Page 245 - WaterSense at Work

WaterSense at Work: Best Management Practices for Commercial and Institutional Facilities
Fume Hood Filtration and
Wash-Down Systems
A fume hood is a ventilated enclosure where hazardous materials can be handled
safely to limit exposure. Fume hoods draw contaminants within the work area away
from the user to minimize contact and exhaust fumes through a ventilation system
to remove contaminants from the building.
As a first step, a facility should determine if treatment is needed prior to exhausting
fumes through the building ventilation system. Dry exhaust fume hoods use a fan
to draw in air containing hazardous contaminants before expelling it without provi­
ding contaminant treatment. These systems might be appropriate depending upon
the hazard level associated with the exhaust being ventilated. If minor treatment
of exhausting fumes is necessary, a facility should consider using condensers, cold
traps, or adsorbents such as activated charcoal, or neutralizing or converting toxic
substances into other less hazardous species.
When dealing with certain hazardous substances requiring more intensive treat-
ment, a fume hood with a filtration system might be needed. There are two types of
fume hood filtration systems typically used to handle hazardous substances: gas-
phase filtration (includes wet scrubbers) and particulate filtration.
Wet scrubbers
require the consumption of water to remove hazardous substances. Other gas-phase
filtration or particulate filtration systems might be suitable alternatives to wet scrub-
bers in certain circumstances, as discussed below. In all cases, laboratories should
follow manufacturer instructions and facility health and safety guidelines in order to
ensure safe operation of fume hoods.
This section focuses on fume hood filtration systems, including those that use water
e.g., wet scrubbers) and fume hood wash-down systems. It also describes systems
that do not use water that could be considered as an alternative to wet scrubbers.
Fume Hood Filtration Systems
Wet Scrubbers
Fume hoods with wet scrubbers that use water to capture and trap hazardous sub-
stances are also known as liquid fume hood scrubbers. Contaminated air enters the
scrubber system from below and passes through a packed bed. The packed bed is
wetted from above with a liquid spray. As the contaminated air comes into contact
with the water, water-soluble gases, vapors, aerosols, and particulates become dis-
solved. The trapped contaminants fall with the water and are discharged into a scrub-
bing liquor sump. The “scrubbing liquor” is recirculated, with make-up water added as
needed to replace water that has evaporated. The scrubbing liquor is removed peri-
odically through a blowdown valve to control total dissolved solids. The treated air is
released through an exhaust system. See Figure 7-4 for a schematic of this process.
Hitchings, Dale T. September 1993-January 1994. “Fume Hood Scrubbers—Parts I, II, and III.”
Laboratory Building Design Update
Page 1.
Pages 6-8.