DRAINAGE OF WASTEWATER AND SEWAGE

Wastewaters coming from 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 a wastewater.

27.4.1 Need for Treatment of Sewage

Nearly 75% of the water supplied returns back in the form of wastewater. As there is a need for water supply system, there is an equal necessity to dispose 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 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.

27.4.2 Sanitary Fittings and Appliances

Sanitary fittings and appliances comprise of traps, water closets, flushing cisterns, etc., which are discussed below.

1. Traps

Traps are fittings which 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 trap. Depth varies in practice from 25 to 75 mm and 50 mm depth is quite common.

Qualities of good traps are:

1. They should have adequate water seal at all times which is possible only when they are non-absorbent.
2. There should not be any projections which may obstruct or retard the flow of water. Further it should be smooth.
3. They should retain minimum quantity of water consistent with providing deep water seal with large area.
4. Apart from self-cleansing, provision should be there for 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.

Figure 27.1 of traps based on shape.

Figure 27.1 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 top so as to retain coarse solid matter. Thus preventing the matter to go along with wastewater and thereby cause blockade (Fig. 27.2).

Figure 27.2 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. 27.3).

Figure 27.3 Gully trap

Intercepting traps or also called as interceptors. These are installed at the junction of the house drain and house sewer. The primary object is to prevent the foul gases in the public sewer entering the house drainage system (Fig. 27.4).

2. Water Closets

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 27.5(a) and (b) show the two different types.

Figure 27.4 Intercepting drain

Figure 27.5 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 which is used flushing out water closets, urinals, etc. There are two types one is valveless siphonic type and the other is valve-fitted siphonic type. The former is preferred and largely used in practice.

4. Washbasins

Washbasin is also called as lavatory basin. It is made of 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 kitchen or laboratory for draining of 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 siphonic type of flushing cisterns. In the bowl type, the cistern may be hand operated and of 5 l capacity. In the stall type the automatic flushing cisterns are used.

27.4.3 Plumbing System of Drainage

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 water seal with the assurance that the entry of gases from sewers are blocked. The advantages of the system are:

1. Simplicity in design, layout and easy plumbing of sanitary fixtures.
2. Better external appearance of the building.
3. More compact system and
4. More economical.

The main disadvantage with this system is easy breaking up of water seal 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 roof top such that to allow a natural outlet to the foul gas. This system requires a difficult arrangement of pipe work and 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 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.

27.4.4 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 pipeline may affect the natural flow.
4. In the points of 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.

27.4.5 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 27.4.

Table 27.4 Recommended gradient of sewer discharge pipes

Source: IS: 1742–1972.

2. Positioning of Pipes

The following points shall be kept in view while deciding the positioning of pipes:

1. 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.
2. All the pipes needed for different purposes have to be adequately provided for during the construction stage itself at their appropriate positions.
3. When pipes are not embedded it should run clear of the wall with a minimum clearance of 5 cm.
4. Waste pipes are to be separated from the house drains by using gully traps. This arrangement is essential to prevent entry of foul air or gas, vermin, etc., into the building.
5. 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 nesting of birds or inadvertent falling in of objects inside the pipe.
6. 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 mmConnecting waste pipe 40 mm

27.4.6 Sludge and Effluent Disposal

Sedimentation is carried out with the object of removing such suspended mineral 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 as 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:

1. Dumping into waste bodies
2. Shallow Burial
3. Lagooning
4. Mechanical dewatering and
5. Drying in Beds

If such a general disposal system is not available one has to go in for a sedimentation tank. Generally sedimentation tank adopted is a septic tank which is a horizontal continuous flow sedimentation tank. A description of septic tank is explained in 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 also gets reduced for easy disposal. The effluent from septic tank has very bad colour. It is dark in colour with fine solid particles and has to be disposed with utmost case. A schematic layout of a septic tank is shown in Fig. 27.6.

Figure 27.6 Septic tank

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 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. 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 to keep 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 which is exposed to 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:

1. Sub-surface irrigation employing absorption field method.
2. Discharge into other soil absorption system as soak pit or seepage pits and leaching cess-pools.

Sub-surface irrigation is also termed as 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 (Fig. 27.7).

1. Brick lining with dry joints

2. Outer casing with coarse aggregates (7.5 cm thick min)

Figure 27.7 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 solid wall which functions as a septic tank (Fig. 27.8).

1. Outer casing with coarse sand (30 cm thick).

2. Lining with coarse aggregate (15 cm thick).

3. Brick work with mortar joints.

Figure 27.8 Leaching cesspool