iSSF (Intermittently Operated Slow Sand Filtration System)
The iSSF is a gravity-fed slow sand filtration system.
The iSSF is a large slow sand filtration system that is designed to be intermittently operated as water becomes available. Without requiring electricity, water is gravity fed through sand where bacteria and turbidity are removed.
Cambodia, Philippines, Nicaragua, Kenya, and the Democratic Republic of the Congo (DRC) Interview with CAWST
This product has been implemented by Centre for Affordable Water and Sanitation Technology (CAWST), Samaritan’s Purse Canada, and Clear Cambodia.
SDG 6: Clean water and sanitation
Target users include schools in rural settings and communities.
The iSSF is built from locally-available materials such as water supply tanks, PVC pipe, and sand. The iSSF Manual is free to download.
This product is provided by aid organizations who work with communities and/or schools to build the system.
Due to the open source nature of the product, an exact number of distributions to date is unknown. However, Clear Cambodia have installed 900 school iSSF filters as of July 2020.
Is this filter designed for individual, household, or community use?
Manufacturer-specified flow rate (L/hr)
Laboratory-evaluated log scale removal rate of bacteria
Laboratory-evaluated log scale removal rate of protozoa
Laboratory-evaluated log scale removal rate of viruses
Reduction levels of heavy metals and/or arsenic through this treatment system
Manufacturer-specified maximum level of inlet turbidity (NTU)
Range or value of outlet turbidity levels (NTU)
Recommended treatment capacity between cleanings
Description of materials of construction
The iSSF is operated by filling the reservoir tank with dirty water. Water flows from the reservoir tank into the filter tank via PVC pipe. The water is gravity fed though the biolayer, sand, and under drain layers in the filter tank. The water then flows through a PCV pipe to the storage tank.
Technical support to organisations and individuals is provided by CAWST.
Replacement components are locally sourced.
The filter was designed for a Hydraulic Loading Rate of 1.6 L/min for a cylindrical tank of 1 m diameter, to provide sufficient yield and quality of water.
This product has been tested by CAWST.
The filter should be monitored and maintained regularly to ensure safety. Instructions for maintenance are provided in the iSSF manual.
Adding chlorine to the outlet tank can increase the safety of the water.
Bédard, G., et al., 2015, Performance of Intermittently Operated Slow Sand Filters in Rural Schools in Cambodia, 2015 Water and Health Conference: Where Science Meets Policy.
This product complies with the WHO turbidity reduction standards.
This product has been evaluated for E. Coli and turbidity reduction through field testing in Cambodia.
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