Vaccari D.A., Strom P.F., Alleman J.E. Environmental Biology for Engineers and Scientists

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The concluding step in this three-step sequence is methanogenesis, by which methane

is produced. Methanogenesis converts low-molecular-weight organic precursor species

into gaseous end products at both ends of the carbon oxidation state range, including

fully reduced methane and fully oxidized carbon dioxide. At this point, therefore, the

overall process of anaerobic digestion will have biochemically converted a sizable frac-

tion of an initial sludge residua l into a far more benign, possibly even energetically useful

gaseous product.

Whereas the hydrolytic and acidogenic participants in this process are all members

of the eubacteria domain, methanogens are classiﬁed as archaea. Methanogens include

17 genus membe rs, typically subdivided into seven different groups based on their

gramstain reaction status, morphology, DNA makeup, and methanogenic substrate form.

On the one hand, these methanogens exhibit considerable variety in terms of their genetic

structure (with C þ G% values ranging from 26 to 62%), shape (short and long rods,

plate-shaped cells, various cocci arrays, and ﬁlamentous forms), and temperature toler-

ances (including both mesophilic and thermophilic preferences).

Contrasted with their apparent physiological diversity, though, methanogens rely

on three fairly narrow metabolic options for usable substrate forms, as depicted in

Figure 16.41. The dominant substrate option, which includes a large number of methano-

genic genus members, involves CO

-type substrates where carbon dioxide serves as an

acceptor for electrons donated by H

, carbon monoxide, or formate. The second option,

which includes only two genuses (Methanosarcina and Met hanothrix) uses acetate in a

metabolic process known as acetoclastis, a form of fermentation whereby this singular

substrate (i.e., acetate) serves as both an electron acceptor and an electron donor. The

third, methylotrophic, substrate option generally follows an electron ﬂow scheme similar

to that of the acetoclastic group, with substrates such as methan ol or methylamine both

–Type Options

(includes many

methanogengenus)

, CO, or formate

Oxidation

Reduction

or CO

Acetate

(CH

COO

)

Acetoclastic Option

(includes only 2 genus:

Methanosarcina and

Methanothrix)

Oxidation

Reduction

Methanol (CH

OH)

Amines

methylamine,

dimethylamine,

trimethylamine,

methyl mercaptan,

dimethyl sulfide

Methylotrophic Options

(includes several

methanogengenus)

Oxidation

Reduction

–Type Options

(includes many

methanogen genus)

Oxidation

Reduction

Acetoclastic Option

(includes only 2 genus:

Methanosarcina and

Methanothrix)

Oxidation

Reduction

Methylotrophic Options

(includes several

methanogen genus)

Oxidation

Reduction

Figure 16.41 Methanogenic substrate options.

638

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

supplying and scavenging electrons. However, there are several more methanogenic

genuses capable of pursuing this third ‘‘methyl’’ substrate option than are involved

with acetoclastis, and in some cases they may also use hydrogen gas as a source of redu-

cing power. Spread among these three catabolic options, most methanogens are only able

to use one or two substrate forms, although Methanosarcina is far more facile given its

ability to metabolize seven different substrates. In addition, there are methanogens whose

usable substrate options lie outside the standard norm, including those able to subsist with

various alcohol forms, including ethanol, 1- and 2-proponal, and 1-butanol.

Methanogenic metabolism naturally requires a highly reduced environment and is sus-

tained only by microbes whose life-style is strictly anaerobic. Although the reductive

metabolism of carbon dioxide practiced by most methanogens can be viewed as an anae-

robic respiration pathway (using CO

as an electron acceptor in lieu of O

), methanogens

do not employ the same sort of cytochromes, quinones, and ﬂavoproteins as are involved

in a normal respiratory electron transport chain. Instead, methanogens use a number of

highly specialized reducing enzym es and coenzymes for their reductive metabolism,

sequentially including coenzyme factor F

420

, methanopterin, methanofuran, coenzyme

M (2-mercap toethanesulfonic acid), coenzyme factor F

430

, and a terminal coenzyme

HS-HTP (7-mercaptoheptanoyl threonine phosphate).

Aside from the important biochemical roles played by these metabolic compounds,

two of these enzymatic forms also provide a unique means of selectively identifying

the presence of methanogen cells. Methanopterin, which resembles folic acid structurally,

exhibits a bright blue ﬂuorescence when illuminat ed with light at 342 nm, and coenzyme

420

projects a similarly distinct ﬂuorescent blue-green hue when exposed to 420-nm

epiﬂuorescent illumination. Coenzyme factor F

430

also absorbs light when irradiated at

430 nm, but unlike F

420

, this coenzyme does not ﬂuoresce. An environmentally important

aspect of F

430

activity, though, is that this coenzyme requires nickel at a level that gives

methanogens a distinct anabolic requirement for this trace element.

The multistep and metabolically complex nature of these anaerobic mechanisms, there-

fore, introduces a set of concerns when applied to the pragmatic business of sludge diges-

tion, particularly those systems designed for single-stage processing. Indeed, there is an

extremely delicate balance that must be achieved in these single-stage anaerobic reactors

between the involved acidogenic and methanogenic sequences in terms of their respective

production and use of the acidogenic fermentation intermediates. Speciﬁcally, two differ-

ent forms of key intermediates are involved, including both the low-molecular-weight

volatile fatty acids (e.g., acetic, butyric, proprionic, etc.) and hydrogen gas.

The ﬁrst issue with fatty acid intermediates revolves around system pH and the

negative impact that acidic pH levels have on these anaerobic reactions. Methanog ens

are sensitive to pH outside the range 6 to 8 (Figure 16.42). A decrease in pH below 6

reduces the activity of the methanogens more than that of the acidogens. This causes a

buildup of organic acids, further reducing pH. Thus, anaerobic digestion is unstabl e

when confronted with pH disturbances. An important part of the inhe rent instability of

the acidogenic-to-methanogenic linkage, therefore, stems from the fact that short-term

lags in methanogenic activity (e.g., perhaps triggered by other environmental stress fac-

tors) can lead to upset pH transients from which the methanogens cannot readily recover.

The second crucial aspect with the metabolism of an anaerobic digester is that the

reductive intermediate production of reduced hydrogen gas via the fermentative acidogens

must be simi larly balanced by its consumption during methanogenic metabolism. Should

hydrogen gas levels rise within the digester much above trace values (i.e., above

SLUDGE TREATMENT 639

10

3

atm), the metabolic harmony between acidogenic and methanogenic metabolism

will again be disrupted, although in this case the negative impact is on the acidogenic cells.

This phenomenon reﬂects yet another delicate thermodynamic circumstance with the

second set of fatty acid fermentation reactions, where the ambient hydrogen gas level

must be kept sufﬁciently low to effect a net release of free energy. The free-energy values

given in le 16.12 aptly qualify this dependence. In the absence of a sink (i.e., methano-

genic consumption) for hydrogen gas during anaero bic digestion, the free-energy release

for the fermentation of either proprionate or butyrate would be positive in value (with

standard values of þ76.2 and þ48.2 kJ for these respective conversions), at which

point these reactions would simply not occur.

Here again, a successfully maintained anaerobic digester system must achieve a bal-

ance between fermentative and methanogenic reactions. By holding the intermediate

hydrogen gas levels at an acceptably low level (just as the fatty acid buildup had to be

0.25

0. 50

0.75

1.00

0.00

6789

pH Preference from

pH→ 6 to 8+

Reactor pH

Methanogenic Activity (in relative %)

pH Preference from

pH → 6 to 8+

Figure 16.42 Approximate pH response by anaerobic methanogens. (Adapted from Speece,

1996.)

TABLE 16.12 Fermentation Free-Energy Changes for Standard vs. Typical Reactor Values

(kJ/Reaction)

Fermentation Type Reaction G

G

Proprionate fermentation

to acetate, CO

, and H

Proprionate þ 3H

O ! acetate þ H

þH

þHCO



þ76.2 5.5

Butyrate fermentation to

acetate, CO

, and H

Butyrate þ 2H

O ! 2 acetate þ H

þH

þ48.2 17.6

G

, Standard free-energy release with 1 M fatty acid and H

at 1 atm.

G, free-energy release with typical reactor conditions (1 mM fatty acid and H

at 10

3

atm).

640 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

kept in check) the acidogenic conversions will realize energetically favorable (i.e., nega-

tive free-energy release) conditions during fatty acid fermentation.

This interplay between H

-producing and H

-consuming anaerobes, known as inter-

species hydrogen transfer, represents an exceedingly vital biochemical alliance, without

which the overall process of anaerobic digestion would not be feasible. In this context, the

bacteria responsible for fatty acid fermentations and preparative H

production are also

referred to as syntrophs (literally translated as ‘‘eating together’’), whereby their coopera-

tive life-style synergistically supports the necessary metabolic exchange of this key elec-

tron donor.

Sulfur metabolism, particularly that of sulﬁde release and transformation, also repre-

sents an important biochemical aspect with the fermentation reactions found in anaerobic

digesters. These systems have three possible sulfur sources, including that of incoming

soluble sulfates being converted directly to sulﬁdes, due to the highly reducing conditions

within the digester. Sulﬁdes may also be introduced to these digesters via the sludge

streams, either as constitutive cellular elements (e.g., present within sulfur-bearing

amino acids, etc .) released directly from lysed microbial cells found in the secondary

sludge or as reduced metallic-sulﬁde (e.g., FeS) precipitates found within reduced primary

sludges. Whatever their source, sulﬁdes within anaerobic digestors may then undergo a

variety of important transformations. Sulﬁde-based precipitation may occur to an extent

that could reduce the net solubility of many metal species (e.g., iron, copper, nickel, zinc),

possibly even to a degree that would reduce their availability as metabolically necessary

trace elements. Hydrolysis of the sulﬁde may also take place given the slightly acidic con-

ditions found in most anaerobic digestors (i.e., due to the low, 10

7:25

, acid ionization con-

stant for sulﬁde hydrolysis, H

S ! HS



þ H

), thereby leading to gas exchange (i.e., via

stripping) into the headspace and contamination of the predominant methane product to

an extent that could neces sitate subsequent removal prior to using the methane as an

energy-rich feed gas. The highly reduced environment of an anaerobic digester may

also transform inorganic sulﬁdes into various malodor ous organic sulﬁdes collectively

known as mercaptans. le 16.13 provides a general synopsis of these various biochemical

products and their associated chemical structures, ranging from simply methyl mercaptan

to dimethyldisulﬁde. These chemical composites of reductively generated organic methyl

groups and inorganic sulﬁde collectively bear an intensely malodorous nature whose

repugnant smell, especially when blended with that of classic ‘‘rotten egg’’ smell of

hydrogen sulﬁde, is well known to anyone who has ever dealt with any of these anaerobic

materials.

Perhaps in part due to the latter complexity of these metabolic pathways and transfor-

mations, anaerobic digestion systems are designed and operated in a wide range of phy-

sical conﬁgurations. These variations can be subdivided into one of three generic options:

conventional high-rate, phase-separated, and anaerobic contact reactors. Even then, there

may be further reﬁnements with regard to mesophilic (20 to 35



C) and thermophilic

(45



C and higher) operating temperatures.

Conventional high-rate anaerobic digesters are typically built in either single- or dual-

tank (i.e., a mixed ﬁrst tank and stratiﬁed second tank) forms, which provide mixing, and

possibly heating, to improve their overall effectiveness (Figures 16.43 and 16.44). These

operations receive smaller, sem icontinuous loads of incoming sludge rather than larger

batch-type loadings spaced farther over a longer period, with the overall intent of fostering

a more stable balance with the acidogenic and methanogenic reactions. The latter

SLUDGE TREATMENT 641

approach toward infrequent, intermittent loadings might impose a ‘‘feast-to-famine’’

regime that would then tend to disturb the desired metabolic harmony considered optimal

for anaerobic digestion.

Taking into account the intermittent nature of these incremental inputs, the recom-

mended loading rates [in terms of the daily applied volatile suspended solids (VSS)

load per unit tank volume] range from 1.6 to 4.8 kg VSS/m

day. Ba sed on the typical

TABLE 16.13 Mercaptan Species Generated by Anaerobic Sludge

Metabolism

Chemical Name Chemical Structure

Methyl mercaptan

CSH

Isopropyl mercaptan

CH SH

Isobutyl mercaptan

CH CH

N-Butyl mercaptan

CCH

Dimethyl sulﬁde

CSCH

Dimethyl disulﬁde

CSSCH

Figure 16.43 Conventional anaerobic sludge digestor: (a) with internal mixing; (b) without mixing.

642

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

characteristics of municipal wastewater sludge, these loading rates correspo nd directly

to hydraulic retent ion times of 15 to 45 days, at which point these systems should be

able to maintain volatile solids reductions on the order of 45 to 50% while producing

0.75 to 1.2 m

of methane-rich gas per kilogram of volatile solids removed. Federal

stipulations for the operation of these systems in a fashion to reduce their pathogen con-

tent signiﬁcantly mandates minimal solids residence times of 15 days at a temperature of

35 to 55



C, and 60 days for sludge digested at 20



Example 16.5: Preliminary Anaerobic Digester Design After completing the preced-

ing series of four optional wastewater treatment designs, the Deer Creek, Illinois , consult-

ing engineer is then asked for another set of preliminary design estimates relative to

sludge processing. In this example, a preliminary design is developed for an anaerobic

digester as might be used in conjunction with a conventional activated sludge system. Com-

pletion of this preliminary design involves four basic assumptions regarding sludge produc-

tion and character. First, conventional wastewater treatment facilities typically generate

Figure 16.44 Conventional anaerobic sludge digestion process schematics.

SLUDGE TREATMENT 643

about 0.25 kg total dry suspended solids per cubic meter of processed wastewater. Second,

in its original wet state, raw primary plus secondary sludge generally contains a rather small

solids fraction, approximately 1.2% by weight, or 12 kg/m

. Third, raw sludge is commonly

prethickened prior to digestion, and for this example a reasonable value of 6% (60 kg/m

)

will be assumed. Fourth, blended primary plus secondary wastewater sludge typically has

a volatile solids fraction of 70%, the remaining 30% being inert solids.

Preliminary Design Details

Sludge solids mass

and volume,

reactor sizing,

and retention

time

Design average wastewater ﬂow: 605.7 m

/day

Estimated daily total sludge solids mass: 151.43 kg TSS/day

ð605:7m

=dayÞð0:25 kg=m

Estimated raw wet sludge volume: 2.5 m

/day

ð151:43 kg=dayÞ=ð60 kg=m

Estimated daily volatile sludge solids mass: 106 kg VSS/day

ð151:43 kg=dayÞð0:7Þ

Design anaerobic digester solids loading rate: 1 kg VSS /m

 day

Design anaerobic digester volume: 106 m

ð106 kg=dayÞ=ð1kg=m

 dayÞ

Design anaerobic digester HRT: 42 days

ð106 kg=dayÞ=ð2:5m

 dayÞ

Related notes 1. The estimated raw wet sludge volume represents more than a

200-fold concentration of contaminants from the original was-

tewater ﬂow into this residual sludge stream (i.e., 2.5 vs. 605.7

/day, or a ratio of 1:242).

2. The design solids loading rate represents a typical norm for

conventional low-rate anaerobic digesters.

3. Here again, this single reactor design does not afford the

desired system redundancy associated with multiple units.

Digester off-gas generated by these conventional high-rate operations typically con-

tains about two-thirds methane and one-third CO

, with a net heating value of approxi-

mately 22,400 kJ/m

. By comparison, natural gas (i.e., a mixture of butane, methane, and

propane) has an energetic value roughly 50% higher. Moderate-to-large wastewater treat-

ment operations will, therefore, typically collect and use this gas to run boilers and inter-

nal combustion engines that then supply heat and power, although many smaller plants

just burn their gas in a ﬂare as a waste product. Prior to using this gas within a boiler

or diesel engine, though, low-level gas contaminants such as water vapor and H

S will

routinely be removed to minimize their corrosive impact.

Although the vast majority of anaerobic systems currently in use for sludge digestion

qualify as high- rate operations, experimental study and limited full-scale evaluation have

644 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

demonstrated that further improvements in process efﬁciency and reliability can be

obtained with appropriate revisions in the reactor staging, temperature and/or solids

contact scheme. For example, even as early as the 1930s, it was noticed that thermophilic

reactor temperatures from 50 to 55



C might provide distinctly higher metabo lic

rates for anaerobic fermentation, thereby resulting in signiﬁcant reductions with

necessary detention time s. Unfortunately, though, these early thermophilic studies also

revealed corresponding problems with process instabilities (e.g., higher levels of off-

gas odor, as well as elevated foaming tendencies) that would largely negate any metabolic

gains.

Recognizing these relative trade-offs (i.e., beneﬁts vs. shortcomings) with mesophilic

and thermophilic systems, several new schemes have been developed in recent years to

combine the advantages of both. Two such temperature-phased processing options are

shown in Figures 16.45 and 16.46, with one using a thermophilic-to-mesophilic sequence

and the other using a mesophilic-to-thermophilic arrangement. In either case, the opera-

tional premise is that the ﬁrst phase serves as a solids preprocessor, which then expedites

the efﬁciency of the trailing, second phase. The resulting improvement in processing

efﬁciency achieved with these temperature-phased designs is reﬂected in their reduced

HRT requirements, which on average (at approximately 15 days) are both comparable

to the lowest permissible values with standard high-rate designs.

Yet another version of these phase-separated design schemes was developed not only to

provide different sequential reactor temperatures but also to secure a beneﬁcial separation

of the acidogenic and methanogenic reactions. These types of acid-to-gas phased proces-

sing options (Figure 16.45), are designed with even smaller hydraulic retention times

(e.g., 1 to 3 days) for the initial, ‘‘acid’’ phase, such that the slower-growing methano-

gens simply cannot be retained within the ﬁrst reactor. At that point, therefore, the ﬁrst

reactor’s hydrolytic and acidogenic roles are uncoupled from that of the concluding,

second-phase methanogenic transformation, thereby promoting optimal conditions for

each of these subdivided reactions.

This type of mesophilic-to-thermophilic acid-gas scheme has been used with consider-

able success on a full-scale level (i.e., at the Woodridge facility in DuPage County,

Illinois) with less than a two-week total retention time. Such acid-gas-phased anaerobic

digestion processes typically carry a far higher level of volatile acids (i.e., commonly ran-

ging from 6000 to 8000 mg/L, and sometimes as high as 18,000 mg/L) within the ﬁrst

tank, and as a result the average pH in this reactor (5.6) is considerably below the

desired, let alone tolerable level for the methanogenic conversion desired. Another telltale

indicator to the success of this two-phase digestion scheme is that the vast majority (typi-

cally, 95%) of the system’s overall gas production takes place within the concluding

thermophilic reactor. Conversely, gas production, particularly that of hydrogen, takes

place at only a nominal level within the ﬁrst reactor, such that the acidogens are not con-

strained by the thermodynamic difﬁculties (i.e., at higher hydrogen levels) that might

otherwise be imposed.

Further efforts to secure better, or more stable, performance with anaerobic digestion

systems have also been made using changes in the mode of initial contact (e.g., with

upﬂow passage through a ﬂuidized bed of anaerobic granules) and/or ﬁnal separation

of the solid–liquid matrix (e.g., using mechanically induced separation as opposed to nat-

ural settling and ﬂotation). In whatever fashion these systems might be constructed,

energy savings represent a signiﬁcant beneﬁt with anaerobic dige stion, including the

SLUDGE TREATMENT 645

twin economic advantages of eliminating the aeration energy cost of aerobic processes

and of recovering a good fraction of the energy content of the organic waste as gas-

phase methane fuel.

16.2.2 Aerobic Digestion

Whether collected within wastewater treatment plants, solid waste landﬁlls, or simply on a

forest ﬂoor, organic solids will naturally degrade and decay under aerobic conditions

through the natural process of oxidative breakdown. The microbial cells found in raw

sludge will therefore undergo much the same progressive sequence of cell lysis, release

of cytoplasmic mater ials, and ﬁnal oxidative conversion within an aerobic digester,

whereby the organics released from lysed cells serve as a source of substrates for other

living aerobic cells. The metabolic conversion of wastewater sludge through aerobic

digestion, such as the unit depicted in Figure 16.47, subsequently converts an initially

organic-rich sludge matrix into carbon dioxide and residual solids that include not only

both inorganic and recalcitrant organic forms but also the remaining complement of meta-

bolically active aerobic oxidizers. The resulting product has the advantage not only of

7-15 day

HRT

(~35

3-5 day

HRT

(~55-60

>5-7 day

HRT

(~35

gas

Raw

sludge

Digested

sludge

Raw

sludge

Digested

sludge

>7-10 day

HRT

(~55-60

Temperature-Phased

‘ → ’

Total HRT

~10-20 days

Temperature-Phased

‘’

Total HRT

~ 12-17 days

7-15 day

HRT

Mesophilic

(~35

3-5 day

HRT

3-5 day

HRT

Thermophilic

(~55-60

>5-7 day

HRT

Mesophilic

(~35

gas

>7-10 day

HRT

Thermophilic

(~55-60

Temperature-Phased

‘‘Thermo→Meso’’

Total HRT

~10-20 days

Temperature-Phased

‘‘Meso→Thermo’’

Total HRT

~ 12-17 days

Figure 16.45 Temperature-phased anaerobic sludge digestion process schematics.

646

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

being well stabilized, but produces much less of an odor problem than that of anaerobi-

cally digested sludge. The main disadvantages of aerobic digestion is the signiﬁcant

energy cost of operation; the loss, compared to anaearobic digestion, of the energy

value of the waste; and the production of high levels of oxidize d nitrate–nitrogen,

which may complicate subsequent land application options.

Compared to the complicated process of anaerobic digestion, aerobic sludge digestion

represents a far more simplistic and in large measure a single-step process. However, there

are still several important biological issues with aerobic digester operations that must be

considered and accommodated. The level of oxygen uptake maintained within these reac-

tors provides a useful indication of the level of organic solids stability and digestion efﬁ-

cacy. This parameter is quantiﬁed as a speciﬁc oxygen utilization rate (SOUR) by which

the measured oxygen uptake rate is divided by the existing volatile suspended solids con-

centration to derive the speciﬁc (i.e., per mass of solids) value. The generally accepted

benchmark for suitably stabilized biosolids is a SOUR value of no more than 1.5 mg

of oxygen consumed per gram of total solids per hour.

The pH levels with standard, mesophilic aerobic sludge digestion operations can typi-

cally be expected to drop with time given the fact that the organic nitrogen released from

1-3 d ay

HRT

>10 day

HRT

‘Gas Phase’

Mesophilic

‘Acid Phase’

gas

Raw

sludge

Digested

sludge

Acid-Gas Phased

‘Meso→ Thermo’

Total HRT

~12+ days

Thermophilic

‘Acid-Phase’

Mesophilic

‘Gas-Phase’

Acid-Gas Phased

‘ →Meso’

Total HRT

~12+ days

>10 day

HRT

1-2 d ay

HRT

Raw

sludge

Digested

sludge

gas

1-3 day

HRT

>10 day

HRT

Thermophilic

‘Gas Phase’

(Methanogenesis)

Mesophilic

‘‘Acid Phase’’

(Hydrolysis & Acidogenesis)

gas

Acid-Gas Phased

‘‘Meso→Thermo’’

Total HRT

~12+ days

Thermophilic

‘‘Acid-Phase’’

(Hydrolysis & Acidogenesis)

Mesophilic

‘Gas-Phase’

(Methanogenesis)

Acid-Gas Phased

‘

Total HRT

~12+ days

>10 day

HRT

1-2 day

HRT

gas

‘Thermo→Meso’’

Figure 16.46 Acid-gas phased anaerobic sludge digestion process schematics.

SLUDGE TREATMENT 647