DRAINAGE OF WASTE WATER AND SEWAGE
Wastewaters coming from the kitchen sink, washbasins and
urinals are not containing any solid matter. But sewage a solid material has to
be treated such that it also could be converted as wastewater.
NEED FOR TREATMENT OF SEWAGE
Nearly 75% of the water
supplied returns back in the form of wastewater. As there is a need for a water
supply system, there is an equal necessity to dispose of the wastewater. The
night- soil and urinary content of domestic sewage (originating from residential
areas) consists of potentially most dangerous organisms. Such organisms are
responsible for the occurrence and spread of water-borne diseases.
Sewage treatment is meant by the various steps undertaken in the process of transferring sewage into a harmless
liquid. That is to render sewage inoffensive without causing nuisance or odour
and to reduce or eliminate any possible contaminations of water supplies,
bathing areas, etc., by the untreated or un-disposed sewage.
SANITARY FITTINGS AND APPLIANCES
Sanitary fittings and appliances comprise traps,
water closets, flushing cisterns, etc., which are discussed below.
1. Traps
Traps are fittings that
are installed in drainage pipes to prevent the passage of foul air or gases
through drains, waste or soil pipes. This is possible because traps are
equipped with water seals having a minimum depth of 25 mm. Higher the depth
greater the effectiveness of the trap. Depth varies in practice from 25 to 75 mm
and 50 mm depth is quite common.
Qualities
of good traps are:
(i) They should have adequate water seal at all times
which is possible only when they are non-absorbent.
(ii) There should not be any projections that may
obstruct or retard the flow of water. Further, it should be smooth.
(iii) They should retain a minimum quantity of water
consistent with providing deep water seals with a large area.
(iv) Apart from self-cleansing, provision should be
therefore cleaning manually or otherwise.
Traps are classified based on shape as R-trap, Q-trap
and S-trap and based on use as floor trap, gully trap and intercepting trap.
The figure below of traps based on shape.
Traps
based on shapes (P-Q-S)
Floor traps are intended to
collect surface wash or wastewater from the floors of the bath and kitchen.
These are provided with cast iron grating at the top so as to retain coarse solid
matter. Thus preventing the matter to go along with wastewater and thereby
cause blockade - figure below
Floor trap
Gully
traps are intended to receive sullage water from baths, sinks and washbasins.
Further, they are also used to receive rain or surface water from house-tops or
back yards. The water seal is 50–75 mm. Waste pipes and rain-waste pipes are
invariably connected to drains through the gully trap (Fig. below).
Gully trap
Intercepting traps or also
called interceptors. These are installed at the junction of the house drain
and house sewer. The primary objective is to prevent the foul gases in the public
sewer from entering the house drainage system (Fig. below)
2. Water Closets
The water closet is a water-flushed-plumbing
figure designed to receive human excreta directly from the user. There are two
types of water closets, viz., Squatting or the Indian type and pedestal or the
European Type. Figures below (a) and (b) show the two different types.
Intercepting drain
Water closets
(i)
Indian Type
This has a squatting pan of
45–63 cm in overall length and 45–50 cm in height. The W.C. is connected to
soil pipe through a P-trap or S-trap.
(ii)
European Type
It is a wash down water
closet provided with a seat and cover and fitted with P-trap and S-trap. It has
a maximum overall height of 63 cm and a height of 34–40 cm.
Both the types of WCs are
provided through flushing cisterns and flushing pipes.
3. Flushing Cistern
This is a sanitary
appliance that is used by flushing out water closets, urinals, etc. There are two
types one is the valveless siphonic type and the other is valve-fitted siphonic
type. The former is preferred and largely used in practice.
4. Washbasins
The washbasin is also called as
lavatory basin. It is made of the white-glazed earthen wave, enamelled iron, etc.
There are two types, viz., flat back and angle back. Washbasins are fixed
either on brackets secured to the walls or on pedestals rising from the floor.
5. Sink
It is a rectangular shaped
receptacle used in the kitchen or laboratory for draining water. It is to be
located as far as possible near a window so as to get adequate light. The
height from the floor is about 90 cm to the top edge of the sink.
6. Urinals
There are two types, viz.,
bowl type and the slab or stall type. Both the types are flushed through a siphonic type of flushing cisterns. In the bowl type, the cistern may be hand-operated and of 5 l capacity. In the stall type, automatic flushing cisterns
are used.
PLUMBING SYSTEM OF DRAINAGE
A plumbing system of drainage
is one through which discharge from sanitary fixtures or appliances is
conveyed. The three principal systems are single stack system, the one-pipe
system and the two-pipe system.
1. Single Stack System
In this system, all waste
matters such as from bath, kitchen and sink and foul matter from urinals or
excreta from water closets are discharged into a single soil pipe. This pipe
also acts as a ventilating pipe. In this system complete reliance is built
entirely on the effectiveness of the water seal with the assurance that the entry
of gases from sewers are blocked. The advantages of the system are:
(i) Simplicity in design,
layout and easy plumbing of sanitary fixtures.
(ii) Better external
appearance of the building.
(iii) More compact system
and
(iv) More economical.
The main disadvantage with
this system is the easy breaking up of water seals in taps.
2. One-Pipe System
In this system, separate
vent pipes are provided. All the traps of water closets, basins, baths etc.,
are adequately ventilated to preserve the water seal. In this system, a caged
dome is provided projecting above the rooftop such that to allow a natural
outlet to the foul gas. This system requires a difficult arrangement of pipework and is also costlier than the single-stack system.
3. Two-Pipe System
In this system all soil
appliances such as water closets and urinals are connected to a vertical soil
pipe. All wastewater appliances such as baths, washbasins, sinks, etc., are
connected to a separate waste pipe which is disconnected from the drain by means
of a gully trap.
Thus in this system, there
are two sets of vertical pipes, viz., the soil pipe and the waste pipe, each
one is provided with a vent pipe. This system is the best plumbing system which
is most efficient in the conveyance of sanitary waste and also largely favoured
for adoption. However, this is the costliest of all systems.
LAYOUT OF BUILDING DRAINAGE
The following aspects have
to be considered in the design and construction of a drainage layout:
1. This layout should be
simple.
2. All pipes should be laid
in straight lines both in horizontal and vertical directions.
3. Abrupt changes in the direction of the pipeline may affect the natural flow.
4. In the points of the intersection of pipes and wherever bends occur, it is essential to provide
inspection chambers or manholes.
5. The contained angle
between the intersecting pipes should be less than 45° so as to ensure a
gradual and smooth flow.
6. Sewers should not be
ordinarily laid under a building. If it is inevitable a cast iron pipe should
be laid in straight reach with a uniform gradient.
PIPES FOR DRAINAGE
1.
Pipes and Gradients
Pipe sizes for house drains
and sewers are 100, 150 and 230 mm and in some cases 300 mm. In order to avoid
deposition of solid matter, the pipes should be laid as per the recommendation
of I.S. Code (IS: 1742–1972), Table below.
Recommended the gradient of sewer discharge pipes
|
Sl. No. |
Diameter (Pipe) |
Gradient |
Sewer Discharge |
|
(A) |
300
mm |
1 in
200 |
1.66m3mt |
|
(B) |
230
mm |
1 in
120 |
1.93m3mt |
|
(C) |
150
mm |
1 in
65 |
0.42m3mt |
|
(D) |
100
mm |
1 in
35 |
0.18m3mt |
Source: IS: 1742–1972.
2. Positioning of Pipes
The following points shall be kept in view while deciding the
positioning of pipes:
(i) All soil pipes, waste and ventilating pipes should be conveniently
grouped in shafts or ducts with adequate capacity. This provision is needed to
allow for inspection and repair work.
(ii) All the pipes needed for different purposes have to be adequately
provided for during the construction stage itself at their appropriate
positions.
(iii) When pipes are not embedded it should run clear of the wall with a
minimum clearance of 5 cm.
(iv) Waste pipes are to be separated from the house drains by using
gully traps. This arrangement is essential to prevent the entry of foul air or
gas, vermin, etc., into the building.
(v) The soil, waste and vent pipes should be vertically carried above
the top of the building. They are to be covered by copper, plastic or
galvanised iron wire domes. The arrangement is needed to prevent the nesting of
birds or inadvertent falling in of objects inside the pipe.
(vi) Diameter of pipes used for various purposes are given below
(Duggal, 1988)
|
(a)
Soil pipe |
100
mm |
|
(b)
Waste pipe – horizontal |
32–50
mm |
|
(c)
Waste type, vertical |
75
mm |
|
(d)
Vent pipe |
50
mm |
|
(e)
Anti-siphonage pipe: |
|
|
Connecting
soil pipe |
50
mm |
|
Connecting
waste pipe |
40
mm |
SLUDGE AND EFFLUENT
DISPOSAL
Sedimentation is carried out with the object
of removing such suspended minerals and organic matter from sewage. Sedimentation
tanks are units in which sedimentation is brought about. The lighter organic
sewage solids which settle in the sedimentation tanks are termed sludge. The
sewage that has been partially classified by the settling out of the solids is
known as the effluent.
In a general set up the disposal of sludge
can be done by adopting any one of the following methods:
(i) Dumping into waste bodies
(ii) Shallow Burial
(iii) Lagooning
(iv) Mechanical dewatering and
(v) Drying in Beds
If such a general disposal system is not available one has to go in for
a sedimentation tank. Generally, the sedimentation tank adopted is a septic tank
which is a horizontal continuous flow sedimentation tank. A description of the septic tank is explained in the next section. The putrescible and highly odorous
efficient from the septic tank requires to be properly treated and disposed of
sub-surface irrigation field method or discharging into other soil absorption
system such as soak pit and leading cell pool which is described later.
1. Septic Tank
A septic tank is a horizontal continuous flow
sedimentation tank. Here the sewage is allowed to move very slowly so as to
retain for a period sufficient to develop 60–70% of suspended matter to settle
in the form of sludge. Lighter solids float to the surface and combine with
grease and fat and form floating scum. The scum and sewage are allowed to stay
for a period of 7 months during which period complete decomposition takes place
through a process called sludge digestion. The volume of sludge is also gets
reduced for easy disposal. The effluent from the septic tank has a very bad colour.
It is dark in colour with fine solid particles and has to be disposed of with the utmost care. A schematic layout of a septic tank is shown in Fig. below.
A septic tank is constructed in such a way so
as to prevent direct current between the inlet and the outlet. This provides a better sedimentation and is achieved by using T-pipes
Septic tank
with submerged ends as inlet and outlet. As
an alternative, baffle walls may be provided. The T-pipe or the baffle at the
outlet also helps in retaining the scum in the tank. A certain quantity of scum
is needed to hold back odours and to create a form of heat insulations which in
turn aids the bacterial action. Usually, a manhole is fixed on the RCC cover
slabs. The tank cover allows keeping the sewage warm, lessening odour, etc.
Gases are separately removed through a vent pipe. Sludge is removed
periodically.
Septic tanks are to be located at a place
that is exposed to the sky and accessible for cleaning. The sewage in a septic
tank can be taken as only a primary treatment effect. Before disposal, the
effluent needs a secondary treatment. The septic tank has only a restricted use
in practice, such as schools, hospitals, small residential colonies and other
public institutions where sewers are not yet laid.
2.
Soak Pit and Leaching Cess Pool
The putrescible and highly odorous effluent
from the septic tank requires to be properly treated and disposed. The methods
of disposal are:
(i) Sub-surface irrigation employing
absorption field method.
(ii) Discharge into other soil absorption
systems as soak pit or seepage pits and leaching cess-pools.
Sub-surface irrigation is also termed land
infiltration. It is the application of sewage or its effluent into the land
through a system of open-jointed pipes or drains placed near the surface of the
ground and thereby enabling the effluent to percolate into the surrounding
soil. A soak pit is a covered pit through which the effluent is allowed to be
soaked or absorbed into the surrounding soil. The pit may be empty or filled up
with brick or stone aggregates
1. Brick
lining with dry joints
2. Outer
casing with coarse aggregates (7.5 cm thick min)
Soak
pit
A leaching cesspool is a pool such that the top portion acts as an absorption
field and the bottom as a septic tank. This is done by providing open-jointed
lining at the upper portion which enables the sewage effluent to the easily
dispersed to the surrounding soil. The bottom portion is of the solid wall which
functions as a septic tank
SOURCE:
BUILDING CONSTRUCTION MATERIALS AND TECHNIQUES | P. PURUSHOTHAMA RAJ
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