where Re = effective filtration rate (m/ hr or gpm/sq ft), Rd = design filtration rate (m/hr or gpm/sq ft), UFRV = unit filter run volume (m/run or gal/sq ft/ run), and UBWV = unit backwash volume (m/run or gal/sq ft/run). Figure 1 employs this equation to show the relationship between the production efficiency (Re/Rd) and the UFRV for various UBWVs. A rapid decrease in production occurs when the UFRV drops below approximately 200 m/run (5000 gal/sq ft/run). In order to develop a design having adequate efficiencies, it is recommended that a UFRV goal be set between 200 and 400 m/run (5000 and 10 000 gal/sq ft/run). This would correspond to run lengths of between 16 and 33 hr at a filtration rate of 12 m/hr (5 gpm/sq ft). In plants having dual media, water backwash, and water scour, unit backwash volumes generally range from 4-8 m/wash (100-200 gal/sq ft/wash). Although the UFRV is a convenient tool, evaluating the overall efficiency of filtration operation is not quite as simple