TYPES OF FLOW

Dry Weather Flow

1. Domestic waste water

2. Industrial waste water

3. Waste water from public facilities

4. Ground water infiltration

Storm Water Flow

Factors Affecting Dry Weather Flow

Rate of water supply Population growth

(Design period) Type of area served Infiltration of ground water.

Factors affecting infiltration of subsoil water into sewer lines

Depth of sewer below ground water levelSize of sewerLength of sewer through infiltration prone zoneNature and type of soil through which the sewer is

laidType of joints, workmanship etc.Sewer material

Storm water flow

The storm water flow depends upon the following factors

Catchment areaGround slopePermeability of groundExtent of impervious areaExtent of vegetation growthRainfall intensity

Method for estimating storm water flow

Rational method

Q=K*A*I*R Here Q= runoff or storm water flow K= constant A= catchment area in hectares R= intensity of rainfall in mm per hour I= impermeability factor.

Hydraulic Design Of Sewers

1) Chezy’s Formula:

V=C(RS)^0.5

where,

C = Chezy’s coefficient

(depends upon shape, size, roughness of

sewer, etc)

2. Manning’s formula:

V=1/n*R^(2/3)*S^(1/2).

where,

n = manning’s coefficient

(the Value of manning’s coefficient n varies

from 0.01 to 0.05 depending upon material of

sewer line)

By comparing Chezy’s & Manning formula

C=1/n*(R^1/6)

3) 3) Hazen–William’s formulaHazen–William’s formula : :

V=0.85*H*(R^0.63)*(S^0.50).V=0.85*H*(R^0.63)*(S^0.50).

where,where, H is Hazen- William’s coefficientH is Hazen- William’s coefficient (the value of H varies from 100 to 150(the value of H varies from 100 to 150 depending upon sewer line’s material)depending upon sewer line’s material)

Minimum Velocity of flow

Minimum or self cleansing velocity is the minimum

velocity required to prevent deposition & clogging of

sediments.

It depends upon the shape, size & specific gravity of

particles.

Required at least once in a day.

Maximum velocity of flow

The maximum velocity

at which no scouring

action or abrasion takes

place is known as non

scouring velocity

Such a velocity depends

upon the material used

for the construction of

sewers

Material of sewerMaterial of sewer Non scouring Non scouring velocity(cm/sec)velocity(cm/sec)

Earth channelsEarth channels 60-12060-120

Ordinary brick-lined Ordinary brick-lined sewerssewers

150-250150-250

Cement concrete Cement concrete sewerssewers

250-300250-300

Cast iron sewer pipesCast iron sewer pipes 350-450350-450

Vitrified tile and Vitrified tile and glazed bricksglazed bricks

450-500450-500

Rectangular

Horse-shoe

U-shape

Parabolic

1. Circular Sewers

2. Non Circular Sewers :

Forms of sewer

Significance of Circular Sewers

I. Circular section has least perimeter & max R for given area so it is most efficient.

II. It requires minimum quantity of materials, making it economical too.

III. Having uniform curvature, it prevents the possibility of deposition.

IV. They are easily manufactured

Crown Corrosion

H2SO4 DropsAir

H2S

Wastewater

Cement concrete pipes

Measures to overcome Crown Corrosion

1.Ventilating the sewers

2.Operating sewers to full run

3.Pretreatment to reduce sulphate

4.Prohibit entry of sulphides

5.Lining the concrete pipes by vitrified clay blocks.

6.Neutralization of sulphides by chemicals.