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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 8: The privy method of excreta disposal design for a village
View the document Overview:
View the document The sanitary survey
View the document The pit privy
View the document Example privy designs
View the document Latrine for village use
View the document Thailand water-seal privy
View the document Lesson plans

Thailand water-seal privy

This concrete water-seal slab is most useful for wide-scale privy programs. It is used to cover an ordinary pit privy. This method represents the collected experience of a long established privy program in Thailand. The general method should be applicable to other water-seal slab designs.

Tools and Materials

Master molds - Can be purchased from Village Health and Sanitation Project, Ministry of Public Health, Department of Health, Bangkok, Thailand. This aluminum master mold weighs 24 pounds and costs $7.50 plus shipping charges.

Concrete making materials

Wood for platform forms

Reinforcing rod and wire


Crankcase oil

Beeswax and kerosene (optional)

3/4" x 3/4" x 5" steel bars

The baste method used for making these water-seal slabs is to cast the slab, bowl, and water-seal trap using three forms :

1. A wooden form for shaping the slab.

2. A concrete bowl core for shaping the inside of the bowl.

3. A concrete core for shaping the inside of the water-seal trap.

Fig. 82 Sketch of Finished Privy

Since the three parts of the stab are all cast at one time, the finished privy slab is quite strong. The water-seal trap is curved hack under the bowl as shown in Fig. 83 A

This makes flushing more difficult, but prevents erosion of the back of the pit on loose soil. The same general method could be used to make a forward flushing trap, Fig. 83 B.

The forms used when making a slab must stay in place till the concrete has gained enough strength to allow their removal. This is usually 24 hours. For this reason, many sets of forms are necessary if a reasonable number of slabs are to be cast every day. Here is where the master molds are needed. One is used to cast the bowl core, and two are needed to cast the trap core.

Casting the Bowl Core

1. Oil the inside of the master bowl mold and insert a 3/4" x 3/4" x 5" steel bar into the bottom.

2. Add a fairly loose mixture of cement and water, called neat cement, to a depth of about 6". Then fill to brim with a 1:1 cement sand mixture. The 1:1 should be firm, not runny, and should be laid into the loose neat cement without stirring to insure a smooth finish on the bow, core.

3. After the bowl core has become firm enough, scoop a depression into the surface to install the two steel hooks made from the reinforcing rod. They should be about 9" apart, and should not protrude above the surface of the concrete. See Fig.

4. Allow the concrete to set at least 24 hours before removing the bowl core from the master mold. The bowl core can be used to make another master mole and vice versa.

Fig. 83A Backward Flushing Trap

Fig. 83B Forward Flushing Trap

Casting the Trap Core

Make the trap core using the pair of master molds, which consist of the trap master mold and the insert mold.

1. Add about 1" of 1:1 cement sand mix to the oiled trap master mold and put in some wire for reinforcing. Then fill it with 1:1 almost to the brim.

2. Put the oiled insert mold into place and scrape off excess.

3. After 45 minutes remove the insert and add a square sheet metal pipe 3/4" high made by wrapping sheet metal around a 3/4" x 3/4" steel bar.

4. Remove the finished trap core by gently tapping the master mold with a wooden block

Construction of the Wooden Slab Form

1. Make a frame of 1 1/2" x 1 1/2" wood with an inside diameter of 80 cm x 80 cm. A notch and single nail on each corner works we 1. See Fig. 86 .

2. Make a wooden platform 90 cm x 90 cm out of 1" thick planks. Gouge '/2" deep footrests if these are desired. See the outline in Fig.

Fig. 84 Bowl Core Handles

Casting the Slab

With these three forms finished you are ready to cast the first water-seal slab.

1. Use a paint brush to coat the bowl core and the trap core with a layer of wax about 1/8" thick. Prepare the wax by dissolving 1 kilogram of melted beeswax in 1/2 liter of kerosene. The wax coating will last 5 or 6 castings adding it to the cost of each slab. Wax makes removing the cores much easier, but isn't absolutely necessary.

2. Place the bowl core on the wooden slab form and fill all cracks with clay.

3 Oil the bowl, platform and frame.

4. Apply a 1/4" thick coat of pasty cement and water mixture to the bowl core and platform. (Many Thai people prefer to spend 25ยข more for an attractive polished slab. To do this, instead of using a mixture of cement and water, use a mix of 5 cement : 5 color : 1 granite chips. After tile forms are removed, polish with a carborundum stone and plenty of water).

5. Cover the bow' core with a mixture of l cement : 2 sand, to total thickness of 1/2". Notice the smooth lip made on the cement 3/8" from the top of the bowl core. This lip is your water-seal. Use fairly dry cement and allow it to set for 15 minutes before cutting this lip.

6. Place the trap core on the bowl core and seal the crack with clay. Also add a little clay on each side of the form to prevent cement from getting to 'he front lip.

7. Cover with 1 : 2 cement sand mixture to a thickness of l/2". Do not exceed the 1/2" thickness below the trap core or you will not be able to remove this core.

8. Fill the slab form with a mixture of 1 cement : 2 sand 3 clean gravel or crushed rock almost to the top. In preparing the concrete, first mix cement and sand, then add gravel and water. Use water conservatively. The looser the mixture, the weaker the concrete will be.

9. Press In 4 pieces of 1/4" steel rod reinforcing.

10. Fill to top of frame and smooth. Allow at least 24 hours for setting.

11. Remove the frame by tapping lightly with hammer.

12. Turn the slab form over on a wooden stand and use simple levers to remove the bowl core. You must remove the bowl core before the trap core.

13. Tap the trap core gently and slip it out. Add a little water and check to see if your seal is 3/8".

14. Keep the slab damp and covered for a minimum of 3 days and preferably a week to gain strength.

Fig. 85 Privy Frame

Fig. 86 Privy Slab Outline