In this article we will discuss about the pipes and traps used in house drainage system.
Pipes Used in House Drainage System:
In a house drainage system pipes may be designated depending on the functions carried by them as indicated below:
ADVERTISEMENTS:
1. Soil Pipe:
It is a pipe through which liquid waste carrying human excreta flows.
2. Waste Pipe:
It is a pipe which carries only the liquid waste which does not include human excreta.
3. Vent Pipe:
It is a pipe line installed to provide flow of air to or from a drainage system, or to provide circulation of air within the drainage system to protect the water seal of traps against siphonage and back flow (see Fig. 20.1a).
ADVERTISEMENTS:
Siphonage may lead to the removal of water from the trap and thus results in the breaking up of the water seal of the trap. Siphonage may be either self-siphonage or induced siphonage. Self-siphoange may be caused as a result of removing water from the trap by the discharge of the fixture to which the trap is connected. Induced siphonage may be caused as a result of removing water from the trap by the sudden flush of water from a fixture on the upper floor.
This causes air in the short branch drain pipe of the fixture on the lower floor to be sucked out resulting in siphoning out the water and thus breaking the water seal (see Fig. 20.1 b). An anti-siphonage pipe is used to prevent siphonage.
4. Ventilating Pipe:
It is a pipe which enables the foul gases in the drains or sewers to be safely discharged into the atmosphere.
ADVERTISEMENTS:
5. Anti-Siphonage Pipe:
It is a pipe which is installed to preserve the water seal of traps through proper ventilation (see Fig. 20.1).
6. Rain Water Pipe:
ADVERTISEMENTS:
It is a pipe which carries only the rain water.
Fig. 20.1 (a) Soil, Vent and Anti-Siphonage Pipes; (b) Induced Siphonage.
The sizes of the various pipes commonly used in the drainage system of houses and buildings are as indicated below:
Soil pipe – 100 mm
Waste pipe—horizontal – 30 to 50 mm
Waste pipe—vertical – 75 mm
Vent pipe – 50 mm
Rain water pipe – 75 mm
Anti-Siphonage Pipe:
(i) Connecting Soil Pipe – 50 mm
(ii) Connecting waste pipe – 40 mm
Traps Used in House Drainage System:
Traps are the fittings placed at the ends of the Soil pipes as well as waste pipes to prevent the entry of foul gases from the drains into the interior of the houses or buildings. This is made possible because a trap essentially consists of a U-tube which always remains full of water and thus maintains a water seal. As shown in Fig. 20.2 the vertical distance between the crown and the dip of the trap is known as the depth of water seal.
The depth of water seal represents the strength or effectiveness of the trap. The strength of trap is the ability of the water seal to resist the passage of air or gas through it and is determined by the vertical height to which water in the trap can rise above the dip to resist this passage. Thus greater is the depth of water seal more effective is the trap. In practice the depth of water seal varies from 25 to 75 mm, with 50 mm being quite common.
Causes of Breaking of Water Seal:
The water seal in the trap may break due to following reasons:
(i) Faulty joints,
(ii) Crack in the bottom of the trap,
(iii) Creation of partial vacuum in the sewer fittings,
(iv) Increase in the pressure of the sewer gases, and
(v) Non-use for a prolonged period.
Characteristics of Traps:
The following are the characteristics of a good trap:
(1) It should have adequate water seal at all times to fulfill the purpose of its installation. However, it should retain minimum quantity of water for this purpose.
(2) It should be of non-absorbent material.
(3) It should be free from any inside projections, angles or contractions, so that flow is not obstructed or retarded.
(4) It should be self-cleansing.
(5) It should be simple in construction, cheap and readily available.
(6) It should be provided with suitable access for cleaning.
(7) Its internal and external surfaces should have smooth finish so that dirt etc., does not stick to it.
Classification of Traps:
The traps may be classified in two ways as indicated below:
(a) Classification According to Shape:
According to their shape traps may be classified as:
(i) P-trap,
(ii) Q-trap, and
(iii) S-trap
(i) P-Trap:
This resembles the shape of letter P, in which the legs are at right angles to each other (see Fig. 20.2 a).
(ii) Q-Trap or Half S-Trap:
This resembles the shape of letter Q, in which the two legs meet at an angle other than a right angle (see Fig. 20.2 b).
(iii) S-Trap:
This resembles letter S, in which both the legs are parallel to each other, discharging in the same direction (see Fig. 20.2 c).
Fig. 20.2 (d) shows the development of all the three types of traps.
(b) Classification According to Use:
According to their use traps may be classified as:
(i) Floor trap,
(ii) Gully trap, and
(iii) Intercepting trap (or Interceptor)
(i) Floor Trap:
A floor trap is used to collect wash water from floors, kitchens and bath rooms (see Fig. 20.3) and admit the same into the drain. It forms the starting point of the sewage flow. It is made of cast iron with a grating at the top to exclude entry of coarse solid matter into the drain to avoid its blocking. The grating can be removed to do frequent cleaning of the trap. The floor trap has a small water seal. A form of floor trap commonly used in practice is known as Nahni trap.
(ii) Gully Trap:
A gully trap is provided at the junction of a drain carrying sullage from baths, kitchens, wash basins, etc., and the drains carrying rain water from roofs and courtyards as well as room washings. As shown in Fig. 20.4 the foul sullage from baths, kitchens, wash basins, etc., will enter through the side inlet called back inlet, and the unfoul room washings and rain water from roofs and courtyards will enter from the top.
A gully trap may be made of either stone-ware or cast iron. Stone-ware gully traps are of square section at the top on which cast iron grating is fitted. Fig. 20.4 (a) shows a stone-ware gully trap along with its variations. A cast iron gully trap is circular in section which along with its variations is shown in Fig. 20.4 (b). It is also provided with a grating at the top. A gully trap is usually provided with a water seal of depth about 50 to 75 mm and it may have either S-trap or P-trap.
(iii) Intercepting Trap or Interceptor:
An intercepting trap or interceptor is provided at the junction of house drain and house sewer, so as to prevent the entry of foul gases of the municipal sewer into the house drainage system. It is provided with a deep water seal of depth about 100 mm. As shown in Fig. 20.5 the trap has an opening at the top called the cleaning eye or rodding arm having a tight fitting plug. This enables the trap to be periodically cleaned of any obstruction present inside.
It is desirable to use intercepting traps where drains are old and faulty, or where drains pass under buildings and are constructed other than in cast iron and where drains empty into a cesspool such that foul matter remains therein for months together before being removed for disposal. However, where the drainage system consists entirely of cast iron drains and fittings, and where foul matter is constantly discharged into municipal sewers, intercepting traps may not serve much purpose and may be dispensed with.
When the intercepting trap is used, it is located at the extreme end of the house drain adjoining the boundary of the house and it is provided in a chamber with fresh air inlet on the upstream side of the trap. Such a chamber is called the intercepting chamber (see Fig. 20.6). The fresh air dilutes the drain air rendering it less harmful in the event of its escape, and the heavy cold air which fills the chamber and the drain at the lowest end gradually pushes the warm air out through the house soil vent pipe.
The merits and demerits of using intercepting traps or interceptors are as indicated below:
Merits of Using Intercepting Traps:
(i) Foul gases of municipal sewer cannot pass through the intercepting trap, and hence prevented from entering the house drainage system. If the intercepting trap is not provided, these gases will enter the vent pipes of the house drainage system and spread around in the surrounding atmosphere causing serious air pollution.
(ii) The pathogenic bacteria contained in the municipal sewers are prevented from entering the house drains due to the presence of the intercepting trap.
Demerits of Using Intercepting Traps:
(i) Presence of intercepting traps is found to affect seriously the ventilation of the municipal sewers, as in such cases the foul gases of the municipal sewers will find an outlet only through ventilating columns, which are provided at the head of every branch sewer and at other key points in the city sewerage system. Hence if intercepting traps are provided then the municipal sewers will need large number of ventilation columns involving more expenditure and also providing unsightly appearance.
(ii) Deeper water seal may cause obstruction to the smooth flow of sewage. The lighter organic matter may not be submerged at the inlet to the trap and the heavier matter may not be lifted out of the trap, causing stagnation and decomposition.
(iii) Some blocking of the house drain may result due to carelessness in the proper fixing of the plug in the rodding arm, or by pieces of cloth, brushes, etc., getting into the drain and being unable to get past the trap.
(iv) Short length of pipe between the trap and the municipal sewer is difficult to be cleaned.