Design of Facilities for Physical,
Chemical & Biological Treatment of
Waste Water
Design of racks, screens, grit chamber, aeration units,
sedimentation tanks, activated sludge and trickling
filter processes, rotating biological contactors, sludge
digesters and drying beds
RACKS &
SCREENS...
screen is a device with openings for removing bigger
suspended or floating matter in sewage which
would otherwise damage equipment or interfere
with satisfactory operation of treatment units.
Figure Definition sketch for types of screens used in wastewater treatment
Design Consideration
Velocity
 The velocity of flow ahead of and through the screen
varies and affects its operation.
 The lower the velocity through the screen, the greater
is the amount of screenings that would be removed
from sewage.
 However, the lower the velocity, the greater would be
the amount of solids deposited in the channel.
 Hence,
the design velocity should be such as to permit
100% removal of material of certain size without
undue depositions.
 Velocities of 0.6 to 1.2 mps through the open area for
the peak flows have been used satisfactorily.
 Further, the velocity at low flows in the approach
channel should not be less than 0.3 mps to avoid
deposition of solids.
Head loss
 Head
loss varies with the quantity and nature of
screenings allowed to accumulate between cleanings.
 Head loss through screens mainly depends on:
Size and amount of solids in waste water
Clear openings between bar
 Method of cleaning and its frequency
Velocity of flow through the screens
SEDIMENTATION
TANKS...
Solid liquid separation process in which a suspension is
separated into two phases –
 Clarified supernatant leaving the top of the
sedimentation tank (overflow).
 Concentrated sludge leaving the bottom of the
sedimentation tank (underflow).

To remove coarse dispersed phase.
 To remove coagulated and flocculated impurities.
 To remove precipitated impurities after chemical
treatment.
 To settle the sludge (biomass) after activated sludge
process / tricking filters.

 Suspended
solids present in water having specific
gravity greater than that of water tend to settle down by
gravity as soon as the turbulence is retarded by
offering storage.
 Basin in which the flow is retarded is called settling
tank.
 Theoretical average time for which the water is
detained in the settling tank is called the detention
period.
 Type
I settling (free settling)
 Type II settling (settling of flocculated
particles)
 Type III settling (zone or hindered
settling)
 Type IV settling (compression settling)
Types of settling
Overflow rate
m3m2/day
Average
Primary settling only
Primary settling followed by
secondary treatment
Primary settling with
activated sludge return
Secondary settling for
trickling filters
Secondary settling for
activated sludge (excluding
extended aeration)
Secondary settling for
extended aeration
Solids loading
Detentio
Depth
kg/m2/day
n time
Peak Average Peak
25-30
50-60
-
-
2.5-3.5 2.0-2.5
35-50
60-120
-
-
2.5-3.5
25-35
50-60
-
-
3.5-4.5
15-25
40-50
70-120 190 2.5-3.5 1.5-2.0
15-35
40-50
70-140 210 3.5-4.5
-
8-15
25-35
25-120 170 3.5-4.5
-
-
Detention period: for plain sedimentation: 3 to 4 h, and
for coagulated sedimentation: 2 to 2.5 h.
 Velocity of flow: Not greater than 30 cm/min (horizontal
flow).
 Tank dimensions: L:B = 3 to 5:1. Generally L= 30 m
(common) maximum 100 m. Breadth= 6 m to 10 m.
Circular: Diameter not greater than 60 m. generally 20
to 40 m.

Depth 2.5 to 5.0 m (3 m).
 Surface Overflow Rate: For plain sedimentation 12000
to 18000 L/d/m2 tank area; for thoroughly flocculated
water 24000 to 30000 L/d/m2 tank area.
 Slopes: Rectangular 1% towards inlet and circular 8%.

Problem:
Design a rectangular sedimentation tank to treat 2.4
million litres of raw water per day. The detention period
may be assumed to be 3 hours.
Solution: Raw water flow per day is 2.4 x 106 L . Detention
period is 3h.
Volume of tank = Flow x Detention period = 2.4 x 106 x
3/24 = 300 m3
Assume depth of tank = 3.0 m.
Surface area = 300/3 = 100 m2
L/B = 3 (assumed). L = 3B.
3B2 = 100 m2 i.e. B = 5.8 m
L = 3B = 5.8 X 3 = 17.4 m
Hence surface loading (Overflow rate) = 2.4 x 106 =
100
24,000 L/d/m2
GRIT CHAMBER...
Grit chambers are basin to remove the
inorganic particles to prevent damage to
the pumps, and to prevent their
accumulation in sludge digesters.
 Mechanically
cleaned
 Manually cleaned
 In mechanically cleaned grit chamber, scraper blades
collect the grit settled on the floor of the grit chamber.
 The grit so collected is elevated to the ground level by
several mechanisms such as bucket elevators, jet pump
and air lift.
 Manually cleaned grit chambers should be cleaned at
least once a week.
 The simplest method of cleaning is by means of
shovel.
 An
aerated grit chamber consists of a standard spiral
flow aeration tank provided with air diffusion tubes
placed on one side of the tank.
 The grit particles tend to settle down to the bottom of
the tank.
 Settling rates dependant upon the particle size and the
bottom velocity of roll of the spiral flow.
 Recommended
for horizontal flow and aerated grit
chamber.
Flow= maximum
 Detention time= 30-90 s (usually 60 s)
 Flow through velocity, vh= 0.2-0.4 m/s (usually 0.3 m/s)
 Settling velocity= 0.016-0.021 m/s for 0.2 mm dia particle
= 0.01-0.015 m/s for 0.15 mm dia particles
 Liquid depth= 1-1.5 m
 Length= 3-25 m
 Quantity of grits= 0.022-0.075 m3/1000 m3 of flow

AERATION UNITS...
 Unit
process in which air and water are brought into
intimate contact.
 The contact time and ratio of air to water must be
sufficient for exchange sufficient oxygen.
Advantages
 Providing O2 for purification and improving overall
quality.
 CO2 reduction-reduces the corrosion.
 Raising the pH.
 VOC removal
 Effective method for bacterial control

Diffused aeration

Spray aeration

Turbine aeration

Surface aeration
 Providing
maximum water surface per unit volume of
air.
 Air bubbles brought with water in a mixing or contact
chamber.
 A common way to aerate water is via diffused air.
 Air is pumped through some sort of diffuser to
generate small bubbles.
 Usually
gas is injected into the bottom of the aeration
tank and is allowed to rise to the surface in an open
tank.
 The rising bubbles transfer oxygen to the water, as well
as transport bottom water to the surface.
 The bubbles raising through water create turbulence.
 Untreated water is allowed to enter the tank from top
and exit from bottom.
Efficiency of diffused aeration can be improved:
Fine bubbles (0.2 cm dia) as compared to coarse bubble
(2.5 cm dia)
 By increasing water depth (9-15 ft)
 By improving the basin geometry (width to depth ratio
not exceed 2)
 By increasing the retention time (10-30 min)

Typical diffused aeration system looks like:
There are a large variety of diffuser types. For example ceramic
plates
These plates are arranged on manifolds at the bottom of
aeration tanks as shown here.
Other types of diffusers include coarse aerators
Again, these diffusers would be arranged by a manifold
on the bottom of an aeration tank.
Basically there are two types of mechanical aeration.
Turbine Aeration:
 In
this system coarse bubbles are injected into the
bottom of the tank and then a turbine shears the
bubbles for better oxygen transfer.
 Efficiency
of turbine aerators is generally higher than
diffused aeration.
Surface Aeration:
In this case a mixing device is used to disturb the surface
so that there is increased interfacial area between liquid
and air.
 There are many different proprietary types of surface
aerators .

Common surface aerators
ACTIVATED SLUDGE
PROCESSES…
 The
most common suspended growth process used
for municipal wastewater treatment is the
activated sludge process.
Activated sludge plant involves:
1.wastewater aeration in the presence of a
microbial suspension,
2.solid-liquid separation following aeration,
3.discharge of clarified effluent,
4.wasting of excess biomass, and
5.return of remaining biomass to the aeration
tank.
The process involves air or oxygen being introduced
into a mixture of primary treated or screened sewage
or industrial wastewater combined with organisms to
develop a biological floc which reduces
the organic content of the sewage.
 The combination of wastewater and biological mass is
commonly known as mixed liquor.
 In all activated sludge plants, once the wastewater has
received sufficient treatment, excess mixed liquor is
discharged into settling tanks and the
treated supernatant is run off to undergo further
treatment before discharge.

Part of the settled material, the sludge, is returned to
the head of the aeration system to re-seed the new
wastewater entering the tank.
 This fraction of the floc is called return activated
sludge (R.A.S.). Excess sludge is called surplus
activated sludge(S.A.S.) or waste activated
sludge(W.A.S).
 S.A.S is removed from the treatment process to keep
the ratio of biomass to food supplied in the
wastewater in balance.
 S.A.S is stored in sludge tanks and is further treated by
digestion, either under anaerobic or aerobic
conditions prior to disposal.

Diverse; can be used for one household up a huge
plant
 Removes organics
 Oxidation and Nitrification achieved
 Biological nitrification without adding chemicals
 Biological Phosphorus removal
 Solids/ Liquids separation
 Stabilization of sludge
 Capable of removing ~ 97% of suspended solids
 The most widely used wastewater treatment process


Does not remove color from industrial wastes and
may increase the color through formation of highly
colored intermediates through oxidation

Does not remove nutrients, tertiary treatment is
necessary

Problem of getting well settled sludge

Recycle biomass keeps high biomass concentration
in aeration tanks
Plug Flow
 wastewater is routed through a series of channels
constructed in the aeration basin.
 Wastewater Flows to tank & is treated as it winds its
way through the tank.
 As the wastewater goes through the system, BOD
and organics concentration are greatly reduced.
 Variations
to this method include:
adding return sludge and/or in decreasing amounts at
various locations along length of the tank;
wastewater BOD is reduced as it passes through tank,
air requirements and number of bacteria required also
decrease accordingly.
Complete Mix
wastewater may be immediately mixed throughout
the entire contents of the aeration basin (mixed with
oxygen and bacteria).
 This is the most common method used today.
 Since the wastewater is completely mixed with
bacteria and oxygen, the volatile suspended solids
concentration and oxygen demand are the same
throughout the tank.

Contact Stabilization
Microorganisms consume organics in the contact
tank.
 Raw wastewater flows into the contact tank where it
is aerated and mixed with bacteria.
 Soluble materials pass through bacterial cell walls,
while insoluble materials stick to the outside.
 Solids settle out later and are wasted from the
system or returned to a stabilization tank.
 Microbes digest organics in the stabilization tank,
and are then recycled back to the contact tank,
because they need more food.

Detention time is minimized, so the size of the
contact tank can be smaller.
 Volume requirements for the stabilization tank are
also smaller because the basin receives only
concentrated return sludge, there is no incoming
raw wastewater.
 Often no primary clarifier before the contact tank
due to the rapid uptake of soluble and insoluble
food.

Extended Aeration
Used to treat industrial wastewater containing
soluble organics that need longer detention times.
 This is the same as complete mix, with just a longer
aeration.
 Advantage - long detention time in the aeration
tank; provides equalization to absorb
sudden/temporary shock loads.
 Less sludge is generally produced because some of
the bacteria are digested in the aeration tank.
 One of the simpler modifications to operate.

 The
quality or characteristics of raw waste water to be
treated.
 The
desired quality or characteristics of effluent or
treated waste water.
 The
type of reactor that will be used.
 Volumetric
the reactor.
and organic loading that will be applied to
 Amount
of O2 required and the aeration system will
provide to supply O2 and to support mixing.
 The
quantity of sludge that will be generated and
wasted for its further management.

Besides these nutrient requirements of microbes,
environmental conditions under which plant operated.

The design computations require the determination of:
Volume or dimensions of the aeration tank
Amount of O2 required and power needed for aeration
Quantity of sludge that will produced for particular waste and
treatment conditions
 Volume and dimensions of sec. settling tank




No of aeration tanks, N= min. 2 (small plants)
= 4 or more (large plants)
Depth of waste water in tank= 3-4.5 m (usually)
= 4.5-7.5 m (diffuse aeration)
= 1-6 m (surface aeration)
Freeboard= 0.3-6 m (diffuse aeration)
= 1-1.5 m (surface aeration)
Rectangular aeration tank L:B= 5:1 and B:D=3:1 to 4:1
(depends on the aeration system)
 Air
I.
II.

requirement:
20-55 m3 of air/Kg of BOD removed for diffuse
aeration when F/M => 0.3
70-115 m3 air/Kg of BOD removed for diffuse
aeration when F/M <= 0.3
Power required for complete mixing : 10-14
kW/1000 m3 of tank volume for surface aeration
system
TRICKLING FILTER
PROCESSES…

Trickling filter is an attached growth process i.e. process
in which microorganisms responsible for treatment are
attached to an inert packing material. Packing material
used in attached growth processes include rock, gravel,
slag, sand, redwood, and a wide range of plastic and
other synthetic materials.
The wastewater in trickling filter is distributed over
the top area of a vessel containing non-submerged
packing material.
 Air circulation in the void space, by either natural
draft or blowers, provides
oxygen for the
microorganisms
growing as an attached
biofilm.

The organic material present in the wastewater
metabolised by the biomass attached to the
medium.
 The biological slime grows in thickness as the
organic matter abstracted from the flowing
wastewater is synthesized into new cellular
material.

Recirculation= A portion of the TF effluent recycled through the filter
Recirculation ratio (R) = returned flow (Or)/ influent flow (Q)
Recycle
Or
Final
clarifier
Q
Final
effluent
Influent
Primary
clarifier
Trickling
filter
Waste
sludge
 simplicity
of operation
 resistance
to shock loads
 low
sludge yield
 low
power requirements
 relatively
low BOD removal (85%)
 high
suspended solids in the effluent (20 -30
mg/L)
 little
operational control
Trickling filters are classified as high rate or low rate,
based on the organic and hydraulic loading applied to the
unit.
S.No.
Design Feature
Low Rate Filter
High Rate Filter
1.
Hydraulic loading,
m3/m2.d
1-4
10 - 40
2.
Organic loading,kg
BOD / m3.d
0.08 - 0.32
0.32 - 1.0
3.
Depth, m.
1.8 - 3.0
0.9 - 2.5
0
0.5 - 3.0 (domestic
wastewater) up to 8 for
strong industrial
wastewater.
4.
Recirculation ratio
 Hydraulic
loading rate is the total flow
including recirculation applied on unit area of
the filter in a day.
 Organic loading rate is the 5 day 20°C BOD,
excluding the BOD of the recirculant, applied
per unit volume in a day.
 Recirculation is generally not adopted in low
rate filters.
 A well operated low rate trickling filter in
combination with secondary settling tank may
remove 75 to 90% BOD and suitable for
treatment of low to medium strength domestic
wastewaters.
 The
high rate trickling filter, single stage or two
stage are recommended for medium to
relatively high strength domestic and industrial
wastewater.
 The BOD removal efficiency is around 75 to
90%.
 Single stage unit consists of a primary settling
tank, filter, secondary settling tank and facilities
for recirculation of the effluent.
 Two stage filters consist of two filters in series
with a primary settling tank, an intermediate
settling tank which may be omitted in certain
cases and a final settling tank.
Generally trickling filter design is based on
empirical relationships to find the required filter
volume for a designed degree of wastewater
treatment.
 NRC equations commonly used.
 NRC (National Research Council of USA) equations
give satisfactory values when there is no recirculation, the seasonal variations in temperature
are not large and fluctuations with high organic
loading.

ROTATING BIOLOGICAL
CONTRACTORS (RBC)…
Rotating Biological Contactors, commonly called
RBC’s, are used in wastewater treatment plants
(WWTPs). The primary function of these bioreactors at WWTPs is the reduction of organic
matter.
A fixed growth biological treatment processes used to
consume organic matter (BOD) from wastewater.
 Consists of 2-6 m diameter disks, closely spaced on a
rotating horizontal axis.
 Disks are covered with a biofilm.
 The disks are only partially submerged in wastewater.

 As
the disk rotates the biofilm is exposed to the
wastewater only part of the time.
 The
rotation in and out of the wastewater serves to
vary the feeding cycle of the bacteria and
microorganisms that make up the biofilm.
 The
shaft rotates about 1-10 rpm (slowly).
Advantages







Short contact periods
Handles a wide range of
flows
Easily separates biomass
from waste stream
Low operating costs
Short retention time
Low sludge production
Excellent process control
Disadvantages


Need for covering units
installed in cold climate
to protect against
freezing
Shaft bearings and
mechanical drive units
require frequent
maintenance
Flow Diagram of an RBC
 No
of modules
 Dia of flat discs
 Thickness of flat disc
 Discs spacing
 Speed of rotating shaft
 Disc submergence
 Thickness of bio-film
= 4-5
= 2-6 m
= up to 10 mm
= 30-40 mm
= 1-10 rpm
= 40% of dia
= 2-4 mm
 Organic
loading
 Hydraulic
 Sludge
loading
production
 Hydraulic
retention time
= 3-10 gm BOD/m2 of
disc surface area
= 0.02-0.16 m3/m2-d
= 0.5-0.8 Kg/Kg BOD
removed
= 0.5 -2.0 h
SLUDGE TREATMENT…



Sludge refers to the residual, semi-solid material left
from industrial wastewater, or sewage treatment
processes.
Waste water sludge is the mixture of waste water
and settled solids.
Depending upon the source it may be primary,
secondary, excess activated sludge.

To reduce the volume of the material to be handled
by removal of liquid portion.

To decompose the organic matter and inorganic
compounds for reduction in the total solids.
GOALS OF SLUDGE TREATMENT…
Volume
reduction
• Thickening
• Dewatering
Elimination of
pathogenic
germs
• If used in agriculture as fertiliser or
compost
Stabilisation of
organic
substances
• Gas production
• Reduction of dry content
• Improvement of dewatering
• Reduction of odour
Recycling of
substances
• Nutrients, fertiliser
• Humus
• Biogas
Sludge handling and disposal includes: Collection of sludge
 Transportation of sludge
 Processing of sludge to convert it to a form suitable for
disposal
 Final disposal of the sludge





Sludge from plain sedimentation tank- settable solids
(raw sludge)
This gray in color contain garbage, fecal solids,
debris.
Bad odor.
From sec. settling tank following a trickling filter
consists of partially decomposed organic matter.
Dark brown in color, less odor than raw sludge.

Primary sludge
 3 to 8 % solids
 About 70% organic material

Sec. sludge
 Wasted microbes and inert materials
 90% organic material

Tertiary sludge
 If sec. clarifier is used to remove phosphate, this sludge
contain chemical precipitates.
Overview
Wastewater treatment
Process water
Primary, secondary, tertiary sludge
Thickening
Stabilisation
Biogas
Thickening
Agriculture
Dewatering
Disposal site
Drying
Incineration
Construction industry
Atmosphere
Thickening (volume reduction) by Gravity
Gravity separation, similar to settling tank
Additional mechanic stirring to enhance flocculation and
extraction of water and gas
Supernatant is introduced to primary clarifier or – if floatables
and grease contents are high – to grid chamber
Thickened sludge is withdrawn from hopper and introduced to
sludge treatment
For an efficient thickening process the development of gas
bubbles must be prevented
Gravity Thickener
Inflow
Scum scimmer
Sludge
liquor
Thickened sludge
Thickening by Flotation
Pre treatment: mostly chemical flocculation
Sludge is placed in contact with air-saturated water
(full flow or recycle pressurization)
Air bubbles attach to solid particles
Floating Sludge bubble composite is collected at the
surface
Water is recovered under a scum baffle and removed
Thickening by Flotation


Aerobic digestion
Anaerobic digestion
Aerobic sludge digestion may be used to treat only
 Waste activated sludge
 Mixtures of waste activated siudge and primary siudge
 Activated sludge treatment plant without primary settling





Volatile solids reduction is equal that obtained
anaerobically
Lower BOD concentrations in supernatant liquor
Production of an odorless, humus-like, biologically
stable end
Operation is relativeluy easy
Lower capital cost



A high power cost is associated with supplying the
required O2
A digested sludge is produced with poor mechanical
dewatering characteristics
A useful by-product such as methane is not
recovered
Factors taht must be considered in designing aerobic
digesters include;

Solid reduction

Hydraulic retention time

Oxygen requirements

Energy requirements for mixing

environmental condition such as pH, temperature.




Sludge held without aeration for 10-90 days
Process can be accelerated by heating to 35-40oC
These are called High Rate Digesters (10-20 days)
Advantages
 low solids production
 useable methane gas produced

Disadvantages
 high capital costs
 susceptibility to shocks and overloads
Basic Components of
Anaerobic Digesters
Digester Gas
Digested
Sludge
Raw Sludge
Mixing
Heat
Exchanger Circulating
Pump
Complex
Organics
Acid producing
bacteria
(acidogens)
Organic acids
and
H2
CH4 and
CO2
Methane producing
bacteria
(methanogenics)
Three Mechanisms Occurring:
Hydrolysis Process – conversion of insoluble high
molecular compounds (lignin, carbohydrates, fats) to
lower molecular compounds
Acidogenesis Process – conversion of soluble lower
molecular components of fatty acids, amino acids and
sugars (monosaccharide) to lower molecular
intermediate products (volatile acids, alcohol, ammonia,
H2 and CO2)
Methanogenesis Process – conversion of volatile acids &
intermediate products to final product of methane and
CO2
Steps in anaerobic (oxygen-free) digestion:
Particulate and complex organics Hydrolysis
organics
Soluble simple organics
acids
Short organic acids
Acidogenesis
Methanogenesis
Soluble simple
Short organic
CH4 & CO2
Conventional anaerobic digester
High rate anaerobic digester




Mean Cell Residence Time
Volumetric Loading Factor
Observed Volume Reduction
Loading Factors Based on Populations




Dewatering aims to reduce the water content
further.
The sludge can then be handled like a solid.
Dewatering can be done mechanically using a filter
press (employing pressure or vacuum), or a
centrifuge.
Also be done using drying beds.



Most popular methods.
A drying bed consists of a 30 cm bed of sand with
an under-drainage .
Sludge is applied on the sand bed and is allowed to
dry by evaporation and drainage of excess water
over a period of several weeks depending on
climatic conditions.


Bacterial decomposition of the sludge takes place
during the drying process while moisture content is
sufficiently high.
During the rainy season the process may take a
longer time to complete.