency for larger particles was noticed.