Water and Wastewater Engineering
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WASTEWATER PLANT RESIDUALS MANAGEMENT 27-33
270 38 000 0 3 55200
33 3
m /d g/m g/m[ ( )( )]
(
,.,
338 000 0 7 0 06 0 80 1 60
3 1
,...dg/md/])( )[( )( )
108 10
172 10
625 10
7
3
33
.
.
. m
V
c. From Examples 27-2 and 27-3, the mass of sludge is
M
sl
()( )33461 10 383cycles/d kg/cycle kg/,,dd
d. The required oxygen is estimated using the design parameter of 2.3 kg O
2
/kg VSS
destroyed from Table 27-12 .
VSS TSS kg/d)8306 4 ()()()(0 80 0 80 10 383. . , , . kg/d
VSS kg/d) 3322 5
dest.
.,. ,.()(040 8306 4 66 kg/d
Orequired kg/d)(2 3 kg O
2
3322 56 (, . .
22
/kg VSS) kg/d 7 641, .89
e. Assuming a 10% oxygen transfer efficiency, an air density of 1.204 kg/m
3
, and a mass
fraction of oxygen in air of 0.232, the daily volume of air is estimated as
air
kg/d
kg/m
7 641 89
0 10 1 204 0 232
3
,.
.. .()( )(
))
min 273 581 274 000 190
33
,,or m /d or m /
V
f. Check the air required for oxygen and mixing.
Q
air
m /
m
m / m
190
625 10
0030
3
33
3
min
min
.
.
33
This is within the recommended aeration ranges of 0.024–0.030 m
3
/ min · m
3
for mixed
primary and activated sludge and 0.02–0.040 m
3
/ min · m
3
for diffused air mixing.
Comments:
1 . Two digesters are provided for redundancy.
2. The aeration system is independent of that used for the activated sludge process.
27-9 ANAEROBIC DIGESTION
Process Description
The anaerobic treatment of biological sludges involves three distinct stages. In the first stage,
complex waste components, including fats, proteins, and polysaccharides, are hydrolyzed to their
component subunits. This is accomplished by a heterogeneous group of facultative an
d anaero-
bic bacteria. These bacteria then subject the products of hydrolysis (triglycerides, fatty acids,
amino acids, and sugars) to fermentation and other metabolic processes leading to the formation
of simple organic compounds and hydrogen in a process called
acidogenesis or acetogenesis.
The organic compounds are mainly short-chain (volatile) acids and alcohols. The second stage is
27-34 WATER AND WASTEWATER ENGINEERING
commonly referred to as acid fermentation. In this stage, organic material is simply converted to
organic acids, alcohols, and new bacterial cells, so that little stabilization of BOD or COD is real-
ized. In the third stage, the end products of the second stage are converted to gases (mainly meth-
ane, CH
4
, and carbon dioxide, CO
2
) by several different species of strictly anaerobic bacteria. It
is here that true stabilization of the organic material occurs. This stage is generally referred to as
methane fermentation. The stages of anaerobic waste treatment are illustrated in Figures 27-12
and 27-13 . Even though the anaerobic process is presented as being sequential in nat
ure, all stages
take place simultaneously and synergistically. The primary acid produced during acid fermenta-
tion is acetic acid (CH
3
COOH). The significance of this acid as a precursor for methane forma-
tion is illustrated in Figure 27-13 .
Microbiology
The microorganisms responsible for hydrolysis and acid fermentation include both facultative
and obligate anaerobic bacteria. Examples of genera found in anaerobic digesters includ e Clos-
tridium, Corynebacterium, Actinomyces, Staphylococcus, and Escherichia.
The microorganisms res
ponsible for methane fermentation are strict obligate anaerobes.
Examples of genera found in digesters include Methanosarcina, Methanothrix, Methanococcus,
Methanobacterium, and Methanobacillus. Only the first two genera are able to use acetate to
produce methane and carbon dioxide. The others oxidize hydrogen with carbon dioxide as the
elec
tron acceptor.
Syntrophic Relationships. The methanogens and acidogens form a mutually beneficial
( syntrophic ) relationship. The methanogens convert ferm entation end products such as acetate
(CH
3
COO
), formate (HCOO
), and hydrogen to methane and carbon dioxide. Because the
Lipids
Fatty acids
Polysaccharides
Monosaccharides
Nucleic acids
Purines &
pyrimidines
Simple
aromatics
Other fermentation products
(e.g., propionate, butyrate
succinate, lactate
ethanol etc.)
Methane carbon dioxide
Protein
Amino acids
Theoretical
Stages
Hydrolysis
Acidogenesis
Methanogenesis
Methanogenic substrates
H
2
, CO
2
, formate, methanol,
methylamines,
acetate
FIGURE 27-12
S chematic diagram of the patterns of carbon flow in anaerobic digestion. (Source: Davis and Cornwell, 2008.)
WASTEWATER PLANT RESIDUALS MANAGEMENT 27-35
methanogens are able to maintain a very low partial pressure of H
2
, the equilibrium of the reactions
is shifted toward more oxidized end products, that is, CH
3
COO
and HCOO
. In effect, the meth-
anogenic organisms serve as a hydrogen sink that allows the fermentation reactions to proceed. If
the methanogens do not utilize the hydrogen rapidly enough, propionate and butyrate fermenta-
tion ( Figure 27-12 ) will be reduced and volatile fatty acids will accumulate. This will result in an
adverse reduction in pH (Metcalf & Eddy, 2003).
Nuisance Organisms. Sulfate-reducing bacteria are nuisance organisms because they can
reduce sulfate to sulfide. In high concentrations sulfide is toxic to methanogens.
Stoichiometry
A limited number of substrates are used by methanogens. Madigan et al. (1997) outlined the
following reactions:
O x idation of hydrogen
42
22 42
HCO CH HO
(27-15)
O x idation of formic acid
44 3 2
422
HCOO H CH CO H O
(27-16)
O x idation of carbon monoxide
42 3
242
CO H O CH CO
(27-17)
O x idation of methanol
4 3 2
3 422
CH OH CH CO H O
(27-18)
76%
4%
28%72%
24%52%
20%
Stage 1:
Hydrolysis and
fermentation
Stage 2:
Acetogenesis and
dehydrogenation
Stage 3:
Methane
fermentation
Higher organic
acids
CH
4
H
2
Acetic acid
Complex
organics
FIGURE 27-13
Steps in anaerobic digestion process with energy
flow. (Source: Davis and Cornwell, 2008.)
27-36 WATER AND WASTEWATER ENGINEERING
O x idation of trimethylamine
41293 64
33 24223
()CH N H O CH CO H O NH
(27-19)
O x idation of acetic acid
CH COOH CH CO
3 42
(27-20)
The COD loss in anaerobic digestion is accounted for by methane production. The COD
equivalent of methane is the amount of oxygen needed to oxidize methane to carbon dioxide and
water:
CH O CO H O
42 22
22
(27-21)
Thus, the COD per mole of methane is 2(32 g O
2
/ mole) 64 g O
2
/ mole CH
4
.
The ideal gas law provides a method of estimating the volume of methane production for a given
sludge bsCOD. At standard conditions (0 C and 1 atm) the volume of methane is 22.414 L/mole.
The volume of CH
4
equivalent of COD converted under anaerobic conditions is
22 414
64
0 35
.
.
L/mole
g mole
L/g COD
(27-22)
CO
2
i s also produced in substantial amounts in the anaerobic degradation process. Typically, this
is estimated as 35 percent of the gas production.
E xample 27-5 illustrates the method of estimating the gas production from the COD oxidized.
Example 27-5. Determine the daily
volume of methane and total gas produced in an anaerobic
digester that is operated at 35 C under the following conditions: biosolid s flow 300 m
3
/ d;
bsCOD 5,000 g/m
3
. Assume Y 0.04 g VSS/g COD and 95% bsCOD removal.
Solution:
a . Use a steady-state mass balance to determine the amount of the influent COD converted
to methane:
0 COD COD COD COD
in eff vss CH
4
b. Determine the individual COD values.
COD g/mm/d)g/d
COD
in
3
()(5 000 300 1 10
3 6
,.5
eeff
g/d g/d ()( )1095 1 5 10 7 5010
64
.. .
Noting from Equation 23-42 in Chapter 23, that the COD of waste ac tivated sludge is
1.42 ( P
x
),
COD gCOD/gVSS gVSS/gCOD
vss
()( )142 004..(()( )095 1 5 10
809 10
6
4
..
.
g/d
g/d
WASTEWATER PLANT RESIDUALS MANAGEMENT 27-37
c. Solve the mass balance for
COD
CH
4
.
COD C D COD COD
g/d
CH in eff vss
4
O
1.5 10
6
7750 10 8 09 10 1 3410
44 6
.. . g/d g/d g/d
d. Correct the theoretical methane gas production for the actual anaerobic digester tempera-
ture. From Equation 27-22, the theoretical production is 0.35 L of CH
4
/g COD.
Actual gas production L of CH /g COD ()0 35
4
.
(()273 15 35
273 15
0 395
4
.
.
.
KK
K
LofCH /gC
OOD
e. The methane production is then
CH production gCOD/d Lo
4
6
1 3410 0395()(..ffCH /gCOD m /L
or 530 m /d
4
33
3
10
531
)( )
f. Assuming the methane is 65% of the gas volume, the total gas volume is
Total
m /d
or m /d
gas
531
065
817 820
3
3
.
V
Growth Kinetics
The bacteria responsible for acid fermentation are relatively tolerant to changes in pH and
temperature and have a much higher rate of growth than the bacteria responsible for methane
fermentation. As a result, methane fermentation is generally assumed to be the rate controlling
step in anaerobic waste treatment processes.
T ypical synthesis yield and endogenous dec
ay coefficients for fermentation and methano-
genic anaerobic reactions are given in Table 27-13 . In the range of 20 C to 35 C, the kinetics
of methane fermentation of long- and short-chain fatty acids will adequately describe the over-
all kinetics of anaerobic treatment (Lawrence and Milnes, 1971). Thus, the kinetic equations
presented to describe the completely
mixed activated sludge process are equally applicable to the
anaerobic process.
Environmental Factors
Most anaerobic diges tion systems are designed to operate in the mesophilic temperature range
between 30 C and 38 C. Other systems are designed to operate in the thermophilic range of 50 C
to 57 C. While the design operating tem perature is important, maintaining a stable operating
temperature is more important because the m
ethane formers are sensitive to temperature changes.
Changes greater than 1 C/d can result in process upset. A design that avoids a change greater than
0.5 C/d is recommended (WEF, 1998).
Methanogenic activity is inhibited at pH values less than 6.8. Because the CO
2
content of the
gas produced is high, a high alkalinity is required to maintain the pH near neutral.
Inhibitory substances include ammonia, hydrogen sulfide, metals such as copper, chromium,
and zinc, and a long list of organic compounds that includes aldehydes
, benzene, and phenol
(Metcalf & Eddy, 2003).
27-38 WATER AND WASTEWATER ENGINEERING
Anaerobic Digester Design Principles
A s with alkaline stabilization and aerobic digestion, the design objective is to meet the regulatory
requirements specified in the Code of Federal Regulations (40 CFR Part 503). These are outlined
in Chapter 18. The processes to meet the Class B criteria for a process to significantly reduce
pathogens (PSRP) and to reduce vec tor attrac
tion serve as a basis for this discussion of design
principles. These may be summarized as treatment of biosolids in the absence of air or oxygen to
maintain anaerobic conditions for a solids retention time (SRT) and temperature between 15 d at
35 C to 55 C and 60 d at 20 C (40 CFR 503.33(b)). There are
several options given to verify that
a significant reduction in pathogens and vector attraction reduction have been achieved. Among
a
Safety factors range from 2.5 to 5 times minimum SRT.
A dapted from Metcalf & Eddy, 2003.
TABLE 27-13
Summary of design parameters for completely mixed suspended growth
reactors treating soluble COD
Value
Parameter Unit Range Typical
Solids yield, Y
Fermentation g VSS/g COD 0.06–0.12 0.10
Methanogenesis g VSS/g COD 0.02–0.06 0.04
Overall combined g VSS/g COD 0.05–0.10 0.08
Decay coefficient, k
d
Fermentation g/g · d 0.02–0.06 0.04
Methanogenesis g/g · d 0.01–0.04 0.02
Overall combined g/g · d 0.02–0.04 0.03
Maximum specific growth
rate,
m
35C g/g · d 0.30–0.380.35
30C g/g · d 0.22–0.28 0.25
25C g/g · d 0.18–0.24 0.20
Half-velocity constant, K
s
35C mg/L 60–200 160
30C mg/L 300–500 360
25C mg/L 800–1100 900
Solids retention time (SRT)
35C d
10–20
a
15
30C d
15–30
a
N/A
24C d
20–40
a
N/A
Methane
Production at 35C m
3
/kg COD 0.4 0.4
Density at 35C kg/m
3
0.6346 0.6346
Content of gas % 60–70 65
Energy content kJ/g 50.1 50.1
WASTEWATER PLANT RESIDUALS MANAGEMENT 27-39
the options to reduce vector attraction, the one requiring a 38 percent reduction in volatile solids
by anaerobic digestion is particularly applicable to the discussion of design principles.
The alternative process arrangements for anaerobic digestion include (1) suspended growth,
(2) sludge blanket, and (3) attached growth. Of these, the sus
pended growth processes predomi-
nate. The suspended growth processes are classified as complete-mix, contact, or sequencing
batch reactor. The complete-mix process is found in most applications for municipal sludge treat-
ment in the United States. It i
s the focus of this discussion.
Two temperature regimes are used in anaerobic digestion: mesophilic (30 C and 38 C) and
thermophilic (50 C to 57 C). Although the thermophilic range has the advantages of increased
reaction rates that result in smaller digesters, increase
d solids destruction, and increased destruc-
tion of pathogens and better dewatering, they have not found wide application for municipal
sludges. The reasons for lack of use includ e higher energy requirements, poorer quality super-
natant, and less process stability. Perhaps one of the overrid
ing reasons is that both mesophilic
and thermophilic digestion are classified as a process to significantly reduce pathogens (PSRP).
Therefore, until recently, there has been little regulatory incentive to use thermophilic pro-
cesses. In 2002 the U.S. EPA granted conditional national PFRP equivalency (i.e., a process to
further reduce pathogens) to a two-step process that c
onsists of thermophilic anaerobic digestion
followed by mesophilic anaerobic digestion (Leffler and Bizier, 2009). This process will then
be approved as one that can generate a Class A sludge. If the anticipated change to full national
PFRP equivalency occurs, the thermophilic option will become muc h more attractive. This is
especiall
y true for existing two-stage digestion systems that can be renovated. In the interim the
mesophilic proc ess is considered more ty pical. For this reason, this discussion is limited to the
mesophilic process.
The preferred design principle is one based on the solid
s retention time (SRT) as the
controlling variable. Other bases for design that have been used include volumetric loading,
volatile solids destruction, observed volume reduction, and population (Metcalf & Eddy, 2003).
Only the SRT, volumetric loading, and volatile solids red uction methods are discussed in the
following paragraph
s.
Solids Retention Time (SRT). Substitution of Equation 22-19 into Equation 23-16 yields a
working equation for estimating the SRT or mean cell residence time (
c
):
1
c
m e
se
d
S
K S
k
(27-23)
where
m
maximum specific growth rate, g/g · d
S
e
effluent soluble COD, g/m
3
k
d
decay coefficient, g/g · d
K
s
half-velocity constant, mg/L
When recycle is not practiced, the solids retention time equals the hydraulic residence time, that
is, SRT HRT. This is typical of municipal anaerobic digestion systems.
At 35 C, the washout or SRT
min
for methanogenesis is 3.2 d (Lawrence and McCarty, 1970).
A safety factor of 5 or greater is recommended. Safety factors greater than 5 provide a more
stable process (Parker and Owen, 1986).
27-40 WATER AND WASTEWATER ENGINEERING
The kinetic equations may be used to estimate the methane gas production by utilizing the
stoichiometric relationship between COD and CH
4
production shown in Equation 27-22:
QSSQ P
xCH
4
g/kg
( )[( )( )( ) ]0 35 10 1 42
3
..
o
(27-24)
where
Q
CH
4
flow rate of methane produced at standard conditions, m
3
/ d
0 . 35 theoretical conversion factor from Equation 27-22
S
o
influent bCOD, mg/L
S effluent bCOD
Q flow rate, m
3
/ d
P
x
net mass of cell tissue produced per day, kg/d
Note that this flow rate is at standard conditions. It must be corrected to the digester temperature
and pressure using the ideal gas law.
The mass of biological solids synthesized ( P
x
) may be estimated as
P
YQ S S
k
x
o
dc
()( )
()
10
1
3
g/kg
(27-25)
where the terms are the same as those in the preceding two equations.
The determination of the volume of a complete-mix digester and the volume of gas produc-
tion is illustrated in Example 27-6.
Example 27-6. An anaerobic digester for Omega Three (Examples 27-2, 27-3, and 27-4) is
being considered as an alternative to alkaline
stabilization or aerobic digestion. Determine the
volume of the digester and the flow rate of methane. The following data have been obtained for
this analysis:
T e mperature of digester contents 35 C
Influent bCOD 5,000 g/m
3
D e sign effluent bCOD 500 g/m
3
D e sign safety factor 5
There will be no recycle
Solution:
a . Using Equation 27-23, with the kinetic parameters for 35 C from Table 27-13 , calculate
the SRT and check with requirements of PSRP.
1035 500
160 500
3
3
c
()(). g/g d g/m
g/m g/
mm
g/g d
d
3
1
003
0265 003 0235
.
...
and
c
1
0235
425
.
. d
WASTEWATER PLANT RESIDUALS MANAGEMENT 27-41
With a safety factor of 5, the estimate is
c
()( )5 425 21 2 21..ord
This exceeds the PSRP requirement of 15 d at 35 C (Table 18-15). The SRT of 21 d
governs.
b. Without recycle, the HRT SRT and the volume of the digester is
Qt ()()270 21 5 670
33
m /dd m,
V
c. The mass of biological solids synthesized is estimated with Equations 23-37, 23-38 and
the kinetic parameters from Table 27-13 .
P
x
()()(0 08 270 5 000
33
.,gVSS/gCOD m /d g/m 5500 10
1003 21
596
33
g/m kg/g
d
kg
)( )
()()
.
.//d
d. The methane production is estimated using Equation 23-38 and the corrected theoretical
production of 0.395 L of CH
4
/g COD from Example 27-5.
Q
CH
4
g/m g/mm/d( )[( )(0 3955000 500 270
333
., ))( ) ( )]10 1 42 596
39
3
kg/g kg/d..
553 400
3
.orabout m /d
Comments:
1 . Two digesters are provided for redundancy.
2. Gas storage volume was not considered in the digester volume calculation.
Volumetric Loading. This is an historic method of sizing the diges ter volume. Based on em-
pirical observations, the mass of volatile solids added
to the digester per day per unit volume of
digester was selected in the range 1.6 to 4.8 kg/m
3
· d. The loading criterion was based on a sus-
tained loading condition. Typically, this was the peak two-week or peak month solids production.
Excessively low volatile solids loading rates can result in expensive designs and operating prob-
lems. Dilute sludge results in reduced HRT, redu
ced volatile solids destruction, reduced methane
production, reduced alkalinity, increased volume of solids and supernatant, and increased heating
requirements.
Volatile Solids Reduction. Although the reduction in volatile solid s is more commonly used
as a m
onitoring parameter in the operation of a digester, it can be used for design. An estimate of
the volatile solids destroyed in a high-rate, complete-mix digester can be made with the following
empirical equation (Liptak, 1974):
VS
dc
13 7189..ln( )
(27-26)
where VS
d
volatile solids destruction,%. The suggested value of
c
i s 15 to 20 d.
27-42 WATER AND WASTEWATER ENGINEERING
Anaerobic Digester Design Practice
Pretreatment. Because the preliminary treatment systems do not remove 100 percent of the
rags, grit, and other objects, the uncaptured material ends up in the primary and secondary clari-
fier sludge. Maceration and grit-grinding are typically used to minimize accumulation of these
materials
in the digester.
Process Configurations. The mesophilic anaerobic digesters are described as standard-rate,
two-stage, separate digesters, and high-rate digesters.
The standard-rate process does not employ sludge mix ing, but rather the digester contents
are allowed to stratify into zone
s. The major disadvantage of the standard-rate process is the
large tank volume required because of long retention times, low loading rates, and thick scum-
layer formation. Only about one-third of the tank volume is utilized in the digestion process.
The remaining two-thirds of the tank volume contains the scum layer, stabilize
d solids, and the
supernatant. It is seldom used for digester design today.
The two-stage system evolved as a result of efforts to improve the standard-rate unit. Although
many units are now in operation, it is seldom used in modern digester design. In this process
, two
digesters operating in series separate the functions of fermentation and solids/liquid separation.
The contents of the first-stage, high-rate unit are thoroughly mixed, and the sludge is heated to increase
the rate of fermentation. Because the contents are thoroughly mixed, temperature distribution i
s more
uniform throughout the tank volume. Sludge feeding and withdrawal are continuous or nearly so.
The primary functions of the second-stage digester are solids/liquid separation and residual
gas extraction. First-stage digesters may be equipped
with fixed or floating covers. Second-stage
digester covers are often of the floating type. Second-stage units are generally not heated.
Separate sludge digestion employs a separate digester for primary sludge and for activated
sludge. The goal of this arrangement is to improve the separation of the sludge solids from
the
liquid after digestion. Design criteria for this process are very limited.
The most common design today is a single-stage, high-rate digester ( Figure 27-14 ). It is
characterized by heating, auxiliary mixing, uniform feeding, and thickening of the feed stream.
Uniform feeding of the sludge i
s very important to the operation of the digester. For economical
anaerobic digestion, a feed concentration of at least 4 percent total solids is desirable (Shimp
et al., 1995). The digestion tanks may have fixed or floating covers. Gas may be stored under the
floating cover or in a separate structure. There is no supernatant separation.
Fixed cover
Sludge
inlet
Sludge
heater
Gas storage
Completely mixed
Digester
gas outlet
FIGURE 27-14
S chematic of a high-rate anaerobic digester.
( Source: Davis and Cornwell, 2008.)