Screening and Grit Removal
Content
8
Preliminary Treatment
PURPOSE
The first stage of wastewater treatment is the removal of large floating objects
(such as rags, maize cobs, pieces of wood) and heavy mineral particles (sand
and grit). This is done in order to prevent, for example, floating material
accumulating on the surface of waste stabilization ponds and heavy solids
entering the pond sludge layer, and to protect from damage the equipment
used in the subsequent stages of treatment (for example the floating aerators
in aerated lagoons or any pumps which may be used). This preliminary
treatment comprises screening and grit removal.
Manuals of Practice on preliminary wastewater treatment have been
prepared by the Institution of Water and Environmental Management (1992)
and the Water Environment Federation (1994b).
SCREENING
Coarse solids are removed by a series of closely spaced mild steel bars placed
across the flow. The velocity through the screen should be <1 m/s so that the
solids already trapped on the screen (the ‘screenings’) are not dislodged. The
spacing between the bars is usually 15–25 mm and the bars are commonly of
rectangular cross-section, typically 10 x 50 mm. At small works screens are
raked by hand and, in order to facilitate this, the screens are inclined,
commonly at 60° to the horizontal (Figure 8.1). The submerged area of the
hand-raked screens is calculated on the empirical basis of 0.15–0.20 m2 per
1000 population; this assumes that the screens are raked at least twice each
day.
For flows >1000 m3/day mechanically raked screens (Figure 8.2) are
preferred since they can be cleaned more frequently (every 10–30 minutes) and
they are therefore considerably smaller than hand-raked screens. The channel
dimensions required for a mechanically raked screen are calculated as follows:
flow area =
flow
velocity
Preliminary Treatment 79
Figure 8.1 Simple Manually Raked Screen (flow is from left to right)
The flow area is the channel area corrected for the area of the bars. The flow
is the daily maximum (ie ‘peak’) flow (Chapter 7). The velocity is generally
restricted to 0.6 m/s in order to prevent grit deposition and dislodgement of
screenings. The equation is therefore:
WDmax[s/(b + s)] = Qmax/0.6
(8.1)
that is
W = Qmax/{0.6Dmax[s/(b + s)]}
(8.2)
where W and Dmax are the channel width and depth at maximum flow,
respectively, m; Qmax is the maximum (ie peak) flow, m3/s; b and s are the bar
thickness and spacing, respectively, in millimetres.
A standby hand-raked screen should be provided for use when the
mechanical screen is out of action. This emergency screen is normally the same
size as the mechanical screen and it therefore requires raking at frequent
intervals when in use.
Disposal of screenings
Screenings are generally obnoxious in both appearance and content and should
be disposed of as soon as possible. At small works this is readily achieved by
burial, a small area of land being set aside for this purpose. At larger works
80 Domestic Wastewater Treatment in Developing Countries
Figure 8.2 Mechanically Raked Screen
screenings are commonly dewatered in a hydraulic press and then either buried
(or sometimes incinerated) on-site or taken away to the nearest landfill. Advice
on handling screenings in given by Clay et al (1996*).
The quantity of screenings that is removed varies considerably but, for 10
mm bars at 20 mm spacings, an approximate figure is 0.01–0.03 m3/day per
1000 population.
Fine screening
Fine screens have apertures of 3–15 mm, with 6 mm being the most common,
and very fine screens (‘milli-screens’) are those with apertures of 0.25–3 mm.
They produce very large quantities of screenings which are normally washed
and dewatered before disposal; these processes are generally an integral part
of the fine screen unit – for example, Filtech or Wash-flow screens (Jones and
Attwood, 2002*).
Fine screens are now common in many industrialized countries, but in
developing countries their applicability is much more limited: their high
efficiency is generally too high (‘technology overkill’ – Chapter 4), they are
expensive imported items, and their maintenance is likely to be problematic.
Preliminary Treatment 81
GRIT REMOVAL
Grit (also called ‘detritus’) is the heavy inorganic fraction of the wastewater
solids. It includes road grit, sand, eggshells, ashes, charcoal, glass and pieces of
metal; it may also contain some heavy organic matter such as seeds and coffee
grounds. Grit has an average relative density of ~2.5 and thus it has a much
higher settling velocity than organic solids (~30 mm/s, compared with
~3 mm/s). This difference in sedimentation rates is exploited in grit removal
plants where, for ease of handling and disposal, the organic fraction must be
kept to a minimum (<15 per cent). There are two basic types of grit removal
plant: constant velocity grit channels and the various proprietary grit tanks or
traps available commercially.
Constant velocity grit channels
If the velocity of flow of the wastewater is ~0.3 m/s, grit particles settle out
but organic solids do not. The problem is to maintain the velocity constant at
this value for all rates of flow. The best solution is to locate a standing wave
(ie Venturi or Parshall) flume immediately downstream of a grit channel of
parabolic cross-section (Townsend, 1937; Marais and van Haandel, 1996*).
This solution depends on the following two points:
1
2
provided that it is free-flowing (ie not ‘drowned’), a Venturi or Parshall
flume produces an upstream depth which is independent of conditions
downstream and controlled only by the magnitude of the flow; and
if the geometry of the grit channel is such that its cross-sectional area is
proportional to the flow, then the velocity of flow through the channel will
be constant at all flows – if v = velocity, q = flow and a = cross-sectional
area, then v = q/a; but if a is proportional to q (ie a = kq) , then v is
constant.
In order to comply with (2) the channel should have a parabolic cross-section.
The explanation for this is as follows:
(a) the flow q through a Venturi flume is given by:
q = kbh3/2
(8.3)
where k is a constant; b is the flume throat width; and h is the upstream
channel depth.
(b) differentiating equation 8.3:
1
dq = 3 kbh /2dh
2
(8.4)
82 Domestic Wastewater Treatment in Developing Countries
(c) the flow dq through a cross-sectional element of the channel (Figure 8.3) is
given by:
dq = Vxdh
(8.5)
where V is the velocity of flow and xdh is the area of the element.
(d) equating equations 8.4 and 8.5 and rearranging gives:
x =
h
( 3kb
2V )
1/
2
(8.6)
Equation 8.6 is the equation of a parabola. In practice, for ease of
construction, a trapezoidal cross-section is used (Figure 8.4).
If V = 0.3 m/s and X and H are the channel dimensions (m) at maximum
flow (Qmax, m3/s), equations 8.3 and 8.6 can be rewritten as:
Qmax = kbH
X = 5kbH
3/2
(8.7)
1/2
(8.8)
x
dh
h
Figure 8.3 Flow Elements in a Parabolic Channel
Preliminary Treatment 83
300 mm min.
Figure 8.4 Trapezoidal Approximation to a Parabolic Section
Dividing equation 8.8 by equation 8.7 and rearranging gives:
X = 5Qmax/H
(8.9)
Thus the top width of the channel is simply determined from the maximum
flow and the corresponding depth. In practice at least two channels are
provided so that one may be closed for grit removal. The channel length is
determined by the settling velocity of the grit particles.
Length of channel =
channel depth x velocity of flow
settling velocity of grit particles
Grit particles typically settle at about 0.03 m/s, so that when the velocity of
flow is controlled to 0.3 m/s thus:
Length of channel = 10 x maximum depth of flow
In practice, to allow for inlet turbulence and variations in settling velocity, the
channel length is taken as 20 x maximum depth of flow.
Preliminary Treatment
PURPOSE
The first stage of wastewater treatment is the removal of large floating objects
(such as rags, maize cobs, pieces of wood) and heavy mineral particles (sand
and grit). This is done in order to prevent, for example, floating material
accumulating on the surface of waste stabilization ponds and heavy solids
entering the pond sludge layer, and to protect from damage the equipment
used in the subsequent stages of treatment (for example the floating aerators
in aerated lagoons or any pumps which may be used). This preliminary
treatment comprises screening and grit removal.
Manuals of Practice on preliminary wastewater treatment have been
prepared by the Institution of Water and Environmental Management (1992)
and the Water Environment Federation (1994b).
SCREENING
Coarse solids are removed by a series of closely spaced mild steel bars placed
across the flow. The velocity through the screen should be <1 m/s so that the
solids already trapped on the screen (the ‘screenings’) are not dislodged. The
spacing between the bars is usually 15–25 mm and the bars are commonly of
rectangular cross-section, typically 10 x 50 mm. At small works screens are
raked by hand and, in order to facilitate this, the screens are inclined,
commonly at 60° to the horizontal (Figure 8.1). The submerged area of the
hand-raked screens is calculated on the empirical basis of 0.15–0.20 m2 per
1000 population; this assumes that the screens are raked at least twice each
day.
For flows >1000 m3/day mechanically raked screens (Figure 8.2) are
preferred since they can be cleaned more frequently (every 10–30 minutes) and
they are therefore considerably smaller than hand-raked screens. The channel
dimensions required for a mechanically raked screen are calculated as follows:
flow area =
flow
velocity
Preliminary Treatment 79
Figure 8.1 Simple Manually Raked Screen (flow is from left to right)
The flow area is the channel area corrected for the area of the bars. The flow
is the daily maximum (ie ‘peak’) flow (Chapter 7). The velocity is generally
restricted to 0.6 m/s in order to prevent grit deposition and dislodgement of
screenings. The equation is therefore:
WDmax[s/(b + s)] = Qmax/0.6
(8.1)
that is
W = Qmax/{0.6Dmax[s/(b + s)]}
(8.2)
where W and Dmax are the channel width and depth at maximum flow,
respectively, m; Qmax is the maximum (ie peak) flow, m3/s; b and s are the bar
thickness and spacing, respectively, in millimetres.
A standby hand-raked screen should be provided for use when the
mechanical screen is out of action. This emergency screen is normally the same
size as the mechanical screen and it therefore requires raking at frequent
intervals when in use.
Disposal of screenings
Screenings are generally obnoxious in both appearance and content and should
be disposed of as soon as possible. At small works this is readily achieved by
burial, a small area of land being set aside for this purpose. At larger works
80 Domestic Wastewater Treatment in Developing Countries
Figure 8.2 Mechanically Raked Screen
screenings are commonly dewatered in a hydraulic press and then either buried
(or sometimes incinerated) on-site or taken away to the nearest landfill. Advice
on handling screenings in given by Clay et al (1996*).
The quantity of screenings that is removed varies considerably but, for 10
mm bars at 20 mm spacings, an approximate figure is 0.01–0.03 m3/day per
1000 population.
Fine screening
Fine screens have apertures of 3–15 mm, with 6 mm being the most common,
and very fine screens (‘milli-screens’) are those with apertures of 0.25–3 mm.
They produce very large quantities of screenings which are normally washed
and dewatered before disposal; these processes are generally an integral part
of the fine screen unit – for example, Filtech or Wash-flow screens (Jones and
Attwood, 2002*).
Fine screens are now common in many industrialized countries, but in
developing countries their applicability is much more limited: their high
efficiency is generally too high (‘technology overkill’ – Chapter 4), they are
expensive imported items, and their maintenance is likely to be problematic.
Preliminary Treatment 81
GRIT REMOVAL
Grit (also called ‘detritus’) is the heavy inorganic fraction of the wastewater
solids. It includes road grit, sand, eggshells, ashes, charcoal, glass and pieces of
metal; it may also contain some heavy organic matter such as seeds and coffee
grounds. Grit has an average relative density of ~2.5 and thus it has a much
higher settling velocity than organic solids (~30 mm/s, compared with
~3 mm/s). This difference in sedimentation rates is exploited in grit removal
plants where, for ease of handling and disposal, the organic fraction must be
kept to a minimum (<15 per cent). There are two basic types of grit removal
plant: constant velocity grit channels and the various proprietary grit tanks or
traps available commercially.
Constant velocity grit channels
If the velocity of flow of the wastewater is ~0.3 m/s, grit particles settle out
but organic solids do not. The problem is to maintain the velocity constant at
this value for all rates of flow. The best solution is to locate a standing wave
(ie Venturi or Parshall) flume immediately downstream of a grit channel of
parabolic cross-section (Townsend, 1937; Marais and van Haandel, 1996*).
This solution depends on the following two points:
1
2
provided that it is free-flowing (ie not ‘drowned’), a Venturi or Parshall
flume produces an upstream depth which is independent of conditions
downstream and controlled only by the magnitude of the flow; and
if the geometry of the grit channel is such that its cross-sectional area is
proportional to the flow, then the velocity of flow through the channel will
be constant at all flows – if v = velocity, q = flow and a = cross-sectional
area, then v = q/a; but if a is proportional to q (ie a = kq) , then v is
constant.
In order to comply with (2) the channel should have a parabolic cross-section.
The explanation for this is as follows:
(a) the flow q through a Venturi flume is given by:
q = kbh3/2
(8.3)
where k is a constant; b is the flume throat width; and h is the upstream
channel depth.
(b) differentiating equation 8.3:
1
dq = 3 kbh /2dh
2
(8.4)
82 Domestic Wastewater Treatment in Developing Countries
(c) the flow dq through a cross-sectional element of the channel (Figure 8.3) is
given by:
dq = Vxdh
(8.5)
where V is the velocity of flow and xdh is the area of the element.
(d) equating equations 8.4 and 8.5 and rearranging gives:
x =
h
( 3kb
2V )
1/
2
(8.6)
Equation 8.6 is the equation of a parabola. In practice, for ease of
construction, a trapezoidal cross-section is used (Figure 8.4).
If V = 0.3 m/s and X and H are the channel dimensions (m) at maximum
flow (Qmax, m3/s), equations 8.3 and 8.6 can be rewritten as:
Qmax = kbH
X = 5kbH
3/2
(8.7)
1/2
(8.8)
x
dh
h
Figure 8.3 Flow Elements in a Parabolic Channel
Preliminary Treatment 83
300 mm min.
Figure 8.4 Trapezoidal Approximation to a Parabolic Section
Dividing equation 8.8 by equation 8.7 and rearranging gives:
X = 5Qmax/H
(8.9)
Thus the top width of the channel is simply determined from the maximum
flow and the corresponding depth. In practice at least two channels are
provided so that one may be closed for grit removal. The channel length is
determined by the settling velocity of the grit particles.
Length of channel =
channel depth x velocity of flow
settling velocity of grit particles
Grit particles typically settle at about 0.03 m/s, so that when the velocity of
flow is controlled to 0.3 m/s thus:
Length of channel = 10 x maximum depth of flow
In practice, to allow for inlet turbulence and variations in settling velocity, the
channel length is taken as 20 x maximum depth of flow.
