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

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In the case of a free-water-surface operation, the determination of minimal hydraulic

retention time is based simply on the ratio of the system’s open water volume to the

expected peak incoming wastewater ﬂow. For subsurface-ﬂow designs, however, this cal-

culation must take into account the available pore volume of the medium through which

the wastewater will be ﬂowing, as follows:

HRT ¼

ADP

ð16:3Þ

where A is the wetland surface area (m

), D is the wetland depth (m), P is the porosity

(dimensionless), and Q is the incoming peak wastewater ﬂow (m

/day).

Despite widespread use of the latter hydraulic retention-time para meter, there is con-

siderable uncertainty as to its validity and usefulness. Much of this uncertainty hinges on

the fact that the pore space volume of newly installed gravel will change once it begins to

ﬁll with root material, interstitial microbial growth, and entrapped suspended. As seen in

Figure 16.35, the density of plants placed within a typical wetl and tends to be quite high,

to the point where their root systems undoubtedly ﬁll a sizable fraction of the soil’s avail-

able pore space.

Once the anticipated waste stream extends much above a few thousand liters per day,

constructed wetlands are usually designed with multiple cells arranged to receive the

incoming ﬂow in serial-ﬂow fashion. Within each cell the ratio of length to width used

for the actual layout of constructed wetlands is known as the aspect ratio. Low aspect

values, particularly those below 1 : 1 (L/W, leng th to width), are prone to short-circuiting

hydraulic problems, where the passing ﬂow is not distributed equitably across the entire

cross section of the cell. This problem would then be reﬂected in poor and uneven levels

of treatment (i.e., erratic levels of BOD, TSS, and ammonia removal). Performance will

typically improve with larger aspect ratios, but these beneﬁts level off around a ﬁgure of

3 : 1. Even at this ratio, attention must still be given to ensuring that the incoming waste

stream is distributed uniformly across the face of the wetland.

Various technical references recommend that these organic loadings be kept below 80

to 110 kg BOD/haday [approximately 8 to 11 g (0.008 to 0.011 kg) of carbonaceous

BOD (CBOD) applied per square meter per day] in order to achieve acceptable levels

of treatment. Hydraulic loading rates should also considered in the design of most con-

structed wetlands. These values are inversely related to HRT, whereby longer retention-

time systems will be maintained with a loading range of 150 to 300 m

/haday, whereas

the highly loaded, shorter HRT designs will operate at hydraulic loadings from 300 to

600 m

/haday.

Most wetlands will experience a signiﬁcant loss of water on a seasonal basis due to

evapotranspiration, especially during a warm late spring through early fall time frame.

This behavior is beneﬁcial in the sense that it reduces the neces sary volume of discharge,

but evapotranspiration may also complicate the process of discharge permitting. Indeed,

the current practice of establishing efﬂuent discharge limits on BOD, suspended solids,

ammonia, nitrate, and so on, for any wastewater treatment operation is almost always

based on cont aminant concentrations rather than total mass. If, for instance, a warm-

weather SSF wetland operation were to release one-half of its incoming wastewater

ﬂow via evapotranspiration, the remaining 50% ﬂow rate subject to actual discharge

might well experience seemingly unacceptable efﬂuent concentrations despite the reality

that the total mass of contaminants had actually been reduced. Consideration is therefore

628 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

being given to an arrangement whereby these permits might be written to secure a neces-

sary reduction in inﬂuent mass of BOD, and so on, rather than concentration.

The next set of design issues covers installation of a subsurface liner as well as cell

depth and media placement. SSF and FWS cell bottoms will need to covered with an

impervious, low-permeability liner, typically constructed with overlapped, seamed sec-

tions of high-density polyethylene (HDPE) sheets rolled across the cell ﬂoors and side-

walls. The peripheral edges of these sites are typically edged with an elevated soil berm,

with the wetland’s impervious underlying liner rolled up and over the top of the berm,

where it is then tucked into a stone-ﬁlled trench to secure it in place. This exposed section

of the liner is then covered with a layer of gravel that protects the liner against direct phy-

sical wear as well as blocking sunlight irradiation that could degrade the liner. The key

beneﬁt of installing these berms is that they restrict stormwater runoff from directly enter-

ing the wetland, where it would impose an unnecessary additional hydraulic loading.

Balanced against the aforementioned engineering details and recommendations applied

to the design and construction of these facilities, though, our technical understanding of

wetlands is frankly still evolving, given our relatively brief experience with their use. In

fact, the current state of the art for constructed wetlands is still far less established than

that of conventional wastewater treatment options (e.g., attached- and suspen ded-growth),

to the point where the engineered practice of designing and operating constructed wet-

lands remains a rather inexact science.

Of course, a key factor with wetlands is that of maintaining a healthy population of true

‘‘wetland’’ plants and ensuring their ability to grow under truly wet conditions. Plantings

introduced to these wetland systems during construction will typically encompass a vari-

ety of plants as opposed to just one type, as a means of promoting a more ecologically

stable population (i.e., a diverse population whose varying levels of water uptake, root

depths, root densities, and so on, inherently provided a higher degree of environmental

tolerance, disease resistance, contaminant sensitivities, etc.).

Free-water-surface systems will usually include a combination of free-ﬂoating and

rooted plants and they will naturally distribute themselves across the surface of the wet-

land. However, in the case of subsurface-ﬂow units, a preplanned layout is typically used

for the installed plants, where they are initially placed in successive layers or bands

stretched perpendicularly across the face of the direction of ﬂow. Second, the plants

selected for each such band are often staggered to take advantage of their various root

forms. Shallower rooted plants tend to be placed earlier in the bed to give the incoming

wastewater ﬂow more of a chance to pass downward into the underlying medium, which

helps to negate short-circuiting across the top edge of the bed. Subsequently, deeper-

rooted plants may be placed farther back in the bed as the ﬂow reaches a point where

it has been fully distributed across the vertical proﬁle of the bed.

As for the types of water-tolerant, if not water-loving plants actually planted within

these facilities during initial construction, a number of options are commonly associated

with each of these subsurface-ﬂow and free-water-surface systems Table 16.8. Interest-

ingly, fragmites plants are banned in some regions because of their aggressive nature,

because their seeds might travel away from the site and subsequently colonize areas out-

side the original site. For that matter, each of the various plant types tends to have good

and bad features.

Figure 16.38 depicts schematically the aboveground and root-zone conformations

observed with a few of the more widely used plants associated with subsurface-ﬂow

design. Bulrush plants (such as river bulrush, Scirpus ﬂuviatilis) and sedges (such as

WASTEWATER TREATMENT 629

fox sedge, Carex vulpinoidea) are generally considered to offer superior levels of nutrient

removal, with ideal levels of root-zone penetra tion (typically 0.6 m) and high levels of

root mass and surface area. Conversely, although cattails (Typha latifolia) are often

observed in natural wetland settings, they are generally regarded as a nuisance plant in

constructed wetlands since their shallow-rooting tendency leads to relatively low treat-

ment properties (i.e., much of the wastewater passing through an SSF bed would pass

beneath the cattail root zone).

Duckweed and water hyacint hs (see the insert image on the top of Figure 16.34) are

commonly used in free-water systems. Duckweed has a far smaller root mass than that of

hyacinth, and it extends far less deeply into the FWS system water depth, but even then it

is often considered to be an important contributor to free-water systems, either by itself or

as a secondary plant form. Water hyacinths are even more favored in free-water

TABLE 16.8 Typical Constructed Wetland System Plant Types

Recommended Plant Type Subsurface Flow Free-Water-Surface

Arrowhead (Sagittaria spp.) 

Bulrush (Scipus spp.) 

Coontail (Ceratophyllum spp.) 

Pondweed (Potamogeton spp.) 

Rush (Juncus spp.) 

Sedge (Carex spp.) 

Water plantain (Alisma spp.) 

Water hyacinth 

Duckweed 

0.5

Meters

Shallow

rooting cattail

(Typha latifolia)

Deeper

rooting fox sedge

(Carex vulpinoidea)

Deeper

rooting

river bulrush

(Scirpus fluviatilis)

0.5

Deeper

rooting

(

Figure 16.38 Representative subsurface-ﬂow constructed wetland plant types.

630

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

systems given the sizable extent of their root systems, which extend deeply into the depth

of these ponds. Hyacinths can also sustain high nutrient uptake rates, and as such they are

also used in applications for tertiary-level wastewater polishing wetlands. Large, rapidly

growing plants such as hyacinths, thoug h, can clog a free-water system unless they are

harvested on a regular basis, and in turn this residual plant mass could constitute a

sizable waste materia l. Even then, however, these waste plants may still be put to bene-

ﬁcial use; for example, Disney World in Florida mulches their waste hyacinth plants and

digests the residual organics to produce methane gas for use as an energy-rich fuel.

One of the key uncertainties with wetland systems, though, is that of the true mechan-

ism(s) behind which contaminants are removed within wetlands, and whether the respon-

sible agent be that of plant- or microbial-based pathways. Much the same argument could

also be raised for phyto remediation systems (see Section 16.2.4), in that our understand-

ing of these processes bears a similar degree of lingering fuzziness.

Undoubtedly, plants can contribute directly to the observed uptake and degradation of

contaminants, using a complementary range of plant-based sorption, uptake, accumula-

tion, volatilization, and degradation mechanisms. Furthermore, the medium itself will

provide some measure of complementary treatment, through either simple ﬁltration or

more complex physical–chemical mechanisms (e.g., sorption, ion exchange). Plants

will also play a signiﬁcant role in the metabolic passage of water through a wetland,

and in some instances their rates of water loss through evapotranspiration will prove to

be the dominant path for water movement, even to the point where the wetland may

have little, if any (e.g., 5% or less), actual free-ﬂowing discharge.

However, microbes extensively colonized onto and around plant root surfaces will

clearly play an important, perhaps even dominant role with contaminant removal in

plant-based wetlands. Figure 16.39 depicts this sort of extensive bacterial attachment,

using an acridine orange cell stain to highlight living cells agai nst the darker, unstained

root surface.

Wetland plants nurture this sort of microbial colonization actively on several accounts.

First, plant root and root tip surfaces inherently offer an extensive surface area for this sort

of microbe growth. Second, plants will pump photosynthetically generat ed oxygen

Figure 16.39 Representative image of bacterial colonization on plant root surface. (Courtesy of

Jay Garland.)

WASTEWATER TREATMENT 631

through these same roots, such that this release will nurture the growth and colonization of

aerobic microbes (i. e., largely bacteria plus fungi) within the root’s immediate vicinity.

Third, plants actively release an organic-rich mix of simple sugars from their root,

known as root exudates, which will again sustain and promote active microbial growth.

On a collective basis, therefore, these plants will promote the extensive growth of

microbial biomass within the immediate vicinity of their root structures and, in turn,

these microbes will contribute to the wetland system’s overall treatment efﬁciency. Cyclic

oxygen release and uptake by wetland plants will actually occur on a diurnal cycle com-

mensurate with the metabolic swing between daytime photosynthesis and nighttime

respiration. FWS systems may consequently experience daily shifts in oxidizing and redu-

cing conditions within the bulk wetland ﬂuid, which intermittently promotes aerobic,

anoxic, and anaerobic bacterial metabolism. For SSF systems, though, these same shifts

in day–night plant behavior will similarly affect microbial growth in the root rhizosphere

zone. Microbes immediately adjacent to root surfaces will probably be capable of sus-

tained aerobic activity during daytime periods, but far ther back from this oxygenated

zone, the soil and its microbes will routinely follow a more anaerobic life-style. As

such, there both aerobic and anaerobic microbial mechanisms will be involved in the

degradation process, as well as a variety of interrelated precipitation, sorption, ion-

exchange, ﬁltration, and even volatilization reactions.

Regional climactic conditions will represent another signiﬁcant design factor, both in

terms of selecting an SSF vs. FWS design strategy and in selecting the types of plants to

be used. Whereas free-water wetlands are not suited to geographical areas in which pro-

longed winter weather would freeze much of the system, SSF operations have proven to

be useful in cold-weather regions extending even into the Arctic Circle. During the winter,

an ice layer will develop across the top of SSF wetlands, extending perhaps as much as

20 cm or more in depth. However, overlying ice and snow layers act as an insulating cover

so that the underlying medium and roots remain at a temperature above freezing. Granted,

at these ambient air temperatures the plants themselves will have shifted from an active to

a dormant state, but their remaining vascular stalks are believed to act as physical chan-

nels or conduits that carry oxygen thr ough the ice and into the medium where degradative

metabolism is still maintained by an active microbial consortia. The reduced temperature,

however, results in slower metabolic rates that must be taken into account by providing

longer hydraulic retention times. Conversely, summertime wetland operations not only

enjoy considerably faster rates of microbial degradation, but in many instance the plants

involved will maintain far higher rates of evapotranspiration. In fact, depending on which

plants are used, there may even be periods during summer season operation at which a

wetland’s incoming ﬂow is largely released into the atmosphere rather than being released

as a conventional discharge.

Many of the emergent plant species used in SSF systems tend to be fairly tolerant of

cold-weather exposure. Conversely, hyacinths used in FWS applications are not hardy

(able to tolerate freezing weather), and therefore their use is limited to warm climates.

Duckweed, on the other hand, is able to withstand colder climates, but even then this

plant form is still not widely used in these areas, given the general reluctance to use

free-water systems in cold-weather localities.

Notwithstanding the apparent biological beneﬁts of constructed wetlands, negative

factors can be associated with these operations that must be addressed. For example,

free-water systems, particularly those without supplemental aeration, have been known

to encounter troublesome problems with mosquito growth as well as occasional odor

632 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

generation. In both instances, though, these types of problems can be alleviated or

avoided with either aerated free-water systems or subsurface-ﬂow operations.

Annual maintenance for wetlands is, again, a rather inexact science. Harvesting is

practiced infrequently with plants grown within subsurface syst ems. Instead, carefully

controlled burning is used more commonly to remove the burden of dead, senescent plants

and to help with eradicating invasive plants that have moved into the wetland during

the past growth season. Dandelions and golde nrod are two of the more common invasive

species.

With free-water wetlands, the plants maintained within lowly loaded operations will

probably have to be thinned out with harvesting on an annual basis. Commensurate

with increased loading levels, these harvesting rates will then have to be increased, to

the point where monthly or biweekly plant thinning may be necessary with the more

highly loaded (e.g., aerated) systems.

As for the regulatory requirements and monitoring policies currently stipulated by var-

ious states for constructed wetlands, there is considerable variation. For example, different

stipulations may be placed on the location of ﬂow monitoring equipment, but in most

instances this equipment is situated at the efﬂuent end of the last wetland cell. In some

instances, a second ﬂow monitori ng device (i.e., typically a ﬂow totalizer) is installed at

the head end of the system, by which the operations personnel can then quantify the level

of water loss maintained within the overall operation due to direct evaporation plus plant-

based evapotranspiration.

FWS and SSF systems both typically exhibit fairly high level BOD and SS removal

efﬁciencies, ranging from 750 to 90%, albeit somewhat below the performance levels

that might be achieved with either ﬁxed-ﬁlm or activated sludge processes. For many wet-

land applications, particularly those faced with tight surface or subsurface discharge lim-

its, nitriﬁcation will probably represent one of the most critical issues, and the relative

rates, and efﬁciencies, of removing nitrogenous contaminants in these wetlands can

vary immensely with time and from one site to the next in relation to plant type, organic

loading rate, hydraulic loading, and climate. It is certainly possible for nitriﬁcation to

occur in these wetlands, but the results quantiﬁed to date regarding wetland nitriﬁcation

have been rather erratic, probably due to the fact that the temperatures and dissolved oxy-

gen levels in these wetlands can vary quickly. One recent upgrade on this account has

been that of using parallel SSF cells maintained in a ﬂip-ﬂop sequence of cyclic ﬁll-

and-draw stages, where these periodic, diurnal-type drain steps would hopefully provide

increased oxygenation of nitriﬁers working within the rhizosphere region.

16.2 SLUDGE TREATMENT

Wastewater treatment systems are designed to remove contaminants from the inﬂuent

waste stream, thereby producing a clean liquid efﬂuent as well as a concentrated, semi-

solid residual that contains the extracted contaminants plus newly generated biological

cells. Rather understandably, the latter concentrated residuals, particularly those in raw

form, can have a rather unpleasant, if not altogether foul nature if not handled and man-

aged properly. In fact, one of the most worrisome aspects of these sludge residuals, in

terms of human health, is that of their potential contamination with a wide range of patho-

genic, disease-causing organisms, ranging from viruses and bacteria and extending

upward in scale to protozoans, as shown in Table 16.9.

SLUDGE TREATMENT 633

Up until the past decade, all forms of these concentrated residuals, whether raw or trea-

ted, were commonly described with the same ‘‘sludge’’ label in deference to the typically

negative context associated with words whose spelling begins with ‘‘sl...’’ (e.g., slap,

sleet, slum, slam, etc.). However, despite the unwholesome connotations implied by

sludge, the nature and makeup of many properly treated residuals (i.e., suitably degraded,

stabilized, disinfected, and dewatered) can actually be rather positive, if not altogether

even valuable: harkening back, as it were, to the earliest days of wastewater processing

as a means of generating nutrient-rich fertilizers.

In fact, many properly treated sludges can be used as beneﬁcial soil amendments

on agricultural lands once their negative attributes (i.e., pathogen content, high-level

degradability, etc.) have been effectively resolved. A more positive label for these properly

TABLE 16.9 Principal Pathogens of Concern Associated with Municipal

Wastewater Biosolids

Organism Type Disease/Symptoms

Bacteria

Salmonella Salmonellosis and typhoid

Shigella Bacillary dysentery

Yersinia sp. Acute gastroenteritis

Vibrio cholerae Cholera

Campylobacter jejuni Gastroenteritis

Escherichia coli Gastroenteritis

Enteric viruses

Hepatitus A virus Infectious hepatitis

Norwalk and Epidemic gastroenteritis

Norwalk-like viruses

Rotaviruses

Acute gastroenteritis Enteroviruses

Polioviruses Poliomyelitis

Coxsackieviruses Meningitis, encephalitis

Echoviruses Gastroenteritis and respiratory infections

Reoviruses Gastroenteritis and respiratory infections

Astroviruses Epidemic gastroenteritis

Caliciviruses Epidemic gastroenteritis

Protozoans

Cryptosporidium Gastroenteritis

Entamoeba histolytica Acute enteritis

Giardia lamblia Giardiasis

Balantidium coli Diarrhea and dysentery

Toxoplasma gondii Toxoplasmosis

Helminth worms

Ascaris lumbricoides Ascarids

Toxocara sp. Ascarids

Trichuris sp. Whipworms

Necatur americanus Hookworm

Hymenolepis sp. Taeniasis (tapeworms)

Taenia sp. Taeniasis (tapeworms)

634

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

treated sludges, biosolids, was consequently chosen in conjunction with a national campaign

by a U.S. association representing wastewater operations (i.e., the Water Environment

Federation) during the 1990s to promote a more proactive image for a ﬁnished product

whose prior standing as raw sludge carried a poor reputation among the general public.

The efﬁcient and economical management of these wastewater residuals correspondingly

represents an important engineering task, one that usually relies on the alternative use of

several different biosolids treatment schemes. Granted, physical (e.g., wet air oxidation,

pasteurization, gamma irradiation) and chemical (e.g., lime stabilization, silicate-based

solidiﬁcation) methods are used to stabilize and/or disinfect these residuals, but the major-

ity of sludge proce ssing systems emp loy biological mechanisms whose goal is similarly

focused on degrading, stabilizing, and disinfecting the putrescent (easily degradable) and

pathogenic (i.e., bacteria, viruses, parasites, etc.) problems associated with raw sludges.

The putrescible fraction of wastes can produce both nuisance (e.g., odor generation)

and hazardous (e.g., hazardous gas generation, such as hydrogen sulﬁde) conditions,

and a major goal of sludge treatment is stabi lization with these putrescible materials.

Digestion is a biochemical process by which stabilization can be achieved, such that phy-

sical reductions will then be realized in both the original solids mass and volume. Both

anaerobic and aerobic digestion options are used with liquid-phase sludge processing sys-

tems, and aerobic digestion can also be completed in a more concentrated slur ry- or

solids-type mode using composting procedures.

One additional goal with any of these processes would be that of disinfection,by

which another reduction will be secured in the original pathogen content. Of course,

extending beyond any of these digestion schemes there is usually a ﬁnal processing

step, called dewatering, which is intended to reduce the water content of the biosolids

and produce a semisolid material for subsequent disposal with a total suspended solids

concentration above about 25%. The latter dewatering step is usually completed by phy-

sical processes such as ﬁltration or centrifugation.

Before addressing speciﬁc biological options for sludge processing, further clariﬁca-

tion is warranted as to the fact that tw o versions of raw residuals are produced at most

primary and secondary wastewater plants. In either case, these raw residuals are then

referred to as sludge, while their biochemical transformation and treatment subsequently

generates a ﬁnished biosolids product. The primary sludge fraction includes heavy, parti-

culate organic solids as well as various combinations of grit and inorganic ﬁnes extracted

from the raw wastewater during its primary sedimentation. On the other hand, a large frac-

tion of secondary sludge residuals is simply that of bacterial cells that grew within, and

were then wasted from, the secondary biological treatment operations. Table 16.10

TABLE 16.10 Raw Primary and Secondary Sludge

Characteristics

Organic Grouping Primary % Secondary %

Protein content 20–30

32–41

Grease and fat content 6–35

Low

Cellulose content 8–15

Low

Volatile solids 60–80

59–88

By weight of dry total sludge solids.

Source: Adapted from Metcalf and Eddy (2003); Crites and

Tchobanoglous (1998).

SLUDGE TREATMENT

635

provides a general characterization of primary and secondary sludge forms. Before

using any one of the various biological digestion and stabilization schemes, these primary

and secondary streams will typically be blended and concentrated to achieve a desired

starting point with a total solids content of 1.5 to 6% solids (by weight). In the follow-

ing sections we provide additional details regarding strategies for converting sludge into

biosolids.

16.2.1 Anaerobic Digestion

Although each of the alternative sludge digestion and composting processes have its own

degree of metabolic complexity and operational peculiarities, anaerobic systems are

widely regarded as the most difﬁcult to maintain given the interlinked sensitivity of

their bioc hemical pathways. Furthermore, where the aerobic options (i.e., aerobic diges-

tion and composting) involve only an oxidative breakdown of the sludge solids and a sin-

gle separation (liquid–solid) step, the anaerobic digestion process entails a sequential

series of complex metabolic pathways and concluding separations of gas, liquid, and

solid products. As a result, this digestion strategy has historically tended to require more

attention and care while being more prone to upset. Conversely, though, there are a

number of potential beneﬁts to be gained with this technology. Most notably, the anaero-

bic digestion process tends to have a far lower energy demand (since it does not require

aeration); at the same time, it can be used to generate a useful, energy-rich by-product in

the form of methane gas that is created reductively by these metabolic reactions.

The underlying biochemical mechanism with anaerobic digestion is that of fermenta-

tion rather than the respiration reactions maintained in most aerobic wastewater treatment

systems, as discussed previously. In either case, these options again involve a balanced set

of oxidative and reductive (i.e., redox) reactions that are linked to electron donor and

acceptor compound conversions, respectively. With fermentation reactions, the ele ctron

donor and acceptor compounds are typically both organic in form, although there are

also critical pathways in which inorganic hydrogen and carbon dioxide gas species can

ﬁll the associated roles (Zehnder, 1988). Yet another unique feature of fermentation is

that of a metabolic ﬂexibility which allows a single organic compound to ﬁll both

roles, not only accepting electrons but also serving as the electron donor. For example,

during the fermentative anaerobic transformation of acetate, this single substrate can

simultaneously be oxidized to CO

(CH

COO



þ 2H

O ! 2CO

þ 4e

þ 7H

Þ and

reduced to methane, CH

(CH

COO



þ 4e

þ 9H

! 2CH

þ 2H

OÞ.

Figure 16.40 provides an overview of the three progressive metabolic steps that take

place during the complete anaerobic digestion process: hydrolysis, acidogenesis, and

methanogenesis. During the ﬁrst, hydrolysis step, macromolecular organic compounds

entering the system are transformed hydrolytically from large, and in many instanc es

solid-phase macromolecular materials (e.g., cellulose, grease, protein, microbial cells)

into their smaller, soluble building blocks (e.g., amino acids released from protein, carbo-

hydrates from polysaccharides, fatty acids from lipids and fats). During subsequent fer-

mentation maintained by the anaerobic digestion process, one fraction of these hydrolyzed

organics will eventually be oxidized to carbon dioxide, another fraction will be reduc-

tively converted to methane, and a third, comparatively smaller fraction will be assimi-

lated anabolically into a new anaerobic cell mass.

The two fermentation reactions following hydrolysis sequentially are acidogenesis

and methanogenosis. Acidogenesis involves the formation of acid products by which

636 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

the previously hydrolyzed organics are transformed into three substrate precursors (i.e.,

, and acetate) in preparation for the concluding methanogenes is transformation.

Two different types of acidogenesis reactions are involved: one- and two-step conversions.

The ﬁrst such mechanism produces acetate primarily (i.e., by means of an acetogenic con-

version, encompassing reactions that produce organic acids directly), whereas the second

fermentative conversion involves intermediate production of volatile fatty acids (e.g.,

butyric and proprionic aci d) and alcohols, which are converted subsequently to acetate,

hydrogen, and carbon dioxide. The latter conversions are mediated by a wide range of

gram-positive and gram-negative bacteria, whose classiﬁcations depend on the primary

form of their fermentative products (Table 16.11).

Methane (CH

)

Carbon Dioxide (CO

)

Methane (CH

)

Carbon Dioxide (CO

)

Monosaccharides Purines & PyrimidinesFatty acids Amino Acids & Peptides

Simple Organics

Proteins

Lipids

Polysaccharides

Nucleic acids

Complex Organics

BIOSOLIDS

Short-Chain Fatty Acids

Proprionate Butyrate

Phase I - Hydrolysis

Phase III

Methanogenesis

Phase II - Acidogenesis

Acetate

(CH

COO

–

)

Hydrogen (H

)

Carbon Dioxide (CO

)

Figure 16.40 Anaerobic biosolids digestion metabolic phases.

TABLE 16.11 Fermentative Bacterial Types and Products

Anaerobic Reaction Products

Fermentative

Bacterial Groups

Representative

Bacterial Forms

Proprionate

Butyrate

Acetate

Formate

Alchohols

Proprionic acid

bacteria

Proprionibacterium

Chlostridium propionicum

 

Butyric acid bacteria Chlostridium butyricum   

Mixed acid bacteria Enteric gram-negative bacteria  

SLUDGE TREATMENT 637