What is a Biosand Filter?
A biosand filter (BSF) is a filter based on traditional slow sand filters that helps remove pathogens, iron, turbidity and manganese from water [1]. In a BSF there is a biolayer of small microbes that lives on the surface of the sand [2]. When water is poured into the filter, the microbes in the water start living in the sand. After around 30 days the biolayer of microbes will develop to maturity. In the developed biolayer, the microbes will start to look for food, and thus as water is poured into the filter, the microbes in the biolayer will eat the new microbes and pathogens [2].
How a Biosand Filter Could Be Implemented:
This system can be constructed by the students using online resources and simple materials purchased from a hardware store. We suggest the construction methods provided by CAWST in their manual linked here. Below, we outline a possible method to make the process of constructing these filters accessible to schools within the Navajo Nation. Additionally we included specific sections in the manual that may serve as a starting point for filter construction projects. For a detailed how-to on the construction process, refer to the manual above.
The materials required for the container for this system include [3]:
- Three 5 gallon buckets with lids
- 51 inches of 1/2″ PVC Schedule 40 pipe
- A 90.5″ PVC degree elbow with one threaded female end and one female press fit endÂ
- A 0.5″ PVC sprinkler riser
- A O-ring with a 3/4″ inner diameter
- A 90 degree 1/2″ PVC elbow with one threaded male end and one female press fit end
- A 1/2″ PVC T joint
- Two 1/2″ PVC end caps
- Silicon caulk
- PVC cement
- Super glue
While it may be difficult to get a hold of some of these supplies due to the lack of access to hardware stores in the Navajo nation, it might be possible for the materials to be collected somewhere else and then shipped or delivered to the schools.
Once these items are purchased the container can be constructed following the instructions laid out by James Harper [3]. This method for constructing the container of the filter was picked because it uses materials that should be available at many hardware stores. Many other instructions available online require molds (made from metal or wood) that schools may have a harder time getting.
If a school wants to build numerous biosand filters, they may consider investing in building a wooden mold and using concrete to create the container [4]. This however does not seem like a feasible method for building just a few biosand filters due to the excessive amount of material needed.
Based on information from the Biosand Filter Construction Manual written by CAWST, the materials required for the actual filter system itself are [1]:
- Sieves (4 sizes): 12 mm, 6 mm, 1 mm, and 0.7 mm
- Water: 7-10 liters
- Washed filtration sand (<0.7mm): 30 liters
- Washed separation gravel (0.7-6mm): 3.25 liters
- Washed drainage gravel (6-12mm): 3 liters
While the sand and gravel may be difficult for schools to get, pages 50 – 54 in the document can help with finding sand and gravel.
Other useful sections in the document include:
Pages 91 – 98: provide information on how to layer the sand and gravel
Pages 99 – 107: provide information about how to maintain the filter
Works Cited:
[1] Dangol, B. (2010). Biosand Filter | SSWM – Find tools for sustainable sanitation and water management! sswm.info. https://sswm.info/sswm-solutions-bop-markets/affordable-wash-services-and-products/affordable-water-supply/biosand-filter.
[2] Construction Manual Biosand Filter Construction Manual. (2012). In CAWST. CAWST. https://cdn.sanity.io/files/r8bm187e/production/e84806e136a6ddf9a5eb0cae095c5f10b28350dc.pdf.
[3] Harper, J. (2015, February 27). Biosand Filter. Instructables. https://www.instructables.com/Biosand-Filter/.
[4] Goodier, R. (2016, August 5). How to make a wooden mold for biosand water filters. Engineering for Change. https://www.engineeringforchange.org/news/how-to-make-a-wooden-mold-for-biosand-water-filters/.