In this method, water passes through bed of fine sand (usually <0.5mm diameter) at a low flow-rate. Removal occurs through a combination of chemical, physical and biological processes. Pre-treatment in the form of screening, sedimentation or roughing filtration is required. Used frequently for the removal of microorganisms, algae, suspended solids, iron, and manganese.
Apart from labor and land expenses, construction costs are relatively low for a centralized method. O&M costs are also low since cleaning can be done manually. Requires no backwashing. India (1983): USD 60-130/m2. Colombia (1987): USD 105-215/m2. More recent costs…
Can be applied at local or centralized level. Requires limited amount of labor. Used in combination with other (pre-treatment) processes, persons for these processes can also do sand filtration maintenance. Training might be needed for appropriate management of contaminated materials.
Risk of contamination with open (uncovered) installations. Heavy rains might flush filter material. Removed top layer material is highly contaminated and requires proper handling and disposal. Simply disposing in waste pit may result in leaching of concentrated contaminants into groundwater.
Construction requires technical experience. Initial sand bed 0.8m, minimum height 0.5m. Operation includes flow control for proper infiltration rate (0.1-0.3m/h). Cleaning by scraping top part (1-3cm) of filter. Occasionally re-sanding required. Parallel filtration units required for continuous flow during cleaning. The inflowing water should not contain (residual) chlorine.
Known to deliver high quality drinking water, but full guarantee cannot be given. If the raw water is too contaminated, additional (household-level) treatment may be necessary. Since the method is simple and commonly known, the expected acceptance level is high.
No relevant remarks for Slow sand filtration.