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 ormechanical equipment must be accessible to workers
for maintenance and servicing, so they must be ventilated
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 , however, only requires forcing
the air into the wet well; multiple inlets and outlets
for wet wells over 5 m (15 ft) deep are also recommended.
Of course, interconnections between wet
well and dry well ventilating systems are not allowed.
Fan wheels and shaft seals should be rated nonsparking
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 hazardous
areas. Both automatic heating and dehumidification
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),
• intermittent with 30 air changes per hour with the
high-speed fan ventilation switch interlocked to the
wet pit lighting system. However, intermittent ventilation
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 concentration
of a combustible gas in air at which an explosion
can occur (approximately 5% by volume for methane).
Hydrogen sulfide gas detectors can be set for 10
ppm, which is considered a safe level for 8-h exposure
. 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 maintenance
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 supply
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 system
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 (V8 to V4 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 recommendations
for ventilation are as follows:
• All ventilated spaces are to be served by both supply
and exhaust fans and powered from two independent
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,
• Galleries and tunnels are to be treated the same as
below-grade spaces with a 0.38 m/s (74 ft/min) air
• Below-grade spaces without gas piping are to be
ventilated at the rate of 10 air changes per hour; galleries
are to be ventilated at that rate or by an air
velocity of 0.25 m/s (50 ft/min), whichever is
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 supplied
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
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