present.
There
are
about
a
dozen hazardous environ-
ment
groups.
All
equipment
in
Group
D
must
be
explosionproof
—
an
expensive requirement
for
polyphase motors.
•
Class
1,
Group
D,
Division
2
where there
is
the
pos-
sibility
of an
explosive hazard. Only single-phase
equipment
(switches, lights, motors) must
be
explo-
sionproof.
A
Division
1
classification
is
assigned
to a wet
well
either
not
ventilated
or
only intermittently ventilated
in
accordance with NFPA
820
criteria.
If the
pump
intake basin
is
well ventilated
at all
times,
the
likeli-
hood
of an
explosive atmosphere
is
greatly reduced
and
the
space could
be
reclassified
as
Division
2.
Personnel
Safety
There have been
too
many incidents
of
death
and
injury
to
ignore good practice
for
workers entering
any
room containing wastewater
or the
possibility that
sewer
gases could enter
a
room. Some jurisdictions
require permanently installed meters
for
explosive
gas
(hydrocarbons),
hydrogen
sulfide,
and low
oxygen
levels. However,
the
chance that such equipment will
not
be
calibrated with adequate frequency
or
that
detectors will become fouled with hydrogen
sulfide
poses
a
risk.
It is
better that workers
be
equipped with
portable monitors regularly calibrated
and
tested.
The
monitors
are
expensive,
but
there
is no
good substitute
for
the
protection they
offer.
Confined
Spaces
OSHA
defines
confined
spaces
as
follows:
• A
"confined
space"
is an
area that
is
large enough
and
so
configured
that
an
employee
can
bodily enter
and
perform assigned work,
has
limited
or
restricted
means
for
entry
or
exit [for example: tanks, vessels,
silos, storage bins, hoppers, vaults, trenches greater
than
1.2 m (4 ft) in
depth,
and
pits]
and is not
designed
for
continuous occupancy.
• A
"permit
required confined
space"
is
defined
as a
confined
space with
one or
more
of the
following
characteristics: Contains
or has the
potential
to
con-
tain
a
hazardous atmosphere such
as
lack
of
oxygen
(less than 19.5%),
or
explosive
or
toxic gasses; con-
tains
a
material that
has the
potential
for
engulfing
an
entrant;
has an
internal configuration such that
an
entrant
could
be
trapped
or
asphyxiated
by
inwardly
converging walls
or a floor
that slopes
downward
and
tapers
to a
smaller
section;
or
con-
tains
any
other
recognized
serious
safety
or
health
hazard.
Merely providing ventilation
per
codes, such
as
NFPA
820 or the
Ten-States Standards, does
not by
itself
change
the
classification
of a
confined
space
to a
non-
confined
space.
The
criteria regarding entry, exit,
and
continuous
occupancy must also
be
addressed.
In
practical terms,
it
would seem that
the
following fea-
tures must
be
present
in a
wastewater pumping station
wet
well
to
avoid classification
as a
confined
space:
•
Stairway
access.
Ladder
access
probably would
not
be
considered
to
meet
the
criteria
of
eliminating
"limited
or
restricted means
for
entry
or
exit."
•
Permanently installed, continuous ventilation
suffi-
cient
to
control
the
accumulation
of any
hazardous
(toxic
or
explosive) gasses
and
prevent oxygen deple-
tion below concentrations necessary
to
support
life.
•
Permanently installed lighting.
•
Permanently installed
or (if
always used) portable
detectors
for
combustible gas, hydrogen
sulfide,
and
oxygen concentrations.
Even with
the
above features, there
are
some sanita-
tion agencies that
do not
allow individuals
to
enter
wet
wells alone. They must always
be
accompanied
by an
observer
who
does
not go
with them
into
the wet
well.
Hazards
in Wet
Wells
Proper
ventilation
of
pumping stations
is an
often
mis-
understood
and
neglected subject. Enclosures below
grade, such
as wet
wells
and
vaults,
are
considered
by
OSHA
to be
confined
spaces,
and
there
are
stringent
requirements
for
access
to
them
and
elaborate protec-
tive measures
for
persons entering them. Many deaths
in
both water
and
wastewater pumping stations could
have
been avoided with good ventilation. Although
ventilation
by
itself does
not
change
the
classification
of
a
"confined
space"
to an
"unconfined
space,"
good
ventilation goes
a
long
way in
improving
the
safety
of
wastewater
pumping stations.
In an
accident
in
England, methane
gas
leaked
from
the
forebay into
a
water pumping station
and
collected
over
a
weekend.
The
station
was not
continuously ventilated. When
a
party
of
visitors entered
the
station,
a
spark
from
the
lighting system caused
an
explosion
that killed
14
people.
In
another incident,
an
operator
on his
normal
rounds breathed
a
fatal
concentration
of
hydrogen sul-
fide gas in a wet
well inadequately ventilated
by an
inappropriate
design.
In
subsequent
air
sampling,
hydrogen
sulfide
concentrations
up to 20
times greater
than
the
recommended limit were
found.
These
exam-