ENGI 9605 – Advanced Wastewater Treatment
Chapter 2:
Conventional Wastewater
Treatment
Winter 2011
Faculty of Engineering & Applied Science
1
2.1 Physical treatment processes
1. Screening
(1) Why screening
 Wastewater  frequently contain
suspended and floating debris varying in size
from logs to small rags  those solids can
blog and damage pumps or impede flow in
open channels and pipes
 Screening  the first step in treating
wastewater containing large solids
2
(2) Screens in wastewater treatment
 Coarse bar racks (Screens)
 have openings ≤ 2.5 in.  protect wastewater lift pumps
 set in a channel inclined 22º-45º to the horizontal  to
facilitate cleaning
 Mechanical cleaned medium screens
 have bar openings 5/8 – 1.75 in.
 the maximum velocity through the openings  2.5fps
 Fine screens
 have bar openings 1/32 in.  remove suspended and
settleable solids in pretreating certain industrial waste streams
 high head loss + high installation and operation cost  rarely
employed in handling municipal wastewater
3
Preliminary wastewater treatment
(Viessman et al., Water Supply and Pollution Control, 2009 )
4
Primary and secondary wastewater treatment
(Al-Malack, Water Supply and Wastewater Engineering, 2007 ) 5
Coarse bar racks
Mechanical cleaned medium screen
(Viessman et al., Water Supply and Pollution Control, 2009 )
6
(3) Design equations
 Head-loss across bar screens can be represented by:
Va Aa = Vb Ab
Where
hL = head loss, ft (m)
Va = approach velocity, ft/sec (m/s)
Vb = velocity through bar openings, ft/sec (m/s)
g = acceleration of gravity, ft/s2 (m/s2) (g = 32.2 ft/s2 = 9.8 m/s2)
Aa = Area of channel, ft2 (m2)
Ab = Area of screen, ft2 (m2)
7
 Example 2-1: A mechanical bar screen is to be used in an
approach channel with a maximum velocity of 1 m/s. The
bars are 15 mm thick, and the openings are 25 mm wide.
Determine:
(1) The velocity between the bars
(2) The head loss in meters
(4) Shredding
 Grinders/ barminutors/ comminutors installed before
screening during wastewater treatment in some cases
 Cut solids to about ¼ to ¾ in. in size
8
2. Mixing and flocculation
(1) Why mixing and flocculation
 Mixing  can be classified as
 Continuous-rapid mixing (less than 30s) or rapid
mixing
 blending of chemicals with wastewater (e.g., the addition
of alum or iron salts prior to flocculation + dispersion of
chlorine into wastewater for disinfection)
 blending of miscible liquids
 addition of chemicals to sludge and biosolids to improve
their dewatering characteristics
 microflocs are produced
9
 Continuous mixing (i.e., ongoing)
 used where contents of a reactor or holding tank or basin
must kept in suspension (e.g., in equalization basins,
flocculation basins, suspended growth biological treatment
processes, aerated lagoons, and aerobic digesters)
 Flocculation  one application of continuous
mixing
form aggregates or flocs from finely divided particles and
from chemically destabilized particles
 follow rapid mixing where chemicals have been added to
destabilize the particles  agglomerate microflocs to
larger ones that can be removed readily by settling or
filtration
10
Primary and secondary wastewater treatment
(Al-Malack, Water Supply and Wastewater Engineering, 2007 ) 11
Tertiary wastewater treatment processes
(Al-Malack, Water Supply and Wastewater Engineering, 2007 ) 12
(2) System design for rapid mixing
 Mixing devices
 Mechanical agitators  most common
(e.g., using an impeller in a tank with stator
baffles) type of impellers
 Turbine Impellers
 Paddle Impellers
 Propeller Impellers
 Pneumatic agitators  through hydraulic
action (e.g., a jet injector mixer)
 Static in-line blenders  inserted into a
pipe for blending  normally for gas-water
contact
13
A vertical-shaft impeller-type mechanical rapid
mixer for dispersion of chemicals into water
(Viessman et al., Water Supply and Pollution Control, 2009 )
14
Turbine Impellers
Paddle Impellers
(Al-Malack, Water Supply and Wastewater Engineering, 2007 )
15
Propeller Impellers
(Al-Malack, Water Supply and Wastewater Engineering, 2007 )
16
Jet injection mixer 
Jet injection of chemical
into the water flow in a
pipe
Static mixing elements
that are inserted into a
pipe for in-line blending
of chemicals
(Viessman et al., Water Supply and Pollution Control, 2009 )
17
 The degree of mixing is based on the power provided
 which is measured by the velocity gradient (the
difference in velocity between adjacent layers of the
fluid )
1
 P
G  
 V



2
Where
G = root mean square velocity gradient, S-1
P = power input to water, ft-b/s (N-m/s)
µ = absolute viscosity of water , b-s/ft2 (N-s/m2 or
kg/m-s)  μ=0.00131 N-s/m2
V = volume of tank/pipe, ft3 (m3)
18
The volume of the tank or pipe containing the
water can be computed by the equation
V  Q  tR
Where
V = volume of tank/pipe, ft3 (m3)
Q = discharge rate, cfs (m3/s)
tR = the mean residence time, s
Typical tR and G for rapid mixing basins 
tR (Sec )
G (Sec-1 )
19
 Example 2-2: Chemical addition in a 25-mgd
wastewater treatment plant will be performed in two
parallel mixing basins with mechanical mixers. The
mixing intensity as measured by the root mean velocity
gradient should be a minimum of 750 s-1, and the mean
residence time should be 15 s at the design flow. Find the
volume of the tanks and the power required for the mixers.
The water temperature varies between 15 and 20 ºC
annually. Assume the mechanical mixer is 70% efficient
in transferring power to the water.
20
(Viessman et al., Water Supply and Pollution Control, 2009 )
21
(3) System design for flocculation
 Mixing devices
 The common mechanical mixing devices paddle
(reel) flocculators and vertical-turbine mixers
 The paddle flocculator  consists of a shaft with
protruding steel arms that support wooden, plastic or
steel blades  the paddle shaft could be located
transverse (more common) or parallel to the flow
 The paddle units slowly rotate (1-10 rpm)  collision
among the floc particles that are held in suspension by
the agitation  growth of the suspended particles from
colloids to settable floc
22
The paddle flocculator
The vertical-turbine mixer
(Viessman et al., Water Supply and Pollution Control, 2009 )
23
The paddle flocculators in water treatment
(Al-Malack, Water Supply and Wastewater Engineering, 2007 )
24
The paddle flocculator with the paddle shaft located
transverse to the flow
The paddle flocculator with the paddle shaft located
parallel to the flow
(Al-Malack, Water Supply and Wastewater Engineering, 2007 )
25
 The power dissipated in water by a paddle flocculator
can be calculated as:
Cd A  p 1  k 
3
P
2
Where
P = power dissipated, ft-b/s (N-m/s)
Cd = coefficient of drag
A = Paddle area, ft2 (m2)
ρ = density of the water, b-s2/ft4 (kg/m3)
vp = velocity of the paddle relative to water, ft/s (m/s)
K = ratio of the rotational velocity of water to the
velocity of the paddle, normally 0.25-0.50, with 0.3
being a common value
26
 For a paddle flocculator:
 p  2rN
Where
vp = velocity of the paddle relative to water, ft/s (m/s)
r = distance from shaft to center of paddle, ft (m)
N = rotational speed, rev/s
 For a flocculator tank with n symmetrical paddle arms
with blades at r of r1,r2…rj, rotating at N rev/s, the power
dissipated is:
j
3
3
nCd Ai  1  k  2N   ri3
1
P
2
Where Ai = area of each paddle blade, ft2 (m2)
27
 Example 2-3: A 25-mgd wastewater treatment plant has
a flocculation tank 64 ft long and 100 ft wide with a water
depth of 16 ft. The four horizontal shafts of paddle
flocculator are in compartments separated by baffles. All
the paddle units have four arms with two blades, with
radii of 3.0 and 6.0 ft measured from the shaft to the
center of the 0.80-ft-wide boards. The total length of the
paddle boards across the tank is 90 ft. Assume a ratio of
water velocity to paddle velocity of 0.3, rotational speed
of 3 rev/min, Cd equal to 1.8, and water temperature of
50F. Calculate the velocity gradient.
28
3. Sedimentation
(1) Why sedimentation
 To separate solids from liquid using the force of
gravity  in sedimentation, only suspended solids
(SS) are removed
 Particles  pollutants with adverse impacts to
aquatic life (e.g., damage fish gills, smother coral
reefs)
 Particle settling  clogs river, fills up reservoirs
 Particles  may carry adsorbed chemicals (e.g.,
PCBs in Hudson river)
29
 In wastewater treatment plants  sedimentation is
used to reduce SS in the influent wastewater and to
remove settleable solids after biological treatment
(2) Type of sedimentation (settling)
 Type I settling (free settling)
 Type II settling (settling of flocculated particles)
 Type III settling (zone or hindered settling)
 Type IV settling (compression settling)
30
Primary and secondary wastewater treatment
(Al-Malack, Water Supply and Wastewater Engineering, 2007 ) 31
Tertiary wastewater treatment processes
(Al-Malack, Water Supply and Wastewater Engineering, 2007 ) 32
(3) Type I settling (free settling or discrete settling)
 It means settling of discrete (nonflocculent)
particles (e.g., settling of sand particles in grit
chamber)
 In type I settling  a particle will accelerate
until the drag force (FD) equals the impelling
gravitational (due to weight) force on particle (FI)
 settling occurs at a constant velocity, Vs (m/s)
 The gravitational force on particle (FI, kg-m/s2) 
FI   p   w gVp   p   w g 

d 3 (2.1)
6
Where
ρp = particle density and ρw = water density (kg/m3)
VP = particle volume (m3) and d = particle diameter (m)
g = acceleration due to gravity (9.81 m/s2)
33
 The frictional drag force on spherical particles (FD)
depends on the particle velocity, fluid density, fluid
viscosity, particle diameter and the drag coefficient 
FD 
CD Ap  wVs
2
(2.2)
Where
2
CD = drag coefficient and ρw = water density (kg/m3)
Ap = particle cross-sectional area in direction of flow = ¼
π d2 (m2)
Vs = particle settling velocity (m/s)
 The drag coefficient CD , as a function of the Reynolds
number (NR) takes on different values depending on the
flow regime surrounding the particle  laminar (NR <1),
transitional (NR = 1-2000) and turbulent (NR > 2000)
34
24
3
CD 

 0.34
NR
NR
and
 wVS d
NR 

 Type I settling  assuming spherical particles in a
laminar flow regime 
 wVS d 24
NR 

(2.3)

CD
Where µ = viscosity of water (N-s/m2)
 Then combine equations 2.1 to 2.3  the constant settling
velocity Vs can be calculated as the Stockes’ Law:
FI  FD

(  P   w ) gd
Vs 
18
2
35
 Overflow velocity (V0) in a sedimentation tank
Settling time = ts = H/V0
Detention time = tR = L/V
If ts ≤ tR  SS removed
Where V0 = overflow rate, gpd/ft2 (m3/m-d) or the
surface settling rate ft/s (m/s)
Q
Q
V0  
A LW
Q = average daily flow, gpd (m3/d)
A = surface plan area of the sedimentation tank, ft2 (m2)
 V0  an important design criterion in the sizing of
sedimentation tanks
 To get desired settling with most efficient tank size
 ts = tR
36
An ideal rectangular clarifier settling discrete particles
with an overflow rate of V0
(Viessman et al., Water Supply and Pollution Control, 2009 )
37
 Sedimentation efficiency under different H?
(Shanahan, Water and Wastewater Treatment Engineering, 2006 )
38
(Shanahan, Water and Wastewater Treatment Engineering, 2006 )
 Example 2-4: Four rectangular clarifiers in parallel,
each 200 ft long and 40 ft wide with a water depth of 12ft,
are used to settle a total flow of 20 mgd after a
flocculation process. Calculate the overflow rate,
detention time, and horizontal velocity. Also determine
the length of effluent weir required to meet a specification
of 20, 000 gpd/ft.
39
(4) Type II settling (settling of flocculated particles)
 Type I settling assume uniform settling velocity
 it relatively rare in water, especially wastewater
treatment
 In treatment, many particles are present  as a
particle falls, it collides with other particles and
they stick together to form larger particles
 Chemicals are also added to enhance
coagulation and flocculation  chemicals are
added to (quickly) cause coagulation
(destabilization and initial coalescing of colloidal
particles), which then (slowly) flocculate
(formation of large particles, flocs, from smaller
particles)
40
 Particles flocculate during settling thus they increase
in size and settle at a faster velocity  Type II settling
 Examples of Type II settling
Primary settling of wastewater
Settling of chemically coagulated water and wastewater
 Design of clarifier for Type II settling requires
knowledge of settling velocity distribution
 A batch settling tests are performed to evaluate the
settling characteristics of flocculent suspensions 
Quiescent settling analysis
41
 Lab apparatus is a column
with diameter ≥ 5 inches to
reduce wall effects
 Apparatus has a depth of 8
to 10 ft and sampling ports
every 2 ft.
 Initially  suspended
sediment is poured into
column, well mixed and
allowed to settle
 Samples are taken at each
port (#1-#5) with selected
time intervals  C/C0
determined
 C/C0 vs time and depth is
plotted on a graph
Apparatus for Quiescent settling analysis
(Shanahan, Water and Wastewater Treatment Engineering, 2006 )
42
(Tchobanoglous et al., Wastewater Engineering, 2003)
The percentage removal at certain time t2 
h1 R1  R2 h2 R2  R3 h3 R3  R4 h4 R4  R5







h5
2
h5
2
h5
2
h5
2
43
(5) Type III settling (zone or hindered settling) and
Type IV settling (compression settling)
 Type III settling  the settling of an intermediate
concentration of particles
 At high particle concentration, particles are close to each
other  interparticle forces hinder settling of neighbouring
particles
 Mass of particles settle as a zone
 Type IV settling  compression settling
 Particles touch each other  settling occurs by
compression of the compacting mass
 It occurs in the lower depths of final clarifiers of activated
sludge
44
(Shanahan, Water and Wastewater Treatment Engineering, 2006 )
45
(6) Type of sedimentation tanks
 Rectangular and circular settling tank
 Rectangular tank  usually have chain-drive scrapers
to bring sludge to withdrawal trough in tank bottom 
typically 3m deep for water treatment
 Circular tank  inflow at center and outflow along
perimeter weir or radial collection troughs  circular
rake arm to rake sludge to center (in water treatment) or
with suction pipes (in wastewater treatment)
46
(Viessman et al., Water Supply and Pollution Control, 2009 )
 Rectangular tank  better hydraulic characteristic in
long and narrow settling tank
47
 Circular tank  circular sludge sweep is relatively
(Viessman et al., Water Supply and Pollution Control, 2009 )
trouble free
48
 Sedimentation tanks for wastewater treatment
(Viessman et al., Water Supply and Pollution Control, 2009 )
49
4. Filtration
(1) Why filtration
 After the water has been settled, some fine
solids/flocs may still be in suspension  removal of
these fine solids can be achieved by filtration
 Filtration  follows sedimentation to separate
nonsettleable solids from wastewater by passing
through a porous medium
 Filtration  help to produce high-quality
wastewater effluent
50
Tertiary wastewater treatment processes
(Al-Malack, Water Supply and Wastewater Engineering, 2007 ) 51
(2) Description of a typical granular filter system
(Shanahan, Water and Wastewater Treatment Engineering, 2006 )
52
 During filtration, water level is 0.91 to 1.2 m above sand
 water passes downward through the media  water
passes into the underdrain system
 Flow of filtered water flow is controlled by the rate of
flow controller (RFC)  standard filtration rate is 1.36
l/s-m2 of filter bed
 Influent and effluent valves are open + washwater and
waste washwater valves are closed
 Particles larger than 1µm  captured by sedimentation
and interception
 Particles smaller than 1µm  captured by diffusion
 Most diffcult particles to capture  about 1µm in size
 Dual media provide better capture than single media
53
 Filter media
usually
sand
crushed
anthracite
garnet
Cross section of the media in a dual-media fiter
(Viessman et al., Water Supply and Pollution Control, 2009 )
54
 Specific gravity  relative density = ρmedia/ρwater
 Effective size  size that 90% of the grains by weight
are larger than
 Uniformity coefficient  measurement of variation in
particle size of filter media
= size that 60% of the grains by weight are smaller and
40% of the grains by weight are larger than / effective size
55
 Classifications of filters based on the
selection of media
 Single-medium filters (used in water)
have one type of medium
usually sand or crushed anthracite coal
 length to width ratio is 1 : 1.5 to 1 : 2
 Dual-medium filters (used in water and wastewater)
have two types of media
usually crushed anthracite and sand
almost square with length to width ratio of 1 : 1
 Multi-media filters (used in water and wastewater)
have three types of medium
usually crushed anthracite, sand, and garnet
56
 Underdrain system  to support the filter media, collect
the filtered water and distribute the water for backwashing
and air for scouring
(Viessman et al., Water Supply and Pollution Control, 2009 )
57
Undertrain system for air scouring and water backwashing
(a) cross section of the filter (b) detail of the air-water nozzle
(Viessman et al., Water Supply and Pollution Control, 2009 )
58
(3) Head lost and backwash
 Head loss  caused by accumulation of
particles on top and within the depth of the
filter
 Head loss through a clean granular-media
filter  generally less than 3ft (0.9m)
 With accumulation of impurities head
loss gradually increases until the filter is
backwashed  usually at 8-10ft (2.4-3.0 m)
59
 Objective of backwash  to remove
accumulated particles on the surface and within
the filter medium
 Backwash  performed using wash water or
air scouring
 During backwash, the sand bed expands  bed
expansion is between 20 to 50%
 Backwash flow  between 10.2 to 13.6 l/s-m2
 Backwash continues till the waste washwater is
relatively clear
60