The effect of rainwater tank design on sediment re-suspension and subsequent outlet water quality

This paper reports the results of laboratory experiments investigating the effect of rainwater tank design on re-suspension of accumulated sediment and the resultant water quality. Twenty-eight different configurations that represented different tank designs in combination with various water levels and quantities of stored sediment were investigated. Re-suspension of sediment was observed during all experiments regardless of the position of the top inlet (whether side or centrally positioned), position of the bottom outlet (50, 100, 200, 400 and 600 mm above the base of the tank), the shape of the tank base (flat or conical), the initial water level in the tank (empty, quarter full or half full), the sediment thickness (10 or 20 mm), the particle size (two particle size ranges) or inflow rate (0.5 or 1.0 L/s). The re-suspended sediment contaminated the out-flowing water, and the greatest impact (worst outcome for water quality) was observed for a centrally located top inlet and an outlet located 50 mm above the tank base. The least contamination of the out-flowing water was observed when the inlet was positioned on the side of the tank. To reduce the potential for contamination of the out-flowing water, it is recommended that rainwater tanks preferably have a side inlet and a conical base, that the sediment thickness in the tank be maintained at low levels by regularly cleaning the tank, and that the tank water be not used during or immediately following rainfall events.