Economic Analysis of Granular-Bed Filtration

There is a tradeoff relationship between the capital cost of the filters and the operating costs associated with backwashing. In general, as the size and, thus, the cost of the filter is reduced, the frequency and the costs associated with backwashing increase. The results obtained suggest that high rate filtration is economically justifiable for all practical filtration rates when the filter loading is within conventional limits. Higher loadings, such as may be encountered in direct filtration systems, may increase the backwash frequency to a point that an economic limit is set on the filtration rate. This limit may not be too much greater than the range of values used in conventional practice. Declining rate filtration appears to have inherent characteristics that make it, in general, more economical than the constant-rate approach. In comparing dual and mixed-media beds if the effluent quality is acceptable in both cases; whether or not the more expensive alternative is justified economically is determined by both the absolute and relative magnitudes of the filter-run lengths.