Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

pumping

stations

and

wastewater pumping stations.

Maintaining

a

reasonable minimum temperature

in

cold weather

and

removing excess heat

in

warm

weather

are the

principal reasons

for

ventilating

a

dry

well

or

pump room.

The

following

are

sources

of

unintentional heat gain:

•

Motors

of

pumps

and

other equipment

•

Variable-speed drives

and

inverters

•

Motor controls

•

Solar gain

and

lights

•

Personnel (negligible).

The sum of

unintentional heat gains

often

entirely

offsets

building heat losses

due to

transmission through

the

structure

and

infiltration

of

colder outside air.

Any

remaining

net

heat loss must

be

made

up by

additional

heat supplied

to the

space.

In

cold climates, building

insulation

and

double glazing

can be

used

to

minimize

structure

heat losses

and

reduce condensation.

Heat gain

in

pumping stations usually exceeds their

heat

loss

in

moderate weather.

The

excess heat must

be

removed

by

ventilating with cooler outdoor air;

if the

ambient

air

temperature

is too

high,

it

must

be

removed

by

(1)

direct evaporative cooling,

(2)

indirect evapora-

tive

cooling,

or (3)

refrigeration. Direct evaporative

cooling

is

generally

ineffective

if the

relative humidity

of

outdoor

air

exceeds about 75%. Even then,

the

nearly

saturated

air

supply

may

promote rusting. Spaces with

either high heat gains

or a

noise level that must

be

con-

tained

may

justify

the

higher costs

of

refrigerated cool-

ing or the use of

some

of the

pumped water

as the

heat

transfer

medium

in an air

cooling system.

Aesthetics

Pumping

stations, especially those

in

residential neighbor-

hoods, must

be

acceptable

to the

community. Acceptabil-

ity

may

require

(1)

noise control,

(2) a

building

in

harmony

with

other structures

in the

area,

and (3) for

sta-

tions pumping wastewater, control

of

odors.

The

super-

structure

of

some stations

has

been designed

to

resemble

a

residence. Zoning ordinances

may

affect

station height,

setback,

and

appearance. Local noise ordinances

may

require

sound attenuation

at

openings

in the

building enve-

lope, particularly

if an

emergency generator

is

installed

(see Chapter

22 for a

discussion

of

noise control).

HVAC

Complexity

Station

Type

and

Location

The

required extent

of

HVAC,

its

controls,

and its

cost

increase when:

•

Pumping capacity

and

station size increase

•

Auxiliary spaces

are

added

•

Both heating

and

cooling

are

required

•

Distance

to a

residential area decreases

•

Variable-speed pumps

are

used

•

Stations

are

automated

•

Continuous attendance

is

required.

23-2.

HVAC

Design

Criteria

Codes

and

guidelines

for

good engineering practice

are

presented

in

this section with emphasis

on

safety

for

workers.

Codes,

Ordinances,

and

Standards

Confer

with

the

jurisdictional

authorities

to

learn

which codes, ordinances,

or

adopted standards apply

at

the

site. Examples

of

documents that

may

govern

pumping station design include

•

City ordinances

•

State

or

local plumbing codes

•

State

or

local

fire

codes

•

State

or

local building construction codes

•

State

or

local mechanical codes

•

State

or

local energy codes

•

Federal

or

state

EPA

regulations

•

Occupational Safety

and

Health

Act

(OSHA)

[3]

•

National

Electric

Code (NEC)

•

National

Fire

Protection Association (NFPA) codes

and

standards

•

Ten-State Standards

[4]

•

ASHRAE Standard

90-80

energy code, which includes

recommended heat-transfer

coefficients

for

buildings

and

rules

for

energy expenditure

and

recovery.

Codes

and

standards, whether mandatory

or

not,

generally represent minimum requirements. Good

engineering practice

and

safety

considerations

often

require adherence

to

more stringent criteria.

Wet

Well

Design

Guidelines

There

is a

profound distinction between wastewater

wet

wells designed

for

frequent entry (accessible)

and

those designed

for

nonentry (sealed).

Accessible

Wastewater

Wet

Wells

Wastewater

wet

wells containing either

bar

screens

or

mechanical equipment must

be

accessible

to

workers

for

maintenance

and

servicing,

so

they must

be

venti-

lated mechanically. Continuous ventilation with

air

forced

in and

forced

out is the

best safeguard against

the

buildup

of

hazardous gases. Section 32.7

of the

Ten-State Standards [4], however, only requires

forc-

ing

the air

into

the wet

well; multiple inlets

and

outlets

for

wet

wells over

5 m (15 ft)

deep

are

also recom-

mended.

Of

course, interconnections between

wet

well

and dry

well ventilating systems

are not

allowed.

Fan

wheels

and

shaft

seals should

be

rated non-

sparking

in

accordance with standards

of the Air

Movement

and

Control Association. Electric motors,

wiring,

controls,

and

monitors must meet

NEC

requirements

for

Class

1,

Group

D,

Division

1

hazard-

ous

areas. Both automatic heating

and

dehumidifica-

tion

should

be

considered

for

worker comfort

and

safety

as

well

as for

protection

from

corrosion.

According

to the

Ten-State Standards, ventilation

can

be

either

•

continuous with

12

complete

air

changes

per

hour

(based

on wet

well volume above high water level),

or

•

intermittent with

30 air

changes

per

hour with

the

high-speed

fan

ventilation switch interlocked

to the

wet

pit

lighting system. However, intermittent ven-

tilation

is

prohibited

by

NFPA

820 if the

space

is to

be a

Division

2

location.

Ventilation

at a low

rate

can be

automatically

increased

by gas

sensors that

(1)

detect

the

presence

of

combustible gases

or

hydrogen

sulfide

and (2)

either increase

fan

speed

or

start

an

auxiliary fan. (The

incremental

airflow

is

normally unheated.) Methane

detectors

are

usually

set for 20% or

less

of the

lower

explosive limit (LEL), which

is the

lowest concentra-

tion

of a

combustible

gas in air at

which

an

explosion

can

occur (approximately

5% by

volume

for

meth-

ane). Hydrogen

sulfide

gas

detectors

can be set for 10

ppm,

which

is

considered

a

safe

level

for 8-h

exposure

[5].

If

such detectors

are to be

dependable, they must

be

recalibrated

and

tested

at

least monthly,

and

this

maintenance should

be

specified

in the O&M

manual.

Consider

the

possibility

of

intermittent ventilation

design giving

a

false

sense

of

security because mainte-

nance

may

become unreliable

and

sensors

may

fail.

Personnel should always carry portable detectors

when

entering potentially hazardous enclosures.

Good ventilation

is not

easily achieved because

the

purging

of

gases (that

may be

heavier than air)

is by

dilution

and not by a

"clean

sweep,"

as is

suggested

by

the

phrase "air changes." Thus, even with

the

best sup-

ply

air

distribution,

the

purging

of

dangerous gases

from

odd-shaped areas

is far

from

perfect. Opening

the

door

to a wet

well ventilated only with forced

air

inflow

with gravity relief defeats

the

ventilation sys-

tem

because

the

airflow

escapes through

the

door.

The

best

practice

for

hazardous spaces, such

as

wastewater

wet

wells,

is to

blow

air

into

the

chamber

at or

near

the

ceiling

and to

exhaust

air at or

near

the

lowest

level

at a

rate

of

approximately

5%

more than

the air

intake rate,

thus

producing

a

partial

vacuum

of 30 to

60 Pa

(V

8

to

V

4

in.)

of

water column (WC). Even

though

permitted

by

some codes, such practices

as

providing

air

supply

or

exhaust alone

at a

continuous,

but

minimum,

airflow

rate that

is to be

switched

to a

high rate only

at the

time

of

entry should

be

avoided.

It

compromises

safety

for a

relatively small reduction

of

cost.

Minimum

or

intermittent

air

changes

may not

prevent explosions

and do not

adequately protect

impatient workers

who may not

wait

for the

required

period

of

high-speed scavenging.

According

to

NFPA 820,

the

minimum recommen-

dations

for

ventilation

are as

follows:

• All

ventilated spaces

are to be

served

by

both sup-

ply and

exhaust

fans

and

powered

from

two

inde-

pendent sources

to

ensure operation during power

failure

of a

single source.

•

Continuous ventilation

at the

rate

of 12 air

changes

per

hour with combustible

gas

detectors

is

required.

A

two-

speed

ventilation system

is

recommended

with

high-speed operation initiated

at the

warning

level

of gas

concentration.

•

Equipment rooms

and

other spaces below grade

containing

gas

piping

are to be (1)

ventilated

at the

rate

of 12 air

changes

per

hour

and (2)

equipped

with

combustible

gas

detectors and, preferably,

two-speed

fans.

•

Galleries

and

tunnels

are to be

treated

the

same

as

below-grade

spaces with

a

0.38

m/s (74

ft/min)

air

velocity.

•

Below-grade spaces without

gas

piping

are to be

ventilated

at the

rate

of 10 air

changes

per

hour; gal-

leries

are to be

ventilated

at

that rate

or by an air

velocity

of

0.25

m/s (50

ft/min),

whichever

is

greater.

The

airflow

pattern

can be

controlled only

by

using

a

combination

of

supply

and

exhaust ducts. Without

ducts,

air

follows

the

path

of

least resistance

and

leaves stagnant areas. Suggested duct design velocity

and

friction ranges

are

given

in

Table

23-1.

Ducts

must

be

routed

to

clear equipment access

and

removal

space, hoist rails,

and

hoistways.

The air

quantity sup-

plied should

be

based

either

on the

recommended

air

change rate needed

or on the

required heat removal,

whichever

is

larger. Outside ventilation

air

should

be

filtered

for

cleanliness

and

insect control. Arrange

insect screens

for

easy removal

for

cleaning.

In the

O&M

manual, alert maintenance workers

to the

need

for

filter

replacement. Consider installing

a

filter

gauge

to

show

the

pressure drop

across

the

filter

or,

preferably,

use a

differential

pressure switch

to

ener-

gize

a

"clogged filter" signal

at the

control panel.

In

cold climates, select

a

storm louver type

and net air

velocity

to

prevent

the

entry

of

snow; provide heat

at

louvers

to

keep them

from

being clogged with ice.

Sealed

Wet

Wells

"Sealed"

(not easily entered)

wet

wells require

an

ade-

quate

vent (perhaps only

a

manhole cover)

to

accom-

modate

air

displacement

due to

changes

in the

liquid

level. Except

in

very cold climates, heating

or

heat

tracing

is

rarely required because

the

temperature

of

the

moving water

is

sufficient

to

prevent

freezing.

However,

if the

water temperature

is

near freezing

(especially

if it

contains

frazil

ice)

and the

pumps

run

only

intermittently, some

form

of

heating

is

needed.

Explosionproof

electrical

equipment

is

required

for

sealed wastewater

wet

wells just

as it is for

accessible

wet

wells.

Heat

If

no

equipment

is

located

in a wet

well, heating

the

airstream

is

frequently

omitted, which allows

the wet

well

temperature

to

reach

an

equilibrium between

the

water

temperature

and

outdoor

air

temperature.

Pipes

that

are filled

with

motionless water

for

long periods

and

exposed

to

subfreezing

air can be

"traced"

(wrapped) with

electric

heat tape beneath their insula-

tion

to

prevent freezing. Panel heating

of

stairs

and

walkways

can be

used

to

prevent

ice

formation

in

cold

climates.

Wet

wells with

screens

or

other equipment

that

require inspection

and

maintenance should

be

heated

to

approximately

1O

0

C

(5O

0

F)

for

worker com-

fort,

safety,

and

efficiency

as

well

as for

corrosion pro-

tection.

Any

heat source open

to

wastewater

wet

well

air

must have neither

an

open

flame nor a

temperature

above

26O

0

C

(50O

0

F)

to

prevent

the

ignition

of

com-

bustible

gases.

Explosionproofing

All

electrical

equipment (including motors)

in or

open

to the wet

well must

be

explosionproof

and

should

be

located above

the flood

level. Even submersible pump

motors should

be

explosionproof because they

may

not

always

be

submerged. Their control panels should

not be in the wet

well

but in a

nonhazardous location.

Some engineers recommend explosionproof equip-

ment

regardless

of

location.

23-3.

Odor

Control

Odors

at

wastewater pumping stations constitute

one

of

the

worst

difficulties

that agencies encounter.

If not

resolved quickly, they

can

take inordinate amounts

of

staff

time

in

dealing with irate neighbors.

A

more

advisable

and

cost-effective approach

is to

conduct

appropriate evaluation

of any

potential odor problems

during planning

and

design

of the

pumping station,

and

to

install

all

necessary odor control measures

along with

the

construction

of the

pumping station

itself.

There

are

sufficient

valid engineering

and

scien-

tific

tools

available today

to

allow

fully

workable

solutions

to be

determined during

the

project design.

Odor

Control

Evaluations

Odor control evaluations need

to be

conducted within

the

broadest context possible.

Too

many engineers

believe odor control

is

synonymous with "foul

air

treatment." Actually,

foul

air

treatment

is

often

the

most costly type

of

odor control

and

should

be

avoided unless absolutely necessary.

Other

types

or

categories

of

odor control should normally

be

evalu-

ated

first to

determine

if

foul

air

treatment

can be

avoided.

Considerable information

is

needed

to

conduct

a

proper odor control evaluation,

and

information about

the

wastewater entering

the

pumping station

is

crucial.

The

details

of the

upstream collection system (includ-

Table

23-1.

Air

Velocities

and

Friction

Losses

in

Duct

Design

Air

velocity

Friction

loss

Item

m/s

ft/min

mm

WC/1OO

m

in.

WC/1OO

ft

Supply

duct

5.1-9.1

1000-1800

6.7-10

0.08-0.125

Exhaust

duct

4.1-7.6

800-1500

5.0-8.3

0.06-0.10

Registers,

grilles

3.0-5.1

600-1000

4.2-6.7

0.05-0.08

net

Intake

louvers

1.3-2.0

250-400

2.5-5.0

0.03-0.06

net

ing

operation

of

other pumping stations); sources,

kinds,

and

amounts

of

wastewater; industrial contribu-

tions;

and

other information

is

vital. Specific sampling

and

testing

of

existing wastewater

flows in the

same

area

as the

future

pumping station

are

likely

to be

helpful.

Several chapters

in WEF

Manual

22 [6] are

helpful

in

conducting such

an

evaluation.

See

also

ASCE Manual

69

[7].

Four odor control strategies

are

defined

and

dis-

cussed herein.

In

order

of

effectiveness,

they

are:

•

Minimizing

or

preventing production

of

odorous

compounds

•

Treating odorous compounds within

the

liquid phase

•

Containing

and

treating

foul

air

•

Enhancing atmospheric dispersion

of

foul

air.

A

well-organized evaluation

of

these categories

of

odor control almost always results

in a

successful

project. Operation

and

maintenance costs

as

well

as

first

cost must

be

evaluated

for

each strategy. Odorous

substances include

a

large variety

of

compounds.

The

reduced

sulfur

family

of

compounds

is the

major prob-

lem in

most wastewater systems,

and

hydrogen

sulfide

is the

most common

offender.

Various

effects

and

stan-

dards relating

to

hydrogen

sulfide

are

described

in

Table

23-2.

But

other

sulfides,

disulfides,

and

mercap-

tans

are

also

frequent

problem compounds, because

their odor thresholds

are

almost

all in the

part

per

bil-

lion

range

or

less. Reduced

sulfur

compounds, amines,

aldehydes,

ketones,

and

various organic acids

can

cause problems. Ammonia

is

only rarely

a

problem,

because

its

concentration

is

typically

low

compared

with

its

odor threshold.

The

concentration

of

odorants

can

be

measured

in the

liquid phase

as

well

as the gas

phase

for

almost

all

potential odorous compounds.

Liquid phase analysis

is

often

easier

to

complete

and

more accurate, whereas

gas

phase testing

can be

costly,

especially when scanning

for

large numbers

of

poten-

tially odorous organic compounds.

Minimizing

Odorous

Compounds

The first

line

of

defense against odor problems

is to

design

the

entire system

to

produce

the

absolute mini-

mum

quantity

of

odorous compounds allowed

to

enter

the

pumping station

via the

influent

water. Upstream

controls

may be

beyond

the

purview

of the

pumping

station designer,

but

they

may

need

to be

explored,

because

it

could

be

less costly

to

solve

the

problem

upstream. Control measures could include

the

follow-

ing:

•

Further pretreatment

of

specific

industrial

dis-

charges

to the

system.

•

Minimizing slug loads

of

wastewater

from

indus-

tries

or

other point sources.

• If

hydrogen

sulfide

is the

significant problem, keep-

ing

wastewater

pH

well above

7 to

minimize hydro-

gen

sulfide

off-gassing.

A pH of 8

would usually

be

adequate,

but pH 9

might

be

required sometimes.

•

Designing upstream sewers

to

maintain aerobic

conditions

in the

wastewater

—

usually

by

keeping

velocities above

1.5 m/s (5

ft/s).

•

Adding chemicals (such

as

ferrous

sulfate

or

ferric

chloride)

to

precipitate hydrogen

sulfide.

At

the

pumping station

itself,

there

should

be

mini-

mum

turbulence

of

wastewater, because turbulence

Table

23-2.

Effects

and

Standards

of

Hydrogen

Sulfide

Gas in the

Atmosphere

3

Concentration,

ppm by

volume

Effect

0.0005

Olfactory

detection threshold

0.003

Max

concentration

for

electronic equipment

per ISA

0.002-0.008

Practical odor threshold range

0.010

Max

concentration

for

electrical equipment

per

NEMA

0.030

Ambient

Air

Quality (odor-based) Standard

in

California

1

Offensive

odor; rotten

egg

smell

5

Deadens

olfactory

senses

10

Max

24-h

exposure

per

OSHA

15

Max 8-h

exposure

per

OSHA

10-50

Headache, nausea,

and

eye,

nose,

and

throat irritation

50 Max

30-min

exposure

per

OSHA

50-300

Eye and

respiratory

injury

300-500

Life

threatening; pulmonary edema

700

Immediate death

for

everyone

46,000

Lower explosive limit

a

Adapted

from

ASCE

Manual

69 [7] and

industry

standards.

promotes

off-gassing

of

odorous compounds. Drop

inlets into

the wet

well

can and

should

be

avoided.

In

stations with

constant-

speed

pumps,

use a

sloping

approach pipe

with

its

invert

at or

slightly below

the

low

water level even though

its

crown

may be

sub-

merged

at the

high water level (see Section 12-7).

Variable-

speed

pumping

is

most desirable (especially

in

larger stations where odor problems

are

more likely

and

the

investment more easily

justified),

because

matching

water elevations

in the

sewer

and wet

well

allows smooth, nonturbulent entry into

the wet

well.

Refer

to

Section 12-7

for

design details.

Wet

wells should

be

kept small enough

to

mini-

mize stagnation

and the

settling

of

solids. These

deposits

are

anaerobic

and

produce odorous com-

pounds

that

diffuse

into

the

liquid above

and

thence

into

the

air. Slime layers

form

on

submerged walls

of

wet

wells

and

also produce odors. Such wall areas

should also

be

minimized.

Wet

wells should

be

fre-

quently

(say, weekly) cleaned.

Refer

to

Section 12-7.

The

velocities needed

to

keep domestic wastewater

aerobic, promote scour,

and

eliminate odor-producing

deposits

in

pipes

are

given

in

ASCE Manual

69

[7].

In

general,

force

main velocities

of

1.1

to 1.2 m/s

(3.5

to

4.0

ft/s) occurring

at

least once

per day are

advisable

(depending

on

pipe diameter)

to

minimize problems.

Treating

Odorous

Compounds

in the

Liquid Phase

There

are a

host

of

chemicals that

can be

added

to

wastewater

to

inhibit

or

treat odorous compounds,

thus

minimizing

off-gassing

and

subsequent odor

problems. Only

the

broad categories

of

such chemi-

cals

are

defined

here:

•

Oxidants, such

as

chlorine, sodium

hypochlorite,

and

hydrogen peroxide. These chemicals oxidize

sulfide

(and perhaps other compounds) that already

exist

in

solution,

and

they minimize additional gen-

eration

for a

limited time downstream.

•

Precipitants (such

as

ferrous

or

ferric

chloride) that

precipitate

sulfide

as

insoluble, dark-colored com-

pounds. Reactions usually take

15

to 20

minutes.

•

Inhibitors, such

as

high

pH

slugs

(pH

greater than

12

for 20

minutes)

or

anthroquinone slugs, greatly

inhibit

sulfate-reducing bacteria densities

for a 1- to

2-week period. Continuous addition

of

nitrate com-

pounds

is

sometimes advantageous

to

promote

nitrate

reduction

and

minimize

sulfate

reduction.

•

Aerobic conditioning

by the

addition

of air or

high-

purity

oxygen.

If

more than about

0.5 to

1

.0

mg/1

of

dissolved oxygen

is

kept

in the

wastewater, little

anaerobic activity

can

take place

to

form

the

reduced,

offensive

compounds.

In

gravity sewers,

it

is

difficult

to add

dissolved oxygen except

by

natu-

ral

re-aeration

of

wastewater

flowing at

velocities

in

excess

of 1 m/s

(3.3

ft/s).

In

some force mains (with

certain

rising

profiles,

sufficient

pressure,

and

com-

patible

pipe materials),

it is

feasible

and

cost

effec-

tive

to add

high-purity oxygen.

•

Bases such

as

lime

or

caustic added continuously

to

keep

pH

above

8.5

minimizes hydrogen

sulfide

pro-

duction

and

off-gassing.

Chemicals, their reaction times,

and

transit times

through

the

facilities must

be

carefully

evaluated

[6,

7,

8],

Chemicals

can be

added

far

upstream,

at the wet

well,

or at the

discharge

of the

force main.

Containing

and

Treating Foul

Air

The first

step

in

designing

a

foul

air

treatment system

is

to

develop

a

reliable containment

and

ventilation

system that brings

all

foul

air to the

treatment device.

Containment

of

foul

air is not

always easy with sewers

bringing gases into

the wet

well,

and

doors

or

hatches

being opened.

But

regulating

fans

on

both inlet

and

exhaust

to

maintain

the

slight vacuum described

in

Section 23-2 helps,

as

does proper dispersal

of

incom-

ing

air at the

ceiling

and

collection

of

foul

air at the

floor.

Various

ventilation, corrosion,

safety,

and

are

discussed separately

in

this chapter.

A

large

variation

in

pollutant concentration over

the

course

of

the day

sometimes results

in

poor treatment, unless

the

system

is

specifically designed

for it.

Therefore,

details about upstream system characteristics

and

operation

are

critical.

Biological

Foul

Air

Treatment

Of

the

various types

of

biological

foul

air

treatment

systems,

the one

most applicable

to

pumping stations

is

bulk media

biofiltration.

That system involves

(1)

adsorbing odorants onto

the

surface

of

organic media

and

(2)

biologically oxidizing

the

odorous compounds

over time

via the

aerobic action

of

micro-organisms

that

live

on the

media surfaces.

The

media

is

usually

a

mixture

of

wood

or

bark chips, compost, soil, sand,

peat moss, and/or other materials.

The

moisture con-

tent

of the

media

is

critical

for the

biomass,

and the

retention time

of the

foul

air in the

media must

be

suf-

ficient

—

usually

30 to

120

seconds

of

empty

bed

con-

tact time.

Biofilters

can be

built within closed containers

or

built

in the

ground with open-air vertical discharge

at

the bed

surface.

The

latter usually consists

of

perfo-

rated

pipe buried about

0.6

m

(2 ft)

deep

in a bed of

gravel,

covered with

a

thin layer

of

sand overlaid with

loamy

soil, seeded with grass

or

other plants.

The

trench

is

designed

for 0.3

m

3

/min

of gas per

m

2

(1

ft

3

/

min

of gas per

ft

2

)

of

ground

surface

area,

so the

area

requirements

can be

significant.

Good performance

is

reported

for a

wide variety

of

odorants with proper

design

and

operation. Hydrogen

sulfide

in the

foul

air

produces

sulfuric

acid

and

causes

the

media

to

become acidic,

so

underdrains

and

leachate piping

must

be

corrosion resistant.

PVC is a

good choice.

Maintenance

costs

are

relatively low, although

the

media

may

have

to be

changed

from

time

to

time. For-

tunately,

the

exhausted media

is not

classified

as a

hazardous

material.

For

further

information

see the

lit-

erature

[6,7,

and

8].

Adsorption

If

odorant concentrations

are

low,

foul

air can be

effec-

tively

treated

with

activated carbon, potassium perman-

ganate,

or

other adsorption materials. When

the

adsorptive

material becomes exhausted,

it

must

be

either regenerated

or

removed

and

replaced.

In-place

regeneration

of

activated carbon

has

proven

difficult

—

practically

unworkable

at

most sites. Disposal

of

spent

material

has

been

a

problem

at

some locations.

The

high

humidity

of

foul

airstreams sometimes causes

poor treatment,

and

large amounts

of

carbon

are

needed

and

quickly exhausted when odors

are

very bad.

Granular

activated carbon treated with caustic

for

extra

hydrogen

sulfide

absorption

is

highly reactive,

and

instances

of

overheating

and fires

have occurred when

moisture

conditions become critical (see Appendix

I).

Activated

alumina impregnated

with

potassium

permanganate

has

also been used

for

light-duty

foul

air

treatment. Although cost-effective, removal

of

spent material

is an

extremely onerous task.

Wet

Chemical

Scrubbing

Wet

chemical scrubbing

is in

widespread

use at

waste-

water

treatment facilities

for

odor control

and is

space-conserving

for

large

foul

air flowrates. The

scrubbing

solution

is

maintained

at a

high

pH

with

caustic,

to

which

an

oxidant such

as

sodium

hypochlorite

or

hydrogen peroxide

is

added.

The

solu-

tion

is

sprayed over

the

packing media,

and

blowdown

to the

sewer

is

required. Demisting

of

exhaust

removes

most mist particles

from

the

airstream,

but

some

can

remain

in the

treated

gas

discharge.

Chemical mist scrubbing

is

another

wet

chemical

technique.

The

same chemicals

are

used

and a fine

mist

of the

chemical solution (particles

of 10 to 20

microns)

is

sprayed into

a

reaction chamber with

the

foul

air. Mist

particles

are

contained

in the

exhaust,

which

is

often

visible. Good treatment performance

is

possible with both systems.

See WEF

Manual

22 [6]

and

ASCE Manual

69 [7] for

more

details.

Other

Treatment

Systems

High-temperature oxidation

of

foul

airstreams

is

rarely used

at

pumping stations because

of its

high

cost. Achieving consistent performance with ozone

systems

has

proven

difficult.

Neutralizing chemicals

and

counteractants

are

also sometimes used. Two-

stage systems

are

occasionally used when

a

very high

degree

of

foul

air

treatment

is

required.

Enhancing

Atmospheric

Dispersion

of

Foul

Air

Occasionally, elevating

the

foul

air

discharge point

to

about

30 or 40

feet

above ground level

can

solve

a

local odor problem.

A

discharge stack

may be

aesthet-

ically objectionable,

but

with small

foul

airflows,

a

hollow light pole

200 to 250 mm (8 to 10

in.)

in

diam-

eter

can be

used

as a

disguised stack.

Discharging

gas at the

roof line

of the

pumping sta-

tion building should

be

avoided because downwash

from

wind blows

the

discharge

to

ground level. Care-

fully

evaluate

how

local wind conditions will disperse

odors.

In

critical

situations, atmospheric odor model-

ing may be

necessary

to

determine downwind

effects

[6,7].

Sealing

wet

wells, with

the

intention

of

avoiding

any

gas

discharge,

is not

normally recommended.

Maintenance personnel need access

to the wet

well

on

occasion

and

resealing

is

difficult.

Also, so-called

gas

tight

structures

often

have leaks.

Safety

and

corrosion

issues normally demand some degree

of

ventilation.

Small

wet

wells need only

a

ventilation pipe

to

allow

gases

to be

discharged either close

to the

ground

ar at

an

elevation

of at

least

3 m (10 ft) as the

situation

requires (see OSHA requirements

[3] and

NFPA 820).

Controlled

Atmospheres

for

Sensitive

Equipment

Motor control centers should

be

placed

in a

separate

room with

an

atmosphere controlled

for

temperature

and

contaminant level.

Sensitive electrical

and

electronic equipment (such

as

PLCs

or AF

controllers) require highly controlled

atmospheres

to

avoid corrosion

and to

keep tempera-

tures

at

least below

3O

0

C

(85

0

F).

ISA-S71.04 (1986)

defines

four

"Severity

Levels":

Gl is

mild,

G2 is

mod-

erate,

G3 is

harsh,

and GX is

severe. Environments

in

wastewater pumping stations

are

typically

G3

-harsh,

an

environment

in

which there

is a

high probability

that

corrosive attack will occur.

HVAC

systems

for

electronic equipment such

as AF

controllers should

be

designed

and

specified

to

produce

a Gl

environment,

as

partially described

in

Table 23-3.

In

addition, temperature should

be

maintained

as

low

as

possible

(22

0

C,

72

0

F

or

lower

if

possible).

Rooms

or

cabinets

are

often

pressurized

to 2.5 mm

(0.

1

in.)

of WC to

minimize leaks

of

outside contami-

nated air. Rooms

or

cabinets need

to be

sealed

to

achieve this limitation. Clean,

filtered air is

required

to

eliminate dust

and

other airborne particles.

To

achieve these standards,

air

cooling

is

almost

always

required, along with

filtration for

particulate

control.

At

wastewater stations, hydrogen

sulfide

is

the

primary contaminant

of

concern, although ozone

concentrations

in

urban areas

are

often

excessive,

and

various

hydrocarbons

can

also

be

present

in the

ambi-

ent

air.

Use

three-stage

filters

(prefilter

at 10

microns,

followed

by a

carbon

filter,

followed

by a

1

-micron

high-efficiency

filter) on air

supplied

from

the

upwind

side

of the

pumping station. Relative humidity control

is

also required

for

many portions

of the

country.

Chemical

filtration

media

are

often

used

to

adsorb

pollutants,

and

sometimes

to

oxidize them. Activated

carbon media, along with potassium permanganate,

are

often

used

[9].

Equipment

for

integration into HVAC systems,

or

separate, stand-alone equipment

is

available. Equip-

ment

is

also available that performs these

functions

on

a

small scale

for

individual cabinets

or for

banks

of

cabinets. These facilities pressurize

the

cabinet

to

allow

it to

operate within

a

room containing harsh

environmental conditions,

and

they include

filtration,

pollutant

adsorption

filters, air

conditioning, and,

sometimes, relative humidity control.

Airflow

capaci-

ties

in the 2.8 to 5.7

m

3

/min

(100

to 300

ft

3

/min)

range

are

typical

for

cabinet-

type

systems.

Table

23-3.

ISA G1

Environmental

Conditions

Atmospheric

contaminant

Maximum

concentration

Relative

humidity

50%

H

2

S

3 ppb

SO

2

,

SO

3

10 ppb

Cl

2

1 ppb

NO

x

50 ppb

HF

1 ppb

NH

3

500 ppb

Os

2 ppb

23-4.

Dry

Well

Design

Guidelines

The

following guidelines

are

recommended

for

designing

the

heating, ventilating,

and

cooling sys-

tems

of dry

wells. They supplement those

briefly

out-

lined

in the

Ten-State Standards

[4].

Other

suggestions

can be

obtained

from

Design

of

Wastewa-

ter

Treatment

Plants

[1O].

Heating

There

are

three basic heating techniques:

•

Space

heaters (gas, oil,

or

electric) with

a

thermo-

stat

and a

summer

fan-

only

switch.

Heater locations

and

airflow

patterns should

be

chosen

to

produce

thorough circulation through

the

space.

In

milder

climates this

may be the

simplest solution.

A

sepa-

rate fresh-air heater

may

also

be

provided. This

approach

has the

least temperature control,

but

usu-

ally

the

stations

do not

require

precise

temperature

control.

•

Infrared

radiant heating. With either

gas or

elec-

tricity, this

is a

simple

and

effective

way of

heating

large, spacious working areas. Comfort conditions

are

achieved

at a

lower

air

temperature with radiant

heat than with

any

other heating method.

•

Ducted systems with temperature controls. Mixing

dampers

can be

used

to

control

the

proportion

of

outdoor

air to

recirculated

air in

response

to

room

temperature.

A

supply-air

low

limit control

is

rec-

ommended.

Building

heat loss

in

cold climates should

be

mini-

mized

by

using wall

and

roof insulation, double glaz-

ing,

and air

dampers properly installed with blade

and

jamb seals

to

limit unwanted

air

infiltration. Follow

a

locally adopted energy conservation code

or

ASHRAE Standard

90-80.

Maintain adequate space

temperatures

to

facilitate essential activities

and

pre-

vent

freezing (see Table 23-4). Install

a

manual-reset,

capillary-type low-temperature thermostat down-

stream

from

the

preheating

or

heating

coils

and set it

at

3

0

C

(37

0

F)

to

stop

the fan and

energize

a

remote

warning

for

100% outside

air

units

in

unattended sta-

Table

23-4.

Recommended

Temperatures

Minimum

Maximum

Space type

0

C

0

F

0

C

0

F

Continually

occupied areas

20 68 26 78

Occasional

work areas

13 55 35 95

Unoccupied

areas

>4 >40 43

110

tions.

For

systems

with

a

recirculating

air

damper,

an

automatic

reset low-temperature thermostat

may be

used.

In

addition

to

energizing

the

warning signal,

it

repositions

the

dampers

for

recirculation only.

It may

either stop

the fan or

leave

the fan

running

at the

designer's

option.

The fan and

dampers automatically

resume

their normal operation when

the

freeze

poten-

tial

is no

longer sensed. Provide extra heating capacity

controlled

by a

thermostat

(or use

portable heaters)

that

can

raise

the

temperature

in the

work area

to a

reasonable, comfortable level when maintenance

and

repairs must

be

done

in the

coldest expected weather.

Locate

air

outlets

and

inlets

to

take advantage

of the

natural

upward

convection currents

from

hot

equip-

ment

as

well

as to

offset

the

heat losses

at

their source.

Introduce

heat near

the

bottom

of

exposed walls.

If

heated

air is

supplied

from

overhead (either

from

ducts

or

from

unit

heaters),

its

downward velocity

must

be

sufficient

to

force

it

down

to the

occupied

level. Consider central heating systems

if

heated ven-

tilation

air is to be

supplied

to

multiple spaces totaling

more than about

300

m

2

(3000

ft

2

)

of floor

area. Spec-

ify

industrial-quality equipment

for

long service

with

low

maintenance.

Ventilating

The

primary

function

of dry

well ventilation

is the

removal

of

excess heat

for the

protection

of

equipment

(specifically,

motors

and

controls)

and for the

comfort

of

personnel. Secondary

functions

are

moisture

removal, corrosion prevention,

and

odor reduction.

The

outdoor

air

ventilation rate

can be

varied

in

cold,

dry

weather

by

using thermostatically controlled mix-

ing

dampers

to

blend

sufficient

outdoor

air

with return

air to

match

the

need

for

heat removal.

In SI

units

the

required ventilation

is

Q

=

(12OS)AT

(23

-

la)

where

Q is the

airflow

rate

in

cubic meters

per

second,

G is the

heat gain

in

watts,

L is the

heat loss

in

watts,

G

- L is the net

heat gain,

and AT is the

temperature

difference

in

degrees Celsius.

In

U.S. customary units,

Q

=

(I

0

O

8

)Ir

(23

-

lb)

where

Q is the

airflow

rate

in

cubic

feet

per

minute,

G

is the

heat gain

in

British thermal units

per

hour,

L is

the

heat

loss

in

British thermal units

per

hour

G - L is

the net

heat gain,

AT

is the

temperature

difference

in

degrees Fahrenheit,

and

1.08

=

(0.075

Ib/ft

3

)(0.2395

BtuAb

-

deg)(60

min/h).

Cold

Weather

The

recommended minimum

air

temperature

in an

occasionally occupied space

is

13

0

C

(55

0

F)

(see Table

23-4).

If the

supply

air is

thoroughly mixed with

warmer

room

air

near

the

ceiling,

the

supply

air

tem-

perature

can be

reduced

to as low as

7

0

C

(45

0

F)

when

necessary

for

heat removal.

Heat recovered

from

the dry

well exhaust

air can be

used

to

help heat

the wet

well

by

using

any of the

heat

exchangers discussed

in

Section 23-5. There must,

however,

be

complete

separation

of the two

airstreams

to

preclude

any

leakage

of

toxic

or

explosive gases

from

the wet

well

to the

pump room.

The

heat pipe coil

is

the

safest

exchanger

in

this regard.

The

heat recov-

ery

device should

be

protected against corrosion.

Hot

Weather

The

required summer air-handling capacity

of

venti-

lating equipment

for

heat removal

is

determined

for a

given

heat load

by the

difference

between

the

outside

air

temperature

and the

allowable maximum indoor

temperature

(AT

in

Equation

23-1).

The

heat gain

to the

space includes:

•

Heat generated

by

pump motors, drivers,

and

con-

trols

•

Heat

from

other miscellaneous motors

•

Heat

from

lights

and

occupants

•

Inward heat transmission through walls, windows,

and

roof

•

Direct solar heat gain through windows

and

sky-

lights,

and

indirect solar gain through walls

and

roof previously heated

by the sun

•

Heat

from

emergency generators (obtain

the

data

on

heat lost

to the

room

and the

recommended

airflow

rate

for

this equipment

from

the

engine

manufac-

turer).

In

hot

climates

the

rate

of

outdoor ventilation

needed

to

remove excess heat

may be

exorbitant

due

to the

small

difference

between

the

outdoor tempera-

ture

and the

allowable indoor temperature.

Try first to

reduce

the

required ventilation rate

by

reducing

the

heat gain

to the

space.

For

example, cooling

air

that

leaves

a

pump motor

is

well above room temperature.

If

that warm

air can be

immediately removed

by a

hood

or by

direct-ducted

exhaust before mixing with

room air,

the

total room heat gain

and

required venti-

lation

are

less.

Of

course, hoods

and

ducts must

not

unduly

interfere with pump

motor

access

and

removal.

Evaporative

Cooling

In

dry

climates, cooling

the air

supply

by

evapora-

tion

is

effective.

Direct

evaporative

cooling

occurs

when

unsaturated

air

passes

through

a

water spray

or

a wet

filter.

The

heat required

to

evaporate part

of

the

water

is

extracted

from

the air

and, thus, lowers

its

dry

bulb (sensible) temperature.

The

added mois-

ture

increases

the

total heat content

of the air as

measured

by its

increased

wet

bulb temperature.

Less

of the

more humid

but

cooler

air is

required

to

remove

the

sensible heat

from

the

space

at a

given

rate.

Indirect evaporative cooling occurs when evapora-

tively

cooled

air is

used

in

turn

to

cool warmer

air by

passing both airstreams through

an

indirect heat

exchanger.

"Indirect-direct"

evaporative cooling units

are

even

more

effective.

The

supply

air is first

cooled

by

passing

it

through

an

indirect heat exchanger

in

coun-

terflow

to a

second (waste) airstream previously

cooled

by

direct evaporation.

The

supply airstream

is

then

further

cooled

by

direct evaporation.

In

this way,

both

the dry

bulb

and wet

bulb temperatures

of the

original airstream

are

reduced.

Due

to the

small

A7

usually available between sup-

ply

and

room

air

temperatures,

the

evaporatively

cooled

supply

air

rate

is

usually

in the

range

of 15 to

30 air

changes

per

hour. Such rates require larger

fans

and

ducts

and

better

air

diffusion

than

a

refrigerated

air

conditioning system

with

a

larger

Ar.

Recircula-

tion

of

evaporatively cooled

air is

impractical because

of

the

resulting buildup

of

humidity.

Be

warned that evaporative cooling systems

increase

the

humidity,

may

cause condensation, make

rusting

more likely,

and

increase maintenance. Unless

winterized

by

complete draining

in

severe climates,

the

evaporative equipment

can be

badly damaged

by

freezing.

Refrigerated

Cooling

Refrigerated

cooling

may be

necessary where outdoor

temperature

and

humidity

are

high

and

where internal

heat gains

are

relatively large,

as in a

control room

containing inverters

for

variable-

speed

motors. Other

cooling

methods should

be

investigated

first

because

of

the

comparatively high operating cost

of

refrigera-

tion compressors.

Chlorination

Rooms

Chlorination

and

chlorine storage rooms require spe-

cial ventilation. Both

the

Chlorine Manual

[11]

and

the

Ten-State Standards

[4]

recommend that these

rooms

be

heated

to

16

0

C

(6O

0

F)

and

have

the

exhaust

fan

capacity required

for 60 air

changes

per

hour.

The

exhaust

fan

should

be

energized

by an

automatic

door

switch

and a

manual switch (located outside

the

door)

that

simultaneously open

an air

intake damper.

Venti-

lation

air

should

be

introduced

at the

ceiling

and

exhausted

at floor

level because chlorine

is

heavier

than

air.

23-5.

Energy

Use and

Conservation

The

high energy prices

of the

recent past years rein-

force

the

importance

of

energy conservation.

The

adoption

of

energy-saving methods

is at

least partially

responsible

for the

present trend toward

increased

energy availability. Awareness

of the

irreplaceability

of

fossil

fuel

resources warrants continued

efforts

by

designers

to

avoid

its

waste.

Energy

Sources

for

Heating

Frequently available heat energy sources include

elec-

tricity,

natural gas, propane,

and

fuel

oil. Compare

the

probable life-cycle costs

and

reliability

of

each before

making

a

choice.

For

stations

in

warm

to

moderate cli-

mates,

the

air-to-air heat pump

may be an

option

for

both heating

and

cooling

as

long

as

these loads

are

rea-

sonably

in

balance.

In an

electric motor-driven air-to-

air

heat pump,

a

refrigerated coil

is

used

to

extract heat

from

outdoor air. That heat, plus

the

heat

of

compres-

sion,

is

then delivered

to the

space

by a

condensing coil

in

the

supply airstream.

The

"coefficient

of

perfor-

mance"

(COP)

of

such

a

system

is the

ratio

of

electric

energy input

to

heat energy output. With

an

outdoor

temperature

of

7

0

C

(45

0

F),

for

example,

a COP of

about

2.5, which

is not

unreasonable, means that

the

heat

energy delivered

is 2.5

times greater than would

be

obtained

from

an

electric resistance heater

with

the

same

electrical

input.

The

greatest economy results

when

the

summer cooling load also approximates

the

cooling capacity

of the

refrigeration unit

selected,

thus

allowing year-round operation

of the

unit.

Solar heating

is

generally unsuitable because

a

full

back-up

heating

system

(or

very

large

solar

heat

stor-

age)

is

necessary

to

meet demand

at the

times when

solar heat

is

unavailable. Furthermore, there

is no

compatible

use for the

greater heat collection

in

sum-

mer

that

may

require separate expensive disposal.

Major

Energy

Uses

and

Conservation

The

major

HVAC

energy

use is for

heating

the

station

and

its

ventilation

air in

cold weather

and for

cooling

the

station

and its

ventilation

air in hot

weather. Con-

tinuously

operating

fans

handling large quantities

of

air

against substantial static resistances

are the

from

(1)

a

well-insulated structure,

(2)

prudent selection

of air

quantities

and

indoor design temperatures,

and (3)

minimized

air-path resistance. Both

the

maximum

and

minimum

indoor design temperatures should

be

bal-

anced between operator

efficiency

and

operating cost.

Select

air

conditioning equipment

to be

fully

loaded

at

peak summer weather conditions

so

that

it can

per-

form

more acceptably

at

partial load conditions. Mul-

tiple

or

two-

speed

fans

allow operation

at

reduced

capacity

and

cost

at

light load conditions. Multistage

thermostats

or

step controllers

can

actuate additional

fan

capacity

as the

space temperature approaches

the

design maximum.

Introducing

ventilating

air at

lower levels

(in dry

wells only)

and

relieving

or

exhausting

it at the

ceiling

augments

natural convection

flow.

Early opening

of

properly sized inlet

and

outlet dampered louvers

allows excess heat

at

partial loads

to be

removed

by

convection, which delays

the

need

for

mechanical

ventilation.

If the

design

of the

station allows, con-

sider passive cooling

and

heating

by (1)

bringing

air

through

ducts buried below grade

to

take advantage

of

the

earth's temperature, which usually ranges

from

10

to

16

0

C

(50 to

6O

0

F)

or (2)

using skylights

and

other

glass openings arranged

to

admit direct sunlight only

in

the

winter months.

Automatic

backdraft

dampers

are

necessary

on

multiple

supply

and

exhaust

fans

to

prevent short-cir-

cuiting

the

air. Intake louvers should preferably

be

located

on the

windward side

of the

building

so

that

prevailing

wind pressure assists station ventilation.

Wall-type propeller exhaust

fans

should exhaust

to the

leeward

side

of the

building. Propeller

fans

move

more

air per

unit

of

power than other

fan

types

if the

air

resistance

is 125 Pa

(0.5

in. WC) or

less,

and

they

offer

low

resistance

to

natural

airflow

when

not

run-

ning.

Propeller

fans

are

usually

less

efficient

than cen-

trifugal

fans

against

the

higher resistances

of

long

duct

runs

or

heat-recovery devices.

Where

wet

wells must

be

heated

for

extended peri-

ods, heat-recovery devices should

be

considered.

Their

efficiency

is the

recovered

fraction

of the

heat

content

difference

between

the two

airstreams. Much

less

"new"

heat needs

to be

added

to the

makeup air-

stream when such devices

are

used.

The

following

are

commercially available types

of

air-to-air heat-recov-

ery

devices. Recovery

efficiency,

ranging

from

70 to

80% for the first

three types, varies

with

the

tempera-

ture

difference

between

the

warm

and

cool airstreams.

•

Plate-type

heat exchangers. Warm

and

cool air-

streams pass

on

opposite

sides

of

metal plates

and

exchange heat

by

conduction

and

convection. Clean-

ing

is

difficult,

condensate drainage

is a

problem,

and

duct arrangements

are

sometimes awkward.

•

Heat

wheels.

These rotate through

the

counterflow-

ing

airstreams.

The

wheel absorbs heat

from

the

warmer

airstream

and

releases

it to the

cooler air-

stream.

The

heat-recovery rate varies with

the

speed

of

rotation.

•

Heat pipe coils. These consist

of a

number

of

indi-

vidual,

sealed,

finned

tubes containing

a

metered

amount

of

refrigerant.

One

half

of

each tube

is

placed

in the

wanner airstream where

it

absorbs

heat

and

vaporizes

the

captive refrigerant.

The

refrigerant then condenses

in the

opposite half

of

the

tube

and

releases heat

to the

cooler airstream.

The

advantages include

no

moving parts

and the

ease

of

corrosion protection

by

applied coatings.

Face

and

bypass dampers

and

other methods

of

heat-recovery control

are

used.

•

Coil

energy-recovery

loops. Coils both

in the

exhaust

and

intake ducts

are

interconnected with

piping

and a

pump

to

transfer heat

from

the

exhaust

air to the

outside makeup air.

A

glycol solution

is

used

as the

heat-transfer medium

if the

coils

are

subject

to

freezing.

A

three-way bypass valve

on the

intake coil

is

controlled

to

keep

the air

temperature

leaving

the

exhaust coil above

O

0

C

(32

0

F)

to

prevent

the

freezing

of

condensate

on the

exhaust coil

in

cold climates.

The

exhaust

coil,

at

least, should

be

corrosion protected. Overall

efficiency

is

about

50%. This heat-recovery method

is

useful

where

supply

and

exhaust

fans

are

widely separated.

These devices

can

recover

a

significant

portion

of

exhaust heat that would otherwise

be

wasted,

but

their

air

resistance, which

is in the

range

of 125 to 250 Pa

(0.5

to 1.0 in.

WC), penalizes

fan

energy. Filters

are

desirable

in

both airstreams

to

prevent dirt

from

clog-

ging

the

closely spaced

fins of the

heat transfer coils.

In

determining

the

cost

effectiveness

of

such

devices, consider

(1) the

installed

first

cost,

(2) the

purchased energy cost,

(3) the

actual energy savings,

(4)

the

additional space required,

(5) the

added com-

plexity

of the

system,

(6) the

increased maintenance

costs,

(7) the

reliability,

and (8) the

higher resistance

Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

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