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

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portion of bed volume open to liquid and air ﬂow rather than being occupi ed by the media

itself ), which subsequently helps to avoid problems with solids clogging that historically

tended to affect the older rock media units.

As compared to the original stationary media mode, there have also been a number of

recent developments based on rotational or ﬂuidized media movement options. One such

version shown in Figure 16.17 is called the rotating biological contactor (RBC). RBC

TABLE 16.3 Attached-Growth Media Options

Bioﬁlm Media

Option

Nominal Size

(cm)

Speciﬁc Media

Surface Areas

)

Media Void

Fraction (%)

Media Density

(kg/m

)

Old rock media 10–12 30–60 40–60 800–1400

Standard plastic

media

60–100 95–97 32–96

Standard random

packed media

3–10 60–120 92–95 48–96

High-high plastic

media

100–200 95–97 32–48

Ultrahigh random

packed media

1–3 200–600 95–97 40–55

Plastic media sheets are typically conﬁgured in 60 60 120 cm bundles.

Source: Adapted from Metcalf and Eddy (2003).

Figure 16.17 Attached-growth rotating biological contactor system: (a) covered tanks and gantry;

(b) interior media view.

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BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

reactors are built as cylindrical media bundles with densely packed, rotating plastic sheets

which may either be partially or fully submerged into the waste stream. During rotation of

the partially submerged units, the attached bioﬁlm is exposed sequentially to the waste-

water and to the air, while submerged units may be operated in an aerated or even an

anoxic or anaerobic mode.

Over the past two decades, several new strategies for attached-growth processing have

been developed, including two options where the involved bioﬁlms are maintained as sus-

pended, millimeter-scale microbial clusters rather than using a traditional sheetlike con-

ﬁguration. The ﬁrst such approach is typically aerobic, with small-diameter, inert med ia

particles (typically, plastic or low-density ﬁred-clay beads, sand, or activated carbon gran-

ules, etc., in the size range 1 to 4 mm) used as a bioﬁlm carrier, with varied media den-

sities used alternatively to secure either settleable or buoyant behavior. Because of the

elevated biomass densities found in the latter options, their elevated oxygen uptake

rates will require high-level aeration, such that these operations are commonly referred

to as biological aerated ﬁlter (BAF) systems. These types of small media bioﬁlm systems

are typically operated in an up-ﬂow operating regime, possibly with partial or full ﬂuidi-

zation of the media during its operation on either a continuous or an intermittent basis and

with intermittent backwashing being used (possibly with coaeration) to ﬂush and extract

entrapped solids. There are several advantages with the latter type of BAF reactor. First,

the media usually has a cumulative surface area that is far higher than would be attainable

had the same bed been packed with standard plastic media, and this increased surface area

raises the mass of bioﬁlm able to grow inside the reactor. To some extent the magnitude of

this increase is tempered by the fact that physical rubbing of granules against one another

keeps the bioﬁlm thinner than what is usually seen in conventional trickling ﬁlter systems.

However, the cumulative effect of having a denser and more active biomass is that the

reactor loadings can be increased dramatically. In fact, volumetric organic loadings sev-

eral times higher than that of standard ﬁxed-ﬁlm systems (Rittman and McCarty, 2001)

have been reported with many of these newer BAF bioﬁlm systems (e.g., at levels mea-

sured in the range 5 to 10 kg CBOD

/daym

Yet another innovative quasi-attached-growth concept uses suspended granular bioﬁlm

clusters whose dense, multimillimeter microbial matrix develops in a self-aggregated

bead conﬁguration rather than relying on the preceding inert support materials. These

granulated bioﬁlm clusters are then suspended in an upﬂow anaerobic sludge blanket

(UASB) process, where inco ming soluble COD is rapidly fermented to gaseous methane

and carbon dioxide, which then rises and is released from the reactor through an overlying

inverted-cone degasiﬁcation cover. Internal recirculation of the granular bioﬁlm cluster

blanket is then achieved by upwelling lift provided via wastewater ﬂow and initial gas

buoyancy followed by subsequent downward settling of the denser microbial beads

after gas detachment. Here again, these UASB systems carry extremely high granular bio-

ﬁlm densities, to the point where their permissible loadings with industrial-type soluble

organics are among the highest possible with wastewater operations, ranging from 12 to

20 kg CBOD

/daym

(Metcalf and Eddy, 2003).

Yet another hybrid approach for these bioﬁlm systems couples attached-growth with

suspended-growth processes using a trickling ﬁlter/solids contactor (TF/SC) scheme.

These designs include a solids aeration tank placed between the bioﬁlm tower and clari-

ﬁer, with settled solids drawn from the clariﬁer underﬂow being recycled back to the inter-

mediate solids contactor unit to enhance the uptake and separation of ﬁne particulates.

These intermediate tanks are typically not all that large, with HRTs usually below

WASTEWATER TREATMENT 599

1 hour, and their suspended solids levels are usually not all that high compared to con-

ventional suspended-growth reactors at 700 to 1500 mg/L, but their ability to facilitate

improve bioﬂocculation elevates the overall efﬁciency of the TF/SC process beyond

that of standard bioﬁlm processes, with expected removal efﬁciencies for BOD and

suspended solids 10 to 15% higher than those of standard bioﬁlm processes.

16.1.3 Suspended-Gro wth Syste ms

Suspended-growth processes used for wastewater treatment, such as that shown in

Figure 16.18, have largely surpassed the attached-growth options over the past half-

century in terms of general application, to the point where they have becom e the accepted

state-of-the-art, best available prac tice for wastewater processing. There are several rea-

sons for this preference, covering both performance and economic issues. These systems

do have a higher energy demand to aerate and mix their suspended biomass, but they have

earned a higher degree of conﬁdence in terms of expected performance levels. These

suspended-growth processes, commonly referred to as activated sludge systems, also tend

to have better economy of scale in construction costs, whereas attached-growth is often used

in smaller installations, due to its ease of operation and simpler mechanical design.

Figure 16.18 Suspended-growth (i.e., activated sludge) wastewater treatment facility.

600

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

The key biological structure within suspended-growth processes is that of an amor-

phous, clustered structure (see Figure 16.19) known as ﬂoc, whose constituent microbial

population sustains a desired biochemical oxidation of wastewater contaminants. Like its

bioﬁlm counterpart, ﬂoc is composed of a com plex aggregate of various bacterial and

higher life biota, plus an added quantity of nonliving organic and inorganics material.

Suspended-growth ﬂoc particles exist as free-ﬂoating structures whose physical, chemical,

and biological exchange with the surrounding bulk solution functions in three dimensions,

as compared to a bioﬁlm sheet which is conﬁned against the inert surface to which it is

attached. Floc suspensions are referred to as mixed liquor, and the suspended solids

concentration of this is denoted mixed liquor suspended solids (ML SS). These ﬂoc con-

centrations are usually maintained at total volatile suspended solids concentrations ran-

ging from 1 to several grams per liter.

As was the case with bioﬁlm development, the circumstance of having bacterial cells

stick together in ﬂoc particles depends on starvation conditions which trigger an exocel-

lular release of adhesive exocellular polysaccharides. In turn, individual cells are able to

self-adhere or ﬂocculate into larger, enmeshed ﬂoc clusters. Floc dimensions are consid-

erably smaller than multimillimeter bioﬁlms, though, with typical diameters of 250 to

500 mm, and this limit is probably controlled by shear forces encountered during routine

mixing, aeration, and pumping. Given that ﬂoc biomass is composed largely of bacterial

cells, the speciﬁc gravity of these particles also tends to be only slightly higher than water

(about 1.03 to 1.05), such that these relatively light ﬂoc particles can be kept in suspension

with only moderate agitation and aeration.

Flocculation behavior also plays a role in terms of bacterial survival as a whole. When

an organism metabolically releases any substance outside its boundaries, there is a cost

that must be accompanied by a return. Only under conditions of scarcity is this investment

worthwhile for the microbe, and in the case of ﬂoc physiology the beneﬁt is that of enhan-

cing the entrapment of particulate substrates. The secretion of exocellular digestive

Figure 16.19 Typical light microscope image of suspended-growth (i.e., activated sludge) ﬂoc

conformation.

WASTEWATER TREATMENT 601

enzymes by a single cell could result in most of that enzyme uselessly diffusing away

from the cell, such that a large fraction of any dissolved substrate formed by dige stion

of a particle would also diffuse away. While clustered within an aggregated ﬂoc commu-

nity, though, bacteria are cooperating by releasing enzymes that beneﬁt them collectively.

The physi cal structures used in suspended-growth or activated sludge systems include

both a reactor basin and a settling tank (Figure 16.20). Within the reactor basin, biode-

gradable substrates are then absorbed by the ﬂoc particles, along with oxygen and other

key metabolic elements (e.g., nutrients, trace metals), and used to sustain a complex set of

catabolic and anabolic reactions by whi ch the wastewater is then cleansed biochemically.

In turn, a subsequent settling tank is used to settle suspended ﬂoc biomass in preparation

for recycle or wastage.

To maintain desired aerobic conditions inside the reactor, oxygen must be added rou-

tinely either by introducing compressed air or oxygen via dispersed air bubbles or

through various mechanical aerat ion mechanisms. Extending beyond these reactor and

clariﬁer components and their afﬁliated aeration equipment, suspended-growth systems

will also require a complementary set of valves, pipes, and pumps used to control the

reactor feed, as well as additional hardware for clariﬁer underﬂow recycling and wastage

plus efﬂuent discharge.

The settling tank, also called the secondary clariﬁer, has one basic function, to sepa-

rate the ﬂoc biomass from the efﬂuent. This operation can be divided into two comple-

mentary functions: clariﬁcation and thickening. Clariﬁcation is intended to produce a

clear efﬂuent that is nearly free of suspended solids. Thickening is the production of a

concentrated underﬂow sludge which is then pumped from the bottom of the clariﬁer.

Most of the thickened sludge is returned to the aeration tank [as a recycle or return acti-

vated sludge (RAS) stream], but a portion of the sludge will also be wasted and removed

from the process for further sludge processing. In turn, this wastage turns out to be the

key control parameter for a suspended-growth process, which by varying the age of this

biomass (its SRT) controls net growth rate, will then regulate overall operational efﬁciency.

In this context of qualifying discrete reactor and settling operations, though, it should

be noted that some suspended-growth systems do not follow this convention. For example,

sequencing batch reactor (SBR) systems use individual tanks as both reactors and clari-

ﬁers, where aeration and settling phases are then sequenced during batchwise operating

cycles. Another suspended-growth system, which is just beginning to see wider utilization

in full-scale systems, is that of the membrane bioreactor (MBR), where semipermeable

Figure 16.20 Suspended-growth design schematic.

602

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

synthetic membranes are used for biomass separation in lieu of a settling tank. In these

units, mixed liquor is passed across and through the membranes to obtain an efﬂuent

that is essentially free of all suspended solids, thereby directly retaining ﬂoc biomass

within the aeration tank.

Numerous design options exist with respect to the manner in which the waste is intro-

duced to these suspended-growth reactors, how the reactor system might be arranged in

physically or temporally separated stages, the condition (i.e., aerobic, anoxic, anaerobic)

of these stages, and the rates employed for hydraulic throughput and solids discharge. The

vast majority of systems are maintained in a fully aero bic state for oxidative biodegrada-

tion of incoming organics, and in recent years an escalating number of these aerobic sys-

tems have also been used to maintain an additional oxidation of reduced nitrogen

(ammonia and organic nitrogen) to nitrate by way of nitriﬁcation. In addition, there are

also a variety of suspended-growth design options by which full nitrogen removal, includ-

ing denitriﬁcation, as well as biological phosphorus removal can be achieved by, respec-

tively, using an additional complement of suitably designed and controlled anoxic and

anaerobic phases.

Six design variables are considered in the development of suspended-growth systems,

including hydraulic residence time, solids residence time, speciﬁc substrate loading (mass

substrate applied per mass cells per time), volumetric loading, reactor conﬁguration, and

recirculation rate (recycle ﬂow relative to raw inﬂuent ﬂow) ratio. Table 16.4 provides a

general synopsis of the typical design criteria relative to four of the basic options for con-

ﬁguring these reactor s, including conventional, high-rate, extended aera tion, and nitriﬁca-

tion systems. In addition, further engineering consideration must be given to the

anticipated reactor solids levels, excess sludge production rates, solids–liquid separation

methodologies, and overall environmental conditions (e.g., operating temperature).

The HRT and SRT were described in Section 16.1.1. The third variable, speciﬁc sub-

strate loading, is often referred to in terms of a food-to-microorganism (F/M ) ratio.Of

course, the form of the food term will vary according to the purpose of the reactor and

whether its intended goal was that of carbonaceous BOD, COD, ammonia-nitrogen, and

so on, removal. Similarly, there are several techniques for quantifying ‘‘mass,’’ but the

most common procedure is to use mixed liquor volatile suspended solids (MLVSS).

At steady state, the F/M ratio is inversely related to SRT. Specifying one is equivalent

to specifying the other, and either one can be used for design purposes.

TABLE 16.4 Typical Suspended-Growth System Design Criteria

Suspended-

Growth

Design

Factors

Hydraulic

Residence

Time (h)

Solids

Residence

Time (days)

Speciﬁc Substrate

Loading (F/M) Rate

(kg/kg VSS day)

Volumetric

Substrate

Loading Rate

(kg CBOD/m

 day)

Recirculation

Ratio Q

Relative to

Inﬂuent Flow

Conventional 4–8 5–15 0.2–0.4 (as CBOD) 0.32–0.64 0.25–0.75

High-rate 2–4 3–5 0.4–1.5 (as CBOD) 1.6–16 1–5

Extended

aeration 18–36 20–30 0.05–0.15 (as CBOD) 0.16–0.4 0.5–1.5

Nitriﬁcation 6–12 >8* 0.05–0.15 (as N) 0.2–0.4 (as N) 0.5–1.5

Operating SRTs necessary for nitriﬁcation vary with temperature: >4 days in summer, >8 days in winter.

Source: Adapted from Metcalf and Eddy (2003).

WASTEWATER TREATMENT

603

The control strategies used with suspended-growth systems are more sophisticated than

those used in attached-growth wastewater processes (Bailey and Ollis, 1977), including

that of controlling the aeration plus clariﬁer recycle and wastage ﬂow rates. The major

operational objectives for suspended-growth systems, though, are to maximize efﬂuent

quality while minimizing costs for disposal of waste biomass or sludge (the production

of which decrease with SRT) and energy used with mixing and aeration (which increase

with SRT), and in this context, controlled biomass wastage represents the key control fac-

tor. In turn, there is a delicate balance with controlling SRT to maximize efﬂuent quality

while minimizing operating costs, to an extent that considerable experience is necessary

for optimal process control given the dynamic impacts of seasonal temperatures, changes

in wastewater character, and so on.

In general, control regimes designed to sustain higher rather than lower SRT conditions

are often preferable, in that their correspondingly reduced (i.e., starvation) growth rates

would correspond with low-level efﬂuent substrate concentrations. As was the case with

similarly starved biomass growth within adhesively bound bioﬁlms, suspended-growth

biomass maintained at these elevated SRT levels will also tend to produce beneﬁcial,

ﬂoc-forming exocellular polysaccharides.

In turn, the development of improved ﬂoc-formi ng biomass would be similarly bene-

ﬁcial in terms of desired settling performance. It is imperative that the suspe nded biomass

be able to settle and compact effectively, forming a sludge blanket, so that these solids

can be recycled back to the aeration tank rather than being lost in the settling tank’s efﬂu-

ent. Settling behavior is, consequently, tracked in most systems. The oldest and still likely

to be the most common method of monitoring settling is that of the sludge volume index

(SVI). The SVI is deﬁned as the volume in milliliters occupied by 1 g of sludge after

30 minutes of settling. A settling test is completed by adding mixed liquor to a settleo-

meter (a vertical glass or plastic cylinder) or standard laboratory graduate cylinder

(Figure 16.21). Although there are a number of variations, the basic test consists of allow-

ing the sludge to settle for 30 minutes, then noting the volume of settled sludge [settled

volume (SV)]. This value is then divided by the MLSS concentration:

SVI ðmL=gÞ¼

SVðmL=LÞ

MLSSðg=LÞ

ð16:2Þ

An acceptable SVI value (i.e., producing good settling) is considered to be in the neigh-

borhood of 100 mL/g. An SVI greater than about 200 indicates bulking conditions, where

settling performance is less than satisfactory. Values in between must be judged depend-

ing on the design and operating conditions of a partic ular plant. The settled 1-L sample

shown on the right-hand side of Figure 16.21, for instance, has a 30-minute settled volume

of 250 mL, and with a solids concentration of 2.5 g/L, the corresponding SVI would be

100 mL/g. Although rather crude, this simple test provides an approximate feel for the

settleability of these solids, and when tracked on a recurring basis, the operating staff

can predictably qualify ‘‘good’’ vs. ‘‘bad’’ behavior.

Beyond the SVI test, though, suspended-growth reactors are monitored in a fashion

that pays a higher level of attention to the biological aspects of the involved biomass

than is the case with the various attached-growth processes, taking into account such

issues as expected rates of cell growth, decay, and respiration, as well as the overall mor-

phology and consortia maintained within the involved biomass. For example, dissolved

oxygen levels in the aeration reactors are monitored routinely, both to ensure appropriate

604 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

oxygen transfer and to track biomass activity qualitatively. More precise measurements of

mixed-liquor oxygen utilization rate (OUR) levels are also completed at many facilities

on a frequent basis, using respirometers to evaluate biomass oxygen uptake such that

they might quickly pinpoint potential problems with incoming wastewater toxins or