S-Tube Gravity Flush Toilets Essay

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S-Tube Gravity Flush Toilets


Toilets are vital in our everyday lives: it is the general solution of mankind for its everyday fecal deposition. Everyday, 4.8 billion gallons of water is used for flushing (and, wastage) (Herrick, Flickety, Krystle, 1995). The humongous amount alone depicts toilet use in a large scale. It is undeniable that having a toilet at home is a general must for sanitation and aesthetics.

The importance of toilets and its common use normally outweigh the physical concepts surrounding the toilet. These concepts are important, however, for the toilet’s correct usage, maintenance, troubleshooting and appreciation. Thus, there is a need to identify these physical concepts and explain how these work hand-in-hand in a general view.

This term paper shall focus on the physical concepts of an S-tube gravity flush toilet. Specifically, it focuses on the physical concepts of the toilet’s water tank, S-tube, toilet bowl, flush handle and flush valve flapper and the float ball.

Overview of the Flushing Process

Water is stored in the tank through the inlet tube (see Figure 1). The flow is controlled through the float ball and ballcock, which blocks the inlet tube when the tank is full and opens the tube when the tank is empty. The water is held by the flush valve flapper. When the flush handle is turned, it pulls the flush valve flapper and releases the stored water into the rim where it discharges into the bowl. The bowl, in the appropriate conditions, then proceeds to discharge the materials into the septic tank through the S-tube.

If, in case the float ball and ballcock is defective, the excess water goes to the overflow tube and is discharged to the toilet.

Water Tanks and Their Use

The water tank not only holds the contraptions needed for flushing the toilet, storing and controlling the quantity of water used for flushing, the tank in itself is a tool for flushing.

Common tank capacities for gravity flush models use about 2 gallons of water per flush. This quantity of water is used in the right conditions for a successful flush.

A simple experiment featured on How Stuff Works consists of pouring any number of cups of water (the experiment was introduced with 25 cups of water, around 6 L) into the toilet one at a time and comparing its result to pouring a bucket of water (around 7.6 L) into the bowl (Brain, 2000). The experiment proves that in order to flush the tank, there should be sufficient amount of water poured in a sufficiently high velocity so that the siphoning action on the S-bend will be attained.

The only energy available for the water from its storage to its flushing is its potential energy. The tank has been physically shaped to provide the right height of water so that its potential energy, when converted into kinetic energy, is sufficient for the water to discharge from the tank in a fast velocity. The designed height should include friction losses from the contraptions inside the toilet, the water tank walls and the sudden contraption of the container at its opening (Young and Freedman, 2004, p.171-174).

Siphon Action on the S-Tube

The toilet is connected to a septic tank which stores the depositions and lets the particles settle. In order to transport the materials from the bowl into the septic tank, a siphoning action should be used. Typically, a toilet uses an S-tube to facilitate the siphon action.

The siphoning action is the explanation why there should be a sufficient amount of water having a sufficiently high velocity. Siphons, in order to function, should allow gravitational force of the material being transported to overcome the surrounding tube pressure (Young and Freedman, 2004, p. 515-535). Thus, having a large volume of water with insufficient velocity or a low volume of water with sufficient velocity is not enough to make the siphon action work. When flushing, the transport of materials is made at a fast rate, considering that the septic tank is of a height below the toilet.

The gurgling sound made by the toilet is an indication of air overtaking the water to surface. The air escapes due to its displacement through the pressure inside the toilet tube. This, in turn, makes the surrounding pressure of the tube greater than the gravitational force on the other end of the tube, making the siphon action stop (Knight, 1998).

There is also a huge difference in the flushing action if the septic tank is already full, or overflowing. A full septic tank requires a higher pressure from the toilet for the siphon action to be activated. However, the failure rate for flushing becomes bigger and bigger over time since the septic tank can no longer hold other materials, and since particles relatively heavier than water start to fill the whole tank.

Retained Toilet Water Blocking the Escaping Sewer Gases

After flushing, the toilet is filled with additional water so that the whole S-tube opening is covered. The water retained in the bowl is not only to prevent sticking of our fecal depositions; it is also important so that sewer gases, except for the gases that stop the siphoning action, will not escape. Since water takes the shape of its container, that is the pipes and a small portion of the bowl, there are no voids for the sewer air to escape.

Why Bowl Shaped?

The bowl has been shaped so that energy loses from its discharge to its flushing will be minimal in its operation. The seating of the users, aesthetic purposes, and the purpose of flushing the entire bowl area for sanitary purposes is also considered in its design.

The bowl has been smoothed in order to reduce friction, which can make dirt on the bowl surface stubborn. Since fecal deposition is sticky, the energy of the water (one of the primary purposes of which is to flush the toilet) is dissipated as the water tries to remove stuck depositions on the bowl.

The shaping of the bowl is also designed as such in order to prevent additional reactive forces from the weight and impact of the water if it had an edge (Young and Freedman, 2004, p.282-313). A large energy loss should be expected if the bowl shape was an edged one, as in a rectangle.

Flush Handle and Flush Valve Flapper: Simple Mechanics at Work

The flush valve flapper is the boundary of the water in the tank and its path into the bowl. Since the weight of the water exerts a force equal to the area above the flapper multiplied by its height, this hydrostatic force must be countered by a force greater than the force exerted by the weight of the water. Since there is no force from the bottom of the flush valve, even a small height of the water exerts a force enough to keep the valve shut, leaving no spillage from the tank to the bowl, during its fill-up stage.

To open the valve, an upward force is applied to the flush valve flapper through its connection with the flush handle. Exerting a downward force at any point on the flush handle creates a rotating motion, a torque, equal to the force multiplied by the lever arm. The lever arm is the length perpendicular to the force applied (Young and Freedman, p.362-389). Since the force is taken as straight downward, the horizontal length from the pivot point to the point where the force is exerted is taken as the lever arm.

The rotation of the flush handle is transferred to the flush valve flapper by a means of the flush valve chain. Again, having a torque on the mechanism makes it easier for the flush valve handle to open the flush valve flapper.

Float Balls and Buoyancy

The seemingly useless float ball is of great value to toilet users, particularly for water bill payers, since the float ball regulates the flow of water into the water tank.

The float ball is attached to a stopping mechanism which plugs the water inflow from the fill valve. When the float ball is in a low position, the stopping mechanism is unplugged and when in a high position, the valve is plugged.

The float ball changes its position through the principle of buoyancy, that is, the force exerted by a fluid at rest to an object that is either submerged or floating (Young and Freedman, p.523-525). The float ball is intended to be a light, hollow, plastic material such that only a small buoyant force is required and such that the float ball will not submerge and take up space.

The buoyant force, in turn, creates a torque on the stopping mechanism. As explained in the Lever and Flapper section, the torque is equal to the force multiplied by its lever arm. Since the buoyant force is constant, the force needed to stop the inflow of water depends on the lever arm. Float balls have long lever arms so that after reaching a desirable height, the lever arm is longer since there is more length perpendicular to the buoyant force (Young and Freedman, p. 362-364).

Some toilet users depend on these principles for their money-saving techniques. These users put on containers of water inside the tank, about a half gallon (Herrick, Flickety, Krystle, 2005): not too large a volume to affect the flushing capability of the water and not to small a volume for it to float.


Looking back to the processes described in the overview, there are a lot of physical concepts involved in a toilet bowl. These physical concepts work hand-in-hand to successfully discharge the deposited in the bowl to the septic tank.

S-tube gravity flush toilets, as the name implies, relies on the kinetic energy of the water in order to initiate the siphon action in the S-tube in order to successfully flush materials out of the bowl. Its over-all design is girded to maximize this energy, also including sanitation and aesthetics.

Works Cited

Brain, Marshall.  “How Toilets Work.”  01 April 2000. <>  28 July 2010.

Herrick, Flickety, Krystle. “How to Convert Any Toilet to a Low Flush Toilet”. 1995. <> 28 July 2010.

Knight, L. “How A Toilet Works.” 1999. MIT. < toilet/toilet.html> 28 July 2010.

Young, H. and Freedman, R. (2004). University Physics. San Francisco: Pearson Education, Inc.


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