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Home > Basic Principles > Technology Options > School Toilets > Water Flush Toilets

Water Flush Toilets

Flush toilets use water to flush human excreta into a leach pit, tank, or sewer. After the toilet is used, a minimum of 2.5 liters of water is poured into the pan to flush the toilet. Flush toilets normally have a U-shaped conduit partly filled with water (U trap) under the pan. The U trap overcomes the problems of flies, mosquitoes, and odor by serving as a water seal. Flush toilets discharge wastewater directly into open water courses. If no specific measures are taken, this can result in pollution of neighboring surface water, which in many cases is also used as a household water source.

The water flush toilet technologies presented in this section are:


Offset single pit toilet with pour flush

Figure 1. Offset single pit toilet with pour flush
Figure 1. Offset single pit toilet with pour flush

The superstructure of an offset single pit toilet with pour flush is half a meter away from the leach pit. A short length of sufficiently sloping (1:10) PVC leads from the U trap down to the pit (see figure 1).

Suitability

    The direct single pit toilet with pour-flush is suitable
  • For areas where the water table is high, if the toilet is raised and connected to a soak-pit.
  • In areas prone to freshwater or tidal flooding, if raised.
  • For loose soils, if fully lined.
  • For soils with low permeability, if built with a soak pit.

Advantages

  • It is easy to construct, operate, and maintain:
    • Operation consists of regular water cleansing of the slab (with soap or detergent, if available) to remove any excreta and urine, and daily cleansing of the floor, squatting pan, door handles and other parts of the superstructure.
    • Maintenance consists of monthly inspections to check for cracks in the floor slab and damage to the vent pipe and fly screen, and digging out of part of the feces at the end of the dry season. These feces should be handled with care and buried in a pit covered with soil. After at least a year, when the contents of the pit have decomposed into harmless humus, the humus can be can be used as fertilizer.
  • It is relatively inexpensive to construct, operate, and maintain.

Disadvantages

  • The U-trap can easily become blocked because of bad design or improper use, or damages by improper unblocking.
  • Pour-flush toilets are unsuitable where it is common practice to use bulky materials for anal cleansing which cannot be flushed through the U-trap. Unless those materials are separately collected and safely buried or burned.
  • The pit sludge is not safe until it has been left to decompose for at least a year.

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Offset double pit toilet with pour flush

Figure 2. Offset double pit toilet with pour flush
Figure 2. Offset double pit toilet with pour flush

An offset double pit toilet with pour flush is an offset single pit toilet with a second pit added (see figure 2). The double offset system enables alternating use of the two pits.

When the first pit is full it should be left for at least twelve months, the period required for adequate pathogen destruction. After this period, the decomposed contents of the first pit can safely be removed by hand and used as organic fertilizer. The first pit can be used again while the contents of the second pit decompose.

Suitability

    The offset double pit toilet with pour flush is suitable
  • For areas where the water table is high, if the toilet is raised and connected to a soak-pit.
  • In areas prone to freshwater or tidal flooding, if raised.
  • For loose soils, if fully lined.
  • For soils with low permeability, if built with a soak pit.

Advantages

  • It is easy to construct, operate, and maintain:
    • Operation consists of regular water cleansing of the slab (with soap or detergent, if available) to remove any excreta and urine, and daily cleansing of the floor, squatting pan, door handles and other parts of the superstructure.
    • Maintenance consists of monthly inspections to check for cracks in the floor slab and damage to the vent pipe and fly screen, and digging out of part of the feces at the end of the dry season. These feces should be handled with care and buried in a pit covered with soil. After at least a year, when the contents of the pit have decomposed into harmless humus, the humus can be can be used as fertilizer.
  • It is relatively inexpensive to construct, operate, and maintain.
  • The pit sludge is safe.
  • The toilet can be connected to a soak pit.

Disadvantages

  • The U-trap can easily become blocked because of bad design or improper use, or damages by improper unblocking.
  • Pour-flush toilets are unsuitable where it is common practice to use bulky materials for anal cleansing which can not be flushed through the U-trap. Unless those materials are separately collected and safely buried or burned.
  • The contents of the pit may not decompose safely when the double pits are too close to each other without an effective seal between them, allowing liquids to percolate from one pit to the other.

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Pour flush toilet with 2-chamber septic tank with soak pit

Figure 3. Pour flush toilet with 2-chamber septic tank and soak pit

This type of pour flush toilet is like the offset single pit toilet, but with a septic tank in place of the pit (see figure 3).

A septic tank is a watertight settling tank to which wastes are carried by water flushed down a short PVC pipe. A septic tank does not dispose of wastes; it only helps to separate and digest the solid matter. The liquid effluent flowing out of the tank is as dangerous as raw sewage from a health point of view and must be dispersed by soaking into the ground through the soak pit. The sludge accumulating in the tank must be removed regularly, usually once every one to five years, depending on site, number of users, and kind of use.

In double-compartment septic tanks the first compartment has twice the volume of the second. The total volume of the tank should be at least three times the average volume of water used daily. Every tank must have a ventilation system to allow explosive gases to escape. Septic tanks are more expensive than other on-site sanitation systems and require sufficient piped water.

A soak pit is a pit into which the liquid effluents from the septic tank flow to be filtered into the ground. The capacity of the pit should not be less than that of the septic tank. The pit may be filled with stones or broken bricks, in which case no lining is needed, or lined with pre-cast reinforced cement concrete rings. The top 0.3 m (the topmost ring) should be a non-perforated ring. If no lining is used, the top 0.5 meter should be lined to provide a firm support for the reinforced concrete cover slab.

Suitability

    The pour flush toilet with 2-chamber septic tank with soak-pit is suitable
  • Where the water table is high, if the toilet is raised.
  • In areas prone to freshwater or tidal flooding, if raised.
  • For loose soils.
  • For soils with low permeability.

Advantages and Disadvantages

    The main advantage of the pour flush toilet with septic tank and soak pit is that it is easy to operate. This type of toilet has a number of disadvantages:
  • It is relatively expensive and difficult to construct.
  • It is relatively expensive and difficult to maintain. On a monthly basis, the floor, squatting pan and U-trap need to be checked, and if necessary small repairs must be carried out.
  • Regular cleaning of the toilet with a bit of detergent is unlikely to be harmful, but the use of large amounts of detergents or chemicals may disturb the biochemical process in the tank. The tank must be emptied when solids occupy between one half and two thirds of the total depth between the water level and the bottom of the tank (at least once every five years).
  • The sludge is not safe to handle. Removal is best done mechanically; if done manually, the sludge must be handled with extreme care. The sludge must be buried in a pit and covered with soil.
  • Many problems are caused by too much disposed liquid. Large flows entering the tank may cause a temporarily high concentration of suspended solids in the effluent owing to disturbance of the solids that have settled out.
  • This type of toilet is unsuitable for areas where water is scarce and where financial resources are insufficient for construction of the system, or where emptying of the tank is too expensive or cannot be carried out safely.

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Pour flush toilet with 2-chamber septic tank with drainage field

Figure 3. Pour flush toilet with 2-chamber septic tank and drainage field
Figure 4. Pour flush toilet with 2-chamber septic tank and drainage field

This type of toilet is the same as the pour flush toilet with septic tank and soak pit, but with a drainage field in place of the soak pit. The drainage field is shown in figure 4.

A drainage field is often used where larger quantities of liquid effluents are produced. A drainage field consists of gravel-filled underground trenches, into which the liquid effluents coming from the septic tank are led through open-joint (stoneware) or perforated (PVC) pipes, allowing the effluents to filter into the ground. Initially the infiltration into the ground may be high, but after several years the soil will clog and an equilibrium infiltration rate will be reached. If the sewage flow exceeds the equilibrium rate of the soil, eventually the sewage will surface over the drainage field.

Suitability

    The pour flush toilet with drainage field is suitable
  • In areas prone to freshwater or tidal flooding, if raised.
  • For loose soils.
  • For soils with low permeability where normal septic tanks cannot work.
  • For toilets that require water for flushing.

The pour flush toilet with drainage field is not suitable where the water table is high.

Advantages

  • It is easy to operate.
  • The drainage field is easy to maintain. The maintenance activities for the drainage field consist of cleaning the tank outflow and ensuring that it is still in order, unblocking the delivery pipe if necessary, cleaning the diversion boxes from time to time, controlling plant growth to prevent roots from entering the trenches, and carrying out any necessary repairs.

Disadvantages

  • On a monthly basis, the floor, squatting pan, and U-trap must be checked, and small repairs carried out if necessary.
  • Regular cleaning of the toilet with a bit of detergent is unlikely to be harmful, but the use of large amounts of detergents or chemicals may disturb the biochemical process in the tank.
  • The tank must be emptied when solids occupy between one half and two thirds of the total depth between the water level and the bottom of the tank (at least once every five years).
  • The sludge is not safe to handle. Removal is best done mechanically; if done manually, the sludge must be handled with extreme care. The sludge must be buried in a pit and covered with soil.
  • The problems that can occur are overflowing leach lines, unpleasant odor, groundwater contamination, and social conflict over location of the drainage fields.
  • A drainage field is unsuitable where insufficient space, water or financial resources for construction are available, or where bedrock or groundwater are at shallow depth.

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Pour flush toilet with 2-chamber septic tank and evapo-transpiration mound

Figure 5. Pour flush toilet with 2-chamber septic tank and evapo-transpiration mound
Figure 5. Pour flush toilet with 2-chamber septic tank and evapo-transpiration mound

This type of toilet is the same as the pour flush toilet with septic tank and soak pit, but with an evapo-transpiration mound in place of the soak pit. The evapo-transpiration mound is shown in figure 5.

Where the soil is impermeable or difficult to excavate, or where the water table is near the surface, a possible solution is the use of an evaporation mound. An evaporation mound is filled with sand and gravel into which the liquid effluents coming from the septic tank are led through perforated laterals allowing the effluents to filtrate into the ground or to evaporate.

This ensures a greater depth and wider dispersion of the effluentand removes much of its water content through evaporation from the plants growing on top of the mound.

Suitability

    The pour flush toilet with evapo-transpiration mound is suitable
  • Where the water table is high, if the toilet is raised.
  • In areas prone to freshwater or tidal flooding, if raised.
  • For loose soils.

Advantages and Disadvantages

The pour flush toilet with evapo-transpiration mound has the same advantages as the pour flush toilet with drainage field, and the same disadvantages with respect to the need to empty the tank and dispose of the sludge with care. The principal advantage of a transpiration mound over a drainage field is that a transpiration mound can be constructed where bedrock or the water table are at a shallow depth.

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