sewer and sewage

SEWER & SEWAGE

Content

• Different types of waste• Carriage systems of waste• Types of sewers on the basis of working• Types of sewers on the basis of material used• Joints in sewers• Shapes in sewers• Hydraulic design of sewers• Pumping of sewage

Types of waste

• Night soil• Garbage• Sullage and storm water• Sewage

Carriage systems (on the basis of type of waste )

Conservancy systemsWater Carriage systems

Conservancy systems

• Old system• Collection in separate vessels• Deposition in pools & pits• Removal once in 24 hours atleast• Dry system• In villages and small towns

Water Carriage system

• Water medium• Dilution ratio should be high• Collection in sewers through WC • Garbage not permitted• Initial cost high• Large amount of water required

collectionPoint of production

Packer - Truck Transportation of Garbage

ComparisonS. No. Conservancy systems Water Carriage systems

1. Unhygienic Hygienic

2. Foul smell No foul smell

3. Compact house design not possible Possible

4. Large labour force required Negligible

5. Water consumption is small High

6. Initial cost small, running cost high High initial cost, running cost small

7. No technical person required Required for operation & Maintenance

8. Acute Pollution problems Rare

9. Risk of spread of epidemic No such Risk

10. Large Land required for disposal Small for treated sludge

11. Final disposal into stream have risk No risk

12. Good quality Manure Small manure value

13. Suitable for rural conditions For urban conditions

Classification of WCS

• Separate system– Sanitary sewer– Storm sewer

• Combined systems• Partially separate systems

Separate Systems• Provides two separate systems of sewer

Sanitary sewer:- for the conveyance of foul sewage such as faecal matter, domestic wastewaters, the washings, draining of laundries, slaughter houses, stables, and waste water derived from industries

Storm sewer:-for the rain water, including the surface water from certain streets, overflow from public baths

Separate system

Separate Systems

• Advantages– Installation cost low– Less load on treatment unit– Easy to putrification– Rain water need not any treatment

• Disadvantages– Difficult cleaning– Foul smell may present due to less air contact

Combined Systems

• One sewer • Carry both foul sewage as well as rain water• Treatment before disposal required• Street surface washing is as impure as sewage

Combined system

Combined Systems

• Advantages– One set of sewer– Less choking– Diluted sewage– Less Foul smell

• Disadvantages– High construction cost– Difficult handling and transportation– Increase in load on the treatment plant– During heavy rains, may overflow

Partially Combined System

• Only one set of underground sewer• Carry foul sewage as well as early washing by

rains• If storm water exceed certain limit, is

collected and conveyed in open drains to the natural streams

• Foul sewage still flow in the sewer

Partially combined system

(Open Drain)

Partially Combined Systems

• Advantages– Sewers are of reasonable size– Combines the advantage of both– No chances of choking– No problem of disposing off storm water

• Disadvantages– During dry weather, velocity of flow is low– Storm water increases load on treatment plant– Storm water also increases cost of pumping

Sewers and its Types

• Under ground Conduit or drain• Conveyance of sewage• Started from point of production of sewage• Terminates at the point of disposal

Types of sewer

• Sanitary sewer• Storm sewer• Lateral sewer• House sewer• Branch sewer• Main sewer• Outfall sewer• overflow sewer

Materials used for sewer

• Asbestos cement• Plain or reinforced cement concrete• Vitrified clay or stoneware• Bricks• Cast iron• Steel• Plastic

Selection of material• Hydraulic efficiency• Resistance to abrasion• Resistance to corrosion• Strength• Durability• Cost• Weight• Imperviousness

Asbestos cement sewer

• Mixture of asbestos fibre, cement and silica• Asbestos fibre as a reinforcing material• Other constituents converts it into a dense

homogeneous material under pressure• Diameter vary from 75mm to 500mm• Length 3 to 4 m• Used only as verticals

Asbestos cement

• Advantages– Considerable strength against internal pressure– Easy handling– Easy cutting and joining– Resistive to salts and other corrosive materials– Relatively smooth

• Disadvantages– Brittle and need careful handling– Low structural strength– Susceptible to sulphide corrosion

P.c.c. & R.c.c. sewers

• P.c.c. pipes diameter vary from 80 mm to 450 mm

• Thickness vary from 25 to 30 mm• For bigger diameters r.c.c. pipes are used• Usual mix 1 : 1.5 : 3• Maximum size of aggregates limited to 6mm• Water cement ratio varies b/w 0.5 to 0.7

P.c.c. & R.c.c. sewers

• Advantages– Strong under internal pressure as well as external

pressure– Can be made of any desired strength– Easily manufactured even at the sight– Used for main sewers and branch sewers

• Disadvantages– Get easily corroded and pitted due to corrosive

action of the contents of the sewers

Vitrified clay or stoneware sewers• Manufactured from clay and shales• Ingredients are taken in desired proportionate• Formed in pipe press at about 0.85 N/mm2 • Vitrification achieved at 1200c• NaCl added to place the surface of pipe

Vitrified clay sewer

• Advantages– Highly resistant to sulphide corrosion– Resistive to erosion – Hydraulically efficient– Highly imperious– High compressive strength

• Disadvantages– Weak in tension– Brittle in nature– Bulky and heavy

Brick sewer• Used since olden days• Plastered from outside• Purpose-made bricks are used• Preferred for large sized Combined sewer• Lined inside with stoneware or ceramic blocks• Resistive against sulphide corrosion

Cast Iron Sewers

• Can withstand in tensile, compression as well as bending stresses

• Coated with paint or cement concrete• Diameter from 150 mm to 750 mm• Length up to 3-3.5 m• High strength• Costlier

Steel sewers

• Light in weight• Perfectly impervious• Can absorb vibrations and shock loads• Used in high external and internal pressure• Used for mains outfall and trunk sewers having

large diameters• Corrosion resistive either due to heavy

galvanization or bituminous coatings• Easily welded• Costlier comparison to others

Plastic sewers

• Still in experimental stage• High hydraulic efficiency• Corrosion resistant• Available in longer lengths• Easy joining and bending• Taken in use, in Netherlands, Scandinavia,

France etc.• In India, recently started as domestic sewers

Joints in sewers

• Used for connecting two segments• Water tight• Enough strong• Durable• Economical

Types of Joints

• Bell and spigot joint• Collar joints• Simplex joints• Flexible or Bituminous joints• Mechanical joints• Open Joints

Bell and Spigot joints

• Commonly use in P.c.c. or R.c.c.• Each has a spigot end and a bell or socket end• Cement mortar of proportion 1 : 1 or 1 : 2• Gasket or jute packing may be placed in inner

side• mortar caulked joint is finished at about 45

on the outer face

Bell and Spigot joints

Collar joints

• The plain ends of the consecutive lengths of pipes are kept near each other

• A collar of bigger diameter is placed around• Annular space is filled with cement mortar of

1 : 1 proportion• Used for concrete pipes of large diameter

Collar joint

Simplex Joints

• Also known as ring tie coupling• Similar to collar joints• Use for asbestos cement pipes• Consists coupling of asbestos cement and two

rubber rings• Flexible in nature

Flexible or bituminous joints

• more over same as Collar joints• used For Making them flexible

Mechanical Joints

• Use mechanical devices ( flanged rings, bolts, screw ends etc.) to keep the two ends together

• Use for metallic sewers made• of cast iron , steel etc.

Open joints

• Used, where is no objection to filtration

Shapes of Sewers

• Circular sewer• Non- Circular sewer

– Standard Ovoid section– New Ovoid section– Horse – shoe section– Parabolic Section– Semi-elliptical section– Semi – circular section– U- shaped section– Basket – handle section

Different Non-circular Shapes

Advantages

• Circular sewers– Easily manufactured– Greatest H. M. D.– Most economical– Less opportunities for deposits

• Non- circular sewers– Gives slightly higher velocity during low flow

(Ovoid shaped combined sewers )

Hydraulic design of sewers

• generally design same as open channels.• Except when it is specially required to design

them as conduits carrying sewage under pressure

• Example- The case of inverted siphons .

Empirical formulas used

• Chezy’s Formula-: V=C√(RS) where

V= velocity of flowC=chezy’s coefficientR= hydraulic radius or hydraulic mean

depth S= slope of sewer

Contd……….

Manning’s formula-: V=1/n×R^(2/3)√S

Where n= manning’s coefficient (from 0.01 to 0.05 )

• Relation between manning’s formula and chezy’s formula :

C=1/n×R^(1/6)

contd….......

• Crimp and Bruge’s formula -:V= 83.47 R^(2/3)√S

• Relation between Crimp & Bruge`s formula and Manning’s formulaSame, if n = 0.012.

• This formula is commonly used in England

Contd……..

• Hazen–William’s formula V=0.85*CH×(R^0.63)×(S^0.50)

whereCH = hazen- williams coefficent(from 100

to 150) • used for the design of conduits carrying liquid

under pressure.

Minimum Velocity • self cleansing velocity of flow• such that the suspended materials in sewage

do not get silted up• required velocity to prevent deposition &

clogging of sediments.• depends upon the shape,size & sp.gravity of

particles.

Contd……..

• Vs=1/n×R^(1/6)×√{β(Gs – 1)ds}Where Vs = self cleansing velocity β=Dimensionless constant(0.04 to 0.80)

Gs = specific gravity of sediments ds = diameter of solid particles

Maximum Velocity

• Essential to limit the maximum velocity in the sewer pipe.

• Because the smooth interior surface of a sewer pipe gets scoured due to continuous abrasion caused by the suspended solids present in sewage.

• Depends upon the material of the sewer.• To prevent erosion of inner surface of sewers,

should not exceed 3.0 m/s

Effects of Flow Variation

• Depth of flow varies

• Hydraulic mean depth (r) varies.

• Flow velocity (which depends directly on r2/3) gets affected from time to time.

Points to remember• Check the sewer for maintaining a minimum velocity

of about 0.45 m/s at the time of minimum flow (assumed to be 1/3rd of average flow).

• Ensure that a velocity of 0.9 m/s is developed atleast at the time of maximum flow and preferably during the average flow periods also.

• Moreover, care should be taken to see that at the time of maximum flow, the velocity generated does not exceed the scouring value.

Hydraulic elements

• Area of flow section • Wetted perimeter• Hydraulic mean depth• Velocity of flow• discharge

Circular sewers running full

1. Area of flow section A= пD2/42. Wetted perimeter P= пD3. Hydraulic mean depth R= A/P= D/44. Velocity of flow

V=0.3968/n×D^(2/3)√S5. Discharge Q= 0.3116/n×D^(8/3)×√S

Circular sewers running partially full

• Depth of flow d= D/2(1 – cosθ/2)• Area of flow section A= пD2/4[θ/360˚-

sinθ/2п]• Wetted perimeter P= пDθ/360• H.M.D r= D/4[1- 360sinθ/2пθ]• Velocity of flow v= 1/n*r^2/3*s^1/2• Discharge q= a × v

SEWAGE PUMPING

WHAT IS SEWAGE PUMPING ? WHY WE NEED IT?

• When some area of a town or city is low lying it may not be possible to drain the low lying area by gravity to discharge into a submain or main located at a higher level.

• In such cases it is more economical to collect the sewage of the low lying area into a sump well by gravity and then pump it into the gravity main sewer of the high level area

PROBLEM IN SEWAGE PUMPING

• Sewage contain a lot of suspended and floating material.

Problem- difficult running of pump and clogging of pumps

• Sewage contain organic or inorganic waste.Problem-corrosion which reduce life of equipment• Sewage may contain biological content including

disease producing germsproblem- delay of pumping, immediate repairing.

• The size of sump or wet well is limited.problem-rate of pumping has to be adjusted

with the rate of entering sewage.• The pumps should be of high order of

reliability.problem- failure of pumps will lead to flooding

which cause unbearable nuisance.

PUMPING STATION

• When the sewage level rises to a predetermined point, a pump will be started to lift the sewage upward through a pressurized pipe system called a sewer force main or rising main from where the sewage is discharged into a gravity manhole.

• From here the cycle starts all over again until the sewage reaches its point of destination – usually a treatment plant.

PUMPING STATION

PUMPING STATION

• Specific speed of centrifugal pumps(in r.p.m.)- Ns=N Q/H^3/4

N=actual speed of the pump in r.p.m. Q=discharge or flow rate

H=head developed by the pump in m

TYPES OF PUMP

• 1.Centrifugal pumps• 2.Reciprocating pumps • 3.Air pressure pumps or pneumatic ejector

1.Centrifugal pumps• Centrifugal pumps is a machine that impart energy to fluid to rise up at desired

elevation.• Types• Radial flow, axial flow, mixed flow and ejector are among the types of centrifugal

pumps.

• The differences are based on the particular methods involved in pumping and extracting fluids.

• Radial flow involves accelerating the sewage at a high pressure. This ultimately results in continuous pumping activity in the suction line.

• Axial flow type pumps involve lifting fluids and • The mixed flow pump is a combination of the

radial and axial method

Reciprocating pumps

• These pumps are used where it is to deal with diffcult sludge and where large quantity of sewage is to be pumped against low head.

• Type –• Plunger type- A plunger pump consists of a

cylinder with a reciprocating plunger in it.• Diaphragm pumps-The plunger pressurizes

hydraulic oil which is used to flex a diaphragm in the pumping cylinder. Diaphragm valves are used to pump hazardous and toxic fluids.

Diaphragm pump

AIR PREESSURE PUMP

• It consists of a receiver or "pot' that allows liquids and solids to enter without restriction. When the pot becomes filled, compressed air is introduced to displace the contents up to a higher discharge line. This pump is used for small quantity of sewage

Pneumatic pump

Prime movers

The various types of movers• Electric motors• Diesel engine• Gasoline (or petrol)• Steam engine

Reference……..

• Environmental Engineering vol. 2 By Dr. P.N.MODI• Environmental Engineering vol. 2 By B.C.Punmia• http://nptel.iitm.ac.in/courses/Webcourse-contents/II

T-KANPUR/wasteWater/Lecture%2018.htm• en.wikipedia.org/wiki/Sewage_pumping• http://www.inspectapedia.com/septic/

SepticPumps.htm• 140.194.76.129/publications/eng-manuals/em1110-3-

174/c-6.pdf