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

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of the consequent biological response. For example, it is quite rare for 100% of the

exposed cells to actually be killed following high-level UV exposure, and we now

know that certain microbes have an inherent degree of disinfection insensitivity or resis-

tance. One such microorganism that exhibits a deﬁnite tolerance for this ionizing impact,

Micrococcus radiodurans, was named to reﬂect this remarkable resistance to UV irradia-

tion, thereby offering yet another demonstration of the facile nature of bacterial cells.

Furthermore, and although presently not well understand, there is evidence that some

microbes are able to rehabilitate themselves following disinfection events. One such

occurrence involves a mechanism known as dark-ﬁeld repair, through which cells pre-

viously exposed to UV light are able to autorepair DNA sites affected by thymine

dimer damage.

Balanced against the h ealth beneﬁts derived by the routine use of disinfectant chemi-

cals and measures, signiﬁcant biological concerns have also been raised regarding the for-

mation and consequent human health-related impacts of potentially carcinogenic

disinfection by-products (DBPs) during disinfection of water and wastewater. DBP com-

pounds can be formed from halogen-based disinfection activities. For example, chlorina-

tion in the presence of natural organic compounds can lead to the formation of

trihalomethane (THM) compounds, including chloroform and bromoform. In such

cases, the desired disinfection beneﬁts of using halogens are offset by problems among

long-term consumers tied to cance r and reproductive effects. After recognizing this

risk, the U.S. EPA identiﬁed a maximum contaminant level (MCL) for total THMs within

community water systems serving at least 10,000 people that added a disinfectant to the

drinking water during any part of the treatment process (whether it be for disinfection,

ﬁlter cleaning, prechlorination, etc.). Speciﬁcally, the U.S. EPA has stipulated the Stage 1

Disinfectants and Disinfection Byproducts Rule (in com pliance with the Safe Drinking

Water Act Amendments) (U.S. EPA, 1998), which limits total THMs to an annual average

value below 0.08 mg/L and total haloacetic acids to values below 0.06 mg/L, as well as

setting additional limits on chlorite (1 mg/L) and bromite (0.01 mg/L) designed to further

reduce carcinogenic risks from halogen-disinfected waters. The inherent irony is that

ozone has been promoted partly in the hope of avoiding the DBPs from chlorination.

Another irony is that some systems have switched to chloramination, which has a

lower tendency to form DBPs. However, this has led to another health effect by increasing

corrosion in plumbing systems, resulting in elevated lead concentrations in tap water.

16.5 SOLID WASTE TREATMENT

Biodegradation of dead plan t, animal, and other residues has played an important role on

our planet’s surface since the inception of life eons ago, recycling valuable nutrients back

into our biosphere while negating or at least reducing the shear physical burden of this

never-ending natural debris stream. At least in theory, therefore, nature provides a highly

useful example of a means by which we might manage our own solid waste residuals,

taking full advantage of much the same aerobic and anaerobic degradative mechanisms.

The level of success presently realized in using biology effectively as a management

process for human solid waste residua ls has, however, considerable room for improve-

ment. At the low end of this spectrum of biochemically engineered solid waste manage-

ment strategies, the majority of municipal detritus generated by the world’s current leader

in per-capita solid waste production (i.e., the United States) follows a least-cost disposal

668 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

pathway with little, if any, thought given to promoting, let alone maximizing, its biologi-

cal degradation, resulting in the consequent loss of potential resources, including nutri-

ents, organic material, and energy. Indeed, expedient burial in subsurface landﬁlls

(Figure 16.56) has in this case become the U.S. norm, with emphasis given to prior source

reduction and recycling of various fractions of the origina l waste as a means of reducing

the shear magnitude of the problem.

However, there are areas in the world that exhibit clear and escalating evidence of a

national shift toward biochemically engineered solid waste processing systems involving

controlled biological strategies. Europe and Asia represent two regions where escalating

land constraints that limit landﬁlling options are matched both by a national motivation to

pursue environmentally friendly disposal strategies and the ﬁnancial wherewithal and

willingness to accept technical solutions considerably more expensive than mere burial.

Commensurate with the cont inuing escalation of our world’s population and its atten-

dant increase in human solid waste production, future generations will no doubt be

aggressively challenged to pursu e a better universal means of managing their solid

waste problems. Although biology may, at best, be presently considered a minor, hidden

aspect of solid waste processing, at least for much of the world, the opportunity at hand to

employ renewable metabolic strategies beneﬁcially will proba bly continue to attract

increased attention (Palmisano and Barlaz, 1996).

One of the fundamental issues with solid waste residues is that of characterizing its

nature and source, principally in terms of how degradable or recalcitrant it might be.

The term solid waste truly covers a wide range of high-volume residues, including not

only municipal waste s of the sort that might be generated in your own home to industrial

(e.g., food-processing residues, spent foundry sands, slag), agricultural (e.g., manure, bed-

ding, plant residues), power (e.g., ﬂy ash, bottom ash), and even mining (e.g., overburden)

wastes. Only two of these fractions, the municipal and agricultural groups, include putres-

cible materials, and even in the latter case, a large segment of these wastes (e.g., manures)

are already being biochemically recycled for the beneﬁcial purpose of rejuvenating farm-

land productivity.

The municipal solid waste (MSW) segment itself is composed of many different frac-

tions whose putrescibility varies widely. At one end of this spectrum, food scraps have the

Figure 16.56 Solid waste landﬁll operation.

SOLID WASTE TREATMENT 669

highest level of potential degradability, due to both their organic makeup and their high

water content. Cellulose-rich residues, whether discarded as lawn clippings or paper pro-

ducts, would also be amenable to biochemical degradation, although perhaps at a some-

what slower rate. Extending beyond these two segments, though, municipal solid waste

includes many other materials whose composition will not be ame nable, and possibly

even antagonistic or inhibitory, to biochemical degradation. For example, MSW generated

within afﬂuent countries includes a sizable proportion of plastic, for which the vast major-

ity will have no susceptibility to biochemical breakdown. Similarly, afﬂuent countries

generate municipal solid wastes with a proportionately higher percentage of metals

(e.g., cans, batteries, used appliances) whose presence may actually lead to the release

via leaching of soluble heavy (e.g., cadmium, chromium, nickel, lead) and transition

(e.g., arsenic, selenium) metal ions that are detrimental to desired biochemical activity.

Yet another key issue is that of available moisture, sinc e there must be adequate water

present to facilitate and sustain the growth of biological populations. Contrasted against

this necessity, though, modern landﬁlls are provided with overlying physical caps (i.e.,

built with impervious clay and/or plastic liners) that intentionally limit the inﬂux of

water into their buried wastes, such that limited moisture presence within the subsurface

waste matrix could well become a constraining factor. Indeed, solid waste materials exca-

vated from decades-old landﬁlls have in many instanc es proven to contain a remarkable

amount of undegraded putrescible residuals (e.g., corncobs, newspapers) whose lingering

structural integrity reﬂects what appears to be a desiccating condition within the waste vs.

a moisture-bearing environment conducive to microbial degradation.

Extending beyond water presence, a number of additional ambient environmental con-

ditions come into play. Nutrient availability will certainly be an issue, not only in regard

to the macroscale distribution of carbon and nitrogen (i.e., for which a C/N ratio of

20 : 1 is typically considered optimal), but also in terms of the available presence of

lower-level essential elements (e.g., phosphorus, sulfur, potassium, iron, calcium).

While the pH of the constitutive moisture must also be suitably conducive to microbial

activity, fermentative metabolism will progressively release weak organic acids that could

well shift the pH to a more acidic, and less optimal, state. In fact, a downward shift in pH

of this sort might accordingly escalate the undesired rate of trace metals leaching from the

waste matrix, thereby leading to inhibitory metal toxicity.

Finally, the relative combination of high solids and low moisture found in solid waste

streams effectively yields a high-level speciﬁc energy content (i.e., cal/kg) that is higher

than that of most other biodegraded waste (e.g., greater than wastewater and sludge), with

the sole exception of agricultural manures. Commensurate with effective biodegradation

of these wastes, therefore, the heat release per mass of degraded solids would not only be

considerably higher, but also apt to be trapped inside the high-density waste, due to its

insulating nature. On the one hand, this heat release and temperature increase could

help to accelerate the ongoing biochemical process. However, as is the case with compost-

ing, this thermal buildup could accelerate evaporative water loss to a degree that even-

tually retards the desired activity.

There are several engineering and operational aspects of the current design and man-

agement of MSW landﬁlls that are frankly contrary to a goal of optimizing their biological

degradation. For example, restricting water egress into and out of this buried waste is rou-

tinely considered a beneﬁcial environmental goal for landﬁll operations, even though this

effort could well end up slowing down biochemical degradation. By reducing the hydrau-

lic pressure gradient that might accrue with an interior water buildup, the transport of

670 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

waterborne contaminants into the adjacent groundwater table can subsequently be

reduced or obviated. At no point during this process, either during active ﬁlling of the

landﬁll or after ﬁnal closure, will the moisture content of the material ever come anywhere

close to an optimal value relative to maximal microbial activity. Unlike composting,

therefore, where moisture control is addressed routinely, landﬁlls have no management

plans that involve steps to proactively increase moisture content. Fu rthermore, efforts

taken to heavily compress the waste to minimize its bulk volume, or of using a daily

cover of 15 cm soil to restrict rodent and other vector access, work against desired

microbial degradation, as these measures impose physical constraints that degrade process

homogeneity. In fact, the mere circumstance of allowing homeowners to isolate much of

their daily MSW within plastic bags contributes yet another complication, further exacer-

bating the waste’s isolation into discrete, semi-isolated packets.

Given these inherent shortcomings with landﬁlls in the context of biological MSW

degradation, several alternative engineering strategies have been developed to secure

higher levels of success. These systems are typically charged with a presorted waste

stream whose degradable content has been enhanced by way of separating out a large

fraction of the relatively nondegradable content (e.g., plastics, glass, metal). The engineer-

ing features of these systems subsequently vary in relation to operational solids content.

In-vessel digester reactors are maintained with the lowest solids content and the highest

water contents, including both low-solids units at 4 to 8% solids and high-solids units with

solids values in the low- to mid-20% range. MSW is shredded and slurried before intro-

duction to these reac tors. These digeste rs include both aerobic and anaerobic options,

much like sludge digestion vessels.

A second MSW processing option, composting systems, is again quite similar to the

equivalent strategies used for sludge treatment, with waste solids levels approximately

double that of high-solids MSW digesters (i.e., 40 to 50%). Both aerobic and anaerobic

composting strategies are available, maintained in static and windrowed piles held either

in open or in-vessel conﬁgurations. Yet another option is that of codisposal systems, treat-

ing a blended mixture of MSW and sludge. As compared to MSW digestion options,

though, and their relative focus on volatile solids destruction, MSW composting reactors

provide an overall volume reduction tied both to volatiles destruction and moisture loss

allowed to accelerate at the end of the processing cycle during a ﬁnal curing phase.

Volume reductions on the order of 50 % have been observed with both aerobic and anae-

robic composting systems at processing times on the order to 3 to 4 weeks.

Finally, it is worth mentioning a process in which animals are the basic organism.

Vermic ulture (raising of earthworms) or vermicomposting uses earthworms to help

decompose organic material, especially food- processing wastes (Datar et al., 1997).

Vermic omposting produces stabilized waste similar to ordinary composting, although

the process must be maintained at lower temperatures. The process has been applied

mostly at small scale, including household use, although large-scale facilities exist that

process up to 20,000 metric tons of organic was te per year.

16.6 AIR TREATMENT

Several abiotic engineering methods are commonly used to treat contaminated air-

streams, including those of incineration, carbon absorption, condensation, and scrubbing

(i.e., either with water, chemicals, or combinations thereof). However, within the past

few decades the idea of using bioﬁltration columns has proven to be a highly attractive,

AIR TREATMENT 671

complementary treatment strategy for air quality remediation. Bioﬁltration consist s of a

closed vessel containing a porous support medium with an attached bioﬁlm through which

a contaminated gas is passe d. The attached bioﬁlm will ﬁrst sorb, and then degrade, the

incoming contaminants. This technology ﬁrst gained signiﬁcant attention in Europe dur-

ing the late 1970s, and there are now a number of relatively recent applications in the

United States. However, as with constructed wetlands and phytoremediation systems,

the involved technology is still in the process of being clariﬁed and optimized.

Bioﬁltration technology is, in fact, rather similar to that of the attac hed-growth media-

ﬁlled bioﬁlters (e.g., trickling ﬁlters) described earlier, although in this case the stream to

be treated is that of contaminated air rather than wastewater. Similarly, the mode of con-

struction and operation used for bioﬁltration columns is also comparable to that of bioso-

lids composting using aerated piles, although with a goal of treating the incoming gas

rather than a solid-phase (sludge) material blended into the pile.

In some cases, the main objective of gas-phase bioﬁltration processing may simply be

that of remediating a malodorous gas stream by removing the chemicals responsible (e.g.,

hydrogen sulﬁde, mercaptans, free ammonia). Figure 16.57 depicts one such full-scale

application used to effectively cle anse these types of cont aminants from the off-gas

discharge generated by an aerobic thermophilic biosolids digester. Several comparable

wastewater-related applications exist for handling contam inated, odor-bearing off-gas

streams, including those released by attached-growth wastewater treatment towers,

activated-sludge reactors, biosolids digesters, and even sewer force mains.

However, bioﬁltration may also be used to biochemically degrade one or more regu-

lated volatile organic compound air contaminants such as thos e generated within a speci-

ﬁc industrial airstream. High-level removals have, in fact, been obser ved with a number of

industrial organic contaminants, including phenol, styrene, formaldehyde, BTEX, and

methanol as well as various aldehydes and ketones. Many of these bioﬁltration applica-

tions have been tested within industrial applications in which the current abiotic off-gas

treatment procedures are too mechanically complex, too expensive, or simply too

inefﬁcient.

Figure 16.58 depicts a typical bioﬁltration system. The various design strategies for

bioﬁltration systems cover a wide range of options, some of which depend solely on

Figure 16.57 Air bioﬁlter system.

672

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

the bioﬁlter for contaminant removal (e.g., typically involving low-level contaminant con-

centrations), while in other instances the process train will include a pretreatment abiotic

scrubber. Abiotic scrubbers are, in fact, often included as an integral part of many sys-

tems, helping to moderate inﬂuent gas temperatures and ensuring full humidiﬁcation as

well as securing preliminary sorption and partial removal of highly soluble contaminants

such as ammonia. Four options are typically seen with these scrubbers: (1) water-only

scrubbing, to humidify and cool an incoming gas stream as well as to initiate desired

gas-to-liquid sorption; (2) acid spray scrubbing, to adjust pH in a downward fashion

that shifts gaseous ammonia to ionic, liquid-phase NH

; (3) caustic spray scrubbing, to

adjust pH upward both to shift gaseous hydrogen sulﬁde to ionic, liquid-phase HS



and to

buffer both the scrubber water and gas-phase moisture against the acidity of incoming

gaseous CO

; and (4) oxidant (i.e., typically bleach) spray scrubbing, to oxidatively attack

any chemically amenable contaminants.

Once this prescrubbed gas stream reaches the bioﬁltration zone, the targeted contami-

nants must be readily transferable from the gas phase to the water phase within which the

biomass is growing. The success of this sorptive phenomenon depends on several factors,

including the Henry’s constant (K

) of the involved contaminant species, ambient tem-

perature, media-ﬁlm pH, and the media-bioﬁlm surface area available for interfacial

gas–liquid exchange. Bioﬁltration-based degradation of contaminants with high K

values

may therefore be difﬁcult, since it is likely that these species will remain in the gas phase

rather than being sorbed and degraded.

Media temperatures between 20 and 40



C appea r to work best, avoiding low or high

extremes that would retard the desired metabolic activity of the bioﬁlm or escalate water

evaporation to an unacceptably high level. For cold incoming gas streams, steam might be

used to raise a reactor’s operating temperature, and hot incoming gases can be cooled by

means of the initial water scrubbing.

In terms of ﬂow schemes and design retention times, both up- and down-ﬂow regimes

have been used, although the up-ﬂow option is probably the most common. The contact

time for gas throughput also represents another key factor, with most units sized to

Figure 16.58 Air bioﬁlter system.

AIR TREATMENT 673

provide 45 seconds. Pile moisture levels in the neighborhood of 50% are considered

optimal, and at the same time, the relative humidity in the gas stream will need to be

above 95þ% to avoid excessive levels of water evaporation from the pile. Many bioﬁl-

tration vessels are ﬁtted with internal humidifying spray misters as a means of ensuring

desired wetting and moisturization of the bioﬁlm, and if necessary, supplemental nutrient

or buffering chemicals may be blended into these wetting streams to enhance biological

activity.

Extending beyond these heuristic details, though, there are at present relat ively few

speciﬁc criteria for designing or operating bioﬁlter units, such that their design and con-

struction represents, more a heuristic art than a codiﬁed engineering science. Indeed, the

state of the art for this technology is still being actively reﬁned, covering a wide range of

design and operational factors (e.g., optimal bed depths and conﬁgurations relative to

minimizing undesired short circuiting, airﬂow per unit volume of media, throughput velo-

cities, necessary rates of water addition and scrubbing, nutrient and buffer requirements,

bed media longevity and replacement practices, tolerance levels with loading variations,

microbial seeding and startup requirements).

Undoubtedly, though, one of the most important variables with any bioﬁltration system

is that of its media form and composition. In general, three different type s of media are

used in bioﬁltration reactors: (1) synthetic random-packed plastic media, (2) inorganic

rock-type media (e.g., lava rock, limestone, clay, shale), and (3) organic plant- or tree-

derived media (e.g., shredded or chipped roots and stems, as well as bark). The issue

of media packing itself can impose a considerable resistance to the passage of the gas

stream, such that it will have to be effectively p ushed through the bed using a mecha-

nical air pump or rotary blower. In many instances, though, coblended columns (see

Figure 16.59) have been constructed successfully with various blends of organic and inor-

ganic materials (e.g., shredded roots, stems, and bark plus limestone and clay admixtures),

with the overall goal of providing an enhanced surface area conducive to bioﬁlm growth,

as well as a higher moisture retention capability and enhanced buffering capacity. In addi-

tion, these blended natural ingredients may contribute secondary beneﬁts that further

enhance metabolic efﬁciency. For example, the concomitant breakdown of plant- and

tree-based suppor t materials may beneﬁcially serve as secondary organic substrates that

promote complementary micro bial (e.g., fungal) growth as well as potentially supplying a

fraction of the metabo lic nutrients required. Hardwood materials passed through shred-

ders and grinders probably appear to represent one of the more desirable natural options,

given their perceived durability and resistance to rotting associated with prolonged

wetting while favoring bioﬁlm attachment and retention. In addition, inorganic media

additives, such as limestone and clay, may also provide a beneﬁcial buffering capacity,

which in the case of ammonia or sulﬁde hydrolysis may comprise a particularly import-

ant beneﬁt.

With time, though, the historical pattern observed with all of these natural media is that

they tend to degrade in terms of media depth and overall performance, at which point they

would have to be supplemented with fre sh media to rejuvenate the bed. During large-scale

bed replacement with new media, partial blending with some of the older media would

help to secure beneﬁcial recycling of microbial seed. Here again, the science behind

media replacement is still in a research stage, but 5 years is generally believed to be a

reasonable time frame.

Yet another aspect that has not yet been fully explored is that of the presence and role of

microscale metabolic niches that develop inside the biomass contained within bioﬁlters. It

674 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

is highly likely that metabolically complementary anaerobic–aerobic mechanisms will

develop in these sites, whereby special biodegradation pathways could be developed to

attack normally recalcitrant chemicals (e.g., reductive dehalogenation of TCE) , fol lowed

by aerobic degradation of the partially dehalogenated products).

16.7 SOIL AND GROUNDWATER TREATMENT

16.7.1 Phytoremediation

Phytoremediation is the use of plants to remove and/or biotransform contaminants. The

process of phytoremediation is comparable to that of constructed wetlands. Both applica-

tions make use of macro- and microscale biology, and both concepts have captured con-

siderable attention, even though they each still qualify as evolving technologies given

their relatively short histories. However, phytoremediation also has several important dis-

tinctions (Schnoor et al., 1995). First, the focus of phytoremediation is that of biologically

remediating contaminated soils, sedime nts, and waters (both surface and ground water) as

opposed to wetland applications focused solely on wastewater treat ment. Second, phytor-

emediation systems may employ trees as well as smaller plants as the pri mary biological

agent. Indeed, for those applications in which they are suitable, phytoremediation systems

may offer a highly attractive ‘‘green’’ means of decontaminating lands and waters.

Figure 16.59 Natural shredded root and stem mass used for bioﬁlter support media.

SOIL AND GROUNDWATER TREATMENT 675

Successful applications have been achieved experimentally with a wide range of inorganic

(e.g., metals, ammonia) and organic [e.g., petroleum hydrocarbon, BTEX, creosote wood

preservative, polycyclic aromatic hydrocarbon (PAH), reﬁnery waste, organophosphate

insecticide, chlorinated pesticide, chlorinated solvent, explosive, cyanate] contaminants

(Jackson, 1997).

The potential beneﬁts are again much the same as those of constructed wetlands,

including that of an apparently simple, solar-driven, aesthetically pleasing, in situ

‘‘green’’ technology with few, if any, complex or energy-intensive hardware or opera-

tional requirements (i.e., as compared to conventional treatment operations that employ

pumps, mixers, aerators, etc. that routinely use energy and require careful operator atten-

tion). These systems can also be self-sustaining in terms of procuring nutrients, they can

make a beneﬁcial contribution to the balance of water in their soils, they can establish a

highly evolved complement of degradative enzymes, and they tend to be inexpensive both

in their initial startup and in subsequent maintenance. Granted, this remediation approach

will not work in all situations, and in even when it is successful, the remediation process

will operate on a time-scale measured in years rather than in hours or days. The public

perception of phytoremediation is extremely high, though, as a natural means of promot-

ing the restoration of chemically contaminated sites.

However, the seemingly simplistic notion of using plants and trees to clean up these

contaminated sites actually involves a far more sophist icated process than what is appar-

ent to the eye. As was the case with constructed wetlands, the visibly ‘‘green’’ above

ground portions of these systems are but a part, and in some cases perhaps even a lesser

part, of an integrated remediation scheme that encom passes a complex array of physical,

chemical, and certainly biological treatment factors. The type, density, and nurt uring of

the plants and trees is important as well as the nature of the soils (e.g., soil type, conduc-

tivity, depth to groundwater, nutrient availability) and climate (e.g., rainfall frequency and

duration, radiation, seasonal climate, windspeed, humidity) in which they are grown.

Finally, the character, concentration, location, and form (e.g., whether it is sorbed, solu-

ble, solid) of the contaminating materials are also important factors.

Before delving into the underlying sophistication of these phytoremediation systems,

though, one must develop a background understanding and appreciation of the vertical

layering of the soils in which these bioremediating plants and trees grow, and their cor-

responding physical and chemical characteristics. Figure 15.2 provides a schematic over-

view of a representative soil–plant system and its associated horizontal layers. There are

two major regions shown in this schematic, situated above and below the groundwater

table, respectively, and known as the unsaturated and saturated zones. Within the unsa-

turated zone the soil water volume does not entirely ﬁll the pore space in the soil. The

unsaturated zone is also called the vadose zone. The remaining (partially or fully) open

void space, however, will also facilitate better levels of aeration and oxygen transfer,

which will then promote more aerobic microbia l activity. The saturated zone is where

the pores are completely ﬁlled with water. The groundwater table is the point in the satu-

rated zone where the hydraulic head is equal to zero. Water is drawn by capillary action

into the capillary fringe slightly above the groundwater table. The top of the capillary

fringe marks the division between the saturated and unsaturated zones.

The level of the groundwater table may vary considerably from one location to another,

and also according to temporal changes in precipitation and climate, but in most instances

it lies many meters below the surface and at a level not usually reached by plant root

systems. As a result, phytoremediation was developed with systems whose remediating

676 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

activity took place almost totally within the uppermost layer of the unsaturated zone (i.e.,

at shallow depths). However, subsequent developments with the nurturing and use of

deep-rooting plants and trees have now expanded this technology down to the saturated

region, at which point phytoremediation could then deal with contaminants extending

fully down to the groundwater table.

With most soil columns there will also be considerable variation in the horizontal and

vertical homogeneity of the top and bottom layers. The top, unsaturated zone may be

further divided into another series of layers vertically from top to bottom:

 O horizon: consisting mostly of leaf litter and othe r dead organic matter

 A horizon: the topsoil, with a high content of mostly degraded organic matt er, well

populated by microorganisms and invertebrates, often coincident with the root zone

 B horizon: less weathered mineral matter plus organic and inorganic matter leached

from the A horizon

 C horizon: partially weathered material from the bedrock below, very low in organic

matter and living organisms

Once the remediating plants and/or trees have been introduced successfully into a

site, the means by which they can attempt to degrade or remove a group of involved conta-

minants can be quite diverse. An important, yet all too easily overlooked aspect of phyto-

remediation is that the plants are generally not the sole means of contaminant treatment.

Granted, the plants themselves may contribute many different and important remediating

effects (e.g., phytovolatilization), as discussed below, but in most instances their remedia-

tion role is metabolically complemented, perhaps even dominated by that of the microbes

that are motivated correspondingly to live within the same soils. Figure 16.60 provides an

overall synopsis of these prospective plant and microbial mechanisms, and in the follow-

ing synopsis we examine each of these metabolic contributions relative to the overall

process of phytoremediation.

Water release

evapotranspiration

Contaminant release via

phytovolatilization

rhizofiltration

Contaminant degradation

viaphytodegradation

Contaminant accumulation

via phytoaccumulation

Phytouptake

andphytotranslocation

Root Zone

Metabolism

- Microbial stimulation

- Organic enrichment

- Microbial degradation

Rhizosphere root system

Water release via

evapotranspiration

Contaminant release via

phytovolatilization

Contaminant degradation

via phytodegradation

Contaminant accumulation

via phytoaccumulation

Phyto uptake

and phytotranslocation

Root Zone

Metabolism

- Organic enrichment via

Rhizosphere root system

Contaminant filtration via

rhizoenrichment

Figure 16.60 Phytoremediation mechanisms.

SOIL AND GROUNDWATER TREATMENT 677