Updated on January 11, 2024


Created on July 20, 2017

iSSF (Intermittently Operated Slow Sand Filtration System)


The iSSF is a gravity-fed slow sand filtration system.

Tested By
  • Centre for Affordable Water and Sanitation Technologies (CAWST)
Content Partners
  1. Centre for Affordable Water and Sanitation Technologies (CAWST)


Product Description

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.

Target SDGs

SDG 6: Clean Water and Sanitation

SDG 3: Good Health and Well-Being

Market Suggested Retail Price


Target Users (Target Impact Group)

Community, Public Sector Agencies, NGOs

Distributors / Implementing Organizations

This product has been implemented by Centre for Affordable Water and Sanitation Technology (CAWST), Samaritan’s Purse Canada, and Clear Cambodia.

Competitive Landscape

Direct competitors include Jal-TARA Water Filters.

Manufacturing/Building Method

The iSSF is built from locally-available materials such as water supply tanks, PVC pipe, and sand. The iSSF Manual is free to download.

Intellectural Property Type


User Provision Model

This product is provided by aid organizations who work with communities and/or schools to build the system.

Distributions to Date Status

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.

Target use case

Community use

Manufacturer-specified flow rate (L/hr)


Bacteria reduction

2 log (E. coli )


Protozoa reduction

3 log

Virus reduction

1 log

Heavy metals and/or arsenic reduction

Yes, 67 % reduction of heavy metals. No arsenic reduction.

Maximum recommended influent turbidity level (NTU)

There is no requirement. However, if the sourced water has a turbidity >50 NTU, the system will need to be cleaned more often.

Effluent turbidity levels (NTU)

Mean filtered water turbidity of 0.56 NTU

Manufacturer-specified volume between cleaning (L)


Material of construction

Plastic tanks

Design Specifications

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.

Product Schematics

Technical Support

Technical support to organisations and individuals is provided by CAWST.

Replacement Components

Replacement components are locally sourced.


10-15 years

Manufacturer Specified Performance Parameters

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.

Vetted Performance Status

Testing performance by CAWST, during a field evaluation in Cambodia, determined 75% of the sampled iSSF filtered water presented zero E. coli and 95 % of stored water presented <10 E. coli. This is further described on page 52 of the iSSF manual.


The filter should be monitored and maintained regularly to ensure safety. Instructions for maintenance are provided in the iSSF manual.

Complementary Technical Systems

Adding chlorine to the outlet tank can increase the safety of the water.

Academic Research and References

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.

CAWST, “Centre for Affordable Water and Sanitation Technology (CAWST),” Centre for Affordable Water and Sanitation Technology (CAWST). Available: https://www.cawst.org

“Samaritan’s Purse Canada,” Samaritan’s Purse Canada. Available: https://www.samaritanspurse.ca/

“Home – Clear Cambodia,” Clear Cambodia – A better life, 12-Jul-2017. Available: https://www.clearcambodia.org/

M.-P. Newswire, “Samaritan Filters now in schools in five countries, protecting children from potentially deadly polluted water,” Multivu. Available: https://www.multivu.com/players/English/7783251-samaritan-purse-biosand-filters/

“School WASH Program – Clear Cambodia,” Clear Cambodia – A better life, 24-Jul-2017. Available: https://www.clearcambodia.org/school-wash-program/

“Slow Sand Filtration,” Sswm.info. Available: https://sswm.info/water-nutrient-cycle/water-purification/hardwares/semi-centralised-drinking-water-treatments/slow-sand-filtration

“Goal 6,” Sdgs.un.org. Available: https://sdgs.un.org/goals/goal6

“Intermittently Operated Slow Sand Filtration (iSSF) Manual,” Cawst.org. Available: https://washresources.cawst.org/en/resources/09b317de/intermittently-operated-slow-sand-filtration-issf-manual

“Home – Clear Cambodia,” Clear Cambodia – A better life, 12-Jul-2017. Available: https://www.clearcambodia.org/

“Bacteria reduction,” Cawst.org. Available: https://blog.cawst.org/wp-content/uploads/2015/11/UNC-POSTER-Performance-of-Intermittently-Operated-Slow-Sand-Filters-F….pdf

“Why BioSand – Clear Cambodia,” Clear Cambodia – A better life, 11-Nov-2021. Available: https://www.clearcambodia.org/why-biosand/

CAWST, “Centre for Affordable Water and Sanitation Technology (CAWST)”.  Available: https://www.cawst.org/

Crystalstream.com. Available: https://crystalstream.com/wp-content/uploads/2010/12/Hydraulic-Loading-Rates.pdf

CAWST, “Why does the filtered water need to be disinfected? Isn’t the filter enough?,” FAQ (Frequently Asked Question): Complementary technical systems. Available: https://www.biosandfilters.info/faq/e27101b8/why-does-the-filtered-water-need-to-be-disinfected-isnt-the-filter-enough

Compliance with regulations

This product complies with the WHO turbidity reduction standards.

Evaluation methods

This product has been evaluated for E. Coli and turbidity reduction through field testing in Cambodia.

Other Information


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