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The process percolates untreated water slowly through a bed of porous sand, with the influent water introduced over the surface of the filter, and then drained from the bottom.


  • Simple and reliable.
  • Large volume water filtering. 
  • Self maintenance (which is low once working properly).
  • Inexpensive to build.
    The basic materials can often be found on scrap yards. Ask for off-cuts of pipe from plumbers (pvc) or farmers (hdpe).
  • No chemicals are added to aid the filtration process.

Properly constructed, the filter consists of:

  • a tank, 
  • a bed of fine sand, 
  • a layer of gravel to support the sand, 
  • a system of underdrains to collect the filtered water, 
  • and a flow regulator to control the filtration rate. 
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Through physical processes:

  • Suspended organic and inorganic matter

Through microbiological processes:

  • Pathogenic organisms
  • Bacteria, parasites, cloudiness, and organic levels

Algae, that grows on the sand surface releases oxygen back into the water

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Basic Design Example


  1. 200L plastic drum: washed
  2. 5-6 buckets fine sand: 0.15 – 0.35mm, washed and free of organic matter
  3. (optional) 2 buckets charcoal: crushed & compacted
  4. 2 buckets of gravel: 6 – 12mm, washed and free of organic matter
  5. enough rocks (2-3 buckets) to cover outlet pipe: 50-100mm
  6. Outlet pipe: PVC or HDPE pipe with 4-5mm holes drilled every 25mm, both sides of the pipe.
  7. Pipecaps
  8. Bulkhead or tank fitting (eg. Hansen fittings)
  9. Valve or garden tap
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Setup & Maintenance

  • The schmutzdecke takes a while to form and ripen, this may take 2 to 3 weeks, depending on the temperature and the biological content (bacteria and organic material) of the raw water. 
  • The sand must be kept wet to keep the essential micro-organisms alive in the biological zone.
  • The biological zone needs food, therefore raw water should be continually fed in and the filter should ideally be run continuously.
  • The biological zone needs adequate oxygen for the metabolism of biodegradable components and the consumption of photogenes. To achieve this ensure there is a continual flow of water through the filter, provide an aeration treatment before, or as, the raw water enters the filter and do not have an excessive head of water above the sand bed.
  • Once functioning the schmutzdecke should remain undisturbed until the filter has to be cleaned, probably 2 to 20 weeks (when the outlet flow decreases)
  • To clean, you remove the top 1-3 cm of sand. You can optionally back wash the sand via the outlet pipe. The schmutzdecke will take a few days to reripen.
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The design can be build out with more features, for example:

  • A slow sand filter is slow 🙂 Therefore it may not be able to keep up with the demand. One solution is to combine multiple tanks together. I recommend to add a tank to collect the filtered water acting as a buffer just like gravity bench top filters.
  • The schmutzdecke needs to be undisturbed. The inlet water should flow slowly and smoothly on the filter. It may be necessary to use a ‘diffuser’, similar to the outlet pipe assembly, to evenly spread the water intake.
  • The schmutzdecke should always be covered with water. If it dries up, you would destroy the microbiological algae life, responsible for an essential part of the filtering. If you have a stream or other continues water supply, this can be used directly as long as the water intake is greater than the filter output.
    Otherwise a good solution is using a float valve (like used in drinking throughs or to refill flush toilet cisterns).
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Images credit: David Tarsi