1) Discuss the relative merits of the separate and the combined systems of sewage, and give the conditions favorable

for the adoption of each one of them.

The advantages of separate systems of sewage are:

Surface run-off and rainwater can be reused (e.g. for landscaping or agriculture)

Surface run-off, grey water and black water can be managed separately

No risk of sewage overflows

Provides a high level of hygiene and comfort

The conditions favorable for the adoption are:

Especially suitable in areas where irregular, heavy rainfall is expected to avoid frequent combined sewer overflows.

Suitable for urban areas with resources to implement, operate and maintain the system.

The advantages of combined systems of sewage are:

Provide a high level of hygiene and comfort for the user at the point of use

No problems related to discharging industrial wastewater

Storm water and wastewater can be managed at the same time

The conditions favorable for the adoption are:

Suitable for urban areas with resources to implement, operate and maintain the system

Manholes are installed wherever there is a change of gradient or alignment

Appropriate when a centralised treatment facility is available

2) Write short notes on: financing the sewage projects and types of sewerage.

Financing the sewage projects:

The functioning of sewerage system require large investment of capital, the initial budget has to be provided by

government, but the interest on the capital investments and the depreciation charges as well as RMO( Running,

Maintenance and Operation) expenses must be recovered from the users of these facilities by properly taxing them.

However there cannot be a very rational system of taxing the users of sewerage facilities because it is not feasible to

estimates the quantity of sewage removed from an individual household and hence cannot be as rationally taxed as can be

done for water supplies. Nevertheless, planning engineer must also develop the suitable and logical financing programme,

indicating the sources of income and balancing them against bond and interest payments and RMO charges.

Types of sewerage:

Combine sewer

Combine sewer are large networks of underground pipes that convey domestic sewage, industrial wastewater

and storm water runoff in the same pipe to a centralised treatment facility. A constant downhill gradient must be

guaranteed along the length of the sewer to maintain self-cleansing velocity.

Separate sewer

Separate sewer systems are designed to convey wastewater and storm water in separate pipes.

A properly constructed separated system the sewage is transported in a closed system directly to the treatment

plant and cannot overflow into the environment.

Simplified sewer

A sewer system that is constructed using smaller diameter pipes laid at a shallower depth and at a flatter gradient

than conventional sewers Allows for a more flexible design associated with lower costs and a higher number of

connected households.

Pressurised sewer

It is different from conventional gravity collection systems, because they use pumps instead of gravity to transport

wastewater. The pumps should be checked regularly and the pipe connections should be controlled for leakages.

Open channel

An open channel is a conveyance in which water flows with a free surface.

Less expensive compared to underground sewer systems.

The main drains are generally connected with natural drainage channels such as rivers or streams.

On steep terrain, erosion could damage the drain and flow retarding structures may be necessary.

3) Note down the various problems of sewer works observed at field. Document how the problems are resolved.

The various Problems of sewer works observe at field are as follow:

i. Laying of sewer network

ii. Gradient of sewer

iii. Pumping

iv. Steep slopes

v. Soil of laying

4) Differentiate between sanitary sewage and storm sewage. Discuss and explain the Rational Formula and its

limitations in calculating the quantities of storm sewage.

Sanitary sewage Storm sewage

The sanitary sewer is a system of underground pipes

that carries sewage from bathrooms, sinks,

kitchens,and other plumbing components to a

wastewater treatment plant where it is filtered, treated

and discharged.

Sanitary sewage is treated in treatment plant

The storm sewer is a system designed to carry rainfall

runoff and other drainage.

It is not designed to carry sewage or accept hazardous

wastes.

The runoff is carried in underground pipes or open ditches

and discharges untreated into local streams, rivers and

other surface water bodies.

The Rational method is based upon the following formula:

QT = C iT A

Where:

QT = estimate of the peak rate of runoff (cfs) for some recurrence interval, T

C = runoff coefficient; fraction of runoff, expressed as a dimensionless decimal fraction, that appears as surface runoff from

the contributing drainage area.

IT = average rainfall intensity (in/hr) for some recurrence interval, T during that period of time equal to Tc.

A= the contributing tributary drainage area to the point of design in acres which produces the maximum peak rate of runoff.

Tc = rainfall intensity averaging time in minutes

The use of the rational formula is subject to several limitations as follow:

The most important limitation is that the only output from the method is a peak discharge (the method provides only an

estimate of a single point on the runoff hydrograph).

The simplest application of the method permits and requires the wide latitude of subjective judgment by the user in its

application. Therefore, the results are difficult to replicate.

The average rainfall intensities used in the formula have no time sequence relation to the actual rainfall pattern during the

storm.

The computation of Tc should include the overland flow time, plus the time of flow in open and/or closed channels to the

point of design.

5) Mention the various aspects you would keep in view when designing a sewer.

Following aspects should be considered while designing the system:

For evaluating proper diameter of the sewer, correct estimation of sewage discharge is necessary

The flow velocity inside the sewer should not be so large as it require heavy excavation and high lift

pumping.

It should not be so small causing deposition of the solid in the sewers.

The sewers should be laid at least 2 to 3 m deep to carry sewage from basement.

The sewage in sewer should flow under gravity with 0.5 to 0.8 full at designed discharge, i.e. at the

maximum estimated discharge.

Treatment plant should be designed taking into consideration the quality of raw sewage expected and to

meet the discharge standards.

The sewers provided should be adequate in size to avoid overflow and possible health hazards.

The sewerage system is designed to remove entire sewage effectively and efficiently from the houses to the

point of treatment and disposal.

6) What are the advantages of the egg-shaped section, and under what conditions of flow, does it become useful?

Will you recommend its use for sanitary sewers, and if not, why?

Figure: egg-shape section

The advantages of the egg-shaped section is egg shape ensures that they are narrower at the bottom, this causes water near

the bottom to flow faster, reducing the likelyhood of solids being deposited. If solids were deposited the sewer would

eventually silt up and become blocked. Egg-shape section is useful when the flow goes less than half full.

I will not recommend egg-shape section to use in sanitary sewers because of following reasons:

a. They are more difficult to construct.

b. Since the smaller base has to support the weight of the upper broader section, they are less stable.

c. They require more materials and are therefore more costly.

7) Explain the importance of (1) self-cleansing velocity; and (2) non-scouring velocity.

Self-cleansing velocity: Sewage consists of considerable amount of organic and inorganic solids, which remain floating or

suspended. If velocity of flow in the sewer is less, these solids get deposited at the invert of the pipe and cause obstruction

to the flow of sewage. Hence, it is necessary to maintain a minimum velocity of flow at which no solids get deposited in

the sewer. The velocity of flow that prevents settlement of solids is known as self-cleansing velocity. This velocity should

be maintained at least once in a day during the peak flow at all sections of the sewerage system.

Non-scouring velocity: The velocity of flow in sewer should not be too high, as the suspended solids will cause wear to

contact surface of the pipe and erode the pipe material of sewer. This will reduce the life of the sewer. The permissible

maximum velocity to prevent eroding is termed as non-scouring velocity and it should be limited to 3.0 m/s.

8) Write detailed notes on characteristics of partially full sewers

9) Describe in order the various stages followed in the laying of sewers

The installation of sewage pipe consist of the following steps.

i. Locate the positions of the manhole on the ground along the longitudinal section of the sewer line. It is common

practice to lay sewer line between two manholes at a time.

ii. The center line pegs of the sewer are driven at a distance of every 7.5 m or 15m.

iii. The center line of the sewer line should be properly maintained by providing an off-set line usually marked at a

distance of 2m to 3m.

iv. The off-setline helps in locating the sewer center line when excavation is carried out to laying of sewer pipe.

10) What should be the characteristics of materials to be used for sewers?

Following characteristics should be considered for selecting materials to be used for sewer pipes:

Resistance to corrosion

Sewer carries wastewater that releases gases such as H2S. This gas in contact with moisture can be converted in to sulfuric

acid. The formation of acids can lead to the corrosion of sewer pipe. Hence, selection of corrosion resistance material is

must for long life of pipe.

Resistance to abrasion

Sewage contain considerable amount of suspended solids, part of which are inorganic solids such as sand or grit. These

particles moving at high velocity can cause wear and tear of sewer material. This abrasion can reduce thickness of pipe and

reduces hydraulic efficiency of the sewer by making the interior surface rough.

Strength and durability

The sewer pipe should have sufficient strength to withstand all the forces that are likely to come on them. Sewers are

subjected to considerable external loads of backfill material and traffic load. They are not subjected to internal pressure of

water. To withstand external load safely without failure, sufficient wall thickness of pipe or reinforcement is essential. In

addition, the material selected should be durable and should have sufficient resistance against natural weathering action to

provide longer life to the pipe.

Weight of the material

The material selected for sewer should have less specific weight, which will make pipe light in weight. The lightweight

pipes are easy for handling and transport.

Imperviousness

To eliminate chances of sewage seepage from sewer to surrounding, the material selected for pipe should be impervious.

Economy and cost

Sewer should be less costly to make the sewerage scheme economical.

Hydraulically efficient

The sewer shall have smooth interior surface to have less frictional coefficient.

References:

Walski, T. M., and T. E. Barnard (2004), Wastewater collection system modeling and design, Haestad Press.

S.K.Garg, Sewage Disposal and Air Pollution Engineering, Vineet Khanna.

Prof. M.M.Ghangreka, sewer materials, IIT Kharagpur