A brief review of the literature on filtrate quality obtained in constant and variable declining rate operation systems is presented, with the general conclusion that both systems usually produce filtrate of similar quality, assuming the flow-rate controllers in the constant flow rate system operate properly. A mathematical model of CR (constant rate) filters and VDR (variable declining rate) filters based on the model by Mackie and Zhao (1999) has been described and used to investigate filtrate quality under different operating conditions. A similar quality of filtrate was achieved in computations carried out for both modes of operation, while the total head loss was much lower in the VDR mode. For the same time of filter runs slightly better filtration quality resulted from CR operation, but for different filter runs resulting from the same head loss of flow just before a backwash, the VDR control system produced lower turbidity filtrate.
- DECLINING-RATE FILTRATION
- deep bed filtration
- water treatment
- non-homogeneous suspension solids
- unit bed element model
- variable declining rate filter