Water and Wastewater Engineering

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23-8 WATER AND WASTEWATER ENGINEERING

is separated from the activated sludge in a settling tank. The wastewater is discharged and the

settled activated sludge is sent to a second reactor (called a stabilization tank ) where aeration is

continued. Here the stored and adsorbed COD is oxidized ( Figure 23 -2g ).

The advantage of this system is the reduction in overall tank volume. The disadvantage is

that the system requires substantial operator skill and attention.

Conventional with Selector. Somewhat like the contact stabilization process, the detention

time in the selector is too short for complete BOD oxidation. The detention time is shorter than

that provided in the contac

t stabilization process. This system promotes the formation of floc that

will settle. The selector is followed by an aeration tank of conventional design ( Figure 23-2h ).

Tapered Aeration. This is an alternative to step feeding. The air supply is increased at the

head end of the conventional plug-flow tank. It is tapered to lower levels along the tank. Provided

that the inlet organic load is not so high that oxy

gen cannot be supplied to meet the demand, this

system reduces power costs and equipment sizes.

Extended Aeration. This process is used primarily to treat wastewater flows from small

residential communities. Process aeration is extended to 24 hours or

more. Under these conditions

endogenous respiration (Chapter 22, Equation 22-21) governs the oxidation process. This

minimizes the sludge mass.

While these systems can achieve good results, they have experienced problems with poor

settling sludge, low pH due to nitrification, and high suspended soli

ds in the effluent when

operated in a conventional plug-flow reactor. With adequate operator supervision, these problems

have been successfully overcome when extended aeration has been applied in an oxidation ditch.

23-3 PROCESSES FOR DENITRIFICATION

A s discussed in Chapter 22, biological nitrogen removal requires both an aerobic and an anoxic

zone. It is also referred to as nitrification-denitrification (NDN or BNDN). In addition, nitrate re-

duction requires an electron donor. This can be supplied by the influent wastewater, endogenous

respiration, or an external (exogenous) carbon sour

ce. Table 23-2 lists some of the configurations

used for biological denitrification. These are discussed in the following paragraphs.

Modified Ludzack-Ettinger (MLE). This preanoxic process is one of the most commonly

used for denitrification. It relies on the return of nitrate formed in the aerobic zone to the anoxic

zone ( Figure 23-4a ). The provision of an internal recyc

le is the “mod ification” to the original

process. Both the denitrification rate and the overall nitrogen removal efficiency are increased by

this modification.

TABLE 23-2

Selected biological denitrification configurations

Preanoxic Postanoxic

Modified Ludzack-Ettinger (MLE) Single sludge

Step feed Bardenpho

(4-stage)

Sequencing batch reactor (SBR) Oxidation ditch

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-9

Aerobic Aerobic

Return activated sludge

Mixed-liquor return

Effluent

Sludge

Influent

Anoxic Anoxic

Secondary

clarifier

Anoxic

Return activated sludge

Effluent

Sludge

Secondary

clarifier

Influent

Aerators

Aerobic

Sludge

Effluen

Secondary

clarifier

Return activated sludge

Aerobic

Anoxic

Aerobic

Anoxic

Aerobic

Influent

Anoxic

Aerobic

Anoxic

Fill React/aerationFill

anoxic/anaerobic

mix

Settle IdleDecant

Effluent

Influent

Air

Secondary

clarifier

Sludge

Return activated sludge

Effluent

Influent

Anoxic

Secondary

clarifier

Sludge

Return activated sludge

Internal recycle

Effluent

Influent

AerobicAnoxic

(a) Modified Ludzack-Ettinger (MLE)

(b) Step feed

(d) Single-sludge

(e) Bardenpho™ (4-stage)

(f) Oxidation ditch

Preanoxic

Postanoxic

Aerobic

FIGURE 23-4

Processes for denitrification.

(Adapted from Metcalf &

Eddy, 2003.)

Step Feed. The step feed process can be modified to perform biological nitrogen removal (BNR)

as shown in Figure 23-4b . The final flow portion to the last anoxic/aerobic zone is critical in

defining the final effluent concentration of NO

-N as the nitrate in that zone will not be reduced.

Sequencing Batch Reactor (SBR). The SBR system for BOD oxidation and nitrification is

modified by an additional operational step ( Figure 23-4c ). Although sufficient BOD and fill time

are available to remove almost all of the nitrate after the settle and decant steps, a separate mixing

step without aeration provides more flexibility a

s well as improved nitrogen removal.

Single Sludge. In this process a mixed anoxic tank follows the aerobic tank ( Figure 23-4d ).

To achieve a high nitrate removal efficiency, a long detention time in the anoxic tank is required

because the denitrification rate is proportional to the endogenous respiration rate.

Bardenpho

(4-Stage). Both preanoxic and postanoxic processes are incorporated in the

Bardenpho four-stage process ( Figure 23 -4e ). The hydraulic detention time of the postanoxic

stage is about the same or longer than the preanoxic stage. In actual practice it was discovered

that phosphorus rem

oval also occurred. The process name is derived from the inventor’s name

(Barnard, 1974) and the truncation of “denitrification” and “phosphorus” removal.

Oxidation Ditch. By increas ing the length of the oxidation ditch to provide an anoxic zone

after the aerobic zone, BNR can be achieved in a single tank ( Figure 23-4f ). Most of the BOD is

removed in the aerobic zone. Nitrate is us

ed for endogenous respiration. A large tank volume and

long sludge retention times are required for efficient BNR.

23-4 PROCESSES FOR PHOSPHORUS REMOVAL

A s discussed in Chapter 22, biological phosphorus removal (BPR) requires an anaerobic zone

followed by an aerobic zone. The alternating exposure to anaerobic and aerobic conditions can

be accomplished in the main biological treatment process (called mainstream ) or in the retu rn

sludge stream (called side stream) There are several mod

ifications to the basic process. Among

the most common are those that incorporate biological nitrogen removal. Table 23-3 lists some of

the mainstream configurations used for BPR. These are discussed in the following paragraphs.

Phoredox. This is the name given by Barnard (1975) to represent any anaerobic/aerobic

sequ

ence to promote BPR. It is shown in Figu re 23-5a . A version of this process with multiple

stages is patented as A/O ™ (anaerobic/aerobic). Thes e processes are not designed to promote

nitrification/denitrification (Metcalf & Eddy, 2003). The anaerobic detention time is 30 min to 1 h.

The SRT of the aerobic zone is 2 to 4 d.

TABLE 23-3

Selected mainstream biological

phosphorus removal configurations

Phoredox

Anaerobic/anoxic/aerobic (A

/O™)

Bardenpho™ (5-stage)

University of Cape Town (UCT)

Sequencing batch reactor (SBR)

23-10 WATER AND WASTEWATER ENGINEERING

Fill Anaerobic

react (mixed)

Aerobic Anoxic

(mixed)

Settle Decant

Effluent

Influent

Air

Secondary

clarifier

Sludge

Effluent

Return activated sludge

Influent

Anaerobic Aerobic

Secondary

clarifier

Recycle

Sludge

(containing P)

Effluen

Return activated sludge

Influent

Anaerobic

Aerobic

Anoxic

Secondary

clarifier

Recycle

Sludge

(containing P)

Effluen

Return activated sludge

Influent Anaerobic Aerobic

Aerobic

Anoxic Anoxic

Secondary

clarifier

Anoxic recycle

Anoxic (nitrate) recycle

Sludge

(containing P)

Effluent

Return activated sludge

Influent Anaerobic Aerobic

Anoxic

(a) Phoredox (A/O

)

(b) A

(5-stage)

(d) UCT (standard and modified)

(e) SBR with biolo

ical phosphorus removal

FIGURE 23-5

Processes for biological phosphorus

removal.

(Adapted from Metcalf & Eddy,

2003.)

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-11

23-12 WATER AND WASTEWATER ENGINEERING

/O ™ . This is a proprietary modification of the A/O ™ process that provides internal recycle

and an anoxic zone for denitrification ( Figure 23-5b ). The detention period in the anoxic zone is

approximately 1 h.

Bardenpho ™ (5-Stage). This modification of the four-stage process provides for both denitri-

fication and phosphorus removal (

Figure 23-5c ). The staging and recycle differ from the A

™

process. The five-stage process uses a longer SRT than the A

/O ™ and thus increases the carbon

oxidation capability.

University of Cape Town (UCT). The UCT process was developed at the University of Cape

Town in South Africa. It is similar to the A

/O ™ process with two exceptions. The return sludge

is recycled to the anoxic stage instead of the aeration stage, and the internal recycle is from the

anoxic stage to the anaerobic stage ( Figure 23-5d ). By returning the sludge to the anoxic stage,

the introduction of nitrate to the anaerobic stage is avoided. This improves the phos

phorus uptake.

The internal recycle feed provides increased organic utilization in the anaerobic stage.

Sequencing Batch Reactor (SBR). The six operational steps of the SBR denitrification process

are retained, but the conditions are modified ( Figure 23-5e ). One alternative is to provide an anoxic

period after sufficient aerobic time has elapsed

for nitrification to occur. Another alternative is to use

cyclic aerobic and anoxic periods during the react period. This minimizes the nitrate concentration

before settling. Little nitrate is available to compete for rbCOD during the fill period so that rbCOD

uptake and storage by PAOs can occur instead of rbCOD consumption by nitrate reducing bacteria.

23-5 BIOLOGICAL TREATMENT WITH MEMBRANE SEPARATION

M e mbrane biological reactors (MBRs) consist of a biological reac tor with suspended biomass

and solids separation by mic rofiltration (MF) or ultrafiltration (UF) membranes. They may be

used with any of the processes described in the previous sections of this chapter. Membranes are

intro

duced in Chapter 9. MF filtration theory, practice, and design are discussed in Chapter 12.

The following discussion focuses on the application to wastewater systems in contrast to the water

treatment applications discussed in Chapter 12.

Process Description

MBRs have two fundamental process arrangements: (1) integrated systems that have membranes

immersed in an activated sludge reactor and (2) separate systems that have a membrane module

placed outside the reactor ( Figu re 23-6 ). Immersed membranes

using hollow-fiber or flat sheet

membranes are the most popular for several reasons. They operate at lower pressures, readily

accommodate variations in the types of biosolids found in activated sludge bioreactors, concen-

trate biosolids without settling concerns, and, typically, have a lower life cycle cos

t for municipal

systems. Separate systems use pressure-driven, in-pipe cartridge membranes. These are more

prevalent in industrial settings (Metcalf & Eddy, 2003; WEF, 2006b).

Process arrangements for implementation of MBR for nitrification, nitrogen removal,

and complete biological nutrient removal (BNR) are illustrated in Figure 23-7 . In contrast to

onventional activated sludge or typical BNR processes, the volume of s ludge returned to the

aeration basin is on the order of 400 percent of the wastewater flow.

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-13

(a)

(b)

Air

Influent

Effluent to

disinfection

Effluent to

disinfection

Membrane

module

Membrane

separation

unit

Bioreactor

FIGURE 23-6

S chematic diagram of membrane bioreactors: ( a ) immersed

membrane, ( b ) external membrane. ( Source: Metcalf &

Eddy, 2003.)

Air

Influent

Return activated sludge

Permeate

Aeration tank

Membrane tank

Air

Influent

Air

Return activated sludge

Permeate

Anoxic

tank

Aerobic

tank

Membrane

tank

Anoxic

tank

Anaerobic

tank

Aerobic

tank

Membrane

tank

Air

Permeate

(a) Car

bonaeous BOD oxidation and nitrification

(b) Nitrogen removal

FIGURE 23-7

Process arrangements for membrane bioreactors (MBR).

23-14 WATER AND WASTEWATER ENGINEERING

23-6 SUSPENDED GROWTH DESIGN PRINCIPLES

Overview

The design principles of suspended growth processes may be broadly separated into two categories:

those based on experience and those based on microbial biochemistry and microbial population

dynamics. Facultative oxidation ponds fall into the first category. The re

maining suspended growth

treatment systems fall into the second category. In the following discussion the basic biological reac-

tions of the facultative oxidation pond are discussed as well as the empirical basis for their design.

The completely mixed, conventional plug-flow, and batch reactor

models that are presented

in the next section serve as means of showing the relationship between several design variables

for suspended growth processes. Mass balance relationships and applied microbial biochemistry

relationships provide the basis for other process design relationships.

Oxidation Ponds

Although empirical expressions have been developed for the design of oxidation ponds, the design

of facultative ponds is generally governed by prescriptive requirements of state regulatory agencies.

Facultative Ponds. A schematic representation of a facultative pond operation is given in

Figure 23-8 . Raw wastewater may enter at the center of the pond

or at one end. Suspended

solids contained in the wastewater settle to the pond bottom, where an anaerobic layer develops.

Microorganisms occupying this region do not require molecular oxygen as an electron acceptor

in energy metabolism, but rather use some other chemical species. Both acid fermentation and

methane fermentation occur in the bottom sludge deposits.

The facultative zone exists just above the anaerobic zone. This means that molecular oxygen

will not be available in the region at all times. Generally, the zone is aerobic during the daylight

hours and anaerobic during the hours of darkness.

Energy

Aerobic

zone

Anaerobic

zone

Sludge

Inlet

1.5 m

Organic Acids CH

 CO

Organic Acids

Organic compounds

Organic compounds  O

 H

Algae  O

 H

O  Light

FIGURE 23-8

S chematic diagram of facultative pond relationships. ( Source: Davis and Cornwell, 2008.)

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-15

Above the facultative zone, there exists an aerobic zone that has molecular oxygen present at

all times. The oxygen is supplied from two sources. A limited amount is supplied from air diffu-

sion across the pond surface. However, the majority is supplied through algal photosynthesis.

Completely Mixed Reactor Model

In 1970, Lawrence and McCarty proposed a model of the activated sludge proc ess based on

microbial biochemis try and microbial population dynamics. It serves as starting point for

understanding the design principles used in biological treatment reactor design. It is ba

sed on the

Monod equation (Chapter 22, Equation 22-16) and mass balances across a defined volum e for

specific constituents of interest such as biomass and substrate.

Mass Balance for Biomass. A mass balance diagram for the completely mixed system (CSTR)

is shown in Figu

re 23-9 . The mass balance equations are written for the system boundary shown

by the dashed line. Two mass balances are required to define the design of the reactor: one for bio-

mass and one for substrate ( readily biodegradable soluble chemical oxygen demand, rbsCOD).

U n der steady-state conditions, the mass balance for biomass

may be written as:

Biomass in Net biomass Biomass in Biomass

inffluent growth effluent wasted

(23-1)

The biomass in the influent is the product of the concentration of microorganisms in the

influent (X

) and the flow rate of wastewater ( Q ). The concentration of microorganisms in the

influent (X

) is measured as suspended solids * ( mg/L). The biom ass that grows in the aeration

tank is the product of the volume of the tank (

) and the Monod expression for growth of micro-

bial mass (Equation 22-16):

()







⎛

⎝

⎜

⎞

⎠

⎟

(23-2)

The biomass in the effluent is the product of flow rate of treated wastewater leaving the plant

(Q  Q

) and the concentration of microorganisms that does not settle in the secondary clarifier

( X

). The flow rate of wastewater leaving the plant does not equal the flow rate into the plant

because some of the microorganisms must be wasted. The flow rate of wasting ( Q

) is deducted

from the flow exiting the plant.

(Q + Q

)

, X

, S

Return sludge

(Q  Q

), S, X

, X

, S

Q, S

, X

X, S

X, S, V

Aeration tank

Secondary

settling tank

FIGURE 23-9

Completely mixed biological reactor with solids

recycle. ( Source: Davis and Cornwell, 2008.)

*Because the model is for soluble COD, the COD of the suspended solids in the influent is not considered. This problem is

addressed in Example 23-1.

23-16 WATER AND WASTEWATER ENGINEERING

The biomass that is wasted is the product of the concentration of microorganisms in the WAS

flow ( X

) and the WAS flow rate ( Q

). The narrative mass balance equation may be rewritten as

kX Q Q X QX

dwewr0



 ( ) ( )



⎛

⎝

⎜

⎞

⎠

⎟

(23-3)

The variables are summarized as follows:

Q  wastewater flow rate into the aeration tank, m

/ d

 microorganism concentration (volatile suspended solids or VSS) * entering aeration

tank, mg/L

 volume of aeration tank m,



 maximum growth rate constant, d

 1

S  readily biodegradable soluble COD (rbsCOD) in aeration tank and effluent, mg/L

X  microorganism concentration (mixed-liquor volatile suspended solids or MLVSS) **

in the aeration tank, mg/L

 half velocity constant

 soluble BOD

concentration at one-half the maximum growth rate, mg/L

 decay rate of microorganisms, d

 1

 flow rate of liquid containing microorganisms to be wasted, m

/ d

 microorganism concentration (VSS) in effluent from secondary settling tank, mg/L

 microorganism concentration (VSS) in sludge being wasted, mg/L

Mass Balance for Substrate. At steady-state, the mass balance equation for substrate (rbsCOD)

may be written as

Substrate in Substrate Substrate in Substratte in

influent consumed effluent WAS 

(23-4)

The substrate in the influent is the produ

ct of the concentration of rbsCOD in the influent ( S

) and

the flow rate of wastewater ( Q ). The substrate that is consumed in the aeration tank is the product

of the volume of the wastewater in the tank

( )

a n d the expression for the rate of substrate uti-

lization

( )

()

YK S





⎛

⎝

⎜

⎞

⎠

⎟

(23-5

)

The substrate in the effluent is the product of flow rate of treated wastewater leaving the plant

( Q  Q

) and the concentration of rbsCOD in the effluent ( S ). The concentration of rbsCOD in

*Suspended solid s means that the material will be retained on a filter, unlike dissolved solids such as NaCl. The amount of

suspended solids that volatilizes at 500  C  50  C is taken to be a measure of active biomass. The presence of nonliving organic

particles in the influent wastewater will cause some error (

usually small) in the use of VSS as a measure of biomass.

**Mixed-liquor volatile suspended solids is a measure of the active biomass in the aeration tank. The term “mixed liquor”

implies a mixture of activated sludge and wastewater. The phrase “volatile suspended solids” has the sa

me meaning as in the

definition of X

SECONDARY TREATMENT BY SUSPENDED GROWTH BIOLOGICAL PROCESSES 23-17

the effluent ( S ) is the same as that in the aeration tank because it is assumed that the aeration tank

is completely mixed. Because the rbsCOD is soluble, the secondary settling tank will not change

the concentration. Thus, the effluent concentration from the secondary settling tank is the same

as the influent con

centration.

The substrate in the waste activated slu dge flow is the product of the conc entration of

rbsCOD in the effluent ( S ) and the WAS flow rate ( Q

). The narrative mass balance equation for

steady-state conditions may be rewritten as

YK S

QQSQS





 ( )

()



⎛

⎝

⎜

⎞

⎠

⎟

(23-6)

where Y  yield coefficient (see Equation 22-17).

To develop working design equations the following assumptions are made:

1 . The influent and effluent biomass concentrations are negligible compared to that in the

reactor.

2. The influent substrate ( S

) is immediately diluted to the reactor concentration in accor-

dance with the definition of a CSTR.

3. All reactions occur in the CSTR.

From the first assumption the following terms may be eliminated from Equation 23-3 : QX

and

( Q  Q

) X

because X

and X

are negligible compared to X. Equation 23-3 may be simplified to

( )

kX QX

dwr







⎛

⎝

⎜

⎞

⎠

⎟

(23-7)

For convenience, Equation 23-7 is rearranged in terms of the Monod equation:







⎛

⎝

⎜

⎞

⎠

⎟

( )

(23-8)

Equation 23-6 may also be simplified and rearranged in terms of the Monod equation:







⎛

⎝

⎜

⎞

⎠

⎟

()

(23-9)

Noting that the left side of Equations 23-8 and 23-9 are the same, set the right-hand side of these

equations equal and rearrange to give:

QX QY

()

XXV

(23-10)

Two parts of this equation have physical significance in the design of a c ompletely m ixed acti-

vated sludge system. The inverse of Q / V is the hydraulic detention time (  ) of the reactor:

 

(23-11)