Introduction

FUNDAMENTAL Definition :

• Refuse: All the material that is wasted.(Solid + liquid + Semisolid)
• Garbage: Dry refuse, organic in nature. (Eg: Vegetables Grass, Sweepings etc).
• Rubbish: Dry waste from offices, residences,inorganic in nature. It is combustible. (Eg: Paper, Rags, etc)
• Sewage: A liquid waste of domestic, or Industrial origin. ‘Foul’ in nature Consists 99.9% water Domestic and industrial
•  Sullage: Waste  water from bathrooms,Washbasins, Kitchens. Less foul in nature
• Storm Water: “ The run-off from roads,buildings and  ,‘ other catchment areas.Generally called. ‘storm drainage’ or ‘ Drainage’.
• D.W.F: Dry weather flow i.e. normal flow available in any season. It is due to •‘Sanitary sewage’. *D.W.F is .generally 1/2Oth”to 1/25th of max flow during monsoon. 
• Sewer: A pipe carrying sewage

 Sewerage: The process of ‘collection . + conveyance’ of sewage.

SYSTEMS OF SEWERAGE:

• Separate system: Sanitary sewage and storm water are collected, conveyed and disposed off separately.

Suitability:

1. In areas of uneven rainfall in a year.
2. In hilly areas with steep slopes.
3. Where deep excavations cannot be easily done.

• Combined system : Both sanitary sewage and storm water drainage are collected into the same conduit.

Suitability:

1. Where rainfall is evenly distributed through out the year.
2. In plain areas where excavation is easy and less costly. .

 when area is congested or for narrow streets

• Partially separate system: Designed for Carrying sewage discharge plus part of the storm drainage from the roofs and courtyards
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 CLASSIFICATIONS OF SEWERS :

• House sewer:  Sewer pipe carrying sewage from a building to the point of its immediate disposal. The sewerage system originates from it.
• Lateral sewer: A sewer which receives the discharge from a number of independent houses. Also called branch sewers or sub mains

Trunk or Main sewers : These are main sewers to  which lateral sewers arcconnected

Outfall sewer : The sewer which transports the sewage to point of treatment and from the T.P to disposal point.

As far as possible should be designed to flow

Under gravity with 1/2 or 3/4 th fuIl.

Sewage treatment plant should generally be

located in a low- lying area.

ESTIMATION OF DRY WEATHER FLOW

(SEWAGE DISCHARGE):

• Quantity of sewage should be equal to the quantity of water supplied. But certain additions and subtractions may take place. Generally , the Net quantity of sewage = 75 to 80% of the water supplied. It is also known as Dry weather flow

QwF = Population x per capita water supply x factor

Variations of sewage flow:

Max. daily flow     = 2 x Average daily flow

Max. hourly flow  = 1.5 x Max. daily flow

                                =  3 x average daily flow 

Peak flows for lateral sewers are so much greater than that of large trunk sewers because more .

flow time for trunk sewers compared to laterals. Sewers are designed for carrying the max.

hourly flow running 3/4th full.

Minimum flow: At minimum flow, reduces therefore silting  occurs.Slope at which sewer is to be laid is decided based on permissible minimum velocity at minimum flow.

 Minimum daily flow =2/3 x Average daily flow

 Max. hourly flow  = 1.5 x Max. daily flow

                                =  3 x average daily flow 

Sewers must be checked for minimum velocities

At their minimum hourly flow.

 Design periods of different  component:

• Branches, mains and Trunk sewers

        — Generally 30 years

• Treatment units— 15 to 20 years
• Pumping plants — 5 to 10 years

ESTIMATION OF WET WEATHER FLOW

(STORM WATER):

Estimating peak runoff

By the use of rational formula

QWWF = peak rate of runoff in cumec.

I= Coefficient of runoff / Impermeable coefficient

A = Catchments area contributing to runoff in hectares.

R = Critical rainfall intensity in mm/hr corresponding to ‘time of concentration’.

The period after which the entire area starts contributing to the runoff is called the  ‘time of concentration of concentration’ .or It is the maximum time taken by a drop of water to travel from the farthest point or remotest point to the outlet of the catchment.

The maximum runoff will be obtained from the rain having a duration equal to the time of concentration and this is called Critical  rainfall duration.

Coefficient of runoff (K) :

‘K’ increases as imperiousness of the area increases.

For paved areas , k= 0.9 to 1:

For Lawns and gardens, k 0.15

tc = te +tf

where  tc= time of concentration

te= time of entry or inlet time

tf= time of flow = time required to flow in the

sewer from inlet to the point of concentration.

a and b are constant 

a=30     b=10 for if tc =5 to 20 min

a=40     b=20  tc  >20 min

Empirical formulas:

For Larger catchment areas I.e. more then 400 hectares. it is advisable to use empirical formula.

Dickens  formulas: For North Indian catchments

A = Catchment area in Sq.km

Qp= Peak discharge in cumec.

Cd=Constant depending on different factors

Ryvns formula : For south Indian  catchments