1. science
2. ecology
3. pollution

UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
UNIT 3
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preliminary and primary treatment
What this unit is about
This unit looks at the various options available for preliminary and primary treatment
and looks at design criteria and parameters for various options. It also looks at typical
designs for screens, grit channels and primary sedimentation tanks.
A
What you will learn
On completion of this unit, you will:

learn about the aims and purpose of preliminary treatment;

learn about the different types of screens that may be used, and screenings and
their disposal;

learn about comminutors and macerators;

learn about the various types of facility used for grit removal;

learn about other functions of preliminary treatment;

understand the principles surrounding primary sedimentation and settlement;
and

learn about the different types of sedimentation tanks used and their basic
design considerations.
© WEDC Loughborough University UK
3.1
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Contents
1. Introduction
1.1
The aim and purpose of preliminary treatment
1.2
The aim and purpose of primary treatment
3
3
3
4
5
5
9
11
12
13
13
14
18
19
19
21
22
24
25
3. Primary sedimentation
3.1
Principles characteristics of primary settlement
3.1.1 Types of tanks
3.1.2 Functioning principles
3.2
Basis of sedimentation tank design
3.2.1 Description of settling behavior
3.2.2 Theory of sedimentation tank design
3.2.3 Summary of design values 3.2.4 Inlet and outlet design
3.3
Factor affecting the efficiency of sedimentation
3.4
Quantity of sludge and removal
3.5Other developments
25
26
26
27
28
29
32
34
34
36
36
36
4. The scope for preliminary and primary treatment
37
5. Summary
37
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2. Preliminary treatment
2.1
Screens and screenings
2.1.1 General characteristics of screens
2.1.2 Hydraulic aspects
2.1.3 Typical quantities of screenings, and disposal
2.2
Comminutors, macerators and disintegrators
2.3
Grit removal
2.3.1 Grit removal units: operating principles
2.3.2 Techniques for grit removal
2.3.3 Grit quantities and disposal
2.4
Grease separation 2.4.1 Basic principles about grease removal
2.4.2 Static removal (= spontaneous flotation)
2.4.3 Aerated oil removal (=stimulated flotation)
2.4.4 Grease removal
2.5
Flow measurement 3.2
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
1.
Introduction
1.1
The aim and purpose of preliminary treatment
Sewage, while usually fairly uniform in nature, may consist of almost anything.
Flows may contain solid materials, rags, organic matter and grit, in quantities that
depend on whether the flows contain foul sewage, storm water, or both. These can
cause blockages, damage and wear to pipework, valves and pumps, and items of
treatment equipment.
The aim of preliminary treatment is to remove the easily separated components
(mainly bulky solids and grit) to protect the principal treatment processes which
follow. Bulky suspended or floating solids are removed by screens or are chopped up
by macerators or comminutors. Grit are removed by grit channels.
The recommended sequence for preliminary treatment facilities is:
incoming sewer;
b.
removable inclined bar-screen (manually or automatically raked);
c.
concrete slab, sloping to a drain, for collection of screenings;
d.
penstocks (sliding gates, which act as valves in channels), one at the entrance
to each grit channel;
e.
grit channels, of appropriate cross-section, possibly with “ladders” fitted;
f.
drain (typically 50 mm diameter), fitted with a valve, from each grit channel to
the inlet of the next treatment process (this allows water to be drained out of
the grit channel when the channel needs cleaning or maintenance);
g.
approach channel and critical flow weir (Parshall or Venturi); and
h.
inlet to next treatment process.
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a.
1.2
The aim and purpose of primary treatment
Primary treatment provides a period during which the wastewater is stored under
calm conditions. The conditions encourage many light solids to sink to the base of
the storage tank as ‘sludge’ and floating materials to rise to the surface as ‘scum’.
Primary treatment is a physical treatment stage. Sludge and scum can be separated
and removed from the wastewater during primary treatment, reducing the loading for
biological treatment which follows.
© WEDC Loughborough University UK
3.3
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
A typical flow-diagram is shown in Figure 3.1 below
Dewatering
Unit
Screenings
Disposal
Screenings
Water
Grit
Removal
Flow
Measurement
Primary
Sedimentation
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Bar
Screens
Influent
Grit
Disposal
Further
Treatment
or Disposal
Sludge
Handling
Figure 3.1. Typical flow-diagram for preliminary and primary
wastewaterWWT0301
treatment stages.
© WEDC / WWT0301
2.
Preliminary treatment
A
Two main types of materials may be removed during primary treatment, namely
screenings and grit. Screenings are bulky solids which may be added to the
wastewater from houses, drains and manholes. Screenings may include a wide range
of objects and materials: rags, paper, plastics, faeces, condoms, sanitary towels,
disposable nappies, food waste, straw, pieces of wood, animal carcasses, etc.. Grit is
dense material, which may be carried into sewers with domestic wastewater or from
roads. Grit may consists of silt, sand, cinders and ash, small pieces of metal, broken
glass, pieces of bone, and dense food waste such as peas or sweet corn.
The materials present in screenings and grit, if not removed at an early stage of the
treatment, could cause blockage of pipes and channels within the treatment works,
reduce the effectiveness of later treatment processes and cause abrasion and damage
to pipes, pumps and fittings. Rags and pieces of paper can also become wrapped
around automatic sensors, affecting their operation. The easily separated solids
present in wastewater flows are usually removed, using physical treatment methods,
during preliminary treatment.
3.4
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
2.1
Screens and screenings
2.1.1 General characteristics of screens
A screen is a device with uniform openings through which raw sewage is passed.
Sloping bar screens are usually employed to remove larger solid materials, such as
pieces of wood, which can be intercepted by bars that appear vertical when viewed
along the direction of flow. The distance between the bars is often 6 mm, 12 mm, 18
or 24 mm but may occasionally be as great as 50 mm. A series of coarser and finer
screens may be used. Fine-brushed screens are becoming more popular, as they can
remove pieces of paper and plastic which may pass through bar screens. Fine brushed
screens consist of perforated metal sheets which are cleaned by brushes.
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Screenings need to be removed from the screens to prevent the screens from
becoming blocked. Bar screens may be either hand-raked (at small works) or,
more usually, mechanically raked. Mechanically raked screens are activated either
by a time-switch or by an increase in differential head (between the upstream and
downstream water levels) as the screenings collect on the bars. For small works (with
flows up to about 1000 m3/day) the screens can be cleaned manually. The bars are
usually inclined at an angle of 60 degrees, to ease manual cleaning. In larger works,
where there can be adequate technical supervision, mechanical plant can be installed,
but it is desirable to have a manually raked screen as well, in case of mechanical
breakdown.
Various different types of screen are available, and several types of screen are
described below, in sections 2.1.1.1 to 2.1.1.10 inclusive.
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Manually-raked screen (or hand-raked screen)
This unit is used for removing large solids (screenings) from wastewater. It consists
of a series of parallel bars, often tapered with the wider part being on the upstream
side, and with the bars being inclined at about 60 to the horizontal. The bars extend
across the full width of a channel and are usually curved at the top allowing the
screenings to be raked up into a simple channel from which water can drain. Figure
3.2 below illustrates a typical raked screen. The upper part of the figure shows
the screen in vertical section, looking at right angles to the main flow direction.
The lower part of the figure shows the bars in elevation, as seen looking along the
channel parallel to the direction of flow. Spaces between bars can vary from about
25 mm to about 100 mm. Screenings are removed manually using rakes, the tines
(or teeth) of which fit into the gaps between bars. The main advantage is that no
power is required; but fibrous and small solid materials can pass through the screen.
In addition, the screen needs to be cleaned frequently as it will become blocked if it
is not cleared regularly. An operator must be available to clean the screen whenever
necessary.
© WEDC Loughborough University UK
3.5
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Water surface
Flow
Flow
~ 60 deg
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(0.5 to 1.0 m/sec)
Figure 3.2. Manually-raked screen
WWT0302
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© WEDC / WWT0302
Curved-bar screen (mechanically-raked)
This unit is used for removing large solids (screenings) from wastewater. It consists
of a series of curved parallel bars extending across the full width of a channel. The
spaces between bars are often 15 to 20 mm. Screenings become trapped against the
bars, and the screenings are then raked into a collection trough either by a rotating
rake or by a rake which partially rotates but only engages with the screen bars
when moving upwards. Screenings are removed from the rake by a bar known as
the doctor bar. Power is required; and fibrous and small solid materials can pass
through the screen. A manually-raked screen should be provided in parallel with the
mechanically-raked screen in case there is electrical or mechanical failure. Generally,
however, it is not necessary for an operator to be available at all times.
3.6
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Vertical-bar screen (mechanically-raked)
This unit is used for removing large solids (screenings) from wastewater, and is
usually installed in deep channels. A vertical bar-screen consists of a series of parallel
bars, with the bars often being vertical. The screens extend across the full width of
a channel and can be raked either from the front or back. The rake, which collects
screenings, descends into the flow away from the screen bars until it reaches the
bottom of the channel. The rake then engages into the screen bars, and the rake and
screenings are raised above the water surface. Another bar (known as the doctor bar)
then removes the screenings from the rake and deposits them into a trough. Power
is required; and fibrous and small solid materials can pass through the screen. A
manually-raked screen should be provided in parallel with the mechanically-raked
screen in case there is electrical or mechanical failure. Generally, however, it is not
necessary for an operator to be available at all times.
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Band screen (mechanically-raked)
This unit is used for removing large solids (screenings) from wastewater. It consists
of an endless band of perforated metal or other durable material which is installed
across the full width of the inlet channel. The band rotates continually around upper
and lower rollers, and the openings in the screen are usually between 4 and 6 mm in
diameter. Screenings become trapped on the perforated band, and are lifted to the top
of the unit where a water jet removes the solids and forces them into a trough. The
clean screen then passes down again through the flow. Power is required, and fibrous
and small solid materials may pass through the screen. The screenings from this unit
contain a considerable amount of water. The submerged bearings may also cause
maintenance difficulties. A manually-raked screen should be provided in parallel
with the mechanically-raked screen in case there is electrical or mechanical failure.
Generally, however, it is not necessary for an operator to be available at all times.
Drum screen (mechanically-raked)
This unit is used for removing suspended solids (screenings) from wastewater. It
consists of a cylindrical or truncated cone drum, extending across the full width of
a channel, with mesh around the perimeter. Wastewater approaches the outside of
the cylinder, and liquids flow into the cylinder through the mesh perimeter. Solids
become trapped on the outside of the mesh. The drum rotates about a horizontal
axis, and jets of washwater are used to clean the mesh. Solids collect in a pit below
the drum and are removed by pumping or mechanical lifting. Power is required, but
fibrous and solid materials smaller than the mesh size are unlikely to pass through
the screen. The screenings from this unit contain a considerable amount of water. A
manually-raked screen should be provided in parallel with the mechanically-raked
screen in case there is electrical or mechanical failure. Generally, however, it is not
necessary for an operator to be available at all times.
© WEDC Loughborough University UK
3.7
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Cup screen (mechanically-raked)
This unit is used for removing suspended solids (screenings) from wastewater. It
consists of a cylindrical or truncated cone drum, extending across the full width of a
channel, with mesh around the perimeter. Wastewater enters the cylinder, and liquids
flow out through the mesh perimeter. Solids become trapped inside the mesh. The
drum rotates about a horizontal axis, and jets of washwater are used to clean the
mesh. Solids are washed off into a collection hopper. Power is required, but fibrous
and solid materials smaller than the mesh size are unlikely to pass through the screen.
The screenings from this unit contain a considerable amount of water. A manuallyraked screen should be provided in parallel with the mechanically-raked screen in
case there is electrical or mechanical failure. Generally, however, it is not necessary
for an operator to be available at all times.
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Rotating bar interceptor
This unit is rarely used, and may be used for removal of large solids from
wastewater, usually before other forms of screening. When they are used, Rotating
Bar Interceptors are provided to protect other screening equipment from large solids.
The unit consists of several cylindrical bars, installed vertically in a row, fitted
across the sewage flow. The bars all rotate the same way, but the direction of rotation
may be reversed at intervals to remove papers and other materials which may
become wrapped around the bars. Large objects will be trapped by the Rotating Bar
Interceptor, but smaller screenings will be removed at other screens downstream. The
author has no information about how solids trapped by the Rotating Bar Interceptor
are removed from the flow. Power is required, and fibrous and small solid materials
may pass through the screen.
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Disposable bag screen
This unit is used for removing large solids (screenings) from wastewater. The
wastewater flows into a bag of woven synthetic material. Solids become trapped
in the bag, but liquids pass through the fine openings between the woven strands.
Disposable bag screens are not usually used at inlets to wastewater treatment works.
They are more commonly used where treated effluent is discharged; or just before
biological filters (percolating filters) to reduce the likelihood of blockage caused by
papers and plastics which may have passed through screens at the works inlet. At the
inlet to a works these will trap solids of all sizes, and disposal of the bags may be
difficult and unpleasant. Bags are available with a range of opening sizes, and new
bags must be purchased regularly. The woven material may become blocked to the
passage of liquids if fats and oils seal up the openings between strands. No power is
required, and fibrous and solid materials smaller than the openings between woven
strands are unlikely to pass through the screen.
3.8
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Run-down screen
This unit is used for removing fine solids (screenings) from wastewater. It usually
follows conventional screening, or may be used as a form of tertiary treatment. A
run-down screen consists of several fine tapered wires or bars (wedge-wires) placed
horizontally and parallel to one another. Wastewater flows down along or across the
bars, and the liquid flows between the bars. A head of around 1m is required for this
screen. Fine solids become trapped on the bars, and the solids are gradually washed
down to the lower end of the bars for collection and disposal. No power is required,
and fibrous and small solid materials are unlikely to pass through the screen. Grease
and fat may block the gaps between the wedge-wires.
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Fine brushed screen
This unit is used for removing fine solids (screenings) from wastewater. It usually
follows conventional screening, or may be found as a form of tertiary treatment. A
fine brushed screen usually consists of a curved perforated metal plate, in the form of
a trough, which is regularly cleaned by rotating brushes. The holes in the metal plate
are usually fine, between 2 and 6 mm in diameter. They are more commonly used
just before biological filters (percolating filters) to reduce the likelihood of blockage
caused by papers and plastics which may have passed through screens at the works
inlet. Liquids can pass through the small holes in the metal sheet, but solids become
trapped, and are removed by the rotating brushes. Sometimes the holes may be small
rectangular slots; and sometimes the screen is made from a flat perforated sheet cut
into the form of a circle. The screen then rotates within a specially shaped channel,
and the brushes are fixed. Power is required, but fibrous and small solid materials are
unlikely to pass through the screen.
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2.1.2 Hydraulic aspects
The approach velocity to screens should be not less than 0.5 m/s to prevent
settlement of solids. It should also be not greater than 1.0 m/s to prevent any
screenings being dislodged.
Analysis of screens and other treatment units involves calculation of the hydraulic
head-loss through them. The presence of bars across a channel will provide resistance
to flow, because the bars reduce the flow area. The channel width required to
accommodate screens can be estimated using the following equation.
© WEDC Loughborough University UK
3.9
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Channel widths for screens can be calculated by:
t×Q
V ×H
where:
W
= Width (metres),
Q
= Maximum flow rate (m3/sec),
H
= Maximum depth of water (metres),
V
= Velocity (m/sec),
C
= Allowance for side frames (metres),
t
= (b+s) ÷ s , with
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W = C+
b
= bar thickness (mm), and
s
= clear space between adjacent bars (mm).
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The head-loss through a clean screen can be calculated using the formula shown
below, but head loss will increase as screenings become trapped on the screen, and
different designs of screening equipment will have different hydraulic characteristics.
3.10
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
The head-loss through a clean screen is given by:
W
hL = b 
B
4
3
h v (sin q )
W
= total closed width due to bars
B
= total open width due to bars
hv
= approach velocity head = V2/2g
β
= coefficient defining shape of the bars.
θ
= angle of the screen to the horizontal
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where:
The value of β in the equation above typically varies from about 1.7 for streamlined
bars to 2.5 for rectangular bars. Rectangular bars are cheap, but present a flat face at
right angles to the flow, increasing head-loss. Streamlined bars are more expensive,
but reduce head-loss.
A
2.1.3 Typical quantities of screenings, and disposal
The amount of screenings varies widely from situation to situation even between
treatment works of similar sizes and with similarly sized screens. Also, the amount
of screenings at high flow might be up to 7 times that at an average flow. Typical
quantities range from 0.01 - 0.03 m3/1000 people per day [wet weight in the region
of 10-25 kg/1000 people per day]. The quantity of screenings also depends on habits
in society (what people dispose of in wastewater), and effectiveness of the screens,
which depends on the spacing of bars or size of openings.
Screenings can be disposed of by:

directly burying (covered with sufficient depth of earth to prevent problems
with rodents);

pressing, bagging and burying;

pressing followed by incineration (incinerated with difficulty); and

passing through a maceration pump and returning to the wastewater flow
upstream of the screens. The technique of comminution is also sometimes
employed in place of screens.
© WEDC Loughborough University UK
3.11
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Screenings may be disposed of by:
Burial
Incineration
Comminution
Comminutors, macerators and disintegrators
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2.2
Comminutors, macerators and disintegrators describe units which intercept and shred
screenings which are not permanently removed from the sewage flow. They provide
an alternative to removal of screenings. The use of the terms to describe these units
varies. Some units may shred screened materials that have been removed from the
wastewater flow, and the shredded materials are then returned to the flow. Others may
screen and shred the material in the wastewater flow.
A
Typically comminutors are installed in open channels, and solids are shredded until
they are small enough to be washed through slots in the comminutor wall by the
wastewater flow. Macerators are usually installed in pipes, often on the suction
side of pumps to reduce the likelihood of the pumps becoming blocked by solids.
Macerators consist of a set of rotating blades which press against a perforated
metal disk, and solids are shredded until they are small enough to pass through the
perforations in the disk. Disintegrators usually consist of two sets of overlapping
rotating blades, set in an open channel. Solids are shredded until they are small
enough to pass between the blades.
A comminutor consists of a large, hollow cast-iron cylinder set in the sewage flow
and rotating about a vertical axis. The cylinder is covered externally with horizontal
slots which act as a screen. Any material incapable of passing through the slots is
caught up, as the cylinder revolves, by a large number of projecting teeth which carry
it into contact with fixed hardened-steel combs. These create an effective shredding
action normally sufficient to reduce the material down to a size sufficient to pass
through the screen slots.
Macerators, comminutors and disintegrators are not frequently used because of
the increasing use of plastics, treated papers, and other materials which do not
decompose readily. If macerators or comminutors are used, grit removal should
precede them, because grit will quickly blunt the edges of the cutting teeth.
3.12
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Now read: chapter 5 sections 5.1 to 5.4 pages 241 - 255 in Crites & Tchobanoglous
to learn more about screens, screenings and comminution.
2.3
Grit removal
In many low and middle-income countries, the inorganic grit load in sewer-borne
wastewater is high. This is due to the common practice of using soil, sand or ash as
scouring aids for cleaning domestic utensils. Squatting slabs also allow soil from the
users’ feet to be washed into the latrine, and flushed into the sewer, during cleaning.
Grit loads of up to 0.2 m3 per 1000 m3 of wastewater flow are not unusual. An
average figure for Africa is 0.05 litres per 1000 litres, or about 125 mg/l. In higherincome countries, some grit may be washed into road drains.
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When waste stabilisation ponds are used, grit which is not removed settles around
the inlet pipe to the first pond. With high grit concentrations, banks of grit build up
to the surface fairly quickly. These banks can hinder mixing, and produce unsightly
conditions and bad smells. The pond capacity is also reduced.
The simplest way of dealing with grit in pond systems is to provide a small anaerobic
pond which is desludged as necessary. Anaerobic ponds can cause odours, however,
and may not be used close to residential areas.
A
Degritting facilities involving mechanical plant are often used in industrialized
countries to save high labour costs. In low and middle-income countries, a simple
manually-cleaned grit channel is usually best.
2.3.1 Grit removal units: operating principles
The principle for operation is to reduce the wastewater flow velocity so that the grit
settles, but to keep it high enough to retain organic matter in suspension.
The theory of grit settlement is based on Stokes’ Law, which provides the maximum
settling velocity (VMax) for a spherical particle in a liquid.
VMax =
2
9
 r2g 

 (r − r ')
 h 
where:
VMax = Velocity of settling particle (m/s)
ρ
= density of particle (kg/m3)
r
= radius of settling particle (m)
ρ’
= density of liquid (kg/m3)
© WEDC Loughborough University UK
3.13
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
g
= gravitational constant (N/kg or m/s2)
η
= Dynamic viscosity of liquid (N.sec/m2)
For a 0.2 mm grit particle having a density of 2650 kg/m3 the settling velocity in
water at 20°C will be about 21 mm/s. Since their concentration is relatively low, grit
particles will behave as discrete particles and will normally obey Stokes’ Law.
Usually grit particles have a density of between 2000 and 2750 kg/m3, and a bulk
density of about 1600 kg/m3. The bulk density is less than the particle density
because it considers the density of a collection of particles, which will include voids.
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2.3.2 Techniques for grit removal
Grit removal is usually by short-term settlement, and is commonly achieved by one
of the following:

constant velocity grit channels

detritors (detritus tanks)

aerated grit channel or

Pista (or Jeta) grit traps
A
In industrialised countries, the design velocity through grit removal units is usually
0.3 m/s. This allows a high proportion of fine organic solids (0.2 mm equivalent
diameter or less) to pass forward to primary sedimentation tanks. From these tanks
the fine material is removed together with the sludge. It is suggested that with waste
stabilisation ponds the velocity should be 0.23 m/s, and that a retention time of
one minute should be used. This would give a length of about 13.8 metres for the
channels, and it is claimed that 95% of the 0.2 mm material would be retained.
The flow velocity can be controlled by proportional flow weirs, and by using a
channel cross-section that approximates to a parabola. The recommended form for
low and middle-income countries is a rectangular channel, followed by a Parshall
flume. Metal removable ‘ladders’ may be placed at the bases of the channels to
help retain grit. Except in very small works, there should always be at least two grit
channels in parallel.
Degritting facilities involving mechanical plant are often used in industrialised
countries to save high labour costs. In low and middle-income countries, a simple
manually-cleaned grit channel is usually best.
3.14
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
WASTEWATER TREATMENT
Constant velocity grit channel
This unit is for removal of dense solids (grit) from wastewater. It consists of two or
more channels, each about 20 or 25 times as long as they are deep, and with each
channel being of approximately parabolic cross-section, as shown in figure 3.3
below. Two or more channels are used to enable one channel to be cleaned without
interrupting the flow. The flow velocity should remain almost constant in a channel
of parabolic cross-section, and a flume is constructed at the downstream end of
the channel to control the flow velocity at all rates of flow. The grit settles into a
rectangular section along the base of the channel, and is either pumped out or scraped
out by machine, or removed manually. No power is required, but organic materials
are likely to be trapped with the grit, and the unit may occupy relatively large areas
of land.
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The deposited grit is removed either manually as at some small works or, at larger
works, by means of a pump or suction device mounted on a travelling gantry.
Figure 3.3. Cross section of a constant velocity grit channel, showing the ideal
parabolic section (dotted) and a typical channel cross section used in practice.
© WEDC / WWT0303
© WEDC Loughborough University UK
WWT0303
3.15
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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The appreciably long and narrow channels are designed to have a parabolic cross
section. In this manner if the flow through the channel is regulated by a standingwave flume at the outlet end which controls the depth of water in the channel, then
the rate of flow is constant for all water depths, i.e. for a rectangular control flume
Q
= flow rate (m3/s)
Qf = flow rate through flume (m3/s)
Qg = flow rate through grit-channel (m3/s)
b
= width
h
= depth of flow
A
= cross-section area of grit channels
VF = velocity in grit channel
C
= flume coefficient
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Where:
Qf = C.b.h(3/2)
(i)
and the flow through the grit channel is given by:
Qg = A.VF
(ii)
A
By the nature of the design the same flow passes through both the grit channel and
the flume so
Qg = Qf
Therefore,
C.b.h (3/2) = A.VF
(iii)
The area of flow for the parabolic channel is
A=
2h
3
2
3 a
(iv)
where “a” is a proportionality coefficient.
Substituting (iv) into (iii) and rearranging gives: VF =
3C.b. a
2
so that the velocity of flow is shown to be independent of either the depth or rate of
flow.
3.16
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A relationship between the flow rate and the depth and width of the grit channel can
also be derived.
From the geometric properties of a parabola
A = 2/3 bh
(v)
To allow grit to settle
VF = 0.3 m/s
(vi)
Flow through the grit channel is given by
Qg = VF.A
(vii)
Combining (v), (vi) and (vii)
Qg = (0.2) b.h
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Qg = 0.3 (2/3 b.h)
Normally the design velocity of flow is 0.3 m/s. At this velocity the grit will settle at
about 0.03 m/s, and hence the theoretical length of channel required is:
Depth of flow in channel  Velocity (VF) ÷ (0.03) (with VF ( 0.3 m/s)
⇔
Channel length (m) = 10  (depth of flow in channel)
⇔
(i.e. Length = 10  maximum depth of flow).
A
However, to allow grit of different sizes to settle and to compensate for turbulence
the selected length is normally 20 times the maximum depth of flow. Should the
velocity of flow fall below about 0.2 m/s organic solids may also settle out. With a
velocity greater than about 0.4 m/s some grit will be carried forward.
Detritor
This unit is for removal of dense solids (grit) from wastewater. It consists of a large
shallow unit which is roughly square or circular in plan. The incoming flow from a
narrow channel is slowed down when it enters the wider detritor. Deflector plates at
the inlet direct the flow from one side of the detritor to the other, and help to ensure
even distribution of flow across the width of the detritor. There may be a weir on the
outlet side of the detritor. A slowly rotating scraper pushes the grit to a collection
sump on the outside edge of the detritor. The grit is then removed from the sump into
a collection receptacle from which water can drain. Grit is raised either by a pump
or by a scraper which raises the grit along an inclined ramp. Power is needed to
operate the scraper, and reference should be made to manufacturers’ literature when
determining suitable dimensions for a detritor unit.
© WEDC Loughborough University UK
3.17
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Aerated grit channel
This unit is for removal of dense solids (grit) from wastewater. It consists of a deep
channel, approximately rectangular in cross-section. An air pipe introduces air along
one side of the channel, creating a rolling or spiral current in the wastewater as it
flows from one end of the channel to the other. Flow velocities are approximately
0.3 m/sec close to the bottom of the channel, and grit settles in this zone. Grit is
deposited on the bottom of the channel, and is usually removed and raised by chain
driven scrapers. Power is needed to pump air and to operate the scraper.
Some aerated grit channels are also still employed in which air added along one side
of a channel imparts a spiral motion to the flow of wastewater such that a velocity of
about 0.3 m/s is achieved near to the floor of the unit.
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Pista (or Jeta) grit trap
This unit is for removal of dense solids (grit) from wastewater. It consists of a tank
which is circular in plan. Water enters tangentially, and leaves radially. The unit
creates the correct speed for deposition of grit by means of rotating paddles. Grit
settles into a central sump, and is cleaned because lighter organic matter which may
settle with the grit is disturbed by a stream of air blowing through the solids. An air
lift pump removes the grit, raising it into a collection receptacle from which water
can drain. Power is needed to pump air and to operate the paddles. The unit can
become blocked by grit if there is a power failure.
A
2.3.3 Grit quantities and disposal
Quantities of grit can be very variable, and a study should be made of likely grit
loads when designing new grit removal facilities. Grit loads of 0.2 m3/100m3 of
wastewater flow are not unusual, although in parts of Africa grit loads may be only
about 0.05 m3/100m3 of wastewater flow.
Grit is usually disposed of by burial. Grit is usually relatively inoffensive, and many
of the particles are inert. Care should be taken when handling grit, however, because
all materials associated with wastewater may contain pathogenic and parasitic
organisms.
Now read: chapter 5 section 5.8 pages 292 - 300 in Crites & Tchobanoglous to learn
more about grit composition and quantities, grit chambers and grit removal.
3.18
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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2.4
Grease separation
2.4.1 Basic principles about grease removal
Introduction
Greases usually have an animal or vegetable origin, and are present either in the form
of free particles or attached to different suspended solids.
Producers of greases are households, restaurants, cafeterias (schools, retirement
homes etc.), and food and agricultural industries (e.g., slaughterhouses and the meat
industries).
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If present in significant quantity in the sewage, grease can cause operating and
functioning difficulties in wastewater treatment plants. In the sewer, they may
accumulate and obstruct pipelines. In the pumping station, they can adhere to, or
damage, the pumps. In the works, they can obstruct the pipelines, and/or reduce the
transfer of oxygen into water, and thus decrease the amount of available oxygen for
the microorganisms that are supposed to break down organic matter. In free cultures,
this can happen by fixing on the organic matter; in fixed cultures, by settling on the
biofilm. They can also combine with sludge (in the aeration basin), and lead sludge
particles passing through the sedimentation tank. In exceptional cases, grease on the
water surface may create a physical barrier between the water and the atmosphere.
A
Grease is removed using the flotation process: flotation makes use of the difference
in specific mass between solids or liquid droplets and the liquid in which they
are suspended. This method of solid-liquid or liquid-liquid separation is applied
only to particles whose true or apparent specific mass (the process being called
“spontaneous” or “stimulated” flotation respectively) is lower than that of the liquid
in which they are contained.
“Stimulated” flotation is based on the readiness with which certain solid and liquid
particles attach to gas (usually air) bubbles to form “particle-gas” composites with a
density less than that of the liquid in which they form the dispersed phase.
The resultant of the applied forces (gravity, buoyancy and resistance) causes the
“particle-gas” composites to rise and become concentrated at the free surface of the
liquid.
Formulae for the rising velocity
The “particle-gas” composite rapidly acquires a rising velocity which remains
constant. This is the maximum rising speed which, as in the case of particles in
settling out, can be calculated by applying Stokes’ law (or other formula appropriate
to flow conditions). Depending on the value of the Reynold’s number, it is therefore
possible to define flow systems in which the maximum rising velocity is given by
the specific formulae of Stokes (laminar flow), Allen (intermediate flow) or Newton
(turbulent conditions).
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Stokes’ formula:
where
V =
V
(r e − r s )gd 2
18h
= terminal velocity m/s
ρe and ρs = specific mass of the fluid and of the particle-gas
composite (kg/m3)
d
= diameter of the particle-gas composite (m)
g
= gravitational force (N/kg)
η
= viscosity of the fluid (N.s/m2)
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Application of this equation to air bubbles by themselves in water at 20°C shows
that laminar flow conditions hold good for bubbles with a diameter of less than 120
microns. Their maximum velocity is then 30 m/h. This represents an extreme case as
the difference (ρe - ρs) is at its maximum.
This equation reveals the influence of the various factors: the velocity v varies with
d2, with (ρe - ρs) and with the temperature of the liquid, the latter varying inversely
with the viscosity.
Another factor, which needs to be taken into account, is the shape of the particlegas composite. In the equations of Stokes and Newton the shape of the particle-gas
composite is taken to be spherical. Application of a correction factor, which is easy
to determine for simple geometrical shapes, leads to velocities which are lower than
those which could be obtained with a sphere.
A
The favourable effect of the diameter, or size, of the particle-gas composite should
not make us forget that, where the gas-assisted flotation of particles heavier than the
liquid is concerned, the specific surface area,
i.e. the ratio or surface area ÷ volume or surface area ÷ mass diminishes as the
diameter increases. Given the same quantity of air attached per unit of surface area,
the result is a reduction of the factor (ρe - ρs). The two parameters are therefore
opposed to one another.
Minimum volume of gas to cause flotation
The minimum volume of gas Vg, of specific mass (ρg) needed to bring about the
flotation of particle of mass s and specific mass (ρp) in liquid having a specific mass
of (ρl) is given by the expression:
Vg
S
3.20
=
rP −rl 1
rl −r g r p
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Grease can be removed before biological treatment of sewage. Standardised grease
separators are factory-made for flows up to 20 or 30 l/sec. They have a retention
time of 3 to 5 minutes and an ascendant sedimentation (rising) velocity of about 15
m/h. Properly operated they can retain up to 80% of congealed fatty matter. Regular
cleaning is essential. The water should be less than 30°C at the separator outlet (to
allow the grease to congeal).
Grease can also be removed during preliminary treatment at a wastewater purification
plant. In wastewaters containing substantial amounts of grease (such as wastes from
food and agricultural industries, restaurants, etc.), it may be advisable (and is often
required by by-laws) to have a separate grease separator designed for a hydraulic
loading of 10 to 20 m3/h.m2 of effective surface. Such a separator would protect the
sewer because it should be installed before the wastewater enters a public sewer.
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For household wastewater, grease separation is essential if there is no primary
settling. Options for suitable preliminary treatments are briefly described below.
2.4.2 Static removal (= spontaneous flotation)
The aim of this device is to slow down the influent and assure a stabilising zone and
adequate retention period, during which grease separates the water and floats up to
the surface.
A
Static removal is usually in a tank that can be either circular or rectangular. In the
second case, rectangular tanks are fitted with two vertical partitions, one near the
inlet, the other one near the outlet (see figure 3.4). Wastewater enters at the top and
also leaves near the top in these tanks. The partition near the outlet is known as a
“scum board”. This should extend for the full width of the tank, should be about 250
mm from the outlet weir, extend about 150 mm above water level and project 300
mm below water level.
In the case of circular tanks (see Figure 3.4) wastewater enters near the top and
leaves near the bottom. There is then no need for a scum board.
Once at the surface, grease can be removed with a perforated shovel (to collect the
grease and let the water drain back). Mechanical “skimmers” are often used to collect
grease from the surface of tanks.
© WEDC Loughborough University UK
3.21
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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FOG particles
or layer
Water level
Water level
Grease
Influent
Effluent
Influent
Effluent
Rectangular Static Grease removal
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Circular Grease removal
WWT0304
Figure 3.4. Grease removal
© WEDC / WWT0304
A
2.4.3 Aerated oil removal (=stimulated flotation)
This process consists of the mechanical dispersion of air bubbles, which must be
small enough (0.1 to 1 mm) to enable them to adhere to the particles to be floated.
Aerated oil removal can be in either rectangular, or circular tanks. The operation
of a circular tank will be described here (see figure 3.5). The tanks are cylindrical
with a conical base. A submerged fine-bubbles diffuser assures the aeration. Two
separate zones are generally provided: one for mixing (generally in the centre of
the basin), the other, a calmer zone (in the periphery), is for flotation proper. The
influent is directed to the centre of the basin, inside a baffle. In the emulsion zone,
the suspended solids are stirred and mixed with air. The length of path taken by the
bubbles is increased by the spiral flow thus created. In the separation and collection
zone, flow is slower and turbulence is reduced.
Stimulated flotation processes are generally combined with grit removal, because
diffused bubbles enable separation both of grease from suspended matter, and also
sand. The grit removal unit should be large enough to allow for this additional
treatment. The diameter of a cylindro-conical grit/grease separator is from 3 to 8
m, and its liquid depth (at the centre) is from 3 to 5 m. Rectangular units with a
width of 4 m (single unit) to 8 m (double unit) have a liquid depth of about 4 m and
3.22
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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a maximum length of about 30 m; they are able to treat large flows. The average
retention time in the grit/grease separator unit is about 10 to 15 minutes at average
flow, with a minimum of 5 minutes. The flow of injected air is about 0.5-2 m3/hour.
m3 capacity of the structure. Given the above conditions, it could be able to retain
80% of the grease. The stilling zone should be design for an upward velocity of 15 to
20 m/h with a maximum of 25 m/h.
Floated grease is removed using mechanical surface scrapers.
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Dissolved air flotation units may sometimes be used to produce stimulated flotation.
Water containing supersaturated concentrations of air is released at the base of tanks,
and fine air bubbles form as the air comes out of solution.
A
Nota bene: In section 2.4, the term “grease” has been used to describe
a mixture of fats, oils and grease (FOG) which may be present
in domestic wastewater. These materials are often associated
with food preparation and production. Oil is the name given to
various liquid products such as vegetable oils, mineral oils and
light hydrocarbons. The term oil removal is usually used only
for the removal of oil present in appreciable quantities in
industrial wastewater, especially from the petroleum industry.
Therefore, their removal will not be treated here.
© WEDC Loughborough University UK
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UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Surface scrapers
Grease
Inlet
Outlet
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Grease removal
Grit removal
Figure 3.5. Grease separation
WWT0305
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© WEDC / WWT0305
2.4.4 Grease removal
In general, grease and scum collected at the surface of grease separators, grit/grease
removals or primary settling tanks, cannot be reused. Therefore, they could be:
3.24

sent to anaerobic digestion (after having been fined screened), because they
usually increase gas production. The risk is that they can produce a scum layer;

stored for burial or landfill;

incinerated with sludge or screened matter (if furnace and handling conditions
allow it); or

biologically treated.
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Now read: chapter 5 section 5.11 pages 325 - 328 in Crites & Tchobanoglous to
learn more about oil and grease removal.
2.5
Flow measurement
3.
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Due to the amount of suspended material on the flow, a flume is a better proposition
for flow measurement than a weir. A commonly used flume is the Parshall flume,
designed in the USA in the 1920s. It is available “off the shelf” in a variety of
sizes, with accurate calibrations. Glass fibre inserts into the main channel, forming
a simple venturi flume, are also available. A flume is usually included at the inlet
to wastewater treatment works; so that flows into the works can be measured and
monitored, although flow measurement is not a treatment process. Provision of
a flume for flow measurement also has the advantage of controlling flow depth
upstream of the flume.
Primary sedimentation
Primary treatment is usually effected in continuous flow sedimentation tanks, where
about 35% (usually between 25 and 40%) of BOD loadings can be removed by
settlement of solids and flotation of oil and scum. In addition to the BOD reduction,
suspended solids can be reduced by between 50 and 70%.
At 20°C, for Primary Sedimentation tanks, the Removal Efficiency (R) for BOD and
SS can be estimated using the equation:
A
t
R = a + b t
R
= expected removal efficiency (%)
t
= normal hydraulic retention time (hours)
a, b = empirical constants
(see Crites and Tchobanoglous, section 5.9, pages 303-304)
Parameter
a
b
BOD
TSS
0.018
0.0075
0.020
0.014
Settlement may depend upon the nature of the sewage. Controlling factors include:

particle density (dense particles settle faster than less dense particles);

liquid density;
© WEDC Loughborough University UK
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UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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
liquid temperature;

particle size (larger particles settle more quickly than smaller sizes); and

short-circuiting of flows.
Most settlement tanks are of either rectangular horizontal flow or circular radial flow
design. Scrapers collect sludge from the base of the tanks, and booms collect scum
from the surface.
3.1
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A theoretical analysis for the design of ideal sedimentation tanks exists. In practice,
however, rule of thumb principles are usually applied to sedimentation tank design.
One of the great problems is that quality of sewage is highly variable on an hourly,
daily and seasonal basis. Detailed analysis using settling column tests is difficult
also. Design tends to be chiefly empirical but supported by a moderate amount of
computation.
Principal characteristics of primary settlement
A
3.1.1 Types of tanks
There are basically three types of settlement or sedimentation tanks: Upward flow,
Horizontal flow, and Radial flow.
Influent
Scraper
Effluent
Sludge
Figure 3.6. Schematic of rectangular tank
© WEDC / WWT0306
WWT0306
3.26
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Upward flow tanks can be circular or square. Horizontal flow tanks are rectangular
and compact, but the length of the outlet weir is limited. Radial flow tanks are
circular and then occupy more land. They may have their performance affected by
flow changes, but have a long perimeter outlet weir.
Both circular and rectangular tanks, when of medium or large sizes, will be equipped
with sludge and scum collection device. This device is generally made of an
overhead bridge, on which bottom and surface scrapers are fixed.
Rectangular (horizontal flow) tanks
Uniform flow should occur between the inlet and the outlet. Settled material is
removed either by a continuos belt scraper, or by an overhead bridge scraper, which
moves up and down the tank. The bottom of the tank is usually sloped at about 1 in
100 to assist sludge movement into the collecting hopper, which is usually at the inlet
end, where most deposition occurs.
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Transverse cross collectors are often installed in the sludge collection trough or
hopper, to scrape the sludge to the bled off point.
Rectangular tanks are more compact than radial flow tanks and are less susceptible
to flow disturbances. A disadvantage is the limited length of outflow weir available.
Complicated weir arrangements may be needed, possibly extending for up to one
third of the tank length.
A
Circular (radial flow) tanks
Circular tanks can have diameters up to 50 m. The sewage enters in a central
distribution well in which this is designed to distribute the flow evenly in all
directions. Material which settles out is scraped down the tank bottom (usually
sloped at about 1:15) into a central sump. Scraping equipment is supported and
operated from a central pier in larger tanks (over 12 m diameter), and often simply
spans smaller tanks.
Circular tanks occupy more land and are susceptible to flow disturbance, especially
on larger diameter tanks. They do, however, have an advantage in the long length of
outflow weir available around the perimeter and consequently are more efficient.
3.1.2 Functioning principles
Each tank consists of four stages:
(i)
An inlet zone, in which the flow energy of the incoming liquid is dissipated.
This should take up only a small section but if badly designed and the flow
passing into the settlement zone may still be turbulent. The efficiency of the
whole unit will then be seriously affected.
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UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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Effluent
Influent
Scraper
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Sludge
Figure 3.7. Schematic of circular tank
© WEDC / WWT0307
WWT0307
(ii) A settlement zone which represents the true tank capacity where settlement is
accomplished.
(iii) An outlet weir to collect the settled wastewater. These weirs are sometimes
V-notched and always must be protected by a scum board -partially in and
partially out of the water- on the tank side of the weir, to prevent the loss over
the weir of any floating material.
A
(iv) A zone for the collection and storage of the sludge and from where the sludge
will periodically be withdrawn either by pumping or under hydrostatic head.
3.2
Basis of sedimentation tank design
Sedimentation tank design is based on:
3.28

theory of settling;

surface-loading rate;

settling velocity; and

retention/detention time.
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UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
Settling Depth
WASTEWATER TREATMENT
Discrete particle
Flocculant settling
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Time
Figure 3.8. Difference between flocculant and discrete particle settlement
© WEDC / WWT0308
WWT0308
3.2.1 Description of settling behavior
Generalities
Settling behavior can be classified under four headings:
Unhindered settling of discrete particles in which situation the discrete
particles are in such low concentration that each one may settle freely without
interference from other particles. This is adequately described by Stokes Law.
ii.
Flocculant settling - i.e. the settling of small particles which combine to form
larger particles and settle more quickly. This involves a variety of types and
sizes of particles settling at different rates which may collide and coalesce to
form larger flocs. As the liquid depth increases so too does the likelihood of
particles coalescing.
A
i.
(iii) Zone (or hindered) settling occurs where the concentration of particles is
such as to allow inter-particle forces to bind them together. As a result the
particles no longer settle independently but as a mass and while doing so
produce a distinct solid/liquid interface. Settlement of the particles is hindered
because the blanket obstructs the upward flow of liquid that has to move for
the particles to sink. This type of settlement may occur with the settling of
activated sludge.
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(iv) Compressive settling occurs at the bottom of tanks where all the particles are
in contact with one another. Further settling can only occur by squeezing out
the water and re-aligning the matrix which forms the sludge blanket by packing
the particles more closely together.
Table 3.1 below summarises the 4 different types of settling behavior
Table 3.1. Settling behavior
Name
Description
Occurence
i.
Discrete
Non-interactive settling of
individual particles from a
dilute suspension.
Grit and sand removal.
Sometimes in primary
sedimentation.
ii.
Flocculant
Particles coalesce
or flocculate during
settlement. The increase
in particle size and mass
causes a faster rate of
settlement.
Primary sedimentation tanks.
The particles interact
and hinder the settling of
adjacent particles. The
sludge blanket settles as a
single mass.
Secondary (final)
sedimentation tanks.
Sludge blanket is
structured. Further settling
occurs only through
compression of the
structure caused by the
mass of additional particles
settling on to the blanket.
Thick layers of sludge, and
sludge thickening tanks.
iii.
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Type
Hindered (or Zonal)
iv.
A
Compressive
Rectangular tank
In an ideal rectangular sedimentation tank the critical particle from a design point-of
view is that which only just reaches the floor of the tank at the farthest point from the
inlet. From the geometry of the tank it is appreciated that the time for the particle to
settle is:
L
H =—
t=—
Vp Vh
3.30
But
Vh = Q/WH
and
Vh = Vp.L/H
therefore
Vp = Q/WL
© WEDC Loughborough University UK
UNIT 3: PRELIMINARY AND PRIMARY TREATMENT
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and, since
A
= WL
then
Vp = Q/A (m3/m2)
Where:
H
= tank depth (m)
W
= tank width (m)
L
= tank length (m)
V p
= vertical velocity of particle (m/s)
V h
= horizontal velocity of particle (m/s)
Q
= flow rate (m/s)
A
= tank area (m2)
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hence Q/A (the SURFACE LOADING RATE) is a fundamental parameter affecting
sedimentation. Further explanation of sedimentation theory is given in section 3.2.2.
A
As the rate of particle settlement for class (ii) suspensions cannot be calculated using
Stokes Law it is normal to carry out settlement tests in a column of the height of the
proposed new tank using the sewage that will actually be used - or one which is very
similar. As the column is provided with a number of sampling points throughout
its depth it is possible to create depth/time curves for several percentage removals
of solids. A primary sedimentation tank is usually required to remove 50% to 60%
of all suspended solids. However, as the quiescent settling test does not take into
account such factors as the effects of continuous settlement - as opposed to batch
settlement- the inadequate dissipation of momentum at the tank inlet, the draw-down
effects of the outlet weirs or the occurrence of density currents, it is necessary to
multiple the results obtained by a factor of between 1.7 and 2.5 for practical design.
Horizontal flow tank
For horizontal flow tanks the maximum forward velocity (to avoid sludge scouring)
is usually between 10 to 15 mm/s with a length:breadth ratio of 3:1. The maximum
weir loading is about 200 m3/m.d and the surface loading 30 m3/m2.d. The retention
time is normally 2 hours at a flow rate of three times dry weather flow (3 × d.w.f.).
Given the surface loading and the retention time, the depth can be calculated but to
avoid sludge scouring should not be less than 1.5 m.
Radial flow tanks
They are usually are designed for a maximum surface loading of not greater than 45
m3/m2.d, a retention time (at 3 × d.w.f) of 2 hours, and a weir overflow rate of less
than 100 m3/m.d. The maximum diameter is about 50 m. The depth should not be less
than 1.5 m at the wall, with a floor slope to the central sump of between 7.5° and 10°.
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Upward-flow tanks
They should be designed for 2 to 3 hours minimum retention period, a surface
loading of not greater than 43 m3/m2.d (often 29), an upward flow velocity at the tank
surface of 1.2 m/h at maximum flow, with a floor slope of about 45° to the horizontal
for a circular tank and 60° for pyramidal tank. The weir overflow rate is about 54
m3/m.d at maximum flow, but because, as a result, the rate will be too low for the
self-cleansing of a V-notch or castellated weir system at low flows, it is normal not to
employ them with upward-flow tanks.
3.2.2 Theory of sedimentation tank design
Design theory is based on the theory of settling in a quiescent (calm) fluid (laminarflow conditions). Consider a rectangular settling tank as shown in figure 3.9 below.
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L
B
Q
Particle
enters
Q/BD
L: length (m)
B: breadth (m)
D: depth (m)
Q: Flow-rate (m3/sec)
Vs:Settling Velocity (m/sec)
Q
D
Vs
Particle settles
Figure 3.9. Theory of settling behaviour in an ideal settling basin
WWT0309
A
© WEDC / WWT0309
Consider a suspended particle entering at the top, and just settling by the outlet.
Assume all particle paths are straight parallel lines.
Speed of horizontal flow =
Q/(BD)
(m/sec.)
Time of horizontal flow =
L
Q/(BD) LBD
Q
(sec.)
Time for falling depth D =
D
VS (sec.)
Time for fall =
=
Horizontal flow time
LBD
= Q
D
VS
3.32
Therefore
Vs = Q/LB = Q/A (m3/m2.day)
(A = surface area of tank)
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Q/A is also called the surface loading rate (m3/m2.day)
1.
All particles with settling velocity V > Vs will settle within the tank.
2. Particles with V < Vs will be removed in amounts proportional to their speed
relative to Vs (e.g. 50% of particles having a settling velocity (0.5 × Vs) will
settle).
3. Note that particle removal is independent of tank depth.
4.
Consider the tank to have four zones, as shown in figure 3.10 below.
Inlet Zone
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Settling Zone
Outlet Zone
Sludge Zone
Allow up to 25% extra
depth for sludge
accumulation
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Figure 3.10. Longitudinal section through a horizontal flow sedimentation tank
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© WEDC / WWT0310
Wastewater is usually retained in a primary sedimentation tank for a minimum
period of two hours, by which time most settlement has been achieved. Design for
retention times may be for peak flows (when the retention time may be between 1.5
and 2 hours) or for dry weather flow (when the retention time may be between 6 and
8 hours). Retention times may be reduced in hot climates to reduce the possibility of
septic (anaerobic) conditions occurring, and the associated odour nuisance.
Now read: chapter 5 section 5.9 pages 300 - 313 in Crites & Tchobanoglous to learn
more about sedimentation.
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3.2.3 Summary of design values
Typical design values are shown in table 3.2 below.
Table 3.2. Typical design values
Wastewater
Grit Removal
Wastewater Primary
Settlement
Settling Velocity (mm/sec)
0.1 - 0.5
10 - 35
0.2 - 0.4
Surface Loading (m3/m2.day)
10 - 40
1000 - 3000
20 - 40
Retention time (hours)
2-4
1 - 3 minutes
1.5 – 2
removes 35% BOD
removes 50% SS
Outflow weir loading
(m3/m.day)
<450
not applicable
<300
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Potable Water
Rectangular tank
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3.2.4 Inlet and outlet design
Careful design of inlets is essential, in order to distribute the flow evenly over the
tank cross section, without causing excessive formation of eddies and turbulence.
Short circuiting occurs when influent passes through the tank in a far shorter time
than the design retention time. This can result from variations in the temperature
or composition causing density differences within the liquid. Figure 3.11 below
illustrates inlet and outlet weirs.
A possible problem is that too much baffling creates excessive turbulence, reducing
the degree of clarification.
Long weirs can go out of alignment affecting the flow over the weir. Wind can also
affect the discharge over weirs. Castellated weirs (“V” notch) are frequently used to
maintain uniform discharges.
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Figure 3.11 (a) Inlet port
© WEDC / WWT0311a
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WWT0311A
Figure 3.11 (b) Outlet weir
© WEDC / WWT0311b
WWT0311B
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3.3
Factor affecting the efficiency of sedimentation
The efficiency of sedimentation tanks depends on several factors such as:
Tank shape: the shape of the tank should be such as to encourage calm
conditions to facilitate settlement.

Depth and plan area: the shorter the depth the better but this must be balanced
against surface area requirements as land may be expensive and sparse.

Capacity or volume: this should be sufficient to prevent turbulence and ensure
calm conditions exist.

Inlet and outlet arrangements: inlets and outlets should be positioned to
ensure proper mixing of contents to ensure settlement time. However too much
mixing will cause turbulence inhibiting settlement.

Wastewater source: the type of wastewater entering needs to be known so that characteristics such as density, strength, temperature, etc. can be assessed
during design.
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3.4

Quantity of sludge and removal
Sludge is a thick viscous liquid. It may be pumped, or flow under gravity, for
separate stabilization, treatment and disposal.
A
Sludge can be either pumped out of the sedimentation tanks, or be bled off utilizing
the available hydrostatic load. In the latter case, a residual head of at least 1.5 m
should be available. The friction loss in pipework and fittings is usually slightly
greater than for sedimentation tanks at water treatment plants, but this obviously
depends upon the consistency of the sludge. The friction losses for a thick sludge
may be 1.5 times those for water.
3.5
Other developments
It has been suggested that variable influent characteristics, and the solids load in the
influent are very important in relation to the degree of clarification which results in a
given time. Some researchers suggest that lower retention times and higher surface
loading rates could be used, in conjunction with more rational design based on
settling column tests.
Now read: chapter 5 section 5.7 pages 267 - 291 in Crites & Tchobanoglous to learn
more about gravity separation theory.
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4.
The scope for preliminary and primary treatment
Screening, grit removal and primary sedimentation are physical treatment processes.
Sensible design can yield a relatively simple plant which will radically improve the
water quality. This applies to foul sewage, storm water, and combined sewerage
systems.
5.
Summary
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Various types of mechanical equipment have been described, yet good design for a
plant which will operate under local conditions satisfactorily need not include any
mechanical equipment for sludge removal. A considerable proportion of capital and
operational costs will go on mechanical equipment. Primary sedimentation tanks
can be operated on parallel so that dislodging can be done manually. It needs to be
remembered that if equipment fails, or if there are power cuts, manual dislodging
will be necessary unless the treatment units are by-passed; which defeats the whole
object of the plant. Complicated plant, especially if designed by those inexperienced
in that type of plant is a recipe for failure.
Sewage while usually fairly uniform in nature may consist of almost anything and
thus the aim of preliminary treatment is to protect the principal treatment processes
which follow. The purpose of preliminary treatment is to remove the easily separated
components (mainly bulky solids and grit) which could reduce the effectiveness of
later treatment processes and cause damage to pipes, pumps and fittings.
A
Various mechanical processes both automatic and manual can be employed to
remove large solids and grit and the choice of system used is dependent on the degree
of preliminary treatment required, the nature of incoming sewage, the size of the
works and resources available.
Primary treatment is usually effected in continuous flow sedimentation tanks, where
up to 40% (25-40%) of BOD loadings and can be removed by settlement of solids
(also there can be a reduction in SS by 50-70%) and flotation of oil and scum.
Most settlement tanks are of either rectangular horizontal flow or circular radial flow
design.
Rectangular tanks are more compact than radial flow tanks and are less susceptible
to flow disturbances. A disadvantage is the limited length of outflow weir available.
Complicated weir arrangements may be needed, possibly extending for up to one
third of the tank length. While circular tanks occupy more land for equivalent flows,
they do have an advantage in the long length of outflow weir available around the
perimeter and consequently are more efficient.
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Screening, grit removal and primary sedimentation are physical treatment processes
and thus sensible design can yield a relatively simple plant which will radically
improve the water quality. Simple plant design also facilitates manual desludging if
equipment fails, or if there are power cuts.
3.38
© WEDC Loughborough University UK