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

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phosphorus contained within the raw wastewater stream into phosphate-rich solid-phase

intracellular storage granules contained within cells eventually wasted from the system.

Further design reﬁnements to secure an overall goal of complete biological nutrient

removal, encompassing both nitrogen and phosphorus, typi cally involve the addition of

yet another separate, anaerobic zone or phase at the head end of the reactor train, as

shown schematically in Figure 16.32. The second and third reactors in this sequence,

offering anoxic and aerobic environments, respectively, will again play much the same

roles, including oxidation of organic carbon in both stages as well as nitriﬁcation in the

aerobic stage and denitriﬁcation in the anoxic stage. By adding an additional internal

feedback recycle loop, though, biomass moved from the anoxic stage to the new head-

end anaerobic stage is able to shift rapidly into an anaerobic condition (since it carries

little, if any, nitrate or dissolved oxygen). In turn, fermentative transformation of incom-

ing organics into volatile fatty acids sets up the initial conditions necessary for Acineto-

bacter’s phosphorus-related metabolism.

16.1.4 Stabilization Lagoon Systems

Stabilization lagoons represent one of the simplest possible options for wastewater treat-

ment, with a level of engineering sophistication and cost below that of the preceding

attached- and suspended-growth processes. As explained in an oft-quoted anonymous

description—that ‘‘the design engineer drives a bulldozer and the rest is left to Mother

Nature’’—most lagoons are built as simple earthen basins, albeit with liners, and only lim-

ited operational oversight is provided. Given their simplicity, though, and ease of opera-

tion, lagoon system s are widely used in small rural communities and industries throughout

the world.

There are three basic lagoon design options, including two that are solely aerobic or

anaerobic, and a third, facultative version that has both overlying aerobic and underlying

anaerobic layers. Whereas the aerobic and facultative lagoon options can be used for com-

plete treatment of most municipal waste streams, anaerobic lagoon systems are more

Clarifier

Aerobic

Organic carbon

oxidation and

nitrate reduction

(Denitrification)

Anoxic

internal feedback recyle

returning nitrates back to

the denitrifying anoxic zone

Organic carbon

Oxidation and

ammonia oxidation

(Nitrification)

Organic carbon fermentation,

VFA formation and update,

Acinetobacterpromotion

Anaerobic

internal feedback recyle

returning biomass back to

the initial anaerobic zone

Raw

wastewater

influent

Treated

effluent

Waste

sludge

Underflow recyle

returning settled biomass back to

the denitrifying anoxic zone

Aerobic

Organic carbon

oxidation and

nitrate reduction

(Denitrification)

Anoxic

Organic carbon

oxidation and

ammonia oxidation

(Nitrification)

Organic carbon fermentation,

VFA formation and update,

Acinetobacter promotion

Anaerobic

1 2

Figure 16.32 Representative three-tank plus three-recycle design scheme for combined biological

nutrient removalm including nitrogen and phosphorus removal. (Note: This particular arrangement

is often referred to as UCT or VIP systems, based on their respective geographic development at the

University of Cape Town and Virginia; there are numerous alternatives using similar sets of

anaerobic, anoxic, and aerobic reactors.)

618

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

commonly used with high-strength industrial wastes (e.g., rendering plants, slaughter-

houses, food processing residuals), and even then as a means of pretreatment prior to a

following polishing step (e.g., followed by an aerobic lagoon, spray irrigation over land,

or some other means of treatment).

All three of these basic lagoon design options have yet another subgroup set of design

alternatives, depending on their projected organic loading rates and the means by which

they will secure their necessary oxygenation and/or mixing.

Aerobic lagoons of the sort shown in Figure 16.33 rely on natural wind-induced mixin g

and/or surface aeration, plus algal photosynthesis, for oxygen resupply, and tend to be

fairly shallow (with typical depths of about 2 m) and less heavily loaded. On the other

hand, forced mixing and aeration using either mechanical agitation or subsurface com-

pressed-air distribution (Figure 16.33) is also practiced with many aerobic lagoons,

such that their design depths can be increased (to the range 3 to 4 m), along with receiving

higher organic loadings. Anaerobic systems will be even deeper, often reaching depths of

4 to 5 m, to provide sufﬁcient depths for the lower strata of the lagoon to fully reach a

Figure 16.33 Stabilization lagoons: (a) unaerated; (b) aerated.

WASTEWATER TREATMENT 619

point of oxygen depletion, and in many instances an overlying ﬂoating scum blanket will

be allowed to collect on the lagoon surface to further constrain surface reaeration.

There is also a considerable range in the hydraulic detention times used with these var-

ious systems. Design HRTs below 1 week in length, at 3 to 6 days, may be used with high-

rate operations, or for anaerobic or facultative pretreatment units ahead of a second,

downstream aerobic poli shing lagoon. However, many stand-alone lagoons, particularly

those not provided with supplement al aeration hardware, are sized to provide detention

times in excess of a week and possibly extending to month-plus periods. Pretreatment

focused largely on screening solids from the inﬂuent waste prior to entering the lagoon

may also be provided, particularly in the case of high-strength industrial or commercial

operations for which prescreening might effectively pull out a substantial level of incom-

ing contaminant solids loading.

Example 16.3: Preliminary Stabilization La goon Design After completing the trickling

ﬁlter and activated sludge preliminary designs, suppose that a third design overview is

requested for the same Deer Creek, Illinois, community, with its 400 homes and 1600

residents. Given that their expected discharge point into a local creek has seasonal low-

ﬂow conditions that warrant high-level ammonia removal levels (i.e., to avoid summer-

time ammonia inhibition of sport ﬁsh in this Midwest city’s regional area), it is assumed

that a controlled-release, unmixed aerobic oxidation pond with a 6-month retention time

will be required to handle the daily average wastewater ﬂow of 605.7 m

/day and its com-

mensurate total BOD loading of 121.2 kg BOD/day (Note: This design will be based on

the community’s full BOD load , since pri mary settling units are not typically included

with these types of stabilization pond designs.)

Preliminary Design Details

Hydraulic loading,

system sizing,

and retention

times

Design hydraulic retention time (HRT): 6 months

Note: HRTs for these systems typically range from 1 to 6 months,

depending on discharge restrictions into the receiving water

body (e.g., a stream).

Design reactor depth: 2 m

(Note: This depth is appropriate for unmixed lagoons; facultative

and mechanically mixed aerobic lagoons have depths of 3 to

4 m, while anaerobic lagoons handling higher strength wastes

are considerably deeper.)

Projected reactor volume: (605.7 m

/day)(6 months) ¼ 110,600 m

Projected reactor surface area: (110,600 m

)/(1.5 m) ¼ 55,300 m

(Note: This area is equivalent to 13.7 acres, which would corre-

spond to a per-area loading of approximately 116 people/acre.)

Organic loading Projected organic loading:

ð121:2 kg CBOD=dayÞ=ð110; 600m

Þ¼0:0011 kg=m

 day

Related notes 1. This lagoon is rather large in surface area given its long HRT

and shallow, unmixed depth; deepe r, mixed ponds with lower

HRTs often have per-capita loadings of 1 acre per 300 to 600

people.

620 BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CO NTROL

2. As compared to either the activated sludge or trickling ﬁlter

design options, this controlled (i.e., low-rate seasonal) dis-

charge oxidation pond design would have a physical footprint

roughly 1000 times larger.

3. Although it does require signiﬁcantly more space, this type of

facility should be less costly to build and apt to require less

operational attention, although the stability of this system’s

efﬂuent quality will probably be much lower.

To some extent, it is reasonable to classify these lagoons as quasi-suspended-growth

systems given their content of suspended bacterial and algal biomass, although in this

case the level of entrained biomass concentration tends to be several fold lower (i.e., gen-

erally measured in 100s of mg/L as opposed to 1000s). At the same time, most aerobic

and facultative lagoon volumetric organic loading rates (ranging from 0.001 to 0.01 kg/

day) are also much lower than the more advanced attached- and suspended-growth

processes, such that the biomass found in these lagoons tends to have a greater proportion

of higher life-forms (e.g., scavenging protozoans, rotifers) and a lower net mass fraction

of lower bacteria than what would be seen in a more highly loaded attached- or

suspended-growth process.

Yet another unique aspect with open, aerobic lagoons (and to a lesser degree in mixed

lagoons, due to the higher entrained solids levels and lower opacities) is that they have

considerably more involvement with algal growth, due to the reduced density of their sus-

pended solids and the fact that light can penetrate much deepe r than is the case with most

other waste treatment systems. With the incoming waste stream continuously introducing

a fresh supply of readily available nutrients, algal and other small-scale plant growth can

undergo large seasonal swings and will have a correspondingly signiﬁcant impact on sus-

pended solids levels in the efﬂuent as well as the soluble oxygen and carbon dioxide

levels.

The preferred circumstance for lagoons in terms of their algal composition it that of

having suspended, submerged algal cells, where their metabolism often has a distinct,

diurnal impact, resulting in sizable swings in dissolved oxygen and pH [i.e., higher DO

and higher pH during the day, when algal photosynthesis and oxygen release offset bac-

terial respiration (at the same time, algae also removes CO

); lower DO and lower pH at

night, while both bacteria and algae are respiring and jointly releasing CO

]. These algae

are also effective scavengers of solu ble nutrients, including both nitrogen and phosphorus,

assimilating them routinely into solid-phase algal cell mass and then removing them once

these cells die and settle onto the lagoon bottom.

However, there is also a smaller number of lagoons whose algal or plant content is

established in a free-ﬂoating fashion, such as that of the classic duckweed plant form

shown in Figure 16.34, where their presence tends to result in a more extensive mat

stretched across the lagoon surface. These mats may appear quite green and healthy,

but their activity and presence are somewhat contrary to conditions normally considered

optimal for these lagoons. Free-hanging duckweed roots, called fronds, extend only a few

centimeters in depth, and they contribute little, if any, dissolved oxygen via photosynth-

esis. Even worse, they physically block wind-induced surface reaeration, to a degree that

their subsurface conditions are more likely anoxic to quasi-anaerobic. BOD and

suspended solids removal can still be secured in these matted lagoons, albeit with

WASTEWATER TREATMENT 621

lower loading rates, and nutrient uptake will still occur, but effective long-term operation

may require surface harvesting measures.

As for the issue of exactly what happens to the accumulation of dead algae, bacteria,

and other waste solids once they settle and accumulate on the bottoms of these lagoons,

this topic still represents a highly variable circumstance for most lagoons. Ideally, the

input of biodegradable solids settling into a lagoon’s bottom sludge blanket would then

undergo decay at a rate not much different from that of their addition. If, indeed, this

rate of accumulation were to be low, the sludge blanket depth would not increase,

there would be no need to clean (i.e., remove sludge from) the lagoon periodically, and

there would be no reduction over time in the lagoon’s active (as opposed to settled

2cm

16-34a

2cm

(a)

(b)

Figure 16.34 Duckweed plant: (a) overview; (b) closeup.

622

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

sludge) volume. Unfortunately, the history with most lagoons is that they tend to accumu-

late settled solids at a rate that warrants their cleaning at multiyear to decade-long

intervals, without which there would be a debilitating, progressive drop in effective

lagoon volume.

Of course, there is also a negative side to large, exposed lagoon surface s, particularly in

regions faced with colder seasons, in that they will experience sizably higher levels of

evaporative cooling that could lead to signiﬁcant drops in temperatures during cold-

weather periods. In turn, these reduced temperatures will then affect bacterial efﬁcacy

in the lagoon, particularly that of ammonia oxidation by highly sensitive nitrifying bac-

teria. From a regulatory perspective, this circumstance of decreased nitriﬁcation during

cold-weather periods is most important for lagoons that discharge into creeks and streams

with a high degree of variation in their seasonal ﬂow, where low-level dilution of a high-

level efﬂuent ammonia discharge might then cause downstream ammonia toxicity pro-

blems for ﬁsh. In fact, many sport ﬁsh, such as trout, which might be found in streams,

rivers, and lakes downstream of these lagoons, are highly sensitive to quite low ammonia

levels, to an extent where 0.1 mg NH

-N/L concentrations might incur serious, possibly

fatal, stress. To resolve this problem, therefo re, there is yet another lagoon option which

incorporates a control led-discharge release, with far longer HRTs extending up to, and

even beyond, 6 months in time, to ensure that the efﬂuent release can be delayed until

the receiving water body provides an acceptable level of dilution that would negate this

type of ammonia toxicity concern.

Lagoon efﬂuent quality as a whole is not usually comparable to that of the more

advanced waste treatment processes (e.g., activated sludg e), with efﬂuent BOD levels

typically in the range 40 to 60 mg/L and even higher solids levels, typically 60þ mg/L.

Algae represent a large fraction of these solids, however, and as a result, regulatory

criteria levels with lagoon suspended solids concentrations are either set higher to accom-

modate this circumstance or ﬁltration hardware (i.e., mechanical screens or sand, etc.

ﬁlters) will need to be installed if deemed necessary to remove the latter solids prior to

discharge. Therefore, disregarding discharge sites whose low-level dilution might incur

nitriﬁcation concerns, lagoon efﬂuents are commonly considered acceptable for general

discharge.

16.1.5 Constructed Wetland Systems

Wetland systems have played an important role in the process of cleansing surface waters

since the original evolution of Earth’s hydrological and biological ecosystems eons ago.

However, our own efforts with extrapolating this natural concept into controlled treatment

systems for remediation of wastewater and stormwater runoff have been far shorter,

roughly covering only the past half-century.

The involved processing strategy is that of densely clustering water-loving plants

within shallow (60 to 75 cm deep) basins and then allowing the incoming waste or run-

off ﬂow to experience a prolonged period of contact with these plants and their extensive

root matrixes. Rather naturally, the public’s qualitative perception of these types of envir-

onmentally ‘‘green’’ plant-based systems inherently tends to be more positive than is the

case with traditional, concrete-intensive wastewater treatment options, and as such, these

systems have recently drawn considerable appeal (Cole, 1998).

Indeed, the purported beneﬁts of green wetland systems, compared to the conventional

approach of hardware-intensive wastewater treatment, are as follows (U.S. EPA, 2000):

WASTEWATER TREATMENT 623

 They can be constructed in highly attractive, environmentally friendly conﬁgurations

(see, e.g., Figure 16.35).

 They may offer lower capital costs (at least for smaller applications).

 They will almost certainly have lower operating costs.

 They should requi re relatively less operating attention.

Clearly, a primary beneﬁt of constructed wetlands, in terms of their engineered design,

construction, and operation, is that of their simplicity. Wetland cells are typically con-

structed to use a gravity-based ﬂow scheme that has few, if any, moving parts; most

systems use only solar energy; and their demands on operator attention and care are

quite low (being limited to occasional ﬂushing of the inﬂuent and efﬂuent lines to

avoid plugging, and seasonal removal of dead, senescent plant debris) .

Figure 16.35 Constructed wetland wastewater treatment systems.

624

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

However, these systems can also have a number of related disadvantages that mus t be

recognized, particularly in terms of performance. One key issue is that their best levels of

efﬂuent quality (e.g., BOD , TSS, ammonia) are seldom realized in a matter of weeks or

months after a new facility is installed, but instead, will probably require a year-plus per-

iod for system maturation. In addition, these systems are also apt to experience a higher

degree of seasonal variability than what is typically the case with conventional wastewater

systems, and these seasonal variations will be difﬁcult to control. At the same time, these

natural systems also tend to be susceptible to problems of a sort that would otherwise not

be an issue with standard wastewater treatment operations. For example, the performance

of the plants could be partially disrupted or completely curtailed by plant infestation and

disease. Given the inherent plug-ﬂow mode of wastewater passage through these units, an

incoming slug of potentially harmful household chemicals (e.g., paint thinner, lawn care

chemicals, bleach) could also kill a substantial front-end area of this biological operation

for an extended period. In the particular case of wetlands with which their waste stream is

open to the atmosphere, they have been known to encounter troublesome problems with

mosquito growth as well as occasional odor generation.

These constructed wetland designs can be roughly subdivided into two major options,

including those that rely on emergent plants and those that use submerged plants either

alone or in combination with emergent plants. The names given to these two options are,

respectively, that of a subsurface ﬂow (SSF) and a free water surface (FWS) constructed

wetland. In either case, it should be noted that some form of pretreatment (e.g., using a

septic tank) is routinely provided ahead of wetlands receiving wastewater ﬂows, to

remove set ttleable solids and ﬂoating oils and grease that might otherwise pass into the

wetlands, where they might degrade system performance. In general, submerged-ﬂow

wetland systems tend to require somewhat less opera tional attention than free-water sys-

tems, and they may also have a slight advantage in terms of required land areas for com-

parable ﬂows.

Figure 16.36 Subsurface ﬂow constructed wetland schematic.

WASTEWATER TREATMENT 625

The SSF options (see Figur e 16.36) are commonly constructed with one or more cells

ﬁlled with various forms of a porous media, such as coarse and ﬁne gravel washed

previously to remove ﬁne solids that would otherwise reduce necessary void volume,

and planted with various forms of emergent wetland plants. Larger-diameter media

(i.e., 4- to 5-cm coar se gravel) is normally placed across the head-end face of the cell

to spread laterally and distribute the incoming waste uniformly, and then again spread

across the back of the cell to recollect the efﬂuent and direct it smoothly to the system’s

outlet piping. Within the wetland itself, a small-diameter medium (e.g., typically a

washed, 0.75- to 1.5-cm-diameter river gravel) is used to sustain plant rooting. Waste-

water introduced into these cells subsequently ﬂows horizontally through the open,

void space of the porous, subsurface medium at a relatively low velocity, during which

it comes into contact with both the plant’s extensive root system and its afﬁliated rhizo-

sphere consortia of microorganisms.

The FWS option (depicted schematically in Figure 16.37) for constructed wetlands is

built much the same as those in the SSF mode, with single or serial open cells that are

similarly lined with much the same barrier material. These free-water cells, however, are

only partially ﬁlled (10 ! 15 cm deep) with soil or a small-diameter (0.5- to 1-cm) gravel

medium to support the plant root systems, such that the remaining depth is then ﬁlled with

water and with ﬂoating plus rooted plant mass. Both emergent and submergent plant vari-

eties can be grown in these cells; emergent plants will root themselves in the bottom med-

ium, while the submerged plants can either exist in a free-ﬂoating mode whose roots

dangle downward or are again embedded into the bottom medium. When given a

means of supplemental aeration (e.g., using compressed air diffusers), the depths used

with free-water systems are sometimes extended somewhat beyond 1-m depths.

Hydraulic residence time (HRT) is a primary design factor for all of these constructed

wetlands, with minimal recommended values of 5 to 7 days (Hammer, 1989). Conserva-

tive designers may opt for considerably longer times, and many systems are built with

HRTs extending into multiweek periods. However, when provided with supplemental

aeration, or in the case of most subsurface ﬂow designs, these residence times are typi-

cally reduced downward to values around 1 week.

Figure 16.37 Free-water surface constructed wetland schematic.

626

BIOLOGICAL APPLICATIONS FOR ENVIRONMENTAL CONTROL

Example 16.4: Preliminary Constructed Wetland Design Having completed their three

prior preliminary designs, one ﬁnal process design overview is requested for the Deer

Creek, Illinois, community, in terms of a constructed wetland system. Given the pro-

longed subfreezing winter temperatures typically encountered at this northern midwest

U.S. community, a subsurface-ﬂow (SSF) constructed wetland will be sized preliminarily

to handle a daily average wastewater ﬂow of 605.7 m

/day and its commensurate total

BOD loading of 121.2 BOD/day (Note: Although these construc ted wetlands are com-

monly built with preliminary septic tank units, this example design will be based on

the community’s full BOD load as a conservative measure in light of potential septic

tank upset conditions).

Preliminary Design Details

Hydraulic loading,

system sizing,

and residence

times

Design hydraulic residence time (HRT): 7 days

Design wetland media depth: 0.61 m (i.e., 24 in.)

Projected wetland liquid depth: 0.46 m (i.e., 18 in.)

(Note: Typically, only the lower two-thirds of the media depth is

saturated.)

Expected media porosity ¼ 0.4

Projected wetland volume:

ð605:7m

=dayÞð7 daysÞð1=0:4Þ¼10; 600 m

Projected wetland surface area: (10,600 m

)/(0.46 m) ¼23,043 m

(1.9 ha)

Projected wetland total media volume: (23,043 m

)/(0.61 m) ¼

14,046 m

(Note: The latter volume would include both coarse inlet

and outlet media plus smaller 0.75- to 1.5-cm river

gravel.)

Organic loading Projected organic loading:

ð121:2 kg CBOD=dayÞ=ð10;600 m

Þ¼0:0069 kg=m

 day

Hydraulic loading Projected hydraulic loading:

ð605:7m

=dayÞ=ð1:9haÞ¼318 m

=ha  day

Related notes 1. As with the stabilization lagoon option, a constructed wetland

system will need to be considerably larger than the activated

sludge or trickling ﬁlter designs for this community, in both

surface area and volume.

2. Construction costs for constructed wetlands can escalate

rapidly with larger facilities (e.g., beyond a daily ﬂow of

500 m

/day) due to the capital cost of the media (e.g., washed

coarse and ﬁne gravel) and its on-site delivery.

3. Constructed wetlands are also apt to require less operational

attention, but their cold-weather ability to secure nitriﬁcation

may require additional treatment steps depending on regulatory

requirements for efﬂuent ammonia.

WASTEWATER TREATMENT 627