Water and Wastewater Engineering

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28-14 WATER AND WASTEWATER ENGINEERING

TABLE 28-10

Advantages and disadvantages of MBR reactors

Parameter Comment

Advantages

Effluent quality Consistently high-quality with effluent solids concentrations less than 1 mg/L

Footprint Footprint is smaller because primary and secondary clarifiers can be eliminated. Aeration

basin can be smaller because MLSS concentration is higher.

Sludge prod

uction Long SRTs result in lower sludge production compared to conventional activated sludge

Ease of expansion Because the membrane systems are modular, they can easily be expanded

Robust operation System can operate within a wide range of SRTs

Reduced disinfection require

ments Because the membrane serves as a physical mechanism to remove microorganisms and

turbidity, the chlorine demand is less and the higher transmissivity means less energy is

needed for UV disinfection

Disadvantages

Limited flow capacit

y Because of the hydraulic limits of the membrane, the range of flows that can be treated is

less than conventional activated sludge. The peaking factor is 2.0 to 2.5.

Increased potential for foam Operating conditions in a MBR system often favor foaming

System monitoring and maintenance MBRs must be closely monitored to d

etect changes in flux rate and permeability.

Cost Although the capital cost of membranes is falling rapidly, the capital cost is still

significantly higher than that of conventional activated sludge

Limited availability of long-term data Because of the limited availability of long-term data, it is difficult to verify

man

ufacturer’s claims

A dapted from WEF, 2006c.

TABLE 28-9 (continued)

Process Advantages Limitations

SBR Both nitrogen and phosphorus removal are

possible

Process is easy to operate

Mixed-liquor solids cannot be washed out by

hydraulic surges

Quiescent settling may produce lower effluent

TSS concentration

Flexible operation

More complex operation for N and

P removal

Needs larger volume than SBR for N removal

only

Effluent quality depends upon reliable decanting

facility

Design is more complex

Skilled maintenance is required

More suitable for smaller flow roles

PhoStrip

™

Can be incorporated easily into existing

activated sludge plants

Process is flexible; phosphorus-removal

performance is not controlled by BOD/

phosphorus ratio

Significantly less chemical usage than

mainstream chemical precipitation process

Can achieve reliable effluent orthophosphate

concentrations

less than 1 mg/L

Requires lime addition for phosphorus

precipitation

Requires higher mixed-liquor dissolved oxygen

to prevent phosphorus release in final clarifier

Additional tank capacity required for stripping

Lime scaling may be a maintenance problem

Source: Metcalf & Eddy, 2003.

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-15

TABLE 28-11

Typical average day effluent concentrations from granular media filtration of secondary effluent

Filter influent type

Without chemical coagulation—

single or multimedia filter,

effluent SS, mg/L

With tertiary chemical coagulation—dual

or multimedia filter

Effluent SS,

mg/L Total P, mg/L

Turbidity,

NTU

High-rate trickling filter effluent 10–20 0–3 0.1 0.1–0.4

Two-stage trickling filter effluent 6–15 0–3 0.1 0.1–0.4

Contact stabilization effluent 6–15 0–3

0.1 0.1–0.4

Conventional activated sludge effluent 3–10 0–5 0.1 0.1–0.4

Extended aeration effluent 1–5 0–5 0.1 0.1–0.4

Aerated/facultative lagoon effluent 10–50

0–30

0.1

N/A

Poor removal efficiency can result from filtering lagoon effluent because of the presence of algae.

Source: WEF, 1998.

Other Considerations

Infiltration and Inflow Reduction. The implementation of comprehensive programs to reduce

infiltration and inflow have a direct impact on process performance and an indirect impact on

process selection. Plant influent flow rates may be reduced between 5 and 25 percent. In general,

this would be expected to have a positive effect on clarifier performanc

e. However, the wastewa-

ter strength is also likely to increase significantly.

Water Conservation. The increasing c ost of water and wastewater has resulted in reductions

in industrial water use. These activities coupled with implementation of water conservation

devices and new “tight” sewer systems may result in significant in

creases in both the BOD and

solids concentration entering the plant. The lower flow rates and higher strength waste will have

a major impact on process selection and residuals management.

Special Considerations for MBRs. In the applic ation of MBRs, the traditional procurement

and delivery methods for equipment are evolving. Because of the manufacturer-specific m

em-

brane design, which prevents the competitive procurement of replacement membranes, and the

very high cost of membranes , owners are requiring performance guarantees. The resulting liabil-

ity issues have resulted in MBR supplier requirements for involvement in the design of the over-

all process scheme. In the m ost controlle

d scenario, the MBR is selected before the upstream

and downstream processes are designed. Thes e processes are then designed “around” the MBR.

There are many variations of this approach including specifying the treatment requirements and

open bidding the membrane supplier with life cycle

cost being the d eciding factor.

Process Selection Examples

The following three case studies were selected to demonstrate the wide range of choices in select-

ing a process and to illustrate some of the logic that was used in making the c hoices. A litera-

ture review of the many other examples that are reported in the Water Environment Federation

journals ( Water Environment Research and Water Environment and Technology )

should be part

of any study to evaluate process alternatives.

TABLE 28-12

Add-on processes—general considerations

Process capability Process control

Operational

factors

Sidestreams

and recycle Solids

Air

emissions

Energy

requirement

Space

requirement

Filtration Suspended solids

removal to  10

mg/L

Headloss

and flow rate

control typically

automated

Three basi

modes: constant

pressure,

constant

rate; variable

declining rate;

backwash

Backwash

water

Sludge from

solids contained

in backwash

None Low to

moderate

Low

Adsorption Dissolved organics

removal to  99%;

some toxic metal

removal

Headloss, flow

rate, and pollutant

breakthrough

control

Backwash,

column

downtime

for carbon

replacement

Backwash

water

None

None Low to

moderate

Low

Chemical

treatment

Phosphate removal

to 1 mg/L; metals

removal to 1

mg/L, acid–base

neutralization

pH, ORP,

chemical dosage

Chemical

storage,

chemical feed,

mixing

Drainage

from chemical

sludge

dewatering

Hydroxide,

carbonate,

and phosphate

precipitates;

other chemical

sludges

Dust from

chemical

handling

and storage

Moderate to

high

Moderate to

high

Membrane

processes

Removal of

TSS, TDS,

microorganisms,

viruses, and some

organic compounds

Pretreatment,

transmembrane

pressure,

concentrate flow,

membrane flux

Pretreatment

requirements,

concentrate

disposal

Concentrate,

cleaning

solutions

Solids from

backwash

on MF/UF

processes

Fugitive

emissions

from

chemical

storage and

use

Low to high Low to

moderate

Air stripping Volatile organic

carbon removal to

 99%

Ammonia removal

to  99%

Minimal air-to-

water ratio

Minimal pH and

air-to-water ratio

Possible

column wash

liquid

Possible

carbonate

precipitates

Volatile

organics,

ammonia, or

both

Low to

moderate

Low

Reox

ygenation

(postaeration)

Effluent dissolved

oxygen increased

to nearly saturation

concentration

Dissolved oxygen

level

Aeration rate None None Minimal,

volatile

chemicals

Moderate to

high

Moderate

Regeneration of spent carbon can produce air emissions.

ORP  oxidation–reduction potential.

TDS  total dissolved solids.

M F/UF  microfiltration/ultrafiltration.

Source: WEF, 1998.

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-17

TABLE 28-13

Advantages and disadvantages of thickening methods

Method Advantages Disadvantages

Gravity Simple

Low operating cost

Low operator attention required

Ideal for dense rapidly settling sludges

such as primary and lime

Provides a degree of storage as well as

thickening

Conditioning chemicals not typically

required

Minimal power consumption

Odor potential

Erratic for WAS

Thickened solids concentration

limited for WAS

High space requirements for WAS

Floating solids

Dissolved air

flotation

Effective for WAS

Will work without conditioning chemicals

at reduced loadings

Relatively simple equipment components

Relatively high power consumption

Thickening solids

concentration

limited

Odor potential

Space requirements compared to

other mechanical methods

Moderate operator attention

requirements

Building corrosion potential, if

enclosed

Requires polymer for high solids

capture or increased loading

Centrifuge Space requirements

Control capability for process performance

Effective for WAS

Contained process minimizes

housekeeping and odor considerations

Will work without conditioning chemicals

High thickened concentrations available

Relatively high capital cost and

power consumption

Sophisticated maintenance

requirements

Best su

ited for continuous operation

Moderate operator attention

requirements

Gravity belt

thickener

Space requirements

Control capability for process performance

Relatively low capital cost

Relatively lower power consumption

High solids capture with minimum

polymer

High thickened concentrations available

Housekeeping

Polymer depend

ent

Moderate operator attention

requirements

Odor potential

Building corrosion potential, if

enclosed

Rotary drum

thickener

Space requirements

Low capital cost

Relatively low power consumption

High solids capture

Can be easily enclosed

Polymer dependent

Sensitivity to polymer type

Housekeeping

Moderate operator attention

requirements

Odor potential if not enclosed

Source: WEF, 1998.

WAS  waste activated sludge

28-18 WATER AND WASTEWATER ENGINEERING

TABLE 28-14

Comparison of stabilization processes

Process Advantages Disadvantages

Anaerobic

digestion

Good volatile suspended solids

destruction (40 to 60%)

Net operational costs can be low if gas

(methane) is used

Broad applicability

Biosolids suitable for agricultural use

Good pathogen inactivation

Reduces total sludge ma

Low net energy requirements

Requires skilled operators

May experience foaming

Methane formers are slow growing; hence, “acid digester”

sometimes occurs

Recovers slowly from upset

Supernatant strong in carbonaceous oxygen demand,

biochemical oxy

gen demand, suspended solids, and ammonia

Cleaning is difficult (scum and grit)

Can generate nuisance odors resulting from anaerobic nature

of process

High initial cost

Potential for struvite (mineral deposit)

Safety issues concerned with flammable gas

Aerobic digestion Low initial co

st, particularly for small

plants

Supernatant less objectionable than

anaerobic

Simple operational control

Broad applicability

If properly designed, does not generate

nuisance odors

Reduces total sludge mass

High energy costs

Generally lower volatile suspended solids destruction than

anaerobic

digestion

Reduced pH and alkalinity

Potential for pathogen spread through aerosol drift

Biosolids typically are difficult to dewater by mechanical

means

Cold temperatures adversely affect performance

May experience foaming

Autothermal

thermophilic

aerobic diges

tion

Reduced hydraulic retention compared

with conventional aerobic digestion

Volume reduction

Excess heat can be used for building

heat

Pasteurization of the sludge, pathogen

reduction

High energy costs

Potential of foaming

Requires skilled operators

Potential for odors

Composting High-quality

, potentially saleable

product suitable for agricultural use

Can be combined with other processes

Low initial cost (static pile and

windrow)

Requires 18 to 30% dewatered solids

Requires bulking agent

Requires either forced air (power) or turning (labor)

Potential for pathogen spread through dust

High operational cost: can be power, labor, or chemical

intensive, or all three

May require significant land area

Requires carbon source

Potential for odors

Lime stabilization Low capital cost

Easy operation

Good as interim or emergency

stabilization method

Biosolids not always appropriate for land application

Chemical intensive

Overall cost very site specific

Volume of bios

olids to be disposed of is increased

pH drop after treatment can lead to odors and

biological growth

(continued)

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-19

Case Study 28-1

S o me design constraints force the design engineer to think “inside the box,” or in this case inside

the tank. This is especially true when the existing process cannot achieve new permit require-

ments for constituents that were not previously regulated, or because of population growth, or a

combination of these events. The lack of land

for plant expansion is often a driving force to think

inside the box.

Discussion.

The paper by Jackson et al. (2007) was selected to illustrate the use of integrated

fixed-film activated sludge (IFAS) and partitioning of the tank to bring a 30-year-old plant up to

a new set of standards.

The City of The Colony, Texas, clean water plant was designed as a contact stabilization

process to remove biochemical ox

ygen d emand (BOD) and total suspended solids (TSS). The

historic influent and effluent concentrations are shown in the table below. The plant was rou-

tinely capable of meeting the discharge limits of 10 mg/L BOD and 15 mg/L TSS. However, the

growth in population increased the mass loading on the stream. To protect the receiving stream,

the State of Texas im

posed additional discharge limits of 3 mg/L ammonia nitrogen (NH

-N) and

1 mg/L phosphorus. The contact stabilization process was not designed to provide nitrification or

phosphorus removal.

TABLE 28-14 (continued)

Process Advantages Disadvantages

Advanced alkaline

stabilization

Produces a high-quality Class A product

Can be started quickly

Excellent pathogen reduction

Operator intensive

Chemical intensive

Potential for odors

Volume of biosolids to be disposed of is increased

May require

significant land area

Sludge dryers Substantially reduces volume

Can be combined with other processes

Produces a Class A product

Not a biological process so it can be

started quickly

Retains nutrients

Some dryers could be labor intensive

Prod

uces an off-gas that must be treated

Source: WEF, 1998.

The Colony influent and effluent characteristics prior to renovation

Parameter Influent Effluent

Average annual flow 8,400 m

/d 8,400 m

BOD

236 mg/L 6.5 mg/L

TSS 324 mg/L 4.2 mg/L

-N 30 mg/L 9.4 mg/L

28-20 WATER AND WASTEWATER ENGINEERING

The existing configuration was a “bull’s-eye” tank with the aeration basin in an exterior

annulus and the final clarifier in the center ( Figure 28-2 a). To accomplish nitrification, the solids

retention time (SRT) was increased by removing the old divider walls and adding fixed media

to form an integrated fixed-film activated s

ludge (IFAS) unit ( Figure 28-2 b). The existing fine-

bubble diffuser system was left in place. To accomplish biological phosphorus removal, new

divider walls were installed to form anaerobic zones. To handle the additional biomass loading

on the clarifier, a splitter box was installed to route some of the flow to an external secondar

clarifier. An external addition included an anoxic tank for denitrification of the return sludge

( Figure 28-3 ).

After these modifications the plant was able to consistently meet effluent discharge require-

ments of 10, 15, and 3 mg/L of BOD

, TSS, and NH

-N, respectively.

Comment. The start-up of this plant was plagued by a number of mechanical malfunctions. The

existing fine bubble diffuser broke at one of the diffuser joints after start-up. This resulted in a

dramatic increase in air buoyancy under one IFAS unit. This caused the unit to work loo

se from

its anchor bolts. Other failures in the air fine-bubble diffuser grid and in the automatic wasting

system also occurred.

The lesson learned is that existing piping and mechanical systems need to be inspected,

tested, and repaired as part of any renovation. This is particularly

true for IFAS systems as they

stand above the diffusers after installation.

Case Study 28-2

The age of an old plant combined with population growth and the increasing likelihood more

stringent standards often brings the reality of the need for renovation. The City of Wyoming,

Michigan, began to address these issues in the early 1990s. The information for this discussion

was provided by Dave Koch, P.E., Project Manager, Black and Veatch.

Final

clarifer

To external final clarifer No. 2

Influent

Anaerobic

Effluent splitter box

IFAS

Final

clarifer

No. 1

Reaeration zone

(a)(b)

Conta

t zone

Aerob

iges

ter

FIGURE 28-2

E xisting ( a ) and proposed ( b ) aeration basin configurations.

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-21

Discussion. The Wastewater Treatment Plant Facilities Plan for the City of Wyoming was com-

pleted in 1995. In scope, it identified four phases of construction that were to begin in 1998:

(1) new preliminary treatment and biosolids facilities, (2) addition of new process facilities to meet

anticipated regulatory and flow projection requirements through 2015

, (3) addition of new biosol-

ids processing and disinfection facilities, and (4) preparation for further expansion beyond 2015.

The 1998 influent conditions and basis for design are shown in the table below. It may be

noted that the projections for 2015 were limited to wastewater flow. This is because of the long

record of data for wastewater charac

teristics and the likelihood that, because of the size of the

community, population growth would not change the characteristics of the wastewater.

City of Wyoming influent wastewater characteristics

Parameter 1998 2015

Population equivalent 183,886 249,869

Flow 60, 560 m

/d 83,270 m

BOD

312 mg/L 312 mg/L

TSS 237 mg/L 237 mg/L

-N 17 mg/L 17 mg/L

Organic nitrogen 11 mg/L 11 mg/L

Phosphorus (PO

-P) 8 mg/L 8 mg/L

VSS/TSS 0.8 0.8

All are annual averages.

The new preliminary treatment facilities were used to try out a designer–client relationship

that had not previously been used at the facility. Significant input from the users was obtained

in the design. Operators and maintenance personnel as well as management personnel were con-

sulted. Teams of experienced personnel m et the the d

esign team before and during the design

process. Operators and maintenance personnel were given the opportunity to review and com-

ment on plans. Of particular note was the completion of preliminary works that avoided confined

space and used stainless steel for critical parts. The new facility reduced the odor im

pact of the

preliminary treatment facility.

Anaerobic #1

0–0.5 hr.

Anoxic

0.5–1 hr.

Carbon

source

Inffluent

Anaerobic #2

0–0.5 hr.

IFAS

4–6 hr.

Clarifler

Effluent

WAS

RAS

FIGURE 28-3

I FAS process flow diagram.

28-22 WATER AND WASTEWATER ENGINEERING

The placement of the biological treatment processes required an analysis of four alternatives.

A rating scheme was used that considered the following criteria:

• Ease of implementation

• Construction cost

• O&M cost

• Wetlands impact

• Engineering feasibility

• Operational flexibility

Of the process options consi

dered, a biological phosphorus removal plant was selected as the

best alternative to meet future regulatory requirements. This alternative required the construction

of three new aeration basins, four new clarifiers, and a new RAS/WAS pumping station. The

same des igner-client relationship that was developed in the preliminary treatment design was

used in laying out the biological treatment process. The plant went on line in 2008.

The existing complete mix activated sludge system remains in place as backup. The existing

tricking filters were demolished.

Comments.

1 . As with all biological processes, the bio-p plant had start-up problems. Among the mos

difficult to resolve was the removal of phosphorus. The proportions of raw sewage

and return-activated sludge must be adjusted to achieve the appropriate volatile acid

fractions. Although the plant had tried to operate only two of the three aeration bas ins

because it was the beginning of its design life, ultimately all three aeration basins had to

be put into service to achieve good

phosphorus removal.

2. The trickling filters were a major source of odor complaints, and the facility was glad to

see them removed.

Case Study 28-3

In rare instances, the design engineer has the opportunity to design a new plant with a client that

has both the long-range view of the trends in regulatory requirements and the resources to allow

the engineer to design for the future. This case study illustrates both common difficulties with the

site and state-of-the-art design for the future.

The owner and operator of thi

s facility is the North Kent Sewer Authority in Kent

County, Michigan. The plant is called the PARCC Side Clean Water Plant. The design firm

was Prein&Newhof. The information for this discussion was provided by Tom Newhof of

Prein&Newhof.

Discussion. After considering five locations, vacant land that had been used for farming was

purchased. One-third of the 15.4 ha s

ite is wetland. Most of the site was below the 100-year flood

elevation. A flood plain and flood way permit was obtained and a compensating cut was made in

the flood plain and flood way.

A grading contract was let one year before construction began. Fill was moved to the site to

raise it above the 100-flood elevation. The sand fill was surcharged in the area

s where the reactor

CLEAN WATER PLANT PROCESS SELECTION AND INTEGRATION 28-23

tanks were to be placed. The design assumption was that the reactor tanks would be filled in the

event of a 100-year flood to compensate for the buoyancy effect. Earth anchors were provided for

the sludge storage tanks.

The NPDES permit requirements are summarized in the table below.

PARCC Side Clean Water Plant NPDES requirements

Parameter Summer Winter Comment

BOD

10 Daily maximum

40 7 day maximum

425 Monthly maximum

TSS 3045 7 day maximum

20 30 Monthly maximum

-N 2.0 N/A Daily maximum

0.5 N/A Monthly maximum

P 1.0 Monthly average

Units are all mg/L.

N/A  There are no winter limits.

The des ign flow rates are 30,000 m

/ d for the average day, 45,000 m

/ d for the maximum

day, and 60,000 m

/ d for the peak hour. The design average influent constituent concentrations

were: BOD

of 330 mg/L, TSS of 300 mg/L, NH

-N of 60 mg/L.

Early in the design process several alternatives were considered. The choices were narrowed

to extended aeration and MBR. The final process arrangement selected is a form of the Modified

Ludzack-Ettinger process (Figure 23-7) with an anoxic tank and a “swing” tank upstream of the

aeration tank. Although the current permit onl

y requires nitrification, this arrangement provides

the plant with the capability to meet potential future requirements for denitrification. To protect

the membranes, fine screens (1 mm) were selected in lieu of primary sedimentation tanks. The

use of the membranes eliminated the need for secondary tanks. Biosolids

are stored in an open

aerated tank before final disposition. Inclined screw presses were selected to dewater the biosol-

ids before disposal to a sanitary landfill.

The plant layout includes ample room for expansion. This includes space to double the

aeration tank capacity and the membrane

chambers. Space is available to build an equalization

basin. The excess aeration tank capacity at start-up currently provides for equalization if needed.

Because the membrane life is less than the time to reach the expected design capacity of the plant,

the initially purchased membranes occu

py only two-thirds of the chambers that were built.

A s part of the project management process, proposals for the membranes were received about

two years before construction started. Bids were evaluated on a life cycle cost basis.

A comprehensive odor control system was integrated into the de

sign. An air handling system

collects off-gases from the major odor producing processes. The air is scrubbed in a biofilter.

To enhance the visual appearance of the plant, the architecture of the major structures gives

the appearance of a farm house with barn and outbuildings.

A s shown in the table below, in its early stages of operation (abo

ut two months after accli-

mation), the effluent is generally meeting design expectations. The average daily flow rate was

about 15,000 m

/ d when the data in the table were gathered.