Stacked Filters: Novel Approach to Rapid Sand Filtration

Rapid sand filters are a familiar and mature technology, but the mechanical sophistication they incorporate in industrialized nations limits their sustainable application in developing countries. Conventional rapid sand filters require pumps, elevated tanks, or multiple filter units to generate high flow rates for backwashing. Stacked rapid sand filtration is introduced here as a more robust and sustainable alternative. A stacked rapid sand filter can backwash itself with no additional flow, which eliminates the need for pumps or other expensive equipment. This study presents laboratory and field proof-of-concept demonstrations of this novel technology. The multilayer configuration of stacked rapid sand filters allowed a laboratory unit to be loaded at $1.4–1.83\mathrm{mm}/\mathrm{s}$ ($120–160\mathrm{m}/\mathrm{day}$) per layer and backwashed at $10–11\mathrm{mm}/\mathrm{s}$ ($860–950\mathrm{m}/\mathrm{day}$) with the same or similar total flow rate. The filtered effluent met U.S. Environmental Protection Agency drinking water standards. The backwash cycle was also demonstrated, and flushing of contaminants from the sand bed was effective even with 5–10 NTU backwash water. A test stacked filter unit also demonstrated satisfactory filtration performance and effective backwashing at several water treatment plants in Honduras.