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

Подождите немного. Документ загружается.

uniform solids breakdown. Although some heat is lost during turning, and some oxygen

is incorporated in the pile, the increase in microbial activity spurred by the mixing quickly

(within hours) reheats the material and depletes this added oxygen. The height and width

of these windrows must therefore be held to values of less than a few meters, such that

oxygen may adequately diffuse into the pile interior from the surface. In fact, the ongoing

release of heat from windrow piles helps to facilitate this aeration process, whereby the

physical air current induced by heat rising upward and out of these hot piles essentially

drafts or pulls in cooler air, and oxygen, at the pile base.

In lieu of intermittently mixed windrow systems, there are also units designed to pro-

vide nearly continuous mixing and/or aeration for com posting wastes. Forced static pile

aeration, as shown in Figure 16.51, represents yet another method developed by the Agri-

cultural Research Service (U.S. Department of Agriculture) at Beltsville, Maryland. An

aeration system (ducts, or a perforated false ﬂoor) in the lower portion of, or under, the

pile is used, and control systems can be provided to either increase aeration (if the pile

starts to get too hot) or decrease aeration (if the pile starts to get too cool), as needed. In

some cases (including that of the piles seen in Figure 16.51) , this supplemental airﬂow is

drawn into and through the pile by applying a vacuum to the duct work, with the exhaust

air then being routed through a secondary smaller pile of aged compost material in order

to screen out potentially problematic gas-phase odors, fungal spores, and so on. However,

forced blower aeration directly into the pile interior has also proven to be effective, par-

ticularly in terms of securing direct oxygen entry to the pile’s hottest and most active

zone. These types of simple, feedback aeration schemes, based on a thermostatically regu-

lated vacuum or blower, have a number of interesting features. First, the pile itself

demands the amount of aeration it needs to remove enough heat to keep temperatures

in the desirable range. This is important because the rate of activity (heat generation)

changes with time as the readily degradable organics are depleted. Second, most of the

cooling results from evaporation of water within the pile into the supplied air, at which

point the pile starts to dry out. With some materials pile drying may be so extensive that

composting rates decrease and supplemental water will need to be added. Often, however,

the drying is beneﬁcial, as it substant ially reduces the remaining mass and volume and

leads to a more stable ﬁnal product that is more easily stored, transported, and ultimately

used with gardening, land application, and so on, measures.

A third feature stems from the fact that the amount of oxygen required for biodegrada-

tion of the organic material is closely related to the amount of heat released [as can be

seen from equation (16.5)]. Thus, an air function ratio can be deﬁned for forced aeration

systems as the amount of air required to remove the heat produced compared to the stoi-

chiometric amount required to provide the oxygen necessary for the oxidation reaction

that releases it. This ratio will vary slightly based on materials, ambient conditions, and

pile temperature, but is typically about 8.5 to 9.0. This ensures that sufﬁcient—in fact,

considerably excess—oxygen will automatically be provided by the aeration required

for cooling. It is interesting to note that the air function ratio must in fact be greater

than 1.0 for self-heating to occur. As a practical matter, some aeration, typically provided

by a timer, also is needed to supply oxygen before and after the phase of the process d ur-

ing which aeration for cooling is required: the come-up and cool-down stages.

The most advanced composting systems combine forc ed aeration for temperature con-

trol and oxygenation with mixing to increase rates and uniformity. Such systems include

agitated beds and ‘‘mushroom tunnels’’ (so-called because they were developed in

the mushroom industry to provide the high-quality compost needed for the commercial

658 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

growing of mushrooms). Tunnels are enclosed, with a small headspace above the material,

and a portion of the used air can be reused for aeration. This requires some cooling of the

air and condensation of the water vapor present but can greatly reduce the amount of fresh

air needed (since based on the air function ratio, little of the oxygen is utilized). This in

turn reduces the amount of spent gas that must be vented, making control of odors and

other volatile compounds simpler.

Example 16.7: Composting Air Function Ratio Sample Calculation Based on equation

(16.5) and the typical energy release of 14,000 J/g O

consumed, an air function ratio can

be calculated for speciﬁc composting conditions. Assume, for example, that the ambien t

air is at 20



C and 50% relative humidity (RH), and the air exiting the composting pile is

at 60



C and 100% RH. From thermodynamic data (found in psychrometric charts) it can

be determined that this represents a change in enthalpy (heat energy) of 362 J/g dry air

(part from the increase in temperature, and part from the increase in the amount of water

vapor). Thus, the amount of dry air required to remove the heat released from 1 g of O

consumption is

14;000 J=gO

362 J=g dry air

¼ 38:7 g dry air=gO

consumed

The amount of dry air required to provide 1 g of O

is only 4.31 g. Thus, the ratio is

air function ratio ¼

air required for cooling

air required for providing oxygen

38:7g=g

4:31 g=g

¼ 8:98

The air function ratio will vary slightly depending mainly on the temperature and RH

of the inlet and outlet air. However, it is usually between 8.5 and 9.0 for conditions that

are likely to be encountered during thermophilic composting.

A number of other composting system designs have also been of initial interest because

of their materials handling approaches, including semicontinuous feed systems such as

silos (in which materials were added to the top and ﬁnished material was removed

from the bottom) and rams (in which material was pushed along in a ‘‘tunnel’’). However,

these particular approaches greatly compacted the material, destroying the porosity that

was essential to movement of air through it. It is important to keep in mind that although

efﬁcient materials handling approaches are important for cost-effectiveness, the system

will fail if the basic biological requirements of the composting process are not sufﬁciently

met.

16.3 POTABLE WATER TREATMENT

There are relatively few engineered applications for biological treatment within potable

water systems. The rare exception to this is that of denitrifying processes intended to

reduce excessive nitrate levels (i.e., above the regulated level of 10 mg/L), as might be

used in association with shallow groundwaters affected by regional far ming and fertiliza-

tion activities. In contrast to beneﬁcial instances of biological treatment with potable

waters, though, biology can, and in many instances often does, become a troublesome

POTABLE WATER TREATMENT 659

issue either in terms of affecting raw water quality or during subse quent dissemination of

these waters via distribution piping.

Excessive biological growth of any sort, whether algal, bacterial, or other, can lead to

elevated levels of turbidity (i.e., cloudiness) that would complicate subsequent treatment

efforts. However, one of the largest biological problems in potable processing of surface

waters is that of the formation of various taste- and odor-inducing chemicals. Two such

compounds, geosmin and methylisoborneol (often referred to as MIB), are depicted in le

16.14, and it is these particular biochemical products that are most frequently cited as the

chemical culprits behind serious aesthetic complaints. These compounds are produced

biochemically by a variety of cyanobacteria (including Oscillatoria and Anabaena), acti-

nomycetes (including Streptomyces, Micromonospora, and Nocardia), and algae (e.g.,

Asterionella), leading to taste and odor complaints that have been likened to ‘‘musty’’

or ‘‘ﬁshy’’ (Darleym, 1982).

The ability of either compound to create problems is readily demonstrated by their

respective aqueous-phase ‘‘threshold odor’’ values, which quantiﬁes the concentration

value at which humans may perceive their presence. As shown in Figure 16.52, geosmin

and methyl isoborneol have remarkably low ‘‘threshold’’ values, in the single-digit parts

per trillion range. This level is nearly 1000 times lower than that of the next-highest com-

pound (i.e., methyl sulﬁde, otherwise known as methyl mercaptan, which as described

earlier, has a considerable odor of its own!).

The typical circumstance of these two products creating a problem is that of seasonal

blooms of cyanobacteria and algae in reservoirs, lakes, and rivers, tied to changes in water

temperature and nutrient availability. One such commonplace trigger is that of diurnal fall

and spring overturns in deeper reservoirs and lakes, at which point the underlying phos-

phate-rich hypolimnion waters are returned to the surface to stimulate phototrophic

microbial growth. Even without the bacterial release of geosmin or MIB, algal activity

stimulated in much the same fashion may also result in the release of various phenolic-

type organics, which can then react with chlorine during the treatment process to produce

the strong taste- and odor-causing chlorinated phenols.

A second biological problem in potable water systems that may occur is that of bac-

terial colonization along the interior surfaces of distribution piping (Bitton and Gerba,

TABLE 16.14 Taste- and Odor-Producing Chemicals Generated by Bacteria and Algae

Chemical Name Chemical Structure Generating Microorganisms

Methylisoborneol (MIB)

Actinomycetes

Oscillatoria

Geosmin

Actinomycetes

Anabaena

Oscillatoria

660

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

1984). These microbes, such as Aeromonas, use low-level substrates in the form of bio-

degradable and assimilable organic carbon that in many instances are produced by the

oxidative conversion of otherwise recalcitrant materials (e.g., naturally occurring humic

acids) by chlorine. More often than not, these growths do not reach population densities

that would unduly affect the healthful quality of the water. Their presence can nonetheless

lead to potential corrosion problems, particularly with cast iron piping. Isolated patches of

bacterial bioﬁlm can accordingly create microsite niches whose isolated underlying strata

may effect a reducing environment that, in time, corrosively degrades the pipe wall.

Neither geosmin nor MIB can be removed easily during the treatme nt of surface

waters, at least using the conventional technology (e.g., coagulation, ﬁltration, disinfec-

tion) employed at most municipal operations. As a result, the only approach to dealing

with this problem is that of attempting to eliminate the microbial source of the problem,

either by switching water sources or by working to eliminate the biological agent respon-

sible. Reservoirs are often treated with copper salts, which inhibit algae growth at levels

of 0.5 to 1.0 mg Cu/L.

The key to preventing growth of the organisms in the ﬁrst place is restricting or elim-

inating the presence of nutrients, such as phosphate, which contribute to the offending

phototrophic bloom. Within many reservoir and lake settings, these nutrients are intro-

duced through a variety of nonpoint sources that are not particularly easy to control or

eliminate, including fertilizer use on lawns bordering homes and cottages, septic tank lea-

chates, or agricultural inputs received via upstream drainage . However, should the domi-

nant phosphorus source be that of diurnal overturn and exchange, this mechanism may be

0.001 0.01 0.1 1 10 100 1000 10000 10000

Ammonia

Skatole

Hydrogen sulfide

Methyl mercaptan

Propyl mercaptan

Trimethylamine

Amyl mercaptan

Ethyl mercaptan

Phenyl mercaptan

Benzyl mercaptan

Allyl mercaptan

Diphenyl sulfide

Indole

Thiocresol

Tert-butyl mercaptan

Crotyl mercaptan

Methylisoborneol (MIB)

Geosmin

Threshold Odor (µg/L)

5 parts per trillion

Figure 16.52 Threshold odor levels for various aqueous-phase chemicals.

POTABLE WATER TREATMENT 661

controlled using low-level aeration of the hypolimnion region in a fashion that effectively

ties up the available phosphate ions through precipitation with iron or calcium. This

strategy can be relatively simple to employ, using ﬂexible plastic piping, laid across the

deepest reach of a lake, that is drilled with holes and supplied with compressed air such

that it releases a steady current of bubbles. Another approach is somewhat more compli-

cated, using skirted mechanical mixers or turbines that create hydraulic drafts to routinely

mix the upper and lower reaches of the lake, but in either case the end result is that of

inducing a degree of supplemental aeration that reduces or eliminates the presence of

soluble phosphates.

16.4 WATER AND WASTEWATER DISINFECTION TREATMENT

One of the most signiﬁcant public health advances over the past century was that of devel-

oping, and then routinely applying, suitable engineering methods for disinfecting potable

waters that could retard, and ideally obviate, the transmission of waterborne disease.

Rudimental disinfection measures based on water ﬁltration (used by the ancient

Egyptians) and heat treatment have long been practiced, but the advent of commercially

available chlorine during the late nineteenth and early twentieth centuries effectively

revolutionized the wide-scale utility and efﬁcacy of this practice (Baker, 1948).

Indeed, chlorination has subsequently served as the dominant disinf ectant ‘‘tool’’ for

potable waters since the early twentieth century, but there are now at least six strategies by

which waters, wastewaters, or sludges might be duly processed to achieve a desired reduc-

tion in their microbial content (Bryant et al., 1992; U.S. EPA, 1999):

1. Physical ﬁltration (e.g., using ultraﬁltration or reverse osmosis)

2. Heat treatme nt (e.g., boiling and pasteurization)

3. Physical sonication

4. Strong oxidant chemical treatment (e.g., using halogens such as chlorine, bromine

or iodine, or ozone)

5. Nonoxidizing chemical treatment

6. Radiation treatment

These disinfection strategies can largely be subdivided into the following four options,

although in many instances there are likely to be signiﬁcant overlaps with the causative

impacts imposed by any one disinf ection procedure:

1. Remove cells physically

2. Alter and disrupt cell membrane permeability

3. Alter and disrupt metabolically essential proteins and enzymes

4. Alter and disrupt genetic ally essential nucleic material

The ﬁrst such approach to using physical ﬁltration to remove microbes depends on cell

size, which is rather fortunate since chemical-based disinfection tends to become proble-

matic with large cells or cysts, such as might be experienced with protozoan forms of

Giardia lamblia and Cryptosporidium. Conventional slow- and rapid-sand ﬁlters have

been used for more than a century to effectively reduce, if not completely eliminates,

662 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

these larger microbes, and as a result these types of ﬁlters have been widely used for pro-

cessing surface water sources susceptible to microbial contamination. However, the suc-

cess of these operations can be compromised by human and technical shortcomings (e.g.,

inadequate ﬁlter backwashing regimes), as has been demonstrated in several U.S. cities

over the past few decades. One such well-publicized incident with mismanaged water ﬁl-

ter operations in Milwaukee, Wisconsin, during 1994 affected 400,000þ residen ts. The

effectiveness of these operations has been improved with tightened stipulations (i.e.,

the U.S. EPA Interim Enhanced Surface Water Treatment Rule) on routine ﬁlter monitor-

ing (U.S. EPA, 1998). In recent years, the disinfection effectiveness of media-based ﬁltra-

tion has been eclipsed by the use of micro- and ultraﬁltration systems with pore sizes in

the double-digit nanometer range (e.g., typically 30 to 100 nm), which are small enough

to prevent the passage of any pathogens.

As for those disinfection strat egies intended to negatively change the permea bility or

perhaps water content of cells, numerous examples can be seen with foods prepared in

percentile-level salt, sugar, or organic-acid-rich conditions (e.g., pickles, candied fruits,

cheeses, vinegar, tomato catsup). These preservation environments, many of which

yield osm otic pres sures intole rable to active cell growth, facilitate a bacteriostatic condi-

tion in which microbial growth has been effectively stopped without speciﬁcally killing

the original cells. Large-scale adjustments to the osmotic pressure within water, waste-

water, or sludge treatment processes would, of course, be infeasible given the necessary

chemical dosage requirement (i.e., at expensive, high percentage levels).

Chemical disinfection agents that alter the form and function of membrane-bound

transport enzymes could disrupt the transmembrane passage of essential substrates or

nutrients. Whether or not the latter membrane-speciﬁc impact is realized in conjunction

with the use of the more widely used antimicrobial chemicals (e.g., chlorine, ozone),

enzyme disruption and denaturation are widely considered to be their dominant disinfec-

tion mechanism. These agents, and particularly those involving either strong-oxidant (e.g.,

chlorine) or superoxide (generated by means of irradiation) chemicals, readily disrupt a

cell’s hydrogen- and covalently bonded three-dimensional enzyme conformation. Having

lost the enzyme’s catal ytic contribution to energy-yielding catabolism, these disinfected

cells subsequently lack sufﬁcient energetic resources to reproduce effectively.

Four different strong oxidants are widely used for disinfection: (1) halogens (chlorine,

bromine, and iodine), (2) halogen-containing compounds (e.g., chlorine dioxide, chlora-

mines, bromochlorodimethylhydantoin), (3) ozone, and (4) hydrogen peroxide (H

). In

each case, the standard engineering application is that of dosing the applied chemical into

a short-term contact chamber (i.e., typical ly designed for 15 minutes’ retention) using a

chemical delivery system preset to achieve a desired disinfectant concentration relative to

measured ﬂow. As shown in Figure 16.53, these disinfection contact chambers are often

designed with a serpentine conﬁguration in an attempt to secure a quasi-plug-ﬂow regime.

Chlorine has been, and remains, the dominant disinfectant chemical with waters and

wastewaters in the United States, applied either in gas (Cl

), liquid (NaOCl), or solid

[Ca(OCl)

] form at what is likely to be the least possible cost (i.e., in the range of pennies

per pound) for any disinfection option (White, 1992). Aside from cost, chlorine’s advan-

tages include its range of delivery options and expected efﬁciency. However, there are also

shortcomings with its use, including the fact that there are signiﬁcant safety issues to be

addressed when storing and metering chlorine gas.

One key aspect of chlorine use is that of its sensitivity to pH. Above pH 7.5 the desired

hypochlorous acid (HOCl) species found in aqueous environments (e.g., produced by the

WATER AND WASTEWATER DISINFECTION TREATMENT 663

hydration of chlorine: Cl

þ H

O ! HOCl þ H

) will disassociate into a hypochlorite

(OCl



) anion (HOCl ! OCl



þ H

) whose antimicrobial efﬁciency is far lower than

that of the hypochlorous acid (i.e., HOCl) form.

Another important fact is that hypochlorous acid reacts read ily wi th reduced ammonia

(NH

), leading to a series of amination reactions and products [i. e., monochloramine,

OCl; dichloramine, NH(OCl)

; and nitrogen trichloride, NCl

] whose bacteriocidal

efﬁcacy is again less that of the original HOCl.

Bromine use as a disinfectant also has its own set of unit features in terms of chemistry,

beneﬁts, and shortcomings (Water Environment Federation, 1996). First, the operative

hypobromous acid (HOBr) species does not disassociate until it reaches a pH of about

8.5, so offering a wider range of serviceability them chlorine. Second, bromine tends to

have a higher level of efﬁcacy at equivalent concentrations, so lower dosage levels might

be used. Third, the bromamine forms are all far better disinfectants than are the chlora-

mines. Indeed, a solid-phase, bromine-bearing compound called bromochlorodimethylhy-

dantoin (BCDMH) is widely marketed for hot tub and spa applications, given the

particular prevalence of urinary ammonia release. One clear disadvantage, though, is

that of cost, which tends to be several times higher than that of chlorine if the latter’s

efﬁcacy attributes are ignored.

Hydrogen peroxide has little, if any, credible role as a disinfectant in environmental

engineering systems, but ozone has found widespread acceptance, particularly in Europe,

as a potable water disinfectant . As compared to any of the halogen-based options, ozone

has the unique ability to dissipate shortly after its addition, without any semblance of a

lingering residual. This lack of a residual is considered a disadvantage in the United

States, where residual chlorine levels are maintained routinely in potable water delivery

systems as a safeguard against subsequent contamination that can occur within the distri-

bution system. However, the conventional wisdom in Europe is that disinfecting chemical

Figure 16.53 Serpentine chlorination reactor for wastewater efﬂuent disinfection.

664

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

residuals are both unwarranted and undesired. Yet another important aspect of ozone use

is that it must be produced on-site, using electrical hardware that is not all that simple and

at a cost that is considerably higher than that of chlorine.

A wide range of nonoxidizing organic and inorganic chemic als are used for, or are able

to provide, disinfecting effects, including aldehydes (formaldehyde and glutaraldehyde),

phenolics, alchohols (ethanol and isoproponal), cationic detergents, nitrites, and heavy

metals (e.g., mercury, silver nitrate, tin, arsenic, copper). Although most of these chem i-

cals have little relevance for the disinfection of waters, wastewaters, or sludges, there are

two noteworthy exceptions. Speciﬁcally, silver-impregnated ﬁlters are sometimes mar-

keted for point-of-use water conditioning devices, such as those that are sometimes

screwed onto the outlets of sink faucets. In this instance, the silver is intended to be slowly

leached from the ﬁlter medium (typically, activated carbon) at a rate that, hopefully, will

retard the opportunistic formation of microbial bioﬁlms intent on using sorbed organics as

their energy source. A second nonoxidizing chemical disinfectant option is that of using

cationic detergents in the form of quaternary ammonium com pounds (formed as organic

salts of ammonium chloride and commonly referred to as quats). One such common

application is that of controlling bioﬁlm growth on cooling tower surfaces, in which

aggressive (i.e., oxidative) disinfection agents such as chlorine, bromine, and ozone

would unacceptably attack exposed wood or metal heat-transfer surface s.

One of the unique disinfection features of the bactericidal quaternary compounds is

that they have a distinctly higher level of effectiveness with many gram-positive bacteria,

probably due to the added depth and complexity of their membr ane structure. Conversely,

gram-negative cells as a whole are often similarly considered to be somewhat more resis-

tant, perhaps due to the added depth and complexity of their membrane structure. Indeed,

Pseudomonas probably tops the list in terms of durability, under conditions that would foil

the vast majority of other cells (e.g., growth in distilled water). Similarly, gram-positive

Mycobacteria species, as well as spore formers, also tend to exhibit this resistant nature

when challenged with quaternary disinfectants, apparently based on the prot ective capa-

city of their respective outer cell coatings. Finally, given their chemical nature, these quat

compounds also bear a unique sensitivity to inactivation when exposed to complexing

soaps, detergents, and organic materials.

The fourth and ﬁnal disinfection mechanism is that of alter ing and disrupting a cell’s

genetic makeup so that the cell is prevented from reproducing even though it may still

have the energy to do so. This disinfection effect is largely associated with the use of

ultraviolet irradiation, and its consequent high-energy cross-linking of adjacent nitrogen

base groups poised side by side at various points within stranded DNA (see the thymine

dimer reaction in Figure 16.54). The resulting impact of polymerizing DNA and forma-

tion of thymine dimers follow much the same path, such that cells are effectively steri-

lized by this UV exposure.

The range of wavelengths associated with ultraviolet irradiation actual ly has consider-

able breadth, from 4 to 300 nm, but the highest level of absorbance by DNA appears to

fall nearly coincident with the maximal output value (i.e., at approximately 254 nm) for

the emission spectrum of light emitted by mercury-vapor light bulbs. The standard engi-

neering application of UV irradi ation involves an array of mercury bulbs placed inside an

irradiation chamber, through which the water or wastewater is passed, with each such

tube being jacketed inside a UV-transparent quartz jacket. These tubes are aligned verti-

cally or horizontally in a fashion where the hydromechanics of the operation provides

maximal opportunity for exposure of cells ﬂowing through the chamber while obviating

WATER AND WASTEWATER DISINFECTION TREATMENT 665

short-circuiting pathways that would degrade process efﬁciency. Figure 16.55 depicts

one such array used for disinfection of a wastewater efﬂuent immediately prior to dis-

charge. UV irradiation with mercury bulbs offers an effective m eans of sterilization

for those engineering applications involving fairly clear or optically transparent

waters and wastewaters. Conversely, sludge is not amenable to UV disinfection

given the unacceptably shallow degree of light penetration into this material.

Thymine dimer

PHOSPHATE

SUGAR

PHOSPHATE

Figure 16.54 Thymine dimer formation along DNA strand produced by polymerizing ionizing or

UV irradiation.

Figure 16.55 Ultraviolet irradiation reactor for wastewater efﬂuent disinfection.

666

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

Ionizing radiation using x-ray and gamma-ray beams with even higher energy levels

than that of UV irradiation, can also be used in water, wastewater, and sludge disinfection.

These ionizing mechanisms displace electrons during beam bombardment (i.e., at which

point they are said to ionize), and in the presence of oxygen these displaced electrons elec-

trochemically form a type of free radical (called hydroxyl radicals) that is highly toxic to

microbial cells. Free radicals are highly reactive, having essentially no activation energy

for their reaction. Given the acutely reactive nature of these radicals, they readily attack

and destroy hydrogen bo nds, double bonds, and ring structures essential to the metabolic

utility of various cellular molecules. Yet another operative mechanism, and perhaps the

key factor behind disinfection with ionizing irradiation, is that of a polymerizing impact

(e.g., DNA thymine dimerization) whereby the biochemical effectiveness of complex

molecules is degraded or termin ated. This technology bears a degree of complexity and

technical hazard that is not typically appropriate for most municipal applications, how-

ever, so that only a limited number of sites, the majority of which involve sludge disin-

fection presently rely on its use. On the other hand, ionizing radiation is used widely for

the disinfection of pharmaceuticals and of disposable dental and medical supplies (e.g.,

syringes, gloves).

In reviewing the various options for disinfection, the mere fact that there is such a

diverse range of disinfecting mechanisms and associated effects demonstrates that there

is no perfect solution for all engineering applications. Excluding heat treatment and osmo-

tic pressure, which have little if any pragmatic utility for large-scale engineered disinfec-

tion, the remaining options exhibit unique differences in their associated costs, technical

complexity, residual impacts, environmental sensitivity, and relative performance, such

that an appropriate assessment of many such site-speciﬁc issues have to be completed

prior to making a ﬁnal decision.

As for deﬁning the precise goal of disinfection, the normal benchmark for potable

water is that of zero residual fecal coliform presence. In the case of wastewater disinfec-

tion, though, efﬂuent standards tend to be targeted for a somewhat higher level of residual

bacterial presence in a fashion that implicitly reﬂects the latter phenomenon of microbial

resistance. Compared against the typical levels of bacterial presence within raw, undisin-

fected was tewaters (such as those shown in le 16.15), therefore, efﬂuent criteria for fecal

coliform usually tend to be somewhat more tolerant (i.e., typically at 200 fecal

coliform colony forming units per 100 mL). However, compliance with this sort of

efﬂuent fecal coliform standard would still require a sizable four- to ﬁve-log reduction

(i.e., based on inﬂuent fecal coliform densities in the range 10

to 10

cells per 100 mL).

Extending beyond the theoretical aspects of these disinfection mechanisms there are

pragmatic uncertainties and inherent regulatory concerns tied to the nature and degree

TABLE 16.15 Bacterial Levels Present in Representative Wastewaters

Observed Bacterial Densities (10

viable cells/liter)

———————————————————————————————————————

Wastewater Total Coliform Fecal Coliform Fecal Streptococci FC/FS Ratio

A 172 172 40 4.3

B 330 109 24.7 4.4

C 19.4 3.4 0.64 5.3

D 63 17.2 2 8.6

Source: Adapted from Water Environment Federation (1996); Crites and Tchobanoglous (1998).

WATER AND WASTEWATER DISINFECTION TREATMENT

667