PART II

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.


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).


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


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


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


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


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


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.


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.


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


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.


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)


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


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.


2.4.3 Generic Flow Schematic for Wastewater Treatment

Figure 2.5 Source: Davis & Cornwell, Introduction

to Environmental Engineering, Fig 5-10

(1998)


Figure 2.6 Schematic layout of a septic tank and tile field.


Figure 2.7 Schematic diagram of a wastewater management infrastructure (Metcalf & Eddy,

2003).


Figure 2.8 Typical Flow Diagram


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


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.


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)


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.


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.


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.


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.


Figure 2.13 Continuous countercurrent solid bowl conveyor discharge centrifuge.

Figure 2.14 Catenary bar screen.


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.


Figure 2.16 Drawing and schematic diagram of comminutor. (Courtesy of Worthington Pump,

Inc.)


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).


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.


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)


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


Figure 2.20 Vortex-type grit chambers: (a) Pista (courtesy Smith & Loveless) and (b) Teacup.

(Courtesy Eutek.)


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.


Figure 2.21 Counterflow Grit Washer


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).


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


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.


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


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.


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


Figure 2.23 Primary clarifier suspended solids removals versus overflow rates

showing idealized curve2 with data.


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.


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.


Figure 2.25 Primary Clarifier Schematic. Source: Davis & Cornwell, Introduction to

Environmental Engineering, Fig 5-14 (1998)


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.


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.).


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


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.


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.]


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.]


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


= 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.


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


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.


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.


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.


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.


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


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


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


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.


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.


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.


Figure 2.32 Schematic diagram showing the form of the biological process in a tricking filter.


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)


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.


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.


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.


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


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.


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.


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.


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


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


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


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.


Figure 2.41


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


Figure 2.43 Generalized biological process reactions in the activated-sludge process.

Figure 2.44 Rate of metabolism versus increasing food/microorganism ratio.


―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).


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


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.


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)


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.


(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


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)


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).


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.


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.)


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.


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).


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.


Figure 2.49 Schematic diagram of a typical activated sludge process


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)


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


Figure 2.51 Sequencing batch reactor operation for carbon, nitrogen, and phosphorus

removal (Metcalf & Eddy, 2003).


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.


Figure 2.52 Sequencing batch reactor (SBR) design principle


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


Figure 2.53 Schematic of settling regions for activated sludge.


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}.


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


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)


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.


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.


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


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).


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


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).


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.


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


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.


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


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.


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.


Figure 2.60 Schematic diagram of a dissolved-air flotation system.


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.


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.


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.


Figure 2.64 Solid-bowl decanter centrifuge. Source: Courtesy of Alfa Laval Sharples, Alfa

Lava Separation Inc.


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.


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.


Figure 2.65 Schematic diagram of the patterns of carbon flow in anaerobic digestion.


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


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.


Figure 2.68 Cross-sectional diagram of a floating-cover anaerobic digester.


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.


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.)


Figure 2.71 Typical reactor configurations used in anaerobic wastewater treatment (Metcalf

& Eddy, 2003)].


Figure 2.72


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.


Figure 2.73 Standard rate and high-rate digestion. Source: EPA Design Manual.


Figure 2.74 Two-stage anaerobic digestion. Source: EPA Design Manual

Primary objectives:

1. Clear supernatant.

2. Sludge thickening

3. Better efficiency


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.


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%

Documents

PART II