Design of Sewer MCQ Quiz - Objective Question with Answer for Design of Sewer - Download Free PDF

Explanation:

The egg-shaped sewer is designed for combined sewer systems, which carry both sanitary sewage and stormwater. It has a narrower bottom and wider top, and is placed with the smaller (narrow) end at the bottom for the following reasons:

• Self-cleansing velocity: During dry weather (low flow), the narrow bottom ensures higher velocity to prevent sediment deposition.

• High discharge capacity: During storms, the upper wider section accommodates large volumes of water.

• The smaller base (narrower section) supports the broader upper portion of the sewer.

Additional InformationEgg-shaped Sewer

Shape and Design:

• The sewer has a distinctive egg-like shape with a narrower bottom (smaller base) and a wider top (larger base).

Purpose:

• Primarily used in combined sewer systems to handle both sanitary sewage and stormwater flows.

Flow Efficiency:

• The design is intended to optimize flow conditions during varying discharge rates.

• Self-cleansing velocity is achieved by the narrow bottom section, which promotes higher velocity during low flow conditions, preventing sedimentation.

Capacity:

• The wider top section provides more capacity during high flow conditions, such as during heavy rainfall or storms, allowing the system to handle increased discharge.

Advantages:

• Minimizes sediment deposition during low flow by maintaining sufficient flow velocity.

• Increases discharge capacity in storm conditions due to the larger upper section.

• Optimized for combined sewage systems by providing efficient flow at various conditions.

Explanation:

A water closet is a water flushed plumbing fixture designed to receive the human excreta directly and is connected to the soil pipe by means of a trap.

Soil pipe:

• A soil pipe conveying to a drain any solid or liquid filth shall be circular and shall have a minimum diameter of 100 mm.
•  Asbestos cement building pipes may also be used as soil pipes only above ground level.
• Soil pipes, whether inside or outside the building, shall not be connected with any rainwater pipe and there shall not be any trap in such soil pipe or between it any drain with which it is connected.

Vent pipe –

• The pipe installed for the purpose of ventilation is known as a vent pipe through which foul gases escape into the atmosphere. It is provided in a septic tank for the discharge of foul gases.
• The diameter of the main ventilating pipe shall not be less than 50 mm.
• A branch ventilating pipe on a waste pipe in both one and two-pipe systems shall be of not less than two-thirds the diameter of the branch waste ventilated subject to a minimum of 25 mm.
• A branch ventilating pipe on a soil pipe in both one- and two-pipe systems shall be not less than 32 mm in diameter

Anti-siphonage pipe –

Anti-siphonage pipe means any pipe provided in conjunction with a trap in the sanitary fitting to prevent unsealing of the trap by siphonage or backpressure.

Back-siphonage causes breaking of the water seal therefore anti-siphonage pipe is required.

Manholes-

Manholes are masonry or R.C.C. chamber constructed at suitable intervals along the sewer lines for providing access into them.

Explanation:

Traps: 

• As the name suggests, traps simply trap the water and sewer gas.
• It is a plumbing device used to prevent smell, bacteria, also insects from entering your home.

Different types of trap mention bellow:

• Gully trap is provided outside the building before connecting it to the external sewerage line. It also collects wastewater from the kitchen sink, washbasin, bath, and wash area.
• Intercepting trap is provided to prevent the foul gases from the public sewer from entering into the building sewer.
• Anti-siphon trap is used to prevent siphoning action.
• p trap is used with an Indian water closet. The traps are made from UPVC or cast-iron sheets. This trap also has a water seal and prevents entry of foul gases to the house.

Concept:

Self Cleansing Velocity:

• self-cleansing velocity is that which does not allow the silting of solid in the sewer more-ever also carry out the removal of solids that have already settled.
• Self-cleansing velocity for 1 mm inorganic particle and for 5 mm organic particle is 0.45 m/sec.
• This minimum self-cleansing velocity of 0.45 m/sec is attained at minimum hourly discharge. If the sewer is designed for design flow condition and velocity at design depth of flow is greater than 0.8- 0.9 m/sec.

Self-cleansing velocity in the sewer is given by,

\({{\rm{v}}_{\rm{s}}}{\rm{ = }}\sqrt {\frac{{{\rm{8kgd}}\left( {{\rm{G - 1}}} \right)}}{{\rm{f}}}} \)

K is a constant which depends upon the type of solids present in the sewage 

f = Friction factor

d = Diameter of the particle

Explanation:

Drop Manhole:

A drop manhole is a type of manhole where the vertical pipe is permitted to flow between the main sewer and branch sewer.

• Drop manholes are provided at the junction of sewer lines when a high-level branch sewer enters a low-level main sewer. If the difference of more than 600 mm in the water lines and inverse level of two sewers are connected with a drop manhole.

• The branch sewer is joined to a manhole through a vertical pipe. The sewage coking through the branch sewer dips in through the vertical pipe, and trickles over the main sewer channel, just above it.

• A plug is provided at the point where the branch sewer if taken straight intersects the wall of the manhole.

• A plug is provided at the point where the branch sewer if taken straight intersects the wall of the manhole.

The laid pipes are usually tested to ascertain the water-tightness of the pipes and joints. There are three tests that are commonly performed to test leakage of the pipes and are as follows:

1. Air Test - Generally used to test underground and vertical pipes. The escape of air from the leaky joint forms bubbles that can be seen visually.
2. Water Test - Most commonly used for underground house sewer pipes. The drain is plugged at the lower end and the water is filled in the pipe through the upper manhole to maintain a pressure of 1 to 1.5 m depth of water. If there is no drop in the water level in 10 minutes, then there is no leakage from the pipe.
3. Smoke Test - Generally used to detect leakage of existing vertical sullage pipe or rainwater pipes. In this test, smoke is forced into the pipe with the help of a blower to test the leakage.

Explanation:

Design of sewer:

• The peak flow can be considered as 1.5 times the annual average daily flow.
• For a design of the treatment facility, the peak factor is considered as 1.5 times the annual average daily flow.
• The minimum flow passing through sewers is also important to develop self-cleansing velocity to avoid silting in sewers. This flow will generate in the sewers during late night hours. The effect of this flow is more pronounced on lateral sewers than the main sewers.
• Sewers must be checked for minimum velocity as follows:
• Minimum daily flow = 2/3 Annual average daily flow
• Minimum hourly flow = 1/2 Minimum daily flow
• Minimum hourly flow = 1/3 Annual average daily flow

Explanation:

Drop Manhole

A drop manhole is a manhole in which a vertical pipe is provided to allow for flow between the branch sewer and the main sewer.

• Drop manholes are provided when there is change in elevation of ground level.
• A drop manhole reduces the turbulence in the manhole when the elevation difference between the incoming and outflow sewers is greater than 0.5 meter.
• Sloping ground, with drop more than 0.6 m is required to control the gradient.
• Therefore they are provided in hilly townships.

Concept:

The hydraulic mean depth or also called hydraulic radius is defined as:

\(R = \frac{{\rm{Wetted}\;\rm{Area}}}{{\rm{Wetted}\;\rm{perimeter}}}=\frac{A}{P}\)

Wetted area = \(A = \frac{\pi }{4}{d^2} \)  

Wetted perimeter P = πd

Hydraulic mean deppth = \(R = \frac{A}{P} = \frac{{\pi {d^2}}}{{4\pi d}} = \frac{d}{4}\)

Calculation

Given:

d = 100 cm.

For circular pipe:

\(R = \frac{A}{P} =\frac{d}{4}\)

\(R =\frac{100}{4}=25\;mm\)

Explanation:

As per IS 1742:1960:

So as per the given options most appropriate answer, for 50 cm diameter of sewer spacing of manhole is 100 m

As, per IS 1972 (1983), Clause 4.6.1.2 The approximate gradients which give this velocity for the sizes of pipes likely to be used in building drainage and the corresponding discharges when flowing half-full are as follows:

Explanation:

For a circular section (a section which is converging on the top), hydraulic efficiency depends upon:

i) Maximum Velocity

For maximum velocity case, R = 0.30 × d and y = 0.81 × d

ii) Maximum discharge

For maximum discharge case, R = 0.29 × d and y = 0.95 × d

Where

d = diameter of the circular pipe

y = hydraulic depth

R = hydraulic radius

Concepts:

The flow in sewer pipe takes place under atmospheric conditions i.e. open channel flow and also its is assumed to be a uniform flow. Hence, Manning’s formula can be applied for the calculation of discharge, which is given as :

\(Q= A (\frac{1}{N}R^{2/3} S^{1/2}) \)

Where

N is roughness coefficient, depends on pipe material

R hydraulic radius depends on pipe geometry

S is bed slope at which pipe is laid

A is a cross-sectional area

Q is the rate of flow in sewer pipe

Calculation:

Since, both sewer pipelines have same diameter and made of same material, they have same hydraulic radius and cross section area and roughness coefficient.

Therefore, it can be concluded that

Q ∝ S^1/2

Now ,

\(\frac{Q_A}{Q_B} = \frac{S_A^{1/2}}{ S_B^{1/2}}\)

\(\frac{Q_A}{Q_B} = \frac{{(1/225})^{1/2}}{(1/625){1/2}}\)

\(\frac{Q_A}{Q_B} = 1.67\)

Concept:

Self-Cleansing Velocity:

• self-cleansing velocity is that which does not allow the silting of solids in the sewer more-ever also carries out the removal of solids that have already settled.

NBO recommendations for minimum velocity for small sewers:

Concept:

IS 1742-1983: Code of practice for building drainage

Normally, the sewer shall be designed for discharging three times the dry-weather flow flowing half-full with a minimum self-cleansing velocity of 0.75 m/s. The approximate gradients which give this velocity for the sizes of pipes likely to be used in building drainage and the corresponding discharges when flowing half-full are as follows:

Important Points 1. In cases, where it is practically not possible to conform to the ruling gradients, a flatter gradient may be used but the minimum velocity in such cases shall on no account be less than 0.61 m/s. 

2. It is undesirable to employ gradients giving a velocity of flow greater than 2.4 m/s. Where it is unavoidable, cast-iron pipes shall be used.