Slow Sand Filtration

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A slow sand filter is a sand filter adapted for household use. The version most widely implemented consists of layers of sand and gravel in a concrete or plastic container approximately 0.9 meters tall and 0.3 meters square. The water level is maintained to 5-6 cm above the sand layer by setting the height of the outlet pipe. This shallow water layer allows a bioactive layer to grow on top of the sand, which contributes to the reduction of disease-causing organisms. A diffuser plate is used to prevent disruption of the biolayer when water is added.

To use the filter, users simply pour water into the top, and collect finished water out of the outlet pipe into a bucket. Over time, especially if source water is turbid, the flow rate can decrease. Users can maintain flow rate by cleaning the filter through agitating the top level of sand, or by pre-treating turbid water before filtration.

Slow sand filter lab effectiveness studies with a mature biolayer have shown 99.98% protozoan, 90-99% bacterial, and variable viral reduction. Field effectiveness studies have documented E. coli removal rates of 80-98%.

The benefits of slow sand filtration are:
•    Proven reduction of protozoa and most bacteria
•    High flow rate of up to 0.6 liters per minute
•    Simplicity of use and acceptability
•    Visual improvement of the water
•    Production of sufficient quantities of water for all household uses
•    Local production (if clean, appropriate sand is available)
•    One-time installation with low maintenance requirements
•    Long life (estimated >10 years) with no recurrent expenses

The drawbacks of slow sand filtration are:
•    Not as effective against viruses
•    No chlorine residual protection – can lead to recontamination
•    Routine cleaning can harm the biolayer and decrease effectiveness
•    Difficult to transport due to weight – high initial cost


These YouTube videos show the installation of a slow sand filter, in a home in Africa and on a homestead in the USA.


How to Install a Bio Sand Filter - https://www.youtube.com/watch?v=grQX4v4E9Ao

NarroWay Homestead - https://www.youtube.com/watch?v=PHQJGRE0y34

Building a slow sand filter and having it operational at your home is highly recommended for any person / family interested in preparedness. 

In this video Kristina Nelson, an environmental engineer from Safe Water and LED Lighting Operatives Worldwide, Inc. (SWALLOW - www.swallowinc.org), explains how to design, build and operate a pause biosand filter for your own home. Biosand Filter Design - https://www.youtube.com/watch?v=SV7NNiZCEAA


Reference:  

Slow Sand Filtration (https://www.cdc.gov/healthywater/global/household-water-treatment/sand-filtration.html)

Slow Sand Filter Guide http://urbansurvivalsite.com/files/Slow_Sand_Filters.pdf

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