Water and Wastewater Engineering

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28-4 WATER AND WASTEWATER ENGINEERING

TABLE 28-1

Important factors to consider in evaluation of process alternatives

Factor Comments

Design flow The process must be able to handle the design flow rate. For example, oxidation ponds

are rarely selected for populations over 5,000.

Flow variation The process must be able to handle the expected range of flows. This includes the long-

term change in flow from the minimum at start-up to the maxi

mum at the design life as

well as diurnal, seasonal, and emergency variations.

Performance Does the process typically meet or exceed effluent discharge requirements?

Reliability Experience at similar facilities operating under similar conditions should be

considered

in determining the answers to the following questions. Is the process easily upset? Can

it meet performance requirements under periodic shock loads? On a more fundamental

level, what is the maintenance history of the hardware?

Flexibility This factor has three com

ponents. Is the process scheme such that it can be operated at

the low flow conditions at start-up as well as the design flows? Does the operator have

the capability to meet performance criteria when extreme changes in flows and/or loads

occur? Does the operator have the ability to “work around” process components during

sched

uled out-of-service maintenance requirements as well as unscheduled maintenance

for repair of failures?

Area required The process must fit on the available land with space for future capacity or process

upgrades.

Complexity What is the degree of difficulty in operating the plant un

der routine or emergency

conditions? Does the plant require a high level of attention, extraordinary staff training,

and outsourced maintenance?

Staffing

requirements

How many people and what level of skills are required to operate the process? Are these

skills readily available? How much training is required?

Safety Safety concerns inc

lude the potential for falls, confined space entry for routine

maintenance, exposed equipment or moving parts, chemical transport, storage, and

application.

Noise Noise impacts on operating personnel are a safety issue. They must be addressed in

equipment selection and housing. Environmental impacts

of noise on the neighboring

community must be addressed in plant location and maintenance activities.

Climatic

constraints

Climatic issues include precipitation induced flow, restricted availability of disposal

sites for biosolids due to wet or frozen ground, and lack of plant appurtenances

necessary to deal with local weather conditions. Warm temperatures will result in

higher odor complaints. Cold temperatures, ice, and snow will impede maintenance and

increase safety issues. Cold temperatures also adversely affect biological treatment.

The frequency of storm events and their impact on the power supply, flooding, and

physical damage should be considered.

Odors What is the potential for odorous emissions? Are they difficult to control?

Wastewater

characteristics

What constituents of the wastewater may inhibit the process?

Che

micals The chemicals to be employed, the quantities required, their safe handling and storage,

and impact on the downstream process and treatment residuals must be addressed.

Treatment

residuals

The quantity and characteristics of the treatment re

siduals affects the treatment

technologies and disposition of the treated residual. Are there any constraints that

influence the processing scheme? An example would be the need or desire to produce

Class A biosolids. What are the impacts of recycle streams on the process?

(continued)

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-5

TABLE 28-1 (continued)

Important factors to consider in evaluation of process alternatives

Factor Comments

Energy With the current trends in energy costs, the energy requirements of the process are a

major consideration in the selection process. Likewise, opportunities to recover and/or

conserve energy are critical to process selection.

Operating and

maintenance

requirements

What special operating and

maintenance requirements must be provided? Some

examples are access ports, elevators, and overhead cranes.

Construction

issues

Special construction issues such as foundations, slope, and site access may be a major

consideration in the proce

ss selection.

Cost Both capital and operating costs must be considered in the selection process.

A dapted from Metcalf & Eddy, 1991 .

TABLE 28-2

Factors affecting performance of activated sludge processes

Aeration capacity

Ammonia loading

bCOD loading

Environmental factors such as pH and temperature

Food-to-microorganism ratio

Hydraulic loading

Hydraulic detention time

Nutrients

Reactor type

Return activated sludge rate

Solids retention time (SRT)

A dapted from Metcalf & Eddy, 1991.

While these requirements are demanding, many utilities choose to produce water that is much

better in quality than that required to comply with the regulations. This includes improving the

aesthetic characteristics of the water. Many of the processes that are needed to meet the regula-

tions m

ay be operated in a manner that yields a higher quality than is required by the regulations.

One way to get higher quality is to operate at lower loading rates than the customary norms.

Another way is to provide additional treatment processes. Because these design considerations

are also cost issues, they mus

t be addressed with the client.

Reliability. As used here, the term reliability includes robustness as well as mean time between

failures. Robustness includes the ability to handle changes in wastewater quality, on-off cyclic

operations, normal climatic changes, adverse weather events, an

d the degree of m aintenance

required to maintain efficient operation. Although minimum redundancy requirements (see, for

example, Table 1-4) help to ensure reliability, they do not take into account failures because

28-6 WATER AND WASTEWATER ENGINEERING

equipment is operated outside of its normal operating range or failure to meet discharge goals

because of frequent or very long downtime for repairs.

Process Flexibility. The ability of the operator to mix and match various processes to adapt to

variations in flow rates ranging from minimum flows at initial start-up of the plant to maximum

flows at the de

sign life is essential to providing consistently good quality effluent. In addition, the

ability to “work around” scheduled out-of-service maintenance requirements as well as unsched-

uled maintenance for repair of failures should be planned in the selection of process options. Both

the plant layo

ut and the hydraulics of the plant play a role in providing this flexibility. These are

discussed later in this chapter.

A more difficult requirement is the flexibility to meet c hanging regulatory requirements

(which, generally, will become more stringent rather than less stringent) or changes in the waste-

water characteristics. F

or a given set of site characteristics, planning for future expansion is one

logical way to provide flexibility. In some cases, it may be possible to provide extra space in the

hardened facilities (i.e., concrete structures) to allow for addition of equ ipment when the need

arises. Providing access d

oors or roof structures to the space is also a good idea. There is, of

course, the risk that the space will never be needed.

Utility Capabilities. The clean water u tility must be able to operate the plant once it is built.

This includes repairs as well as d ay-to-day adjustments, ordering supplies, taking sample

and so on. Processes should be selected that can be operated and maintained by the available

personnel or personnel that can be trained. The plant management must be informed of the

complexities and requirements of the treatment process before plans are adopted. Staff train-

ing as well as availability and access to servic

e are important considerations in selecting a

process.

For many small (501 to 3,300 people) and very small communities (25 to 500 people) and

even some medium (3,301 to 10,000) to very large communities ( 100,000 people), there are

economies of scale in joining with others to provide treatment. The econo

mies of scale are found

primarily in capital cost, outside services, and materials. Energy and, to a lesser extent, labor

costs do not exhibit as significant an economy of scale. However, larger size does not guarantee

lower costs. In addition to the political issues of local control, a carefu

l economic evaluation of

the alternative of joining with another community is warranted.

Costs. The capital cost may be the key factor in selection of a process. As noted in Chapter 1,

the operating cost is, in all likelihood, equally relevant. It may be even more important than capi-

tal cost in the decision process because of the rising cost of energy and labor.

Odor Control. The stench of waste and decay has been associated with disease for centuries.

Long after the scientific recognition of the germ theory of disease, people attribu te diseas e to

miasmas. Malodors still send a psychological warning signal that distresses the recipient.

A s the United State

s has become urbanized and less agricultural, urban, suburban, and

even rural communities lack tolerance for even ephemeral exposure to extremely low odor

concentrations. In addition to psychological stress, they cause a measurable decline in prop-

erty values. Because the publi

c rates odors as a primary concern in the implementation of

clean water facilities, odor control must rank as a m ajor consideration in process selection and

implementation.

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-7

Nutrient and Residuals Management. The regulatory trends in control of nutrients suggest

that planning for new facilities or renovation of old ones must provide for implementation of

more technologically advanced systems than permit requirements impose. Examples include pro-

vision of space for expansion to implement biological nutrient rem

oval and/or tertiary treatment,

and provision of “front-end” systems such as fine screening for membrane bioreactors.

Because it provides greater flexibility in disposition of biosolids, there is a trend toward

selection of processes that can produce Class A biosolids. Current designs that can be m

odified

or expanded to meet processes to further reduce pathogens (PFRP) requirements to produce Class

A biosolids should be among the alternatives considered.

Energy Management. The surging demand for energy and its escalating cost require rigorous

investigation of energy managem

ent in the selection and design of processes. Energy conserva-

tion in facilities construction, the use of wind and solar power, and recovery of energy from

biosolids should be considered.

Initial Screening

Aids for initial screening that may be found in previous chapters are summarized in Table 28-3 .

A dditional aids in screening that were not included in previous chapters are provided in the following

tables. These are organized under the following topics:

• Clarifiers (Table 28-4).

• Secondary treatment with a focus on nutrient removal (Figure 28-1 and Tables 28-5 through

28-10).

• Tertiar

y treatment (Tables 28-11 and 28-12).

• Thickening (Table 28-13).

• Stabilization (Table 28-14).

TABLE 28-3

Summary of tables to aid in screening alternatives

Process Table Remarks

Bar racks and screens 20-3 Typical use

Bar racks 20-4

Fine screens 20-6, 20-7

Grit removal 20-9, 20-10

Primary treatment 21-2 Chemically enhanced primary treatment

Secondary treatment 24-1 Attached growth

Sludge pumps 27-4, 27-5

Thickening 27-6

Anaerobic digestion 27-14 Cy

lindrical versus egg-shaped

Dewatering 27-21

28-8 WATER AND WASTEWATER ENGINEERING

TABLE 28-4

Comparison of rectangular and circular clarifiers

Rectangular clarifiers Circular clarifiers

Advantages Less land and construction cost in a

multiple unit design

Longer flow path and less chance

for short-circuiting than center-feed/

peripheral overflow circular clarifiers

More even distribution of sludge loads

on collectors

Short detention ti

me for settled sludge

Better effect of dynamic filtration

Can be shallower

Low headloss for flow distribution

Can be easily covered for odor control

More effective foam/scum trapping

and positive removal

Not proprietary

Simple and more reliable sludge-

collecting system

Low maintenance requirements

Disad

vantages Longer detention time for settled

sludge (except for Gould-type designs

which have very short detention times)

Possibly less effective for high solids

Increased maintenance of collectors

Center feed/peripheral units have higher

potential for short-circuiting

Lower limits for effluent weir loading

Generally proprietary

More susceptible to wind effects

High headloss for flow distribution

Gould-type tanks have sludge hopper in middle of tank.

Lack of data at high loadings; most rectangular clarifiers are operated at lower solids loadings.

Source: WEF, 2006a.

TABLE 28-5

Nitrogen and phosphorus removal process selection

Process Nitrification Nitrogen removal Sensitivity to TBOD

/TP ratio

A/O No

Moderate

Phostrip No

Low

A2/O

™

Yes

6 to 8 mg/L

High

UCT/VIP Yes

6 to 12 mg/L

Moderate

Modified UCT Yes

6 to 12 mg/L

Moderate

Bardenpho Yes

3 mg/L

High

All processes except Phostrip can benefit from using fermenters. TBOD

 total biochemical oxygen demand at 5 days;

TP  total phosphorus.

Same degree as achieved in conventional activated sludge.

U sed in particular if wastewater is fresh and low in readily biodegradable organic matter; can be used with any of the other

processes.

Approximate effluent concentration.

Source: WEF, 2006b.

28-9

TABLE 28-6

Conceptual process selection for nutrient removal

Effluent quality

Process Secondary

5 mg/L

BOD

5 mg/L

TSS

Nitrification

10 mg/L

nitrate

nitrogen

3 mg/L

total

nitrogen

1.0 mg/L

total

phosphorus

0.5 mg/L

total

phosphorus

Activated sludge X M X M

Extended aeration (oxidation ditch) X M X X M

A/O

™

XMX M M

Modified Ludzack-Ettinger X M X X X

Operationally modified activated sludge X M X M M M

PhoStrip

™

XMX M M X X

University of Cape Town and VIP X M X X X M

™

XMX X X M

Trickling filters XM

Fluidized bed

MMXX

Postaeration anoxic rank

Two-sludge process

XMX X XX

Three-sludge process with chemical

addition

XMX X XX X X

Denitrification filters

XXX

Bardenpho

™

XMX X M

Modified Bardenpho

™

XMX X M M

Simpre

™

XMX X XM

Bionutre

™

XMX X XM M

OWASA nutrification X M X X M M

Sequencing batch reactors XMX M XM M

Phase isolation ditches XMX M MM M

Chemical addition (alum, lime, or iron salts)XX

X—process capable of producing effluent meeting indicated standard: M—process should be capable of meeting indicaled standard with proper design, acceptable

influent characteristics, and/or tertiory filtration.

20–30 mg/L effluent biochemical oxygen demand (BOD

) and total suspended solids (TSS).

Filtration recommended to meet indicated standard.

Requires methanol addition for denitrification.

Source: WEF, 1998.

28-10 WATER AND WASTEWATER ENGINEERING

TABLE 28-7

Advantages and limitations of activated sludge processes for BOD removal and nitrification

Process Advantages Limitations

Complete mix Common, proven process Susceptible to filamentous sludge bulking

(CMAS) Adaptable to many types of wastewater

Large dilution capacity for shock and toxic loads

Uniform oxygen demand

Design is relatively uncomplic

ated

Suitable for all types of aeration equipment

Conventional plug

flow

Proven process

May achieve a somewhat higher level of ammonia

removal than the complete-mix process

Adaptable to many operating schemes including step-

feed, selector design, and anoxic/aerobic processes

Design and operation for tapered aeration is

ore complex

May be difficult to match oxygen supply to

oxygen demand in first pass

High rate Requires less aeration tank volume than conventional

plug flow

Less stable operation; produces lower-quality

effluent

Uses less aeration energy Not suitable for nitrification

Sludge production is higher

High peak flows can di

srupt operation by

washing out MLSS

Contact stabilization Requires smaller aeration volume

Handles wet-weather flows without loss of MLSS

Has little or no nitrification capability

Operation somewhat more complex

0%(40)

Conventional activated sludge

(10%–30%)

—MLE

—A

—PhoStrip II

—Oxidation ditch

—Biodenitro

—Simpre

—UCT and VIP

—4-Stage Bardenpho

—Modified Wuhrman

—Dual sludge

—Three sludge

—Postaeration anoxic

tank with methanol

—Denitrification filters

—Fluidized bed reactors

—Phase isolation ditches

—Dual sludge with chemicals

—Modified Bardenpho

with

chemicals

—A

/O with denite filters

and chemicals

—Three sludge with chemicals

—PhoStrip

—Chemical precipitation

plus filters

—Modified Bardenpho

—A

/O with denite filters

—Biodenipho

—PhoStrip

—Operationally modified

activated studge

—UCT

—PhoStrip II

—A/O

—PhoStrip

—Sequencing batch

reactors (SBRs)

—OWASA

Conventional activated

sludge

Conventional activated sludge

(10%–25%)

Nitrogen

removal

Nitrogen and

phosphorus removal

Phosphorus

removal

30%(28)

80%(8)

Approximate percentage removal (typical concentration, mg/L)

95%(2)

0%(10)

30%(7)

80%(2)

98%(0.20)

Filters suggested

99%

FIGURE 28-1

Process selection matrix for nutrient removal (MLE  modified Ludzack-Ettinger and UTC  University of Cape Town).

(Source: WEF, 1998.)

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-11

TABLE 28-7 (continued)

Process Advantages Limitations

Step feed Distributes load to provide more uniform oxygen

demand

Peak wet-weather flows can be bypassed to the last

pass to minimize high clarifier solids loading

Flexible operation

Adaptable to many operating schemes including

anoxic/aerobic process

More complex operation

Flow split is not usually measured or known

accurately

More complicated design for process and

aeration system

Extended aeration High-quality effluent possible Aeration energy use is high

Relatively uncomplicated design and operation Relatively large aeration tanks

Capable of treating sho

ck/toxic loads Adaptable mostly to small plants

Well-stabilized sludge; low biosolids production

High-purity oxygen Requires relatively small aeration tank volume

Emits less VOC and off-gas volume

Generally produces good settling sludge

Operation and DO control are relatively

complicated

Adaptable to many types of wastewater

Limited capability for nitrification

More complex equipment to install, operate, and

maintain

Nocardia foaming

High peak flows can disrupt operation by

washing out MLSS

Oxidation ditch Highly reliable process; simple operation

Capable of treating shock/toxic load

s without

affecting effluent quality

Economical process for small plants

Uses less energy than extended aeration

Adaptable to nutrient removal

High-quality effluent possible

Well-stabilized sludge; low biosolids production

Large structure, greater space requirement

Low F/M bulking is possible

Some oxidation ditch process modifications are

proprietary and license fees may be required

Requires more aeration energy than conventional

CMAS and plug-flow treatment

Plant capacity expansion is more difficult

Sequencing batch

reactor

Process is simplifie

d; final clarifiers and RAS

pumping are not required

Compact facility

Operation is flexible; nutrient removal can be

accomplished by operational changes

Can be operated as a selector process to minimize

sludge bulking potential

Quiescent settling enhances solids separation

(low effluent suspended s

olids)

Applicable for a variety of plant sizes

Process control more complicated

High peak flows can disrupt operation unless

accounted for in design

Batch discharge may require equalization prior

to filtration and disinfection

Higher maintenance skills required for

instruments, monitoring devices, and automatic

valves

Some designs use less efficient aeration devices

Countercurrent

aeration

High-quality effluent possible

Oxygen transfer efficiencies are higher than

conventional aeration systems

Well-stabilized sludge; low biosolids production

Process design

can be modified to accommodate

nutrient removal

Fine screening is required to prevent diffuser

fouling

Process is proprietary

Significant downtime of aeration unit for

maintenance will affect plant performance

Good operator skills required

Source: Metcalf & Eddy, 2003.

28-12 WATER AND WASTEWATER ENGINEERING

TABLE 28-8

Advantages and limitations of nitrogen-removal processes

Process Advantages Limitations

Preanoxic—general Saves energy; BOD is removed before

aerobic zone

Alkalinity is produced before nitrification

Design includes an SVI

selector

MLE Very adaptable to existing activated sludge

processes

5 to 8 mg/L TN

is achievable

Nitrogen-removal capability is a function of

internal recycle

Potential Nocordia growth problem

DO control is required before recycle

Step feed Adaptable to existing step-feed activated-

sludge processes

With internal recycle in last pass, nitrogen

concentrations less than 5 mg/L are possible

5 to 8 mg/L TN is achievable

Nitrogen-removal capability is a function of flow

distribution

More complex operation than MLE; requires

flow split control to optimize operation

Potential Nocordia growth problem

Requires DO control in each aeration zone

Sequencing batch reactor Process is flexible and easy to operate

Mixed-liquor solids cannot be washed out by

hydraulic surges bec

ause flow equalization is

provided

Quiescent settling provides low effluent TSS

concentration

5 to 8 mg/L TN is achievable

Redundant units are required for operational

reliability unless aeration system can be

maintained without draining the aeration tank

More complex process design

Effluent quality depends upon reliable decanting

facility

May need effluent equalization of batch

disc

harge before filtration and disinfection

Batch decant 5 to 8 mg/L TN is achievable Less flexible to operate than SBR

Mixed-liquor solids cannot be washed out by

hydraulic surges

Effluent quality depends upon reliable decanting

facility

Bio-denitro

™

5 to 8 mg/L TN is achievable Complex system to operate

Large reactor volume is resistant to shock

loads

Two oxidation ditch reactors are required;

increases construction cost

Nitrox

™

Large reactor volume is resistant to shock

loads

Easy and economical to upgrade existing

oxidation ditch processes

Provides SVI control

Nitrogen-removal capability is limited by higher

influent TKN concentrations

Process is susceptible to ammonia bleed-through

Performance is affected by influent variations

Bardenpho

™

(4-stage) Capable of achieving effluent nitrogen levels

less than 3 mg/L

Large reactor volumes required

Second anoxic tank has low efficiency

Oxidation ditch Large reactor volume is resistant to load

variations without affecting effluent quality

significantly

Nitrogen-removal capability is related to skills of

operating staff and

control methods

Has good capacity for nitrogen removal; less

than 10 mg/L effluent TN is possible

Postanoxic with carbon

addition

Capable of achieving effluent nitrogen levels

less than 3 mg/L

Higher operating cost due to purchase of

methanol

May be combined with effluent filtation Methanol feed control required

(continued)

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-13

TABLE 28-9

Advantages and limitations of phosphorus-removal processes

Process Advantages Limitations

Phoredox (A/O

™

) Operation is relatively simple when compared

to other processes

Low BOD/P ratio possible

Relatively short hydraulic retention time

Produces good settling sludge

Good phosphorus removal

Phosphorus removal declines if nitrification

occurs

Limited process control flexibility is available

™

Removes both nitrogen and phosphorus

Provides alkalinity for nitrification

Produces good settling sludge

Operation is relatively simple

Saves energy

RAS containing nitrate is recycled to anaerobic

zone, thus affecting phosphorus-removal

capability

Nitrogen removal is limited by internal recycle

ratio

Needs higher BOD/P ratio than the A/O process

UCT Nitrate loading on anaerobic zone is red

uced,

thus increasing phosphorus-removal

capability

For weaker wastewater, process can achieve

improved phosphorus removal

Produces good settling sludge

Good nitrogen removal

More complex operation

Requires additional recycle system

VIP Nitrate loading on anaerobic zone is reduced,

thus

increasing phosphorus-removal

capability

Produces good settling sludge

Requires lower BOD/P ratio than UCT

More complex operation

Requires additional recycle system

More equipment required for staged operation

[Bardenpho

™

(5-stage) Can achieve 3 to 5 mg/L TN in unfiltered

effluent

Produces good settling sludge

Less efficient phosphorus removal

Requires larger tank volumes

TABLE 28-8 (continued)

Process Advantages Limitations

Simultaneous nitrification/

denitrification

Low effluent nitrogen level possible (3 mg/L

lower limit)

Large reactor volume; skilled operation also

required

Significant energy savings possible Process control system required

Process may be incorporated into existing

facilities withou

t new construction

SVI control enhanced

Produces alkalinity

SVI  sludge volume index

TN  total nitrogen

TSS  total suspended solids

Source: Metcalf & Eddy, 2003.

(continued)