To evaluate backwash performance, it was necessary to dirty the filter bed of the analyzed filters (F1, F2 and F3) by applying four filtration rates (TF20-20 [m.sup.3] [m.sup.-2] [h.sup.-1], TF40-40 [m.sup.3] [m.sup.-2] [h.sup.-1], TF60-60 [m.sup.3] [m.sup.-2] [h.sup.-1] and TF75-75 [m.sup.3] [m.sup.-2] [h.sup.-1]) in three silica sand granulometries (G1, G2 and G3), with a 4-h filtration in each experimental combination.
The sand granulometries, referred to as G1 (fine: effective diameter--[D.sub.10] = 0.55 mm and uniformity coefficient--UC = 1.34), G2 (medium: [D.sub.10] = 0.77 mm and UC = 1.28) and G3 (coarse: [D.sub.10] = 1.04 mm and UC = 1.36), were used with a fixed filter bed height of 0.35 m, according to the recommendations of the manufacturer.
After the filtration step, backwash trials were performed in each filter individually and the procedure consisted in applying a flow rate from the fourth filter of the module (FR) in the opposite direction of the filtration process, in order to generate a filter bed expansion of 25%, according to the guidelines of Brouckaert (2004), which corresponded to the height of 0.44 m.
Backwash was evaluated based on the collection of sand samples at different depths inside the filter bed (four sampling points in the filter bed profile).
1 and, based on its sum, the total mass of solids present in the filter bed ([MS.sub.Ftotal]) was estimated.
[MS.sub.Fi]--mass of solids present in the filter bed after filtration trial (degree of soiling) in each fraction of volume of the filtering layer i (g); and
2 was used to determine the mass of solids removed in each backwash trial in the filter bed individual layers ([MS.sub.Ri]) and, based on its sum, the total mass of solids removed in the backwash process ([MS.sub.Rtotal) was estimated.
[MS.sub.DRi]--mass of solids present in the filter bed after backwash trials, in each fraction of volume of the filtering layer i (g);
[E.sub.L] - cleaning efficiency for the entire filter bed (%).