part ii
TRANSCRIPT
2009
CIVL 406
PART II
Municipal Wastewater Treatment
Text: Water Supply and Pollution Control, 7th
Ed.
Viessman and Hammer
Course Instructors:
Dr. K. V. Lo
Office: CEME 2004D;
E-mail: [email protected];
Tel: 604-822-4880
Wastewater Treatment 1
2.0 MUNICIPAL WASTEWATER TREATMENT
2.1 An overview of Wastewater Engineering
WASTEWATER: A combination of the liquid- or water-
carried wastes removed from residences, institutions, and
commercial and industrial establishments, together with such
groundwater, surface water, and storm water as may be
present.
WASTEWATER ENGINEERING: The branch of
environmental engineering in which the basic principles of
science and engineering are applied to the problems of water
pollution control.
ENVIRONMENTAL ENGINEERING: A branch of
engineering in which the basic principles of science and
engineering are applied to the problems of pollution control. It
may be subdivided into:
Wastewater engineering
Solid waste engineering
Air pollution control engineering
2.2 Sources of Pollution
2.2.1 Point Source
Pollution being emitted from a single, identifiable source.
Given its nature, point source pollution is quantifiable.
Examples include:
Municipal sewerage systems.
Wastewater Treatment 2
Industrial waste discharges (two types: on-site
treatment and disposal/discharge, and treatment to
appropriate standard and discharge to sewerage
system).
Pipe into a receiving water.
2.2.2 Non-Point Source (NPS)
Pollution with no single, identifiable source. NPS is not
quantifiable, and most often arises from the transport of
contaminants by runoff and overland flow. According to the
USEPA, non-point source pollution is the greatest threat to
water quality. Examples include:
Agriculture (feedlot runoff, pesticide spraying,
fertilizer runoff, etc).
Mining activities (heavy metal leachate, acid drainage,
silts).
Poor construction practices (silt transport and
deposition).
Forestry clearcutting (silt transport and deposition).
Irrigation return flow (agri-chemicals, salts, silts,
fertilizers).
Boating and marinas (nutrients from bilge water,
hydrocarbons, metals).
Urban runoff (silts, grit, hydrocarbons, nutrients,
metals).
Septic tanks and tile fields (dysfunctional tanks release
untreated sewage).
Acid rain (NOx and sulphur compounds).
Wastewater Treatment 3
2.3 Urban Runoff and Wastewater Composition
2.3.1 Comparison of Urban Runoff with Municipal Wastewaters
Table 2.1 Wastewater characteristics
SS, BOD5, N, P in mg/L
Fecal coliforms in MPN/100 mL
Suspended
Solids (SS)
Biochemica
l Oxygen
Demand
(BOD5)
Total
Nitrogen
Total
Phosphorus (P)
Fecal
coliforms
Background levels 5-100 0.5-3 0.05-0.5 0.01-0.2 ..........
Stormwater runoff 415 20 3-10 0.6 14,500
Combined sewer
overflow 370 115 9-10 1.9 670,000
Untreated
Municipal 200 200 40 5 1,000,000
Primary Effluent 80 135 35 8 200,000
Secondary Eff. 15 25 11 4 1,000
Tertiary Eff. < 10 < 5 < 3 < 2
Adapted from US EPA, 1977
Wastewater Treatment 4
2.3.2 Typical Composition of Domestic Sewage
Total Solids (TS) is the sum of Total Suspended Solids
(TSS) and Total Dissolved Solids (TDS), i.e. TS = TSS +
TDS
Suspended solids are particulate and can be removed by
settling or filtration. Dissolved solids may be atoms or
compounds that are in solution, for instance sugar may be
dissolved in water.
When placed in a muffle furnace at 550°C for one hour, a
portion of the solids will volatilize (burn off) and the remainder
will not. This is how volatile and fixed solids are defined.
Volatile solids are typically smaller organic compounds.
Table 2.2
Strong Medium Weak
Solids, total 1200 700 350
Dissolved, total 850 500 250
Fixed 525 300 145
Volatile 325 200 105
Suspended, Total 350 200 100
Fixed 75 50 30
Volatile 275 150 70
Settleable Solids (ml/liter) 20 10 5
Biochemical oxygen demand, 5-day, 20°C 300 200 100
Total organic carbon (TOC) 300 200 100
Chemical oxygen demand (COD) 1000 500 250
Nitrogen, (total as N) 85 40 20
Organic 35 15 8
Free ammonia 50 25 12
Nitrites 0 0 0
Nitrates 0 0 0
Phosphorus (total as P) 20 10 6
Organic 5 3 2
Inorganic 15 7 4
Chlorides†
100 50 30
Alkalinity (as CaCO3)†
200 100 50
Grease‡ 152 100 50
All values except settleable solids are expressed in mg/liter† Values should be increased by amount in carriage water
‡ Problematic for treatment plant operation
Constituent
Concentration
Wastewater Treatment 5
2.3.3 Biochemical Oxygen Demand from Selected Industries
Recall that the biochemical oxygen demand (BOD) is defined
as ―the quantity of oxygen used by microorganisms in the
aerobic stabilization of wastewaters and polluted waters‖
(Viessman & Hammer, page 317). In other words, biochemical
oxygen demand is an indirect measure of the quantity of
organic compounds in the sample that can be degraded
aerobically by microoganisms.
Table 2.3
Source of Waste5-day, 20C BOD of
waste, mg/liter
Beet sugar refining 450-2,000
Brewery 500-1,200
Beer slop 11,500
Cannery 300-4,000
Grain distilling 15,000-20,000
Molasses distilling 20,000-30,000
Laundry 300-1,000
Milk procesing 300-2,000
Meat packing 600-2,000
Pulp and paper
Sulfite 20
Sulfite-cooker 16,000-25,000
Tannery 500-5,000
Textiles
Cotton processing 50-1,750
Wool scouring 200-10,000
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Biochemical
Oxygen Demand
Suspended
Solids
Domestic sewage 1 1
Paper-Mill waste 16 - 1330 6100
Tannery waste 24 - 48 40 - 80
Textile-mill waste 0.4 - 360 130 - 580
Cannery waste 8 - 800 3 - 440† persons per unit daily production
Population Equivalent†, in terms of:
Origin of the Waste
2.3.4 Population Equivalent Defined
Population Equivalent is the per capita per day BOD5 loading
(approx 0.2 lb/day·cap). This measure is used to easily
compare the organic pollution potential of industries presenting
large BOD loads to the system. This is important in
determining treatment plant capacity – waste streams include
both household and industrial wastewaters.
Table 2.4
Wastewater Treatment 7
2.3.5 Variations in Waste Characteristics for Some Industrial Wastes
Table 2.5
10 50 90 10 50 90 10 50 90
Pulp and paper1
11,000 43,000 74,000 17 58 110 26 105 400
Paperboard1
7,500 11,000 27,500 10 28 46 25 48 66
Slaughterhouse2
165 800 4,300 3.8 13 44 3 9.8 31
Brewery3
130 370 600 0.8 2 44 .25 1.2 2.45
Tannery4 4.2 9.0 13.6 575 975 1,400 600 1,900 3,200
1 tons paper production
2 1000 lb live weight kill
3 bbl beer
4 pounds of hides
Flow, gal/production unit BOD, lb/production unitSuspended Solids,
lb/production unitWaste
% frequency% frequency % frequency
Wastewater Treatment 8
2.3.6 Physical Characteristics of Domestic Wastewater
Table 2.6
As a rule of thumb, every increase in temperature by 10°C
increases the rate of reaction by 100%.
Characteristic Cause Significance Measurement
Temperature Ambient air temperature.
Hot water discharged into
sewer from home or
industry.
Influences rate of biological
activity. Governs solubility of
oxygen and other gases.
Affects magnitude of density,
viscosity, surface tension, etc.
Standard centigrade or
Fahrenheit scale.
Turbidity Suspended matter such as
sewage solids, silt, clay,
finely divided organic
matter of vegetable origin,
algae, microscopic
organisms.
Excludes lights, thus reducing
growth of oxygen-producing
plants. Impairs aesthetic
acceptability of water. May be
detrimental to aquatic life, e.g.
algae.
Light scatter and
absorption on an arbitrary
standard scale.
Colour Dissolved matter such as
organic extractives from
leaves and other vegetation
(tannins, glucosides, iron,
etc.), industrial wastes.
Harmless generally, but
impairs aesthetic quality of
receiving waters.
Light absorption on an
arbitrary standard scale.
Odour Volatile substances,
dissolved gases, often
produced by decomposition
of organic matter. In water it
may result from the essential
oils in microorganisms. E.g.
mercaptans, H2S, CH4.
May indicate presence of
decomposing sewage. Affects
aesthetic quality of water. As a
test of sewage it may serve, for
example, as a guide to
condition of sewage when it
reaches the treatment plant.
Human sense of smell,
qualitative scale, and
concentration at threshold
of odour.
Taste Materials producing odors.
Dissolved matter and
various ions.
Impairs aesthetic quality of
water.
Not measured in
unpotable water.
Solid Matter Dissolved and suspended
organic and inorganic salts.
Measures amount of organic
solids, silts, etc., hence is a
measure of the extent of
sewage pollution or the
concentration of a sewage.
By gravimetric analysis
techniques for the
following: total solids,
total volatile solids, total
fixed solids, suspended
solids and dissolved
solids.
Wastewater Treatment 9
SOLIDS (TS,TSS,TDS,VSS,FSS,TVS,VDS,FDS,&TFS)
Total Solids: All the matter that remains as residue upon
evaporation at 104C. [can be further classified as nonfiltrable
(suspended) or filtrable].
Settleable Solids: Solids that will settle to the bottom of
an Imhoff cone in a 60-minute period (mL/L).
Filtrable Solids: This fraction consists of colloidal and
dissolved solids.
Colloidal Fraction: The particulate matter with a size
range of 0.001 – 1 m. [Since it cannot be removed by
settling, bio-oxidation or coagulation followed by
sedimentation is required to remove these particles from
suspension.]
Dissolved Solids: Both organic and inorganic molecules
and ions that are present in true solution in water.
Volatile xxxx: Based on the volatility at 550C, each
category can be further classified into ―volatile xxxx‖ and
―fixed xxxx‖, referring, respectively, to the organic and
inorganic (or mineral) content of the xxxx.
Wastewater Treatment 10
Figure 2.1 Interrelationships of solids found in water and wastewater. In much of the
water quality literature, the solids passing through the filter are called dissolved solids.
(Tchobanoglous and Schroeder, 1985.)
(2.0 m)
103C
550C
Wastewater Treatment 11
Figure 2.2 Imhoff cone used to determine settleable solids in wastewater. Solids that
accumulate in the bottom of the cone after 60 min are reported as mL/L.
Figure 2.3 Apparatus use for the determination of total suspended solids. After wastewater
sample has been filtered, the preweighted filter paper is placed in an aluminium dish for
drying before weighing.
Wastewater Treatment 12
2.3.7 Harmful effects of Domestic and Industrial Waters
Table 2.7
Source: PH McGauhey, Engineering Management of Water Quality, Table 5-8 (1968)
Wastewater Treatment 13
2.4 Wastewater Treatment Processes
2.4.1 Steps in the Development of Wastewater Treatment Systems
Figure 2.4 Source: Viessman & Hammer, Water Supply and Pollution Control, 5
th Ed., Figure 11-19
Wastewater Treatment 14
2.4.2 WASTEWATER TREATMENT
BACKGROUND: “The solution to pollution is (was) dilution”?
CURRENT STATUS: Primary, secondary and advanced (tertiary)
treatment.
UNIT OPERATIONS: methods of treatment in which the
application of physical forces predominates.
UNIT PROCESSES: methods of treatment in which the
removal of contaminants is brought about by chemical or
biological reactions.
Primary Treatment: physical operations such as screening and
sedimentation are used to remove the floating and settleable solids
found in the wastewater.
Secondary Treatment: biological and chemical processes are
used to remove most of the organic matter.
Advanced Treatment: additional combinations of unit
operations and processes are used to remove other constituents,
such as N and P that are not reduced significantly by secondary
treatment.
Natural Systems: "Land treatment" and/or "wetlands"
combining physical, chemical and biological treatment
mechanisms can produce water with quality similar to or better
than that from advanced wastewater treatment.
Wastewater Treatment 15
2.4.3 Generic Flow Schematic for Wastewater Treatment
Figure 2.5 Source: Davis & Cornwell, Introduction
to Environmental Engineering, Fig 5-10
(1998)
Wastewater Treatment 16
Figure 2.6 Schematic layout of a septic tank and tile field.
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Figure 2.7 Schematic diagram of a wastewater management infrastructure (Metcalf & Eddy,
2003).
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Figure 2.8 Typical Flow Diagram
Wastewater Treatment 19
Table 2.8 Approximate Composition of an Average Domestic Wastewater (mg/l)
Before After Biologically
Sedimentation Sedimentation Treated
Total solids 800 680 530
Total volatile solids 440 340 220
Suspended solids 240 120 30
Volatile suspended solids 180 100 20
BOD 200 130 30
Ammonia nitrogen as N 22 22 24
Total nitrogen as N 35 30 26
Soluble phosphorus as P 4 4 4
Total phosphorus as P 7 6 5
Wastewater Treatment 20
Refer to Viessman & Hammer, Chapter 9.
Waste Segregation
The separation of different types or strengths of waste.
Sometimes necessary for industrial wastes, where
process, cooling and sanitary wastes are all generated
on-site.
May involved individual pre-treatment, controlled
mixing, separate disposal, etc.
Note exclusions: Wastes that would
1) create fire/explosion hazard (e.g., gasoline or cleaning
solvents;
2) impair hydraulic capacity (e.g., manure, or sand)
3) cause hazard to people or the treatment system (e.g.,
toxic metal ions, hazardous organic chemicals)
Waste Equalization
Used by both industries and many municipalities.
Municipal systems experience diurnal fluctuations in
flows corresponding with people’s activities, with
morning (e.g. showering, toilet flushing) and evening
(e.g. cooking, washing) peaks.
Involves a ―holding basin‖ that will generate a
stable/uniform effluent for further treatment in the
process train.
Extremely important for municipal secondary treatment
plants.
Wastewater Treatment 21
Possible Benefits:
Uniform hydraulic/organic loading
Possible acid/alkaline neutralization
Removal/Pretreatment of specific toxic compounds
(e.g. heavy metals).
Figure 2.9 Typical Diurnal Flows. Source: Dept. Civil Engineering, Monash University website
(2002)
Wastewater Treatment 22
Combined Sewers (see Sect. 9.2, p 333)
Very common in North America
One pipe collects domestic wastewater, stormwater,
light commercial and industrial wastewaters (etc).
Usually only dry weather flow (domestic, street
drainage and commercial/industrial wastewaters) is
intercepted for treatment.
During major storm events, combined volumes may
exceed the capacity of the treatment plant, requiring
bypass directly to the receiving water: ―Combined
Sewer Overflow.” (CSO)
Vancouver City sewers built before in the first half of
the 20thC were typically combined sewers. 38%/675 km
of Vancouver’s sewers were built before 1930;
31%/565 km were constructed between 1930-59.
Stormwater and wastewater collection systems are
typically separated when old combined sewers are
replaced.
Water quality problems on Vancouver’s beaches are
often linked to CSOs.
Many North American cities prohibit the construction
of combined sewers in new subdivisions.
Wastewater Treatment 23
2.4.4 Preliminary Treatment (Municipal Wastewater Only)
May include:
Coarse screens (aka bar racks)
Medium screens
Comminution
Flow measure (e.g. Parshall flume)
Grit removal
Pumping
Pre-air, pre-Cl2, etc.
Refer to Viessman & Hammer, Figure 9.2
for pre-treatment unit arrangement.
Figure 2.10 Bar rack
Figure 2.11
Possible
arrangements of
preliminary
treatment units
in municipal
wastewater
processes.
Wastewater Treatment 24
Screening Devices (see Sect. 10.4)
Purpose: to remove large solids, and protect pumps if
possible
Coarse Screens
Also called ―bar racks.‖
Comprised of steel bars, <2½‖ (6 cm) apart (clear
openings).
Usually to protect lift pumps.
Bars are inclined to flow channel @ 30-45º to assist
in cleaning
Usually hand-cleaned.
Wastewater Treatment 25
Medium Screens
Popular alternative to above unit.
Clear openings between bars of only 5/8 – 1¾‖ (1.5
4.5 cm).
Mechanically cleaned by ―rake‖ or rotating blades.
Screenings are typically landfilled.
Figure 2.12 Screening unit in operation at Lions Gate WWTP. Seen from above, the unit is
about 5 m high. The screen is on a conveyer and screenings are cleaned at the top of the unit.
Waste water flows in at the bottom and screenings are conveyed to the dumpster seen in the
upper left corner of the photo. There are two parallel screening units at Lions Gate. The room
in which they are located is quite malodorous.
Wastewater Treatment 26
Figure 2.13 Continuous countercurrent solid bowl conveyor discharge centrifuge.
Figure 2.14 Catenary bar screen.
Wastewater Treatment 27
Comminutors
Sophisticated shredding devices.
Reduce solids size further to ¼ - 3/8‖ (0.6 – 1 cm)
Sometimes installed directly in the flow-measuring
channel, with separate by-pass ―screen & channel‖
combination (for emergency use or maintenance
purposes).
In small plants, treating sanitary wastes only (i.e. not
combined sewer), the unit is not preceded by any
other screening device.
Figure 2.15 Wastewater grinders: (a) channel unit and (b) in-line unit.
Wastewater Treatment 28
Figure 2.16 Drawing and schematic diagram of comminutor. (Courtesy of Worthington Pump,
Inc.)
Wastewater Treatment 29
Parshall Flume (refer to p. 352-353)
Wastewater measurement system.
Free-flow, Venturi-type flume.
Has smooth hydraulic flow, no solids deposition and
low head loss.
Under free-flowing (unsubmerged) conditions, the Parshall
flume is a critical-depth meter that establishes a mathematical
relationship between the stage h and discharge Q. For a flume
with a throat width of at least 1 ft but less than 8 ft, flow can be
calculated as
026.0522.14 BWhQ (1)
where:
Q = flow, cfs
W = throat width, ft
h = upper head, ft
Figure 2.17. Parshall Flume schematic for measuring flow in an open channel by measuring
the free-flowing upper head h.(Viessman & Hammer).
Wastewater Treatment 30
Grit Chambers (refer to p. 382)
Grit is defined as gravel, sand, broken glass, bone
chips, coffee grounds, etc.
Grit chamber is designed to protect mechanical
equipment and pumps from abrasive wear, to reduce
pipe clogging and reduce accumulation of settled
material in clarifiers.
Type I – discrete particle sedimentation.
Chamber usually sits between lift pumps and primary
clarifier for sanitary waste systems; if there is a large
solid waste component, then it is placed before lift
pumps. If the chamber is placed after the pump units,
increased pump wear is accepted to afford the
convenience of having the grit chambers at ground
level.
Wastewater Treatment 31
Many types or designs are available, depending on the
specific treatment situation, e.g.:
Figure 2.18 Aerated Grit Chamber. Source: Davis & Cornwell, Introduction to Environmental
Engineering, Fig 5-12 (1998)
long, channel-type settling (sloped)
aerated unit with steep, hopper-like bottom
conventional clarifier-type, with scraper arms
cyclone grit separators, with water jet, used
to wash the grit (esp. small plants) (see
Figure 2-9, next page)
Wastewater Treatment 32
Figure 2.19 Typical section through an aerated grit chamber.
Table 2.9 Typical design information for aerated grit chambers.
U.S. customary units SI units
Item Unit Range Typical Unit Range Typical
Detention time at peak flowrate
min 2-5 3 min 2-5 3
Dimensions:
Depth ft 7-16 m 2-5
Length ft 25-65 m 7/5-20
Width ft 8-23 m 2/5-7
Width-depth ratio Ratio 1:1 to 5:1 1.5:1 Ratio 1:1 to 5:1 1.5:1
Length-width ratio Ratio 3:1 to 5:1 4:1 Ratio 3:1 to 5:1 4:1
Air supply per unit of length
ft3/ftmin 3-8 m3/mmin 0.2-0.5
Grit quantities ft3/Mgal 0.5-27 2 m3/103m3 0.004-0.20 0.015
Wastewater Treatment 33
Figure 2.20 Vortex-type grit chambers: (a) Pista (courtesy Smith & Loveless) and (b) Teacup.
(Courtesy Eutek.)
Wastewater Treatment 34
Grit Chambers, cont.
For design purposes:
grit = fine sand
= 0.2 mm dia.
= specific gravity of 2.65
= settling velocity = 0.075 fps
For large, channel-type systems, the horizontal velocity
designed ~ 1 fps (0.3 m/s).
Settled grit removal: via gravity, bottom scrapers,
screw conveyors, air-lift pumps, etc.
Wastewater Treatment 35
Figure 2.21 Counterflow Grit Washer
Wastewater Treatment 36
Special Installations:
A. Sometimes air diffusers are used to keep organics in
suspension, while grit settles out; this also degases and
freshens the sewage. (e.g. large municipal plants like
Lions Gate in North Vancouver.)
Figure 2.22 Preaeration and Grit Chamber, Lions Gate WWTP
B. Sometimes the grit is washed, to remove organics and
return them to the influent (e.g. small ―package‖ plants
like Squamish; see Fig 2-9).
Wastewater Treatment 37
Purposes of Preliminary Treatment
1. Remove large and small objects
2. Shred some of the solids (minimize wear on pipes and
pumps)
3. Remove grit/sand/other inert waste
4. Freshen incoming sewage (aeration)
5. BOD5 removal
Wastewater Treatment 38
2.4.5 Primary Treatment
Refer to Sect. 10-11
Target performance:
Suspended solids (SS) removal 60 – 70%
Bonus: BOD5 removal 30 – 40%
Actually refers to preliminary treatment plus further
settling of raw wastewater, to remove suspended
organics.
Requires the addition of primary clarifiers to the
process train.
Follows laws of Type I and Type II settling, especially
the latter, in the top half of the tank. (Refer to Water
Treatment section for review of settlement types).
* Settleable solids, free oil & grease,
other floating materials.
Wastewater Treatment 39
Analysis of Discrete Particle Settling (Type 1)
The setting of discrete, nonflocculating particles can be
analyzed by means of the classic laws of sedimentation formed
by Newton and Stokes. Newton’s law yields the terminal
particle velocity by equating the gravitational force of the
particle with the frictional resistance, or drag. The
Gravitational force is given by
Gravitational force = gVs (2)
where s = density of particle
= density of fluid
g = acceleration due to gravity
V = volume of particle
The frictional drag force depends on the particle velocity, fluid
density, fluid viscosity, and particle diameter and the drag
coefficient CD (dimensionless) and is defined by Eq. 6-14.
Frictional drag force = 2
2ACD (3)
where CD = drag coefficient
A = cross-sectional or projected area of particles at
right angles to
= particle velocity
Wastewater Treatment 40
Equating the gravitational force with the frictional drag force
for spherical particles yields Newton’s law:
2/1
3
4
D
sc
C
dgV (4)
where Vc = terminal velocity of particle
d = diameter of particle
In the design of sedimentation basins, the usual procedure is to
select a particle with a terminal velocity Vc and to design the
basin so that all particles that have a terminal velocity equal to
or greater than Vc will be removed. The rate at which clarified
water is produced is then
Q = A Vc (5)
where A is the surface of the sedimentation basin. Equation 5
yields
A
QVc = overflow rate, gal/ft2 d (m3/m2 d) (6)
which shows that the overflow rate or surface-loading rate, a
common basis of design, is equivalent to the settling velocity.
Equation 5 also indicates that, for type 1 settling, the flow
capacity is independent of the depth.
Wastewater Treatment 41
Liquid settling/detention times = 1 to 3 hours.
Liquid ―overflow rates‖ = 300 – 1000 gpd/ft2
but GLUMRB (Great Lakes Upper-Mississippi
River Board of State and Provincial Public Health
and Environmental Managers) recommends:
600 gpd/ft2 for (24 m3/m2-d) plants < 1 mgd
(U.S.) (4000 m3/d) in size, as a guideline.
effluent weir loading limits (GLUMRB):
10,000 gpd/ft (124 m3/m-d) for plants < 1 mgd
15,000 gpd/ft (190 m3/m-d) for plants >1 mgd
BOD5 removal is a function of overflow rate, as per Fig
2-11
Wastewater Treatment 42
Figure 2.23 Primary clarifier suspended solids removals versus overflow rates
showing idealized curve2 with data.
Wastewater Treatment 43
Primary Clarifier Sizing
depth = 7 – 15 ft (GLUMRB minimum is 7ft or 2.1m)
for circular tanks dia. = 30-200 ft
for square tanks dia. = sides < 100 ft
for rectangular tanks dia. = length up to 150 ft, with
L/W ratios of 3:1 to 5:1
number of tanks: at least 2 in operation, with 1 backup
avail.
bottom slopes for organic sludge removal:
o up to 1% for rectangular tanks
o up to 8% for circular and square tanks
sludge removal is usually achieved via mechanical
bottom scrapers, pushing material to a collection hopper
at one end (or in the middle for a circular tank).
Note Example Problem 10-6 & 7, text.
Wastewater Treatment 44
Figure 2.24 Lions Gate WWTP Primary Clarifier. The system consists of numerous
rectangular operating in parallel. Visible at the surface are some of the mechanical scrapers,
which move floatables to a collection pipe as well as settled material to the sludge hopper.
Wastewater Treatment 45
Figure 2.25 Primary Clarifier Schematic. Source: Davis & Cornwell, Introduction to
Environmental Engineering, Fig 5-14 (1998)
Wastewater Treatment 46
Figure 2.26 Typical primary settling tank for wastewater treatment. (a) Circular settling tank
with an inboard weir trough. (b) Stilling well and sludge-collecting mechanism in a circular
clarifier. Source: Courtesy of Walker Process Equipment, Division of McNish Corp.
Wastewater Treatment 47
SPECIAL TOPIC: BIOCHEMICAL OXYGEN DEMAND (BOD) (Ref. Text 8.11)
Quantity of oxygen used by micro-organisms in the
aerobic stabilization of liquid wastes and polluted waters.
Standard 5-day BOD test value is used; standard-sized
samples (300 mL) are incubated at 20ºC for 5 days. Initial
and final dissolved oxygen (DO) levels are measured.
BOD5 is calculated as follows:
BOD5 = L· (1 – 10-5k) (7)
Students are responsible for more detailed knowledge of
the BOD test than is presented here, i.e. lab procedures,
basic equations, theory, fundamental calculations and
example problems. See Viessman & Hammer, Sect 8.11,
pp. 317-321 (7th Ed.) or pp. 289-293 (8th Ed.).
Wastewater Treatment 48
The BOD test is used to:
1. define the organic carbon strength of municipal
wastewaters;
2. evaluate the ―efficiency‖ of treatment by measuring
the oxygen demand remaining in the effluent;
3. determine the amount of organic pollutant in
surface waters, e.g. Fraser River.
Figure 2.27 Source: Viessman & Hammer, 4th
Ed
Wastewater Treatment 49
BIOCHEMICAL OXYGEN DEMAND (BOD):
Biochemical oxygen demand (BOD) is usually defined as
the amount of oxygen required by bacteria while stabilizing
decomposable organic matter under aerobic conditions.
The BOD test is used to determine the pollution strength of
domestic and industrial wastes in terms of the oxygen that they
will require if discharged into natural watercourses in which
aerobic conditions exist.
The 5-day BOD (BOD5) is the most widely used parameter of
organic pollution applied to wastewater and surface water. It
involves the measurement of the dissolved oxygen _ used by microorganism in the biochemical oxidation of organic matter.
The BOD test results are used to
1. Determine the approximate quantity of oxygen that will be
required to biologically stabilize the organic matter
present,
2. Determine the size of waste treatment facilities,
3. Measure the efficiency of some treatment processes, and
4. Determine compliance with wastewater discharge permits.
Wastewater Treatment 50
The BOD test: This is essentially a bioassay procedure
involving the measurement of oxygen consumed by living
organisms (mainly bacteria) while utilizing the organic matter
present in a waste, under conditions as similar as to those occur
in nature.
The test is usually conducted in standard BOD bottles
(300 mL).
Pending on the wastewater samples, dilution [< 7 mg/L]
and seeding may be needed [untreated wastewater, animal
wastes may already contain a large population of
microorganisms].
The samples must be protected from the air to prevent re-
aeration.
The environmental conditions must be suitable for the
living organisms to function in an unhindered manner at
all times. [Absence of toxic substances and all nutrients
needed for bacterial growth, N, P, and certain trace
elements, must be present.]
Wastewater Treatment 51
Limitations of the BOD test:
1. A high concentration of active, acclimated seeded bacteria
is required;
2. Pretreatment is needed when dealing with toxic wastes;
3. Only biodegradable organics are measured;
4. The test does not have stoichiometric validity after the
soluble organic matter present in solution has been used;
5. An arbitrary, long period of time is required to obtain
results. [The 5 day period may or may not correspond to the
point where the soluble organic matter that is present has
been used.]
Wastewater Treatment 52
The carbonaceous oxygen demand curve can be expressed
mathematically as
BODt = L(1 – 10-kt) (8)
where
BODt = biochemical oxygen demand at time t; mg/I
L = ultimate BOD, mg/l
k = deoxygenation rate constant, day-l
t = time, days
The equation for calculating BOD from a seeded laboratory
test is
P
fBBDDBOD 2121
(9)
where
BOD = biochemical oxygen demand, mg/l
D1 = dissolved oxygen (DO) of diluted seeded
wastewater about 15 min after preparation,
mg/l
D2 = DO of wastewater after incubation, mg/l
B1 = DO of diluted seed sample about 15 min
after preparation, mg/l
B2 = DO of seed sample after incubation, mg/l
f = ratio of seed volume in seeded wastewater
test to seed volume in BOD test on seed
P = decimal fraction of wastewater sample used
Wastewater Treatment 53
= wastewaterpluswaterdilutionofvolume
wastewaterofvolume
If the sample is unseeded, the relationship is
P
DDBOD 21 (10)
Example
BOD tests were conducted on composited samples of a raw
wastewater and a treated wastewater after chlorination.
1. The BOD tests for the raw wastewater were set up by
pipetting 5.0 ml into each 300-ml bottle. For one pair of
bottles, the test results were: the initial dissolved oxygen
(DO) was 8.4 mg/l, and after 5 days of incubation at 20°C
the final DO was 3.7 mg/l. Calculate the BOD and estimate
the 20-day BOD value assuming a k of 0.10 day-1.
2. The treated wastewater sample was dechlorinated prior to
conducting a seeded test. The BOD bottles were set up with
50.0 ml of treated wastewater and 0.5 ml of raw wastewater
for seed added to each bottle. For one pair of bottles, the test
results were: the initial DO was 7.6 mg/l, and the final DO
was 2.9 mg/l. Calculate the BOD.
Wastewater Treatment 54
Solution:
1. Using Eq. 10,
lmgBOD /282300/0.5
7.34.85
Using Eq. 8,
lmgL /412101
2820.51.0
BOD20 = 412(1 - 10-0.1x20) = 408 mg/l
2. Using Eq. 9,
lmgBOD /25300/50
0.5/5.07.34.89.26.7
Wastewater Treatment 55
2.4.6 Secondary Treatment
Viessman & Hammer, Chapters 11 & 12
Primary Effluent: contains ~30-40% residual suspended
organics
high in colloidal and dissolved organics,
60-70% BOD
thus, high in BOD and nutrients
Therefore, must further treat this effluent to remove residual
―oxygen-demanding‖ material and keep it out of the receiving
water (a desirable goal in N. America and Europe).
Treatment Methods
A. Chemical not commonplace for municipal WW due to
high chemical costs, chemical sludge
handling problems and low dissolved BOD5
removal efficiency.
if used, chemicals and processes are similar
to water treatment, i.e., Fe salts, lime, alum
and used as polishing treatment only.
Wastewater Treatment 56
Chemical Precipitation
Chemical precipitation in wastewater treatment involves the
addition of chemicals to alter the physical state of dissolved
and suspended solids and to facilitate their removal by
sedimentation.
In the past, chemical precipitation was used to enhance the
degree of suspended solids and BOD removal:
(i) where there were seasonal variation in the
concentration of wastewater,
(ii) where an intermediate degree of treatment was
required,
(iii) as an aid to the sedimentation process.
Now chemical precipitation is also used to provide more
complete removal of organic compounds and nutrients
(N&P).
The common precipitants (coagulants or flocculants) area:
Alum [Al2(SO4)3 18 H2O],
ferric chloride [FeCl3],
ferric sulphate [Fe2(SO4)3 or Fe(SO4)3 3 H2O],
ferrous sulphate [FeSO4 7H2O] and
lime [Ca(OH)2].
There are other chemicals, mostly polymers, such as PERCOL
728, and chitosan, which can be used effectively to achieve
the precipitation.
The removal efficiency can be in the range of 80 to 90% for
TSS, 40 to 70% for BOD5, 30 to 60% for COD, and 80 to
90% for bacteria.
Wastewater Treatment 57
B. Biological
(p. 520-527) 98-99% of all 2º treatment processes are
biological.
can be aerobic and/or anaerobic systems.
usually municipal wastewater main stream
treated aerobically.
Biological treatment is comprised of a living system which
relies on mixed bio-cultures to break-down waste organics and
remove them from solution.
e.g. bacteria & soluble C-organics and N, P etc. + O2
CO2 + H2O + E + more bacteria
(=sludge or biosolids)
where E heat and chemical energy
Micronutrients include Fe, Mn, Zn, Co, Mg, Ca.
Bacteria are single-celled micro-organisms. They can be
classified as heterotrophs or autotrophs.
Autotrophs use carbon dioxide as a carbon source and
oxidize inorganic compounds for energy. They are used
primarily in specialized treatment systems as nitrification.
e.g. NH3 + O2 NO3
Heterotrophs, sometimes called ―saprophytes,‖ use
organic carbon (organic defined as having C—H bonds)
as their energy source and are used in conventional waste
treatment systems.
Wastewater Treatment 58
Two subclasses of heterotrophs:
1. Aerobic: use oxygen as electron receptor
e.g. organic-C + O2 CO2 +H2O + E + biomass
2. Anaerobic: fermentation, i.e. break-down organic-C
e.g. organic-C acid intermediates (aka volatile
fatty acids) + CO2 +H2O + E +
biomass
subsequently the volatile fatty acids are broken
down by a class of anaerobes called methanogens,
i.e. VFA CH4 + CO2 + H2S + N2 (etc.) + E +
more biomass
Anaerobic processes are used if the wastewater is high
in BOD (e.g. 20,000 mg/L). Though lower efficiencies
are achieved than in aerobic systems, energy costs for
anaerobic processes are lower.
Wastewater Treatment 59
Bacterial Growth and Factors
*Reading Assignment*
§12-6 Text – Kinetics
§12-7 Text – Growth
Note especially the concept of F/M Ratio – food-to-micro-
organism ratio.
Bacteria are classified as psychrophilic, mesophilic, or
thermophilic, depending on their optimum temperature range
of growth.
Psychrophilic (cold-loving): 4 - 10C
Mesophilic (moderation-loving): 20 - 40C
Thermophilic (heat-loving): 50 - 65C
The rate of biological activity in the mesophilic range between
5 and 35C doubles for every 10 - 15C temperature rise
(Fig. 12.10). The common mathematical expression for
relating the change in the reaction-rate constant with
temperature, developed in Section 11.6, is
K = K20T-20 (11)
where
K = reaction-rate constant at temperature T
K20 = reaction-rate constant at 20C
= temperature coefficient
T = temperature of biological reaction, C
Wastewater Treatment 60
Figure 2.28 General effect of temperature on biological activity.
Table 2.10 Classification of microorganisms by electron donor, electron acceptor, sources of
cell carbon, and end products
Type of
bacteria
Common
reaction name
Carbon
source
Electron donor
(Substrate
oxidized)
Electron
acceptor Products
Aerobic
heterotrophic
Aerobic oxidation Organic
compounds
Organic
compounds
O2 CO2, H2O
Aerobic
autotrophic
Nitrification CO2 3NH ,
2NO O2 2NO ,
3NO
Iron oxidation CO2 Fe (II) O2 Ferric Iron
Fe (III)
Sulfur oxidation CO2 H2S, S, 2
32OS
O2 24SO
Facultative
heterotrophic
Denitrification
anoxic reaction
Organic
compounds
Organic
compounds 2NO ,
3NO N2, CO2, H2O
Anaerobic
heterotrophic
Acid fermentation Organic
compounds
Organic
compounds
Organic
compounds
Volatile fatty acids
(VFAs) (acetate,
propionate, butyrate)
Iron reduction Organic
compounds
Organic
compounds
Fe (III) Fe (II), CO2, H2O
Sulfate reduction Organic
compounds
Organic
compounds
SO4 H+S, CO+, H+O
Methanogenesis Organic
compounds
Volatle fatty
acids (VFAs)
CO+ Methane
Wastewater Treatment 61
MICROGANISMS
Bacteria:
spherical cocci – 0.5 - 1.0 μm in diameter;
cylindrical rods – 0.1 to 1 μm in width by 1.5 to 3.0 J-lm in length;
helical spirilla – 0.5 to 5 μm in width by 6 to 15 J-lm in lendth for
spirilla.
Fungi:
Heterotrophic organisms (classified into four distinct groups
according to their mode of reproduction, encompassing fission,
budding, and spore formation.)
Algae:
Unicellular or multi-cellular photo-autotrophs.
Protozoa:
Usually motile single cells (the majority of which are aerobic
heterotrophs).
BIOLOGICAL UNIT PROCESSES
Table 2.11
Aerobic Anaerobic
Suspended- Activated-sludge (AS) Anaerobic digester
Growth* Aerated lagoon Earthen lagoon
(facultative)
Attached- Trickling filter (TF) Anaerobic filter
Growth * Rotating biological
contactor (RBC) Fixed-film reactor
* Hybrid reactors
Wastewater Treatment 62
Trickling Filter
Text §12-12 to 12-16
Theory: Primary effluent sprayed onto a bed of rock, wood or
plastic media, coated with a pre-formed biological
film. Biofilms are comprised of bacteria, protozoa,
fungi, etc.
TF is an aerobic treatment process but the film itself
is aerobic to a depth of only 0.20 – 0.25 mm. The
layer below the aerobic layer, then, is mostly
anaerobic.
Figure 2.29 Example of Plastic Media. Source:
www.sequencertech.com
Note: Fig 12-17 in Viessman & Hammer and the concept of
sloughing (i.e. wastewater peels off the top, aerobic
layer).
Fig 12-18 in Viessman & Hammer for a section of a
TF.
Wastewater Treatment 63
Basic Design
Filter Media: 2‖ – 5‖ rock or plastic media. Synthetic
media typically offer a greater surface
area (therefore, greater capacity).
Rotary Arm: Designed for uniform hydraulic loading.
Can be electric-driven or reaction-type
header, requiring at least 60 cm of
hydraulic head.
Underdrain: Provides physical support for media.
Distributes fresh air.
Collects/transports wastewater effluent.
Clarification: Must have a secondary clarifier to settle
out and thicken the sloughed solids.
Effluent/supernatant then discharged.
Figure 2.30 Trickling Filter. Source: Wastewater Engineering: Treatment and Reuse, Metcalf & Eddy,
Tchobonaglous, 1998.
Wastewater Treatment 64
Types
Low-rate filter: One pass of wastewater through filter.
No recirculation of anything.
Sloughed solids settled out.
Usually needs a dosing tank to keep a
minimum level of flow to prevent
killing the biofilm.
Figure 2.31 Low-rate trickling filter.
High-rate filter: There is recirculated flow through the
filter system, usually a combination of
effluent and solids.
No dosing tank required.
Refer to Figs 12-21, 12-22, Viessman.
Wastewater Treatment 65
Figure 2.32 Schematic diagram showing the form of the biological process in a tricking filter.
Wastewater Treatment 66
Figure 2.33 Random packing for both shallow and deep trickling filters. Each element is a
plastic cylinder (3.5 x 3.5 in.) with perforated walls and internal ribs. (Courtesy of Norton
Chemical Process Products Corp., Stow, OH.)
Figure 2.34 Typical packing material for trickling filters: (a) rock, (b) and (c) plastic vertical-
flow, (d) plastic cross-flow, (e) redwood horizontal, and (f) random pack. [Figs. (c) and (d)
from Americal Surfpac Corp., (e) from Nepture Microfloc, and (f) from Jaeger Products, Inc.]
Note: the random pack material is often used in air stripping towers. (Metcalf & Eddy, 2003)
Wastewater Treatment 67
Table 2.12 Physical properties of trickling filter packing materials. (Metcalf & Eddy, 2003)
Packing material
Nominal
size, cm
Approx. unit
weight,
kg/m3
Approx.
specific
surface
area
m2/m
3
Void space,
%
River rock (small) 2.5-7.5 1250-1450 60 50
River rock (large) 10-13 800-1000 45 60
Plastic—conventional 61 x 61 x 122 30-80 90 >95
Plastic—high specific surface area 61 x 61 x 122 65-95 140 >94
Plastic random packing—
conventional
Varies 30-60 98 80
Plastic random packing—high
specific surface area
Varies 50-80 150 70
aC = BOD removal; N = tertiary nitrification; CN = combined BOD and nitrification.
Note: kg/m3 x 0.0624 = lb/ft
3.
m2/m
3 x 0.0305 = ft
2/ft
3.
Figure 2.35 Profile of a typical single-stage trickling=filter plant with recirculation of
underflow from the final clarifier to the head of the plant.
Wastewater Treatment 68
Figure 2.36 Typical recirculation patterns for single-stage trickling-filter plants. (a)
Recirculation with sludge return. (b) Direct recirculation around the filter.
Two-stage filter: 2 filters in series with or without an
intermediate clarifier.
Used to provide a very high quality effluent
or treat high strength wastewater (e.g.
hospitals, military wastes).
Refer to Fig 12-23.
Note: all schemes include a final or secondary clarifier to settle
out sloughed solids.
Wastewater Treatment 69
Figure 2.37 Typical recirculation patterns for two-stage trickling-filter plants without and
with intermediate sedimentation.
Loading (organic)
Usually given in: lb. BOD5/d/1000ft3 (kg/d/1000m3) of
volume
or: lb. BOD5/d/acre-ft (kg/d/hectare-m) of
plan area
e.g. low rate – 15 lb/d/1000ft3 (240 kg/d/1000m3)
high rate – 35 lb/d/1000ft3
2-stage – 55 lb/d/1000ft3
Also, must consider hydraulic loading onto the bed.
Usually given in mill.gal./acre-day, gpm/ft2 or m3/m2-day.
Wastewater Treatment 70
e.g. low rate – 3 mill.gal./acre-day
high rate – 20 mill.gal./acre-day
Formulae:
1. Many, many empirical equations/formulae avail. to
choose from, to define a suitable mathematical
model; but no universal model exists that can
precisely describe substrate removal.
2. However, a ―common model‖ used in practice does
exist and is comprised of the following general
equations:
1. Contact time - nQ
CDt (12)
where t = mean residence time, min.
D = filter depth, ft.
Q = hyd. loading, mill.gal./acre-day
or gpm/ft2
C & n are constants, depending on
shape and specific surface area of the
media.
2. Soluble BOD5 removal without any recirculation
(empirical equation) – first order kinetics
nQKD
o
e eL
L
(13)
where Le = effluent BOD5, mg/L
Lo = influent BOD5, mg/L
n = empirical coefficient; function of
media type/configuration
Wastewater Treatment 71
K = reaction-rate constant, gpm/ft3,
dependent on specific surface area,
type of waste, etc. (usually found in
pilot plant studies).
3. BOD5 applied to filter when recirculation is used:
1
N
NLLL ea
o (14)
where Lo = applied BOD5 of wastewater
stream, after mixing with recirculated
flow, mg/L (requires mass balance
calculation).
La = raw influent BOD5 before mixing
with recirculated flow, mg/L
N = recirculated flow ratio, Qr/Q
Once Lo is found, it is then substituted back
into eqn 2, to evaluate Le.
Figure 2.38 Illustration of variables for equation 3.
Wastewater Treatment 72
Operational Problems
Organic overloading
Plugs the bed and may cause wastewater ―ponding‖
on the surface.
Reduces BOD removal efficiency.
Cold weather:
Causes ice formation on the surface.
Reduces bio-activity and thus, efficiency.
Foul Odours:
Due to overloading and poor air circulation.
More of an aesthetic problem.
Toxicity:
Trickling filters are very sensitive to ―toxic‖
material in the waste, especially near the top of the
bed.
Filter flies:
Called Psychoda flies.
Grow on the media, along inside of walls etc. and
are more of a ―nuisance‖ than any health concern.
Usually controlled by mild application of an
insecticide or by flooding the bed with treated
wastewater or fresh water.
Advantages of Trickling Filters
1. Simplicity of operation.
2. Low operating costs.
3. Waste sludge generated is easy to process.
Wastewater Treatment 73
Table 2.13 Comparison of different types of trickling filters a (Davis & Cornwell, 1998)
Trickling filter classification
Design
characteristics
Low or standard
rate
Intermediate rate High rate (stone
media)
Super rate (plastic
media)
Roughing
Hydraulic loading,
m/d
1 to 4 4 to 10 10 to 40 15 to 90 b 6o to 180
b
Organic loading,
kg BOD5/d m3
0.08 to 0.32 0.24 to 0.48 0.32 to 1.0 0.32 to 1.0 Above 1.0
Recirculation ratio 0 0 to 1 1 to 3 0 to 1 1 to 4
Filter flies Many Varies Few Few Few
Sloughing Intermittent Varies Continuous Continuous Continuous
Depth, m 1.5 to 3 1.5 to 2.5 1 to 2 Up to 12 1 to 6
BOD5 removal, % 80-85 50 to 70 65 to 80 65 to 85 40 to 65
Effluent quality Well nitrified Some nitrification Nitrites Limited nitrification No nitrification a
Adapted from Joint Committee of the American Society of Civil Engineers and the Water Pollution Federation, Wastewater Treatment Plant
Design (ASCE Manuals and Reports on Engineering Practice No. 36, WPCF Manual of Practice No. 8). Lancaster, PA: Lancaster Press, Inc., p.
285, 1977. b Not including recirculation.
Wastewater Treatment 74
The NRC formula for a single-stage trickling filter is
50
056101
100.
VF/w.E
(15)
where
E = BOD removal at 20C, %
w = BOD load applied, lb/day
V = volume of filter media, ft3 x 10-3
F = recirculation factor
w/V = BOD loading, lb/1000 ft3/day
The recirculation factor is calculated from the formula
2101
1
R.
RF
(16)
where R is the recirculation ratio (ratio of recirculation flow to
raw wastewater flow).
The NRC formula for the second stage of a two-stage filter is
50
21
21056101
100.
VF/wE/.E
(17)
where
E2 = BOD removal of the second stage at 20C, %
E1 = fraction of BOD removed in the first stage
w2 = BOD load applied to the second stage, lb/day
w2/V = BOD loading, lb/1000 ft3/day
Wastewater Treatment 75
The effect of wastewater temperature on stone-filled trickling-
filter efficiency may be expressed as
2020 0351 T.EE (18)
where
E = BOD removal efficiency at temperature T inC
E20 = BOD removal efficiency at 20C
Example (Example 12.4 Text)
Calculate the BOD loading, hydraulic loading, BOD removal
efficiency, and effluent BOD concentration of a single-stage
trickling filter based on the following data:
average wastewater flow = 280 gpm
recirculation rate = 0.5
settled wastewater BOD (primary effluent) = 130 mg/l
depth of media = 2.1 m
wastewater temperature = 18C
Solution:
raw-wastewater flow = 280 gpm = 1530 m3/d
recirculation flow = 0.50 x 1530 = 765 m3/d
BOD load = 1530 m3/d x 130 mg/l x l/mg1000
m/kg 3
= 200 kg/d
Wastewater Treatment 76
surface area of filter = (18.0)2/4 = 254 m2
volume of media = 254 x 2.1 = 533 m3
BOD loading = 3m533
g000,200 = 375 g/m3 d
= 23.5 lb/1000 ft3/day
hydraulic loading = dm/m04.9m254
d/m765d/m1530 23
2
33
= 0.15 gpm/ft2 d
By Eqs. 16 and 15,
36.1
5.01.01
5.01F
2
%1.81
36.1/5.230561.01
100E
5.020
Using Eq. 18,
E18 = 81.1 x 1.03518-20 = 75.7%
Effluent BOD = 130[(100 – 75.7)/100] = 32 mg/l
Wastewater Treatment 77
Figure 2.39 Profile of a combined trickling-filter activated-sludge process.
Figure 2.40 Flow diagram for a domestic wastewater plant using rotating biological contactors
for secondary treatment.
Wastewater Treatment 78
Figure 2.41
Wastewater Treatment 79
Activated Sludge Process
(Viessman & Hammer, p. 577–)
Theory
Wastewater fed continuously into an aerated tank.
―Bugs‖ metabolize and biologically flocculate the
dissolved/colloidal organics.
Bugs and organics known as ―activated sludge,‖ while
entire tank contents known as ―mixed liquor.‖
Settling of the activated sludge is carried out in the
secondary clarifier; part of the settled solids is usually
returned to the aeration tank and the rest is ―wasted‖ to
further processing.
Settled effluent/supernatant is then discharged.
General Schematic
Figure 2.42 General schematic for activated sludge treatment process.
Primary feeders bacteria
Secondary feeders protozoa, rotifers
Wastewater Treatment 80
Figure 2.43 Generalized biological process reactions in the activated-sludge process.
Figure 2.44 Rate of metabolism versus increasing food/microorganism ratio.
Wastewater Treatment 81
―Bug growth‖ in the mixed-liquor is usually maintained
in the declining or endogenous growth phase, for a
variety of reasons, including good settling properties of
sludge.
Figure 2.45 Bacterial log-growth curve for a
pure culture. Source: Davis & Cornwell,
Introduction to Environmental Engineering,
Fig 5-14 (1998)
Continued ―synthesis‖ of organics leads to a large buildup
of microbial mass; thus, ―excess‖ activated sludge must
be ―wasted‖ from the system to maintain proper F/M ratio
and growth phase.
What is the F/M (food-to-micro-organism) ratio?
waterinMLVSSlb
dayappliedBODlbM
F /5 (19)
where MLVSS Mixed Liquor Volatile Suspended
Solids.
The daily mass of food supplied to the microbial
biomass mass. This value is important in terms of
managing stage of growth in the system (i.e.
endogenous).
Wastewater Treatment 82
Loading
(Refer to Viessman & Hammer, 12-19)
Usually given in terms of lb BOD5 applied/day/1000ft3 of
tank (mass loading).
Or, as lb BOD5 applied/day/lb MLVSS = F/M ratio,
Where MLVSS mixed-liquor volatile suspended solids.
e.g., low F/M = 0.10 – 0.30 d-1
med F/M = 0.30 – 0.60 d-1
high F/M = > 0.60 d-1
Hydraulic loading discussed in terms of ―aeration period
AP‖ or ―hydraulic retention time HRT,‖ given in
hours.
Return (recycle) rate = % of influent wastewater flow.
e.g. if return sludge rate = 20% and sewer flow into
the plant = 10 mgd (4 x 104 m3/d).
Return sludge flow = 2 mgd
Total plant flow = 12 mgd
See Table 12-3,
text
Wastewater Treatment 83
Also, have the concept of ―sludge age‖ (average time bugs
live in the tank) = retention time of the activated sludge
solids in the system, days.
e.g.,
EEWW SSQSSQ
VMLSS
SS
APMLSS
daysQSS
VMLSSagesludge
,
where MLSS = mixed liquor SS, mg/L
V = tank volume, m3
QE = effluent WW flow, m3/d
SSE = suspended solids in effluent, mg/L
QW = quantity of waste sludge, m3/d
SSW = waste sludge suspended solids, mg/L
AP = aeration period, days
e.g., low SA = 3-5 days
normal SA = 6-10 days
high SA = >20 days
Can substitute MLVSS and VSS into the above equations,
whereby volatile fraction of solids must be known in
advance, i.e. MLVSS/MLSS 0.70 – 0.80 for municipal
waste water.
Normally, [MLSS] in tank 1500 – 3000 mg/L for
treating domestic sewage.
Review Ex. Problem 12-9 in Viessman.
Wastewater Treatment 84
Treatment Systems
Review Figs. 12-32 (included as 2-20 in notes)& 12-33 in
Viessman & Hammer.
Review Ex. Probs. 12-10, 12-11 in Viessman & Hammer.
Process Stability
Problems include:
Oscillating bug growth, especially in conventional or
plug-flow designs.
Changes in organic/hydraulic flow cycle into plant;
flow equalization and recycle can help to mitigate
this problem.
Shock loading (offset somewhat by CMAS design
approach).
Adequate air supply, especially when ―plugging‖
(filling of diffuser parts) occurs too frequently.
Operation and Control
Operation/control of plant via regulation of:
a) Air supply to tank; min DO = 1-3 mg/L.
b) Rate of sludge return; affects F/M ratio.
c) Sludge wasting from the system; affects sludge age,
according to the formula:
EEWW SSQSSQ
VMLSSSA
(20)
Wastewater Treatment 85
ACTIVATED SLUDGE PROCESS CONTROL
The principal factors used in process control are:
1) Dissolved-Oxygen Control: maintaining dissolved-oxygen
(DO) levels in the aeration tank;
Theoretically: The air supply (the amount of O2 transferred)
must be adequate to
(i) satisfy the BOD of the waste;
(ii) satisfy the endogenous respiration by the sludge
organisms;
(iii) provide adequate mixing; and
(iv) maintain a minimum DO concentration of 1 - 3 mg/L
throughout the reactor. [Sludge bulking - when oxygen
limits the microbial growth]
2) Return Activated-Sludge (RAS) Control: to maintain a
sufficient concentration of AS in the aeration tank so that the
required degree of treatment can be obtained in the time
interval desired. {50 - 100 percent.}.
The control strategies are based on either maintaining a target
MLSS level in the aeration tank or a given sludge blanket
depth in the final clarifier.
(i) settleability: settling test or SVI. [30 min. of settling in
a 1,000 mL graduated cylinder.
(ii) sludge blanket level control: 1 - 3 ft (0.3 - 0.9 m).
(iii) clairifier mass balance.
(iv) aeration tank mass balance.
Wastewater Treatment 86
(v) sludge quality.
3) Sludge Wasting: controlling the waste activated sludge
(WAS).
ACTIVATED-SLUDGE PROCESS
Process Description
Oxidation and sythesis:
COHNS + O2 + nutrients CO2 + NH3 + C5H7NO2
+ other end products
Endogenous respiration:
C5H7NO2 + 5O2 5CO2 + 2H2O + NH3 + energy
If all of the cells can be oxidized completely, the ultimate BOD
of the cells is equal to 1.42 times the concentration of cells.
bacteria
(new cells) (organic matter)
bacteria
(cells)
113 160
1 1.42
Wastewater Treatment 87
Oxygen Requirements and Transfer
Theoretical oxygen requirements for the removal of the
carbonaceous organic matter in the AS system can be
computed as:
kg O2/d = {mass of BODL utilized, kg/d} –
1.42 {mass of organisms wasted, kg/d} (21)
kg O2/d =
x
13eo P42.1
f
kg/g10SSQ
(22)
where f = conversion factor for BOD5 to BODL
(0.45 ~ 0.68)
So = influent BOD5 (mg/L)
Se = effluent BOD5 (mg/L)
Q = Flow rate (m3/d)
Px = net waste sludge produced each day (kg/d)
= Yobs Q(So – Se) x (103 g/kg)-1
Yobs = observed yield (g/g)
Wastewater Treatment 88
a) Plug-flow process:
Tapered air (more at front
end).
Conventional process.
PFAS design.
BOD5 gradient.
b) Step air:
Wastewater injected at no.
of points BOD loading is
uniform.
Uniform air supply.
c) Contact Stabilization:
BOD mostly colloidal.
Mostly used in food
processing (dairy).
d) Extended Aeration
No primary clarifier.
Used mainly for small
communities.
Figure 2.46 Flow diagrams from common
activated sludge processes. Source: Viessman &
Hammer, Figure 12-32.
e) Completed-mixed activated
sludge process (CMAS):
Often used for toxic
substances – diluted (effects
are reduced).
Wastewater Treatment 89
Figure 2.47 Conventional activated-sludge process. (a) Long rectangular aeration tank with
submerged coarse-bubble diffusers along one side (Santee, CA). (b) Cross section of a typical
aeration tank illustrating the spiral flow pattern created by aeration along one side.
Wastewater Treatment 90
Figure 2.48 Fine-bubble diffuser for wastewater aeration. (a) A disc diffuser mounted on top
of an air distributed pipe. (b) A grid of diffusers attached to air pipes mounted on the floor of
an aeration tank. (c) Long rectangular aeration tank with uniform mixing and oxygenation by
a grid of fine-bubble diffusers. (Courtesy of SANITAIRE, Water Pollution Control Corp.)
Wastewater Treatment 91
1. Basic monitoring/testing includes:
a) [D.O.] in mixed liquor – computer-controlled in most
plants, through auto-sensors.
b) Influent/effluent BOD5 – measure of efficiency (also
must meet plant permit value).
c) [MLVSS] in tank.
d) Effluent SS leaving system (also must meet plant
permit value).
e) Activated sludge settleability/thickening capacity –
affects sludge processing operation/efficiency.
f) A well-trained operator is the key to overall plant
performance, regardless of the level of sophistication
of the plant.
Wastewater Treatment 92
THE ACTIVATED SLUDGE PROCESS
Process Design Considerations
In the design of the activated-sludge process, consideration
must be given to the following:
1. Selection of the reactor type,
2. Loading criteria,
3. Sludge production,
4. Oxygen requirements and transfer,
5. Nutrient requirements,
6. Control of filamentous organisms, and
7. Effluent characteristics.
Selection of Reactor Type:
Operational factors that are involved in the bioprocess include:
1. Reaction kinetics governing the treatment process; 2. Oxygen transfer requirements; [modifications of AS: i)
tapered aeration, ii) step-feed, and iii) complete-mix ].
3. Nature of the wastewater to be treated;
4. Local environmental conditions; [temperature, pH and
alkalinity ].
5. Costs [construction. operation, and maintenance costs].
Loading Criteria
The two most commonly used parameters are:
1. Food-to-microorganism ratio (F/M), and
2. Mean cell-residence time (c).
Wastewater Treatment 93
Table 2.14 General Loading and Operational Parameters for Activated-Sludge Processes
BOD LOADING
lb BOD
1000ft3/
day a
lb BOD/
day/lb of
MLSS
SLUDGE
AGE
(days)
AERATION
PERIOD
(hr)
AVERAGE
RETURN
SLUDGE
RATES
(%)
PROCESS
Step aeration 30-50 0.2-0.5 5-15 5.0-7.0 50
Conventional
(tapered
aeration)
30-40 0.2-0.5 5-15 6.0-7.5 30
Contact
stabilization 30-50 0.2":"0.5 5-15 6.0-9.0 100
Extended
aeration 10-30 0.05 0.2 20+ 20-30 100
High-purity
oxygen 120+ 0.6-1.5 5-10 1.0-3.0 30
a1.0 Ib/1000 ft3/day = 16 g/m3.d.
Wastewater Treatment 94
Figure 2.49 Schematic diagram of a typical activated sludge process
Wastewater Treatment 95
Figure 2.50 Definition sketch for suspended solids mass balances for return sludge control:
(a) secondary clarifier mass balance and (b)aeration tank mass balance. (Metcalf
& Eddy, 2003)
Wastewater Treatment 96
Table 2.15 Factors affecting the performance of typical secondary treatment processesa
Process Factors affecting performance
Activated sludge Reactor type
Hydraulic detention time
Hydraulic loading
Organic loading
Aeration capacity
Mean cell-residence time (MCRT)
Food/microorganisms ratio (F/M)
Return sludge recirculation rate
Nutrients
Environmental factors (pH, temperature)
Trickling-filter Media type and depth
Hydraulic loading
Organic loading
Ventilation
Filter staging
Recirculation rate
Flow distribution
RBCs Number of stages
Organic loading
Hydraulic loading
Drive mechanisms
Media density
Shaft selection
Recirculation rate
Submergence
Rotational speed
Wastewater Treatment 97
Figure 2.51 Sequencing batch reactor operation for carbon, nitrogen, and phosphorus
removal (Metcalf & Eddy, 2003).
Wastewater Treatment 98
Table 2.16 Description of the operational steps for the sequencing
batch reactora
Operational
step Description
Fill The purpose of the fill operation is to add substrate (raw
wastewater or primary effluent) to the reactor. The fill
process typically allows the liquid level in the reactor to rise
from 25 percent of capacity (at the end of idle) to 100
percent. If controlled by time, the fill process normally lasts
approximately 25 percent of the full cycle time.
React The purpose of react is to complete the reactions that were
initiated during fill. Typically, react takes up 35 percent of
the total cycle time.
Settle The purpose of settle is to allow solids separation to occur,
providing a clarified supernatant to be discharged as effluent.
In an SBR, this process is normally much more efficient than
in a continuous-flow system because in the settle mode the
reactor contents are completely quiescent.
Drawb The purpose of draw is to remove clarified treated water from
the reactor. Many types of decant mechanisms are in current
use, with the most popular being floating or adjustable weirs.
The time dedicated to draw can range from 5 to 30 percent of
the total cycle time (15 minutes to 2 hours), with 45 minutes
being a typical draw period.
Idleb The purpose of idle in a multitank system is to provide time
for one reactor complete its fill cycle before switching to
another unit. Because idle is not a necessary phase, it is
sometimes omitted. aAdapted from Metcalf & Eddy, 2003.
bSludge wasting usually occurs during the settle or idle phases, but wasting can occur in the other
phases depending on the mode of operation.
Wastewater Treatment 99
Figure 2.52 Sequencing batch reactor (SBR) design principle
Wastewater Treatment 100
Table 2.17 Factors That Can Adversely Affect Settleability of Activated Sludge
BIOLOGICAL FACTORS
Species of dominant microorganisms (filamentous)
Ineffective biological flocculation
Denitrification in final clarifier (floating solids)
Excessive volumetric and food/microorganism loadings
Mixed-liquor suspended-solids concentration
Unsteady-state conditions (nonuniform feed rate and discontinuous
wasting of excess activated sludge)
CHEMICAL FACTORS
Lack of nutrients
Presence of toxins
Kinds of organic matter
PHYSICAL FACTORS
Excessive agitation during aeration resulting in shearing of floc
Ineffective final clarification: inadequate rate of return sludge,
excessive overflow rate or solids loading, or hydraulic
turbulence
Wastewater Treatment 101
Figure 2.53 Schematic of settling regions for activated sludge.
Wastewater Treatment 102
Types of Settling: On the basis of the concentration and the
tendency of particles to interact, four types of settling can occur:
a) Discrete particle (type 1): Refers to the sedimentation of
particles in a suspension of low solids concentration. Particles
settle as individual entities, and there is no significant interaction
with neighboring particles. {Removes grit and sand particles from
wastewater}
b)Flocculant (type 2): Refers to a rather dilute suspension of
particles that coalesce, or flocculate, during the sedimentation
operation. By coalescing, the particles increase the mass and settle
at a faster rate {Removes a portion of the suspended solids in
untreated wastewater in primary settling facilities, and in upper
portion of secondary settling facilities. Also removes chemical floc
in settling tanks}.
c) Hindered (zone or type 3): Refers to suspension of
intermediate concentration, in which inter particle forces are
sufficient to hinder the settling of neighbouring particles. The
particles tend to remain in fixed positions with respective to each
other, and the mass of particles settles as a unit. A solids-liquid
interface develops at the top of the settling mass. {occurs in
secondary settling facilities used in conjunction with biological
treatment facilities}.
d) Compression (type 4): Refers to settling in which the particles
are of such concentration that a structure is formed, and further
settling can occur only by compression of the structure.
Compression takes place from the weight of the particles, which
are constantly being added to the structure by sedimentation from
the supernatant. {Usually occurs in the lower layers of a deep
sludge mass, such as in the bottom of deep secondary settling
facilities and in sludge-thickening facilities}.
Wastewater Treatment 103
Column Test (ASTM/ASCE/WEF)
1. Column diameter > 15 cm
2. Column depth > 300 cm of liquid
3. Sampling ports @ max 60 cm intervals (30 cm recommended.)
4. Uniform suspension of liquid/solids
5. Uniform temperature
6. Quiescent water
7. Up to 3 hours
Wastewater Treatment 104
Example (Example 10.8, Text)
Two circular final clarifiers of the type shown in Figure 2.56, with
100-ft diameter and 12-ft side-water depth, are provided for an
activated-sludge plant designed to treat 12.5 mgd. Calculate the
overflow rate and detention time based on design flow. If the
aeration tank is operated at a mixed-liquor suspended-solids
concentration of 4000 mg/L and a recirculation ratio of 0.5,
calculate the solids loading on the clarifier.
Solution:
Surface area of clarifiers = 2 (50)2 = 15, 700 ft2
Volume of clarifiers = 15,700 12 7.48 = 1, 410, 000 gal
Overflow rate = 2ft/gpd800700,15
000,500,12 (32m3/m2 d)
Detention time =000,500,12
24000,410,1 = 2.7 hr
Flow from the aeration tank to the clarifier with a recirculation
ratio of 0.5 equals 1.5 12.5 = 18.8 mgd.
Solids loading = 700,15
34.840008.18 = 40 lb/ft2/day (195 kg/m2 d)
Wastewater Treatment 105
Secondary/Final Settling
Aka clarification.
Follows all secondary, aerobic treatment units.
Removes sloughed solids from T.F.’s, MLVSS from all types
of activated sludge processes, etc.
Designed according to GLUMRB standards:
i. Overflow rate: >800 gpd/ft2 (33 m3/m2-d).
ii. td = 2 to 4 hr (usually < 3hr).
iii. Minimum SWD = 8’ (2.4 m). 10’ (3.1 m) for FC of
AS. Note : Ex. Prob 10-8, Viessman & Hammer.
iv. Solids loading: 40-60lb/ft2/d (200-290 kg/m2/d)
Classified as Type III or Interface or Zone settling (Ref.:
Metcalf & Eddy, 4th Ed., p. 361-384).
o Intermediate concentration of particles.
o Interface settling (line of separation develops
between particles & fluid).
o Particles settle as structure or ―zone.‖
o Includes extremes of Type II (hindered settling) and
Type IV (compression settling).
Figure 2.54 Final Clarifier Profile.
Wastewater Treatment 106
Such a thick, flocculent suspension must be settled out in
a batch process, using a conventional settling column.
Must plot the position of the ―interface vs time,‖ starting
at t=0, following pattern shown Figure 2-55.
During ―interface‖ settling, actually realize all 4 types of
settling simultaneously, i.e.:
o upper zone of clarified liquid,
o second zone of hindered settling (Type II),
o transition zone key for design,
o compression zone at the bottom.
Design Approach
Surface area of clarifier must be determined for
continuous-flow treatment.
Surface area is a function of:
a) clarification capacity,
b) thickening capacity.
From Fig 2-55 (course notes, p.109), can get both
numbers:
Larger of the two numbers = Design Surface Area.
Wastewater Treatment 107
Clarification Area
s
o
cV
QA (23)
where:
Ac = surface area of settling zone
Qo = flow rate for liquid portion only, and is
proportional to the volume above the sludge
zone.
=
o
uo
H
HHQ
(note that Q0 < Q, always)
Vs = subsidence rate in the zone of hindered
settling = slope AB in Figure 2-55
Thickening Area
o
u
TH
tQA (24)
where:
Q = volumetric flow rate into the basin
AT = surface area of settling zone
Ho = depth of fluid in settling column (= initial
interface height).
tu = time to reach desired underflow or sludge
concentration at the bottom, Cu
Wastewater Treatment 108
How to get tu?
a) Find Hu from
U
OOU
C
HCH
(25)
where Co = initial solids conc., mg/L
Cu = desired underflow conc., mg/L; e.g.
20,000 mg/L or so.
b) Draw tangent at C2 (refer to Fig 2-55).
c) Drop perpendicular from point of intersection of
Hu and C2 tangent = tu.
How to find C2?
o C2 is a point on the transition curve and is known as
the ―critical concentration point.‖
a) Draw tangents for AB and DC portions of the
interface settling curve (Fig. 2-55).
b) Bisect the angle between the two tangents.
c) Extend bisector back to the settling curve = pt C2
(as per Fig. 2-55).
Wastewater Treatment 109
Figure 2.55 Graphical Representation of interface height versus time for a batch-settling test.
Source: After W.P. Tamadge and E.B Fitch, “Determining Thickener Unit Areas.” Ind. Eng.
Chem., 47 (1955).
AB = Contributes for clarification – hindered (zone) settling (type III).
CD = Contributes for compression – type IV.
C2 = Critical concentration point.
Hu = Interface height for the desired underflow concentration SS.
tu = Time to reach the desired underflow concentration SS
Wastewater Treatment 110
Summary
Proper design of the final/secondary clarifier must satisfy 3
criteria:
1. Clarification capacity (area) needed for hindered
settling.
2. Thickening capacity (area) needed to ―remove‖ solids
in transition, into the sludge zone at the bottom.
3. Non-excessive detention time, td, or the bottom,
thickened sludge may turn septic/anaerobic
(producing gas and causing rising sludge).
Wastewater Treatment 111
Figure 2.56 Final clarifier designed for use with activated-sludge processes. Return sludge is
withdrawn through suction (uptake) pipes located along the collector arm for rapid return to
the aeration tank. Sludge flowing from each pipe can be observed in the sight well. (Courtesy
of Walker Process Equipment, Division of McNish Corporation.
Wastewater Treatment 112
2.4.7 Sludge Processing/Treatment
Viessman & Hammer, Chap 13.
45 – 55 % of total costs.
Primary sludge processed by anaerobic digestion.
Secondary sludge processed by aerobic digestion or
combined with primary sludge and digested anaerobically.
Sludge (odorous & putrescible) must be further processed
and reduced in volume before land disposal, incineration
etc.
Basic Sludge Characteristics (Municipal Wastewater)
Raw Primary
Grey in colour.
Very high in suspended organics.
60-80% volatile.
Usually aim for 4-6% thickened product, prior to
digestion – thus, may need separate thickener.
e.g. Lions Gate Plant: raw primary clarifier sludge is settled
to a concentration of 2-3%, then pumped to separate
thickener, with a target concentration of 4½-5%.
N.B. 1% solids = 10,000 mg/L
Wastewater Treatment 113
Raw Secondary – examples
1. Trickling Filter Dark brown (oxidized).
Non-odorous, usually.
Very flocculent, sloughed solids.
70-80% volatile.
Can settle to concentration of 4-5%.
2. Activated Sludge Dark brown (oxidized).
MLVSS is very flocculent.
70-80% volatile & non-odorous.
High water content, slow to settle
and will settle to only 2%, or so.
Both sludges will turn septic ( lack of O2, anaerobic and
malodourous) very quickly if allowed to sit around too long –
because of high bacterial presence.
Anaerobically-Digested Sludge
Very thick, slurry-like material, @ 6-12% solids
concentration.
Very dark, almost black in colour (septic).
Odorous, with entrained gases of CH4, CO2, H2S.
Collected CH4 can be used to create electricity for plant
equipment (e.g. Lions Gate).
Will dewater relatively easily (advantage).
Residue is still 30-60% volatile.
Up to 45% solids reduction.
Wastewater Treatment 114
Table 2.18 Processes for Storage, Treatment, and Disposing of
Wastewater Sludges
Storage prior to processing
In the primary clarifiers
Separate holding tanks
Sludge lagoons
Thickening prior to dewatering or digestion
Gravity settling in tanks
Gravity belt thickening
Dissolved-air flotation
Centrifugation
Conditioning prior to dewatering
Stabilization by anaerobic digestion
Stabilization by aerobic digestion
Chemical coagulation
Heat treatment or wet oxidation
Mechanical dewatering
Belt filter pressure filtration
Plate-and-frame pressure filtration
Centrifugation
Vacuum filtration
Composting
Air drying of digested sludge
Sand drying beds
Shallow lagoons
Disposal of liquid or dewatered digested sludge
Spreading as biosolids on agricultural land
Application on dedicated surface disposal site
Disposal of dewatered raw or digested solids
Codisposal in municipal solid-waste landfill
Burial in dedicated sludge landfill
Incineration
Production of bagged fertilizer and soil conditioner
Wastewater Treatment 115
Figure 2.57 Typical sludge-handling scheme for a trickling-filter plant serving a community of
fewer than 10,000 people.
Figure 2.58 Common disposal methods for waste-activated sludge from small treatment plants
without settling prior to aeration.
Wastewater Treatment 116
Figure 2.59 Alternative schemes for processing activated-sludge plant wastes by thickening in
advance of conditioning and dewatering. (a) Separate thickening of waste-activated sludge
before mixing with primary sludge. (b) Gravity thickening of combined raw primary and
waste-activated sludges.
Wastewater Treatment 117
Figure 2.60 Schematic diagram of a dissolved-air flotation system.
Wastewater Treatment 118
Figure 2.61 Gravity belt thickener. Sludge feed after polymer injection overflows a retention
tank onto the porous belt for gravity drainage. Elements positioned close to the belt open
channels for free drainage and to decelerate the agglomerated sludge solids. The thickened
sludge slurry is released from the belt by a discharge blade. Source: Courtesy of Komline-
Sanderson, Peapack, NJ.
Wastewater Treatment 119
Figure 2.62 Two-belt filter press with a gravity drainage zone, wedge zone, and high-pressure
zone. Source: Courtesy of Komline-Sanderson, Peapack, NJ.
Wastewater Treatment 120
Figure 2.63 Diagrams illustrating the operating principle of a solid-bowl decanter centrifuge.
(a) The bowl represents a clarifier with defined surface area and retention time with overflow
weirs. (b) The bottom of the clarifier is wrapped around a centerline to form a bowl that
rotates to increase the gravitational force for sedimentation. (c) The liquid flows through the
long narrow channel formed by the helical screw conveyor against the bowl and out over the
weirs. (d) as the liquid discharge flows out over the weir (adjustable dam plate), the settled
solids are moved by the conveyor out of the liquid onto a conical drainage area for dewatering
prior to discharge. Source: Courtesy of Alfa Laval Sharples, Alfa Laval Separation Inc.
Wastewater Treatment 121
Figure 2.64 Solid-bowl decanter centrifuge. Source: Courtesy of Alfa Laval Sharples, Alfa
Lava Separation Inc.
Wastewater Treatment 122
Aerobically-Digested Sludge
Thick slurry, 6-8% solids concentration.
Dark brown (oxidized), flocculent material.
Has little odour and can be used as a fertilizer, due to
nitrogen and phosphorus content (e.g. GVRD fertilizer
product ―Nutrifor‖ is sludge; applications are subject to
limitation due to potential of contamination from toxins &
heavy metals).
Difficult & expensive to dewater and thicken further,
because of high water content (may need special organic
polymers).
Residue still 30-60% volatile.
Note:
Most digested sludge is further ―mechanically dewatered‖
before final disposal.
Aim for 25-35% ―sludge cake‖ density, even if polymers
have to be used.
Wastewater Treatment 123
Anaerobic Digestion
(p. 681 Viessman & Hammer)
A two-stage process (see Fig 2-66)
Very temperature (min. 30-35°C) & pH dependent (6.7-
7.4; min 6.2)
o See Table 13.7, Viessman & Hammer for reference.
Trouble signs (i.e., failure of process)
1. Buildup in organic acids – < 800 mg/L; max 2000
mg/L.
2. Decrease in gas production.
3. Drop in % CH4 or increase % in % CO2.
4. Drop in pH.
Disadvantage: start-up of an anaerobic digester is a
long/difficult process, especially after an ―upset‖ (~3-5
weeks).
off gas:
65-70% CH4.
30-35% CO2 (max 40% tolerated).
<5% for H2S, H2, N2, NH3, etc.
Typical gas production: 16-18 ft3 total gas/lb VSS
destroyed. (x 0.0624 = m3/kg)
Supernatant decanted and returned to head of plant.
Wastewater Treatment 124
Figure 2.65 Schematic diagram of the patterns of carbon flow in anaerobic digestion.
Wastewater Treatment 125
Reaction Rates:
In comparison:
1st Step – slower
2nd Step – faster
Figure 2.66 Sequential mechanism of anaerobic waste treatment.
2nd Step
pH
drop
1st step
Wastewater Treatment 126
Types of Digesters
1. Single stage, standard rate.
2. Single stage, high rate.
3. 2-stage system.
Refer to Figs. 2-73 and 2-74, attached (from EPA Design
Manual).
Note – can have ―fixed‖ or ―floating‖ covers, but the must
be air-tight; e.g. Lions Gate Plant.
Figure 2.67 Lions Gate WWTP sealed anaerobic digester (centre), next to a CH4 storage tank.
Wastewater Treatment 127
Figure 2.68 Cross-sectional diagram of a floating-cover anaerobic digester.
Wastewater Treatment 128
Figure 2.69 High-rate digester-mixing systems. (a) Mechanical mixing. (b) Gas mixing using a
series of gas discharge pipes. (c) Gas mixing using a central draft tube. (d) Gas mixing using
diffusers mounted on the tank bottom.
Wastewater Treatment 129
Figure 2.70 Sketch of a two-stage anaerobic digestion system. (a) The first stage is a
completely mixed high-rate digester with a fixed cover. The second stage is a thickening and
storage tank covered with a dome for collecting and storing gas. (b) Photograph of two-stage
digesters. (Lincoln, NE.)
Wastewater Treatment 130
Figure 2.71 Typical reactor configurations used in anaerobic wastewater treatment (Metcalf
& Eddy, 2003)].
Wastewater Treatment 131
Figure 2.72
Wastewater Treatment 132
Aerobic Digestion
(p. 690 Viessman & Hammer)
Purpose is to stabilize waste sludge via ―long-term‖
aeration, thus reducing BOD & destroying VSS.
Similar to completely-mixed, activated sludge but longer
detention times (td ~ 15-25 days).
process theory:
o = oxidation of residual organics (i.e. original BOD5)
+ cellular endogenous respiration.
o i.e. cell mass + O2 CO2 + H2O + H+ + P + NH3,
where NH3 is oxidized to NO3-, pH decreases.
Wastewater Treatment 133
Figure 2.73 Standard rate and high-rate digestion. Source: EPA Design Manual.
Wastewater Treatment 134
Figure 2.74 Two-stage anaerobic digestion. Source: EPA Design Manual
Primary objectives:
1. Clear supernatant.
2. Sludge thickening
3. Better efficiency
Wastewater Treatment 135
Need O2 for both aerobic metabolism and mixing of tank
contents.
but ―mixing power‖ & energy‖ usually controls the
air design.
End product has good potential as a fertilizer (much better
quality, usually, than primary and/or anaerobically
digested sludge).
but usage factor ―governed by‖ and ―regulated for‖
1. Final metal content.
2. Pathogen survival/regrowth potential.
3. Proposed usage area and application rates.
4. Risk of groundwater contamination.
Pathogen-free or sterilized sludge is usually achieved by
―liming‖ to pH 10-11 or through further thermophilic
digestion, @ 50-60°C for 5 to 10 days.
Digesters usually operated on a ―semi-batch‖ basis, with
once/twice per day feeding; supernatant is decanted and
returned to the head of the plant.
Wastewater Treatment 136
Table 2.19 General Conditions for Sludge Digestion
Temperature
Optimum 98F (37C)
General range of operation 85 - 95F (29 - 35C)
pH
Optimum 7.0 – 7.1
General limits 6.7 – 7.4
Gas production
Per pound of volatile solids added 8 – 12 ft3 (230 – 340 L)
Per pound of volatile solids destroyed 16 – 18 ft3 (450 – 510 L)
Gas composition
Methane 65% – 69%
Carbon Dioxide 31% - 35%
Hydrogen sulphide Trace
Volatile acids concentration as acetic
acid
Normal operation 200-800 mg/l
Maximum Approx. 2000 mg/l
Alkalinity concentrations as CaCo3
Normal operation 2000 - 3500 mg/l
Minimum solids retention times
Single-stage digestion 25 d
High-rate digestion 15 d
Volatile solids reduction
Single-stage digestion 50% - 70%
High-rate digestion 50%