Influence of Drain and Operational Parameters on Backwash Cleaning Efficiency in a Commercial Sand Filter Model

ABSTRACT Some studies have demonstrated the influence of the drainage system design on the hydraulic behaviour and on the expansion of the filter bed during the backwashing process, with a presumed effect on removal efficiency. The objective was to evaluate the influence of the drainage system design of a commercial sand filter model, the operating time and the level of filter‐bed expansion on the cleaning efficiency of the backwash process. The experimental tests were based on the combination of two filter‐bed expansions (25% and 50%), three operating times (3.0, 8.0 and 15.0 min) and two drainage system designs (cylindrical drain—the manufacturer's original model; and proposed drain—developed with the aid of computational fluid dynamics). Overall, the proposed drain design performed better than the cylindrical model, particularly at 25% expansion. At 50% bed expansion, the best cleanings were observed for both drain designs, with the proposed drain standing out. Cleaning efficiency was lower in regions close to the filter walls, regardless of drain design, level of bed expansion or backwash duration. Regarding water turbidity, a reduction was observed in the first minutes of the backwash, indicating that the greatest solids removal occurs at the start of the process.