“…In of filters may be optimized (27,43). Other research has been concerned with the characteristics of different filter aids and grades (2, 26,35,49), the evaluation of the filtering properties of various suspended solids (3,15,37,44,61), and the use of polyelectrolytes and aluminum sulfate to improve the filtering characteristics of filter aids (16,17,20,52,56,60). Extensive laboratory studies have been made to determine the applicability of precoat filters for the removal of iron (2,35,43,50,70), various flocculant solids (37, 44), clays (3,20,60,61), and suspended solids from coagulated and settled surface waters (15), raw surface waters, softened waters (15,27), and trickling filter effluent (36).…”