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Glossary         

Key

1. Raw water inlet valve

2. Perforated plate (holes about 2mm diam. every 5 cm)

3. Coarse sand (l-2 mm diam.)

4. Large stone (e.g. cobbles)

5. Plug-holes with plugs

6. Pre-filter overflow (towards filter)

7. Fine sand (0.2-0.5 mm diam.)

8. Gravel

9. Perforated pipe (for collecting filtered water)

10. Flat stone

11. Outlet valve

12. Outlet pipe to distribution

13. Overflow

A. Minimum water level in filter

B. Maximum water level in filter

Resources Needed

-        Two 200 L drums

-        Metal saw, hammer, cold chisel, tape measure

-        Drill and bits

-        Round and half-round files

-        Pipe (PVC or galvanized), 1/2" or 3/4"

-        Pipe threader (for galvanized pipe) or solvent and PVC glue

-        Teflon tape or mastic and tow (for making joints)

-        Elbows and nipples (for fixing pipes)

-        Anticorrosion and household paints

-        Paintbrush and thinners

-        Cobbles (stones)

-        Coarse sand (about 1mm diam.): about 0.12m3

-        Fine sand (0.2-0.5 mm diam.): about 0.3m3

-        Gravel: about 0.03m3

Method

Slow sand filtration allows a highly effective microbiological purification of water in addition to mechanical purification by the sand.

This microbiological treatment is due to the action of a biological layer (called the Schmutzdecke), which develops at the surface of the filter. This layer actively retains and destroys helminth eggs, bacteria and some viruses.

Construction

  1. Paint the insides of two 200 L drums. Fix an inlet pipe about 5 cm from the bottom of one drum which will be used for the pre-filter (as shown in the sketch).
  2. Place a layer of large stones on the bottom of this drum (4), making sure there is a plug-hole at the bottom.  If there is not, then make one. 
  3. Place a perforated plate (e.g. the top cut off the drum) on top of the stones (2), to act as a support for a layer of coarse sand.
  4. Place a 30cm layer of coarse washed sand (l-2 mm diam.) on top of the metal plate.
  5. Fix a pipe connecting the two drums near the top, to take water from the pre-filter to the filter.
  6. Make sure the second drum has a plug-hole in the bottom. Make a hole about 5 cm from the bottom and insert a perforated pipe (9). A length of PVC pipe with many slots cut with a saw is suitable.
  7. Bury the perforated pipe in a bed of gravel (8), which covers it by at least 5 cm.
  8. Then add a layer of at least 70 cm of washed, sieved, fine sand (7). The ideal diameter is 0.2 to 0.5 mm, or in any case, less than 1.5 mm. The Schmutzdecke will develop at the surface of this sand.
  9. Place a flat stone (10) where the water flows onto the filter so it will not to disturb the Schmutzdecke by its turbulence.
  10. Insert and fix the pipework (outlet and overflow). Important: the outlet pipe must rise above the level of the top of the sand, so that the filter surface is always under water, even if the water supply is cut.

 

Important

-        The filter sand should be of uniform size. Sieve it, using mosquito netting for instance.

-         Before the first use, fill the pre-filter and the filter with a solution of l00 mg/L chlorine (10ml of 1% solution per litre); leave it for 12 hours and empty it through the plug-holes. Never chlorinate after this: it would destroy the biological layer. Cover the drums to prevent the growth of algae.

-        It is vital that the Schmutzdecke at the surface of the sand is always covered with water; again, make sure that the outlet pipe rises above the level of the Schmutzdecke.

-        Flow setting; dose the outlet valve. Open the inlet valve (not too much, to avoid putting the pre-filter sand into suspension); when the filter is full, open the outlet valve so as to have an outflow of about 1 L/min.

-        The Schmutzdecke is only effective after about 2 weeks, so for the first 2 weeks of service the water is not safe.

-        Such a filter can treat 1,000 to 1,400 L/24h. If the needs are greater, several filters may be built in parallel (this also avoids having to cut the supply during maintenance).

-        Maintenance: when the yield drops significantly, cut the inflow and undo the plugs of the two drums. Let the water empty completely from the pre-filter. Let the water level in the filter fall to 15-20cm below the filter surface; rake the top l-2 cm of sand from the Schmutzdecke. Replace the plugs and put back into service. After this has been done several times, remove about 10cm of sand and put it to one side. Place a layer of dean sand and then replace the 10cm, to bring the total thickness of fine sand back to 70cm.

-        If the water is not very turbid (<30 NTU), the pre-filter will not be necessary.