che_march-2025 - 35

FIGURE 2. Moved outdoors for maintenance, these bioreactors
are used to treat hydrogen sulfide at a large indoor facility. H2S
contamination must be removed from air because it can corrode
equipment and concrete structures
quire a permit. The units do not use a
filter and are easily cleaned, typically
once every two or three months.
Water is pumped up from the bottom
of the reactor, where it flows
down the membrane, keeping the
media wet. Water is distributed
evenly from above the biosupport,
exposing the contaminants to the
enzymes, which are activated and
begin bio-oxidation upon water
contact. Non-digestible solids, such
as metal dust and fibers from insulation,
are washed down to the bottom
of the tank for later removal.
As water flows into the tank, it
forms an internal water curtain that
follows Bernoulli's Principle of fluid
mechanical attraction, as well as
electrical-field grounding to pull in
aerosols and other contaminants
for biological destruction. Enzymes
subsequently and rapidly
oxidize
contaminants in the water. The biosupport
membrane is designed to
support contaminant contact with
the enzymes due to turbulence
created by cyclone
action.
Bioreactors usually have
a fan to capture those
contaminants that are not
electrically charged by
suction into the unit. The
unit is operated in a scrubber
mode - that is, the
air coming in is washed
and scrubbed by a water
stream created by a pump
at the bottom of the scrubber.
It
is then
cleaned
and exits at the top of
the scrubber. The pump
is electrically grounded.
Water is pumped from the bottom of
the scrubber to a distribution plate
at the top. The distribution plate
spreads the water evenly over a spiral
sheet, which has a large surface
area. The air moves via turbulent flow
with abundant back-mixing. This creates
intimate and extensive air-water
contact. Suction action according
to Bernoulli's Principle increases airwater
contact by forming small bubbles
of air that are sucked into the
water. The water falls back down in
the scrubber, and is re-circulated to
the top.
One of the most important functions
of bioreactors in air-cleaning
applications is the rapid and continuous
destruction of the contaminants
by oxidative enzymes in the water, in
real time. This allows for the cleaned
water to be recirculated inside the
scrubber. The need for adding fresh
process water is eliminated. There is
no wastewater stream.
TABLE 1. DESTRUCTION OF GASOLINE VAPORS IN A SMALL
BIOREACTOR
Time, minutes
from startup
1
2
3
4
5
6
8
12
Hydrocarbons
in, ppm
2,727
2,867
2,963
2,895
2,667
2,679
2,628
2,517
2,693
Hydrocarbons
out, ppm
887
921
858
796
884
827
722
784
710
Hydrocarbon
removal, %
67
68
71
73
67
69
73
69
74
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Example: Charged
contaminants.
Electrically charged
contaminants,
or
contaminants that
behave like aerosols,
are captured by the
electrically grounded
water. Some particles
are so strongly
charged that they
have been observed
to slam into the water
from all directions,
including moving
against the exiting
MARCH 2025
airflow at the top of the scrubber. For
instance, in a styrene off-loading facility
in Dubai, the control-room building
air inlet was protected by a carbon filter
and the entire building was under
high pressure such that all air was discharged
to the outside. Styrene was
entering the building through doors in
spite of the discharging air flow, causing
a nuisance and health issues for
the workers. Bioreactors can help in
such scenarios.
Bioreactor enzymes
Enzymes are the workhorses of the
bioreactors. They can catalyze the
oxidation of contaminants at room
temperature and ambient pressure.
They can be generated on an asneeded
basis by a consortium of
microorganisms, which are typically
naturally occurring, non-genetically
engineered, non-toxic mixtures of
several microscopic entities derived
from healthy soils and lakes.
The microogranisms can naturally
re-arrange their population distribution
to meet the needs at hand.
For example, they may have one
population-distribution profile when
oxidizing hydrocarbons, and can
change their population distribution
to destroy chlorocarbons. The
microorganisms are available from
vendors as a bottled mixture, and
can be added periodically (usually
weekly or monthly) to replenish the
bioreactor. Such microoganisms
have been shown to be exceptionally
stable at all atmospheric temperature
ranges and pH ranges,
just as they are robust in their
natural ecosystems.
Bioreactor
in
enzymes
quickly
act
air-cleaning
very
tasks.
Table 1 demonstrates how rapidly
they can destroy gasoline vapors.
In the example in Table 1, gasoline
vapors from an automotive facility
were ducted into a small (around
30 in. tall) bioreactor. Vapors entering
and exiting the bioreactor were
monitored by a standard VOC analyzer.
The bioreactor residence time
was less than one second. About
2,000 parts per million (ppm) of hydrocarbon
vapors were removed
per pass, averaging around a 70%
removal rate.
35
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