 Sewage treatment is the process of removing contaminants

from wastewater and household sewage.

 It includes physical, chemical, and biological processes to
remove physical, chemical and biological contaminants.
 Using advanced technology it is now possible to re-use
sewage effluent for drinking water, although Singapore is
the only country to implement such technology on a
production scale.
 Sewage is generated by residential, institutional, commercial
and industrial establishments. It includes household waste
liquid from toilets, baths, showers, kitchens, sinks and so
forth that is disposed of via sewers.
 In many areas, sewage also includes liquid waste from
industry and commerce. The separation and draining of
household waste into grey water and black water is
becoming more common in the developed world, with grey
water being permitted to be used for watering plants or
recycled for flushing toilets.
 Sewage can be treated close to where the sewage is
created, a decentralized system (in septic tanks, bio-filters
or aerobic treatment systems), or be collected and
transported by a network of pipes and pump stations to a
municipal treatment plant, a centralized system.
 Sewage treatment generally involves four stages, called
Preliminary, primary, secondary and tertiary
treatment.
 Preliminary Treatment:-
 Pretreatment removes materials that can be easily
collected from the raw sewage before they damage or
clog the pumps and sewage lines of primary treatment
clarifiers. Objects that are commonly removed during
pretreatment include trash, tree limbs, leaves,
branches, and other large objects.
 Screening :-
 bar screen to remove all large
objects like cans, rags, sticks,
plastic packets etc. carried in
the sewage stream.
 Grit removal :-
 include a sand or grit channel or
chamber, where the velocity of
the incoming sewage is adjusted
to allow the settlement of sand,
grit, stones, and broken glass.
 Flow equalization :-
 Basinsprovide a place to
temporarily hold incoming
sewage during plant
maintenance. include provisions
for bypass and cleaning, and
aerators as well.
 Fat and grease removal :-
 passing the sewage through a
small tank where skimmers
collect the fat and grease floating
on the surface.
 Primary treatment :-
 It consists of temporarily holding the
sewage in a quiescent basin where
heavy solids can settle to the bottom
while oil, grease and lighter solids
float to the surface. The settled and
floating materials are removed and
the remaining liquid may be
discharged or subjected to secondary
treatment.
 The solids that collect at the bottom
of the tank are called primary
sludge and are pumped from the
tank for treatment elsewhere in the
plant.
 The sludge which settles in the sedimentation basin is
pumped to the sludge digesters. where a temperature
of 30–350C is maintained.
 This is the optimum temperature for the anaerobic
bacteria (bacteria that live in an environment that
does not contain oxygen).
 The usual length of digestion is 20–30 days but can be
much longer during winter months. Continual adding
of raw sludge is necessary and only well-digested
sludge should be withdrawn, leaving some ripe sludge
in the digester to acclimatize the incoming raw sludge.
 Drying beds :-
 Digested sludge is placed on drying
beds of sand. where the liquid may
evaporate or drain into the soil. The
dried sludge is a porous humus-like
cake which can be used as a fertilizer
base.
 Trickling filters :-
 The liquid effluent from the primary
settling tank is passed to the
secondary part of the system where
aerobic decomposition completes
the stabilization. For this purpose, a
trickling filter is used.
 A trickling filter is a fixed bed, biological filter that operates
under (mostly) aerobic conditions. Pre-settled wastewater is
‘trickled’ or sprayed over the filter. As the water migrates
through the pores of the filter, organics are degraded by the
biomass covering the filter material to carbon dioxide and
water while generating new biomass.
 The Trickling Filter is filled with a high specific surface-area
material such as rocks, gravel, shredded PVC bottles, or
special pre-formed filter-material. A material with a specific
surface area between 30 and 900m2/m3 is desirable. Filter is
usually 1–3 m deep but filters packed with lighter plastic filling
can be up to 12 m deep.
 The bed consists of crushed rock or slag (1–2 m deep)
through which the sewage is allowed to percolate. The
stones become coated with a zoogloea film (a jelly-like
growth of bacteria, fungi, algae, and protozoa), and air
circulates by convection currents through the bed.
 Secondary treatment :-
 It removes dissolved and
suspended biological matter.
Secondary treatment is typically
performed by indigenous,
water-borne micro-organisms in
a managed habitat. Secondary
treatment may require a
separation process to remove
the micro-organisms from the
treated water prior to discharge
or tertiary treatment.
Secondary or biological treatment is performed in a tank
containing a "soup" of starved microbes called activated
sludge.
These microbes require air to live (they are aerobic
organisms) and thus air is pumped into the tank.
 During aeration and mixing, the bacteria form small
clusters, or flocs. When the aeration stops, the mixture
is transferred to a secondary clarifier where the flocs are
allowed to settle out and the effluent moves on for
further treatment or discharge. The sludge is then
recycled back to the aeration tank, where the process is
repeated.
 The liquid portion then flows over a wall at the surface
of the settling tank to be chlorinated and released to a
receiving stream.
 Tertiary treatment :-
 Tertiarytreatment can be
employed to further reduce the
solids and organic content of the
effluent. This treatment can
employ conventional processes
with an increased detention time
to allow for greater removals, or
the operations installed for tertiary
treatment can involve more exotic
and expensive equipment such as
electro dialysis units or ion
exchange columns.
 In tertiary treatment, emphasis is placed on absorptive
processes, such as the use of activated carbon; more
efficient oxidation, as with ozone; foam separation of
impurities; and demineralization using reverse osmosis
or distillation.
 Treated water is sometimes disinfected chemically or
physically (for e.g. by lagoons and microfiltration) prior
to discharge into a stream, river, bay, lagoon or
wetland, or it can be used for the irrigation of a green
areas or park. If it is sufficiently clean, it can also be
used for groundwater recharge or agricultural
purposes.
 Septic tanks may be used to serve small
installations where the effluent can be
disposed of through leaching wells, subsurface
tile systems, or artificial subsurface filter
systems.
 When sewage enters a septic tank an equal
volume of liquid is discharged from the tank.
The primary purpose of the septic tank is to
condition the sewage so that the discharged
liquid will not clog the disposal system.
A septic tank combines two processes.
Sedimentation takes place in the upper portion
of the tank, and the accumulated solids are
digested by anaerobic decomposition in the
lower portion.
 As sewage from a building enters a septic tank,
its rate of flow is reduced so that the heavier
solids sink to the bottom and the lighter solids
including fats and grease rise to the surface.
These solids are retained in the tank, and the
clarified effluent is discharged.
 With good care and efficient operation, removal of
solids maybe as high as 60%, but at times the solid
content of the effluent may equal or exceed that of
the influent. Clogging of the disposal system will
vary directly with the amount of suspended solids
contained in the septic tank effluent.
 Septic tanks do not accomplish a high degree of
bacterial removal. Although the sewage
undergoes treatment in passing through the tank,
this does not mean that the infectious agents will
be removed; hence, septic tank effluent cannot be
considered safe.
 The liquid that is discharged from the tank is, in
some respects, more objectionable than that which
goes in; it is anaerobic and aromatic. However, this
does not detract from the value of the tank.
 Further treatment of the effluent, including the
removal of pathogens, is effected by percolation
through the soil. In order not to disturb the
bacterial action of the septic tank, disinfectants and
bleach must never be flushed down toilets
connected to septic tanks.
Typical septic tanks-dual chamber tank (left)
and single chamber tank (right)
 Septic tank capacity should equal a full days flow
plus an allowance of from 15–25% for sludge
capacity.
 The minimum desirable size of the tank is 2000
liters. The tank’s length should not be less than
two or three times the width; liquid depth should
not be less than 1.2 m for small tanks and 1.8 m
for large tanks.
 Manholes should be provided over the inlet and
outlet pipes for observation and maintenance.
 Baffles should be located approximately 45 cm
from the ends of the tank, and should extend
approximately 45 cm below and 30 cm above the
flow line.
 ‘L’s or ‘T’s may be used in place of baffles. If these
are used they should also extend at least 45 cm
below the flow line. The elevations of the inlet and
outlet pipes should provide free flow through the
tank. This can be done by setting the bottom of the
inlet pipe 8 cm above the water level.
 Some sludge from another operating septic
tank or some amount of fresh animal manure
should be added to the new septic tank to
facilitate its initial operation.
 A septic tank servicing an average size home of
five people will need to be de-sludge every
three to five years.
 The scum layer at the top of the septic tank
contains grease, oils, soap films and other
materials that are lighter than water. Both aerobic
and anaerobic bacteria are found in the scum
layer.
 The anaerobic bacteria are the dominant kind in
this layer.
 As the bacteria digest the scum at the top layer of
the tank, the digested wastes from the bacteria
become heavier than water and sink to the bottom
of the septic tank to settle on the sludge layer.
 Key components of a Septic Tank
 Base:-
 The base is usually constructed of plain concrete
with the thickness of about 100-150 mm. This is the
minimum thickness required to withstand the uplift
pressure when the tank is empty. The base also acts
as a foundation for the side walls. A designer may
also reinforce the base slab in larger tanks.
 Side Walls:-
 The side walls of the septic tank are made of brick,
stone masonry or concrete. The septic tank must be
watertight. The quality can be assessed through a
water tightness test.
 Manhole:-
 The manhole provides an access to the
compartments in the septic tank. The manhole
should be made of medium duty cast iron, with a
minimum dimension of 20 inches or equivalent
and with a removable cover in each compartment.
 Inlet and Outlet:-
 The correct installation of the inlet and outlet
are critical in the performance of the septic tank.
 The wastewater must enter and leave the tank
with minimum disturbance.
 The arrangement of inlet and outlet with the
minimum required dimensions is given below.
 Baffle Wall:-
 The baffle wall prevents the scum
from flowing out of the septic tank
without treatment. It also facilitates in
the smooth settling of the sludge. The
baffle wall has slots or openings that
allow the sewage to flow from one Arrangement of Inlet and
compartment to another. Outlet with Dimensions.
 Ventilation:-
 The decomposition of the organic
The gases coming out of
wastes produce gases and the safe exit septic tank has a strong
of the gases must be provided in the stench and so the height
septic tank. The simplest option is to of the vent pipe should
install a vent pipe with a screen on the be higher than the
roof slab of the septic tank. normal height of a
person.
 Soak Pit / Seepage Pit:-
 A soak pit or seepage pit is a vertical leach line
consisting of either a deep hole with porous walls
or a hole filled with gravel or brickbats.
 Septic tanks should always be connected to a
suitable soak pit or a sewer line. The soak pit can be
constructed of pre-cast concrete-rings (with holes)
or a dry cylindrical wall made of brick, block or
stone.
 The minimum diameter of the pit is 1.5 m with a
minimum depth of 3 m. It must be located at least
10 m away from any waterways in saturated soil
conditions.
A lined soak pit An unlined soak pit
 Given:-No Of Units = 100Nos
 Assumed:-

 Water supply=200lit/per/day
 No of persons per units = 5Nos
 Sewage generation = 80% of water supply
 Detention period = 18hours
 Cleaning period = once in a year
 L:B = 4:1 & Depth of Storage of water = 1.8m
 Sludge deposit = 30lit/person/year
 Min Free Board required = 30cm (Req.= 50cm)
 Total Waste water coming to septic tank
=100*5*200*80/100=80000 lit/day
 Detention period = 18hours
 capacity of tank required =80000/24*18=60000lit
 Capacity req. for sludge accumulation = 30*5*100
= 15000lit / year
 Total capacity req. = 60000 + 15000= 75000lit
 Plan area of the Septic tank = 75/1.8=41.67m2
 L:B taken as 4:1, 4B*B= 41.67,B = ( 41.67/4)
 B = 3.25m. L = 3.25* 4 = 13m.
 Total depth of Septic tank = 1.8+0.5= 2.30m
 Waste water coming out from septic tank=
80000lit / day
 Percolation rate = 1500 lit / m3/ day
 Volume of filter media = 80000 / 1500 = 53m3
 Depth taken = 2.5m
 Area of soak pit = 53/2.5 = 21.2 sq.m.
 Dia. of Soak well req.= 21.2*4 / π= 5.2m