Collection and estimation of sewage

Module III

Collection and Estimation
of Sewage

Bibhabasu Mohanty
Asst. Prof.
Dept. of civil Engineering
SALITER, Ahmedabad

Course Content
Different types of sewers, design
period, variations in sewage
flow, estimation of waste water
discharge.

Introduction…
 Sewage is water-carried waste, in solution or
suspension, that is intended to be removed from a
community.

 Also known as wastewater, it is more than 99% water
and is characterized by volume or rate of flow, physical
condition, chemical constituents and the
bacteriological organisms that it contains.

 The system of pipes used to collect and carry
rain, waste water and trade waste away for treatment
and disposal is called the sewerage or the waste water
system.

 There are three types of public sewers, each classified
according to the kind of wastes it is required to handle.

 Combined sewers

 Sanitary sewers

 Strom sewers

Combined sewers

 Combination public sewers are the oldest variety of
the three types of sewers and they are required to carry
storm and sanitary wastes to some safe terminal.

 Rain water should be carried to some terminal not
associated with the disposal plant

Sanitary sewers

 Sanitary sewers are those which are required to carry
domestic sanitary wastes only. All rain water must be
excluded from them.

 Combination and sanitary sewers generally are placed
about ten feet under the street grade and usually are
found below the centre line of the street.

Strom sewers

Storm sewers are a comparatively new
installation, made necessary because of sewage
treatment.

These sewers are made to carry only storm water and
may terminate at any natural drainage area.

Problems in sewer system

The per capita water supply (including the local and
individual sources) to the population is less than the
minimum requirement of water supply for the efficient
performance of the sewerage system, i.e., 100 Lpcd.

This will cause the operation and maintenance
problems with frequent clogging, due to silting in the
absence of self cleansing velocity;

 The topography of the town is flat necessitating
deeper excavation of trenches and more number of
sewage pumping stations.

 Either ground water table is high or the substrata are
made up of hard rock, leading to difficult construction
of sewerage system.

Sanitary sewer system

Public sanitary sewers perform two primary functions:

 Safely carry the design peak discharge,
 Transport suspended materials to prevent deposition in the
sewer.
3 types of sanitary wastewater collection systems based
on hydraulic characteristics and purpose:

 Gravity sewer system

 Pressure sewer system

 Vacuum sewer system

Gravity Sanitary Sewer
 Most common.

 Wastewater transported by gravity.

 Used to collect wastewater from
residential, commercial, industrial, and institutional
sources.

 Conveyance capacity allowances must be made for
groundwater infiltration and unavoidable inflow.

Pressure (Pumped) Sanitary Sewer
 Economical and environmentally friendly way of
collecting, transporting and disposing of wastewater
from households.

 They are often used in areas when the landscape is
either very hilly or very flat, in areas that regularly
flood or have high water tables, or where it is
impractical to install other types of sewerage systems.

 A pressure sewer system is made up of a network of
fully sealed pipes which are fed by pumping units
located at each connected property.

 The pumping unit processes the household
wastewater and transfers it to the pressure sewer
located in the street via a small pipeline within the
property.

 The pressure sewer system is made of four key
elements. These elements are:

 The pumping unit

 The boundary valve kit

 The house service line

 The control panel

Advantages
 Being smaller in diameter are installed at shallower
depths than conventional sewers.
 They need not be laid on a precise grade as in gravity
sewers, but can often go over hill below the hydraulic
gradient line.
 Since there are no elements such as access manholes
and sewer line is under pressure, the inflow from
storm runoff is virtually eliminated.
 The system is cheaper than conventional sewerage
where unfavourable conditions prevail.

Disadvantages
 They are to be differentiated from the water supply
distribution system with proper colour code.

 Awareness among the public is required about the
hazard risk of the pressure sewer system if they are
tampered.

 Each and every house should have a collection sump
and pumping facilities.

Vacuum Sewerage System
 The wastewater is being delivered by a gravity system
to the pre tank of the domestic shaft.

 While the pre-tank being filled, an electronic sensor
opens the interface valve.

 During the opening air flows into the mixing chamber
and is being mixed with the wastewater and leaves the
valve flowing into the vacuum pipe network as a water-
air mixture.

 There are also pneumatically controlled valves that
open and close depending on the vacuum in the pipe
network.

 The vacuum pump produces a vacuum in the
wastewater collection tank as well as the pipe network
by which the wastewater is sucked from the pipe
network to the collection tank at the pumping station.

Sewer Pipes Profiles
 circular
 egg-shaped (inverted)
 egg-shaped
 mouth-shaped
 square
 trapezoidal

Sewer Pipes Materials
 Concrete, Reinforced concrete, Prestressed concrete
 Ceramics
 Asbestos cement
 Cast iron
 Steel
 Plastics - PVC, PE, PP, reinforced with fibre-glass
 Resins - epoxy resin, reinforced with fibre-glass

PVC CONCRETE

CERAMIC

STEEL CAST IRON

RESIN

Design approach
 Where does the wastewater come from?

 How much wastewater flow is there going to
be?

 How is the wastewater going to be removed
and treated?

Where does the wastewater
come from?

Two main categories:

 Sanitary Wastewater

 Wastewater from residential, commercial, institutional and
industrial sources.
 Storm water Runoff

 Wastewater resulting from rainfall running off streets, roofs,
and other impervious surfaces.

Components of a Community’s Wastewater

 Domestic (sanitary) wastewater – wastewater
discharged from residences and from
commercial, institutional and similar facilities.

 Industrial wastewater – wastewater in which industrial
wastes predominate.

 Infiltration/Inflow (I/I) – extraneous water that enters
the sewer system from the ground through various
means, and storm water that is discharged from
sources such as roof leaders, foundation drains, and
storm sewers.

 Storm water – runoff resulting from rainfall and snow
melt

Infiltration to Sanitary Sewer Systems
 Groundwater/percolating water in the subsurface
entering a sewer system through:
 Defective pipes

 Leaking pipe joints

 Poor connections

 Cracked manhole walls etc.

Inflow to Sanitary Sewer Systems

Water entering a sewer system from surface sources
such as:
 Leaking manhole covers
 Directly connected roof gutters
 Cellar or foundation drains
 Cross connections from storm drains and combined sewers
 Yard and area drains
 Cooling-water discharges
 Drains from springs and swampy areas
 Street wash water

Sanitary Sewer Overflows

 Heavy rains overload the system though inflow and
infiltration into cracks, ill-fitting joints, and leaky
manholes.

 To prevent hydraulic overload of treatment plants, the
excessive sewage bypasses the plant and is discharged
without treatment.

Effects of SSOs
Environmental

Nutrients and toxicants may cause algal blooms and
harm wildlife. Algal blooms remove O2 from
water, smothering aquatic life.

Decrease in water quality reduces number and range
of plants and fish.

Public Health
 Direct contact with water containing sewage can cause
skin and ear infections and gastroenteritis, and cuts
become infected.

 Illnesses result from eating fish/shellfish that swim in
sewage contaminated waters.

Inhalation and skin absorption can also cause disease.

How much wastewater flow is
there going to be?

Sources and Rates of Domestic Wastewater
Flows
 Small residential districts – wastewater flows
determined based on population density and average
per capita contribution of wastewater.
 Large residential districts – wastewater flows
developed based on land use areas and anticipated
population density (typically rates are based on
wastewater flows from nearby areas).
If data is unavailable, estimate 70% of the domestic
water-withdrawal rate is returned to the sanitary sewer
system.

The quantity of sewage produced depends upon the
quantity of water use.
Generally;

Average daily flow = (70 – 80) % average water
consumption i.e.
Average Daily Flow (ADF) of Sewage = 75%
Average Daily Demand (ADD) of water consumption
= 0.75 ADD

The flows in sanitary sewers vary seasonably
monthly, daily, hourly. For areas of moderate sizes be
expressed as;
Maximum Daily Flow = 1.5 x ADF
Where

1.5 varies from place to place
Maximum hourly flow = (2 – 4) ADF
This is actually the peak flow

 Sewers are designed on peak flow basis, however the
minimum flow passing through the sewer is also
important in the design of a particular sewer because at
low flow the velocity will be reduced considerably
which may cause silting.
 So the sewers must be checked for minimum
velocities at their minimum hourly flows.
 Generally for a moderate area the following minimum
flows may be assumed.
 Minimum Daily Flow = 2/3 ADF
 Minimum Hourly Flow = 1/3 ADF

Relation Between water and waste water flow

Daily/Weakly variations in residential waste water flows for
dry/ wet periods

Typical hourly variations in residential area waste water flows

Average Wastewater Flows from Residential
Sources

Average Wastewater Flows from
commercial Sources

Industrial Wastewater Estimation
 Industries without internal reuse programs:
approximately 85 to 95% of water used will be
returned to the sanitary sewer system.

 Large industries with internal-water-reuse
programs: need data on how much water is
reused internally.

Average Wastewater Flows from
Institutional Sources

How is the wastewater going to be
removed and treated?

Types of Sewer Pipes in a Typical Separate
Sanitary Collection System
 Sanitary sewers must be laid near all occupied
buildings in order to collect wastewater.
 Building Connecting Pipes
 Connects the building plumbing to the public sanitary
wastewater collection system.

 Convey wastewater from the buildings to lateral or branch
sewer, or any other sewer except another building sewer.

 Normally begins outside the building foundation

Lateral or Branch Sewers
 Forms the first element of a wastewater collection system.
 Usually in streets or special utility easements.
 Used to collect wastewater from one or more building
sewers and convey it to a main sewer.

Main Sewers
 Main sewers are used to convey wastewater from one or
more lateral sewers to trunk sewers or to intercepting
sewers.

 Trunk Sewers
 Trunk sewers are large sewers that are used to convey
wastewater from main sewers to treatment or other disposal
facilities, or to large intercepting sewers.

Interceptor Sewers
 Intercepting sewers are large sewers that are used to
intercept a number of main or trunk sewers and convey the
wastewater to treatment or other disposal facilities

Collection and estimation of sewage

Collection and estimation of sewage

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