Sewage Types Methods Design Procedures

Sanitary Engineering

That branch of public health Engg dealing with the collection, conveyance, treatment and disposal of wastes(garbage, sullage,sewage).

Various features/aspects of sanitary Engg:

  1. Collection
  2. Conveyance
  3. Treatment
  4. Disposal

Dry weather flow (D.W.F): The rate of flow of sewage(sanitary+industrial) in dry weather is known as D.W.F

The flow of sewage including storm water in rainy season is known as wet weather flow (W.W.F)

  • Sewage: It is the waste or foul water of the community conveyed by a sewer.
  • Three types of sewage
  1. Domestic or sanitary sewage: The sewage from residential buildings, business centres, institutions etc. Is known as domestic or sanitary sewage. Also contains human body wastes(feces and urine) and sullage.
  2. Industrial sewage: The liquid wastes obtained from industrial process such as paper making, leather industry etc are known as industrial sewage.
  1. Storm sewage : It is the surface run-off which is flowing in sewer during or following a period of rainfall.

Municipal sewage: Domestic + Industrial sewage. Sometimes domestic plus indusrial sewage is known as sanitary sewage.

Sullage

It is the waste water resulting from personal washing, bathing, laundry, Food preparation and cleaning of utensils. It does not contain discharge from hospital O.T and slaugter houses which has high content of organic matter. Sullage is not very foul and can be disposed off in open drain without treatment.

Carbage

It is the dry refuse of town containing organic, inorganic solids, semisolids, combustible, non combustible, putricible, non putricible substances. It includes sweeping from homes, streets, markets, public places, gardens , waste papers, leaves, grass, paring of vegetable, decaying fruits etc with small amount of cinder, clay, and gravel. It is collected separately from sewage adn sullage and disposed of separately.

  • Infilteratin: is the water leaked into the sewer from ground through poor joints, cracked pipes and walls of manholes.
  • Exfilteration: is the water leaked into the ground from sewer.
  • Inflow: is the water entering the sewers from the surface sources like craks in manholes, perforated manhole covers, and roof drains or basement sumps connected to the sewers. Inflow occurs only during runoff.
  • Sewer: is a pipe or conduit carrying sewage. Sewers are usually not flowing full. The full flowing sewers are called Force main-as the flow is under pressure.
  • Sanitary sewer: A sewer which carry sanitary and industrial sewage excluding storm sewer. It is also sometimes called separate sewer.
  • Storm sewer: A sewer which carries storm sewage including surface runoff and street wash.
  • Combined sewer: A sewer which carries domestic, industrial and storm sewage together.
  • Sewerage: The term sewerage refers to the collection, treatment and disposal of sewage

Methods of disposal of wastes

  1. Coservancy system: In this system different types of wastes/refuse are collected, separated and disposed of.
  2. Garbage is collected separately in dustbins and conveyed by covered carts or lorries to suitable place. The combustibles are burnt and the non combustibles is burried.
  3. The human and animal wastes(feces and urine): collected in pans from lavatories and carried by labours in carts and lorries for disposal outside the city and buried for manuture.
  4. The sullage and storm water: coveyed separately by close and open channels and discharged into natural streams.
  5. This system is obselete now and can beused in rural areas where there is scaricity of water.

Disadvantages of conservancy system

  1. Cost: Less initial cost but high maintenance cost due to working labours.
  2. Design of building: The lavatory has to be built away from the residential building which causes inconvenience.
  3. Insanitory condition: The night soil is carried once in 24 hrs while becomes insanitory after 5-6 hrs causing bad smell and fly nuisance
  4. Labour problem: If the labour goes on strike the system totally fails.
  5. Land requirement: The night soil trenching gournd requires large area for disposal.
  6. Foul appearance: it is highly undesirable to allow night soil carts to pass through roads of the city.
  7. Open drains: Storm water and sullage flowing in open drains which causes unhygenic condition in the area.
  8. Pollution of water: The liquid wastes from lavatories may seep into the ground polluting groundwater.
  9. Risk of Epidemic: The sewage is conveyed openly which causes risk of epidemic

Water carriage system

In this system water is used as a medium to carry wastes to the point of final disposal. The quantity of water is so high (99.9%) that wastes becomes liquid which is carried by the sewers. The garbage is collected separately as in conservancy system. The storm water may be disposed of separately or combined with sanitory sewage. This system is universally used nowadays and has the following advantages.

  1. cost: Though initial cost is high,the maintenance cost is low
  2. Compact design: The lavotories can be accomodated inside the building which causes compact design of building and also convenience.
  3. Hygenic conditions: The sewage is carried in covered drains thus the risks of epidemic are reduced.
  4. Land requirement: Less land is required for treatment and disposal
  5. Treatment: Proper treatment of sewage is possible to make the sewage suitable for disposal.
  6. The only disadvantage of this system is the wastage of water (99.9%)

Velocity in sewers

Self cleansing velocity: The sewers should be laid at such a gradient that a minimum velocity, which will prevent the silting of particles in sewers is developed. Such a minimum velocity is known as self cleansing velocity.

= 0.6 m/s For sanitary sewer

= 1 m/s For storm water sewer

Max Velocity criteria: The upper limit set by scouring action of sewage. If the velocity of flow exceeds a certain limit, the particles of solid matter start to damage the inside surface of sewers or a scouring action takes place. The max permissible velocity at which no such scouring action will occur is known as Non scouring velocity and it depends upon the material of sewers.

Generally for sanitory sewer = 2.4 m/s for storm water sewer = 3 m/s

Note: Manning formula is generally used for finding velociy

Sewage quantities

To find the design flow in sanitory sewer the following steps are followed.

  1. Forecast the design population (p) of the area
  2. Find the sewage flow per day by multiplying population with flow per day per capita of sewage.
  • If sewage flow per day per capita is not known then
  • The sewage is taken as (70 to 80) % of average water supply.
  • If (q) is the average per capita per day water cosumption of water then
  • Qavg = average sewage flow = (0.7-0.8) x q x p i.e. Average value of 75 % can be used.
  1. Select a peaking factor(P.F) to find the peak sewage flow (Note: sewers are designed for peak flow).

Other P.F

According to WASA Peaking Factors are

Avg flow (m3/sec) P.F
Upto 0.0283 4
0.031-0.057 3.4
0.057-0.113 3.1
0.116-0.283 2.7
0.311-0.566 2.5
0.594-1.132 2.3
1.16-2.83 2.15
2.85-5.66 2.08
Above 5.66 2

P.F = It is the ratio of max rate of sewage

flow to average flow

Peak flow (discharge) = Qpeak = P.F X Qavg

Peaking factor is also given by the formula

M = 1 + 14/(4+√P) Here P is population in thousands

  1. Calculate the allowance for industrial and commercial sewage at a rate of 3734 m3/km2/day
  2. Calculate infilteration allowance of (5-10) % of average sewage flow as given by WASA

Qinf = (0.05-0.1) Qavg

  1. Find the design sewage flow by adding peak flow, industrial allowance and infilteration flow.

Qdesign = Qpeak + Qind + Qinf

Formula used: Normally , Manning’s formula is used for the design of sanitoray sewer, which is

V = 1/n R2/3 √S (m/s)

Where V = velocity of sewage flow

R = Hydraulic radius ( R = Area / wetted perimeter = A/P)

S = Slope of sewer

n = roughness coefficient

n = 0.01 (for small inside surface like PVC etc)

n = 0.013 (for brick masonary, concrete, RCC pipes) n = 0.017 (For stone masonary)

n = 0.025 (For earthen channels)

Some other formula used for design of sewage flow are.i.e Chezy’s Formula

Chezy’s Formual

V= C√RS where C = Chezy constant, value of c can be

obtained by using either kutter’s formula or Bazin’s formula

According to kutter c = (41.66+1.811/n +0.00281/S) /

(1+(41.66+0.00281/S) n/√R)

According to Bazin

c = 157.5/ (1+k/√R)

Where k = constant depending upon the nature of sewer.

Hazen-william formula may also be used for the design of sewer (pressure flow)

Design procedure

  1. Calculate the design flow as already expained (Qdesign)
  2. Select value of self cleansing velocity and compute area of pipe(sewer) by A = Q/V and diameter.
  3. Find the slope of sewer by Manning’s formula as V= 1/n R2/3 √S
  4. Check the velocity. It should be greater than the minimum self cleansing velocity.

 

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