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RESOURCE
MAY
1999
By Rakesh Govind
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Limiting hazardous
air pollutants (HAPs) emission from industrial operations and waste
treatment facilities became a national priority in 1990 with passage of
the Clean Air Act amendments.
The trend towards
stricter air emissions regulation is spreading throughout the world,
leading to developments such as biofiltration for effective air
treatment. Biotrickling filters are a recent addition to the list of
alternatives. These filters use synthetic materials, rather than
organic, with active bacteria immobilized on the surface as biofilms.
Biological
treatment or biodegradation involves converting an organic contaminant
to carbon dioxide and water using natural bacteria. Bacterial cultures
are microorganisms that typically consist of several species coexisting
in a colony. These bacteria are found in soil, peat, compost and
natural water bodies including ponds, lakes, rivers and oceans. They
are environmentally friendly and non-harmful to humans unless ingested.
In the
biofiltration process, live bacteria biodegrade organic contaminants
from air into carbon dioxide and water. Bacterial cultures that use
oxygen to biodegrade organics are called aerobic cultures. Chemically,
the biodegradation reaction for aerobic cultures is written as:
Organic(s) + Oxygen + Nutrients + Microorganisms ð Carbon dioxide +
Water + Microorganisms
The organic(s) are
air contaminants, the oxygen is in air, the nutrients are nitrogen and
phosphorus mineral salts needed for microbial growth and the
microorganisms are live bacteria on the biofilter media. Biodegradation
of organic contaminants results in microbial growth, carbon dioxide and
water.
Biodegradation has
been used for years to treat wastewater while biofiltration emerged
within the past decade as an economically viable process. Initially,
biofiltration involved using naturally bioactive media such as soil,
peat or compost. In this media, microorganisms biodegrade contaminants,
such as when bioremediation is used on contaminated sites.
When contaminated
air passes through soil, peat or compost, the naturally present
microorganisms also biodegrade air contaminants. This finding led to
soil biofilter development. Soil with low clay and high organic carbon
content was packed in a bed and contaminated air passed through it to
biodegrade contaminants.
Biotrickling
filters use synthetic media, such as ceramic, plastic or metal. The
biofilms in a biotrickling filter must be kept moist to maintain
bioactivity. Air flowing through the biotrickling filter draws moisture
from the biofilms. A trickling flow of aqueous nutrients must be
maintained to provide nutrients and water to the active bacteria in the
biofilms.
Synthetic support
media include high surface area pellets with either a porous or
non-porous surface. The support media may be coated with activated
carbon to enhance contaminant adsorption.
The
characteristics of synthetic biotrickling filter support media being
developed by PRD Tech Inc. are: 40- to 80-ft.2/ft.3 (131.3 to 265
m2/m3) surface area, and a bulk density of about 6 lb./ft.3 (100
kg/m3).
The support media
has been tested at the pilot scale for treating odorous emissions from
a sludge thermal conditioning (Zimpro) process at the Sanitation
District No. 1 wastewater treatment plant in Northern Kentucky. It has
also been used to treat ethanol emissions from a bakery stack at the
Wonderbread Bakery, Interstate Brands Corp., in Columbus, Ohio.
In both tests, the
biofilters removed more than 99% of odorous emissions and more than 95%
of ethanol emissions.
Biotrickling
filters using synthetic support media use taller beds with a smaller
cross-sectional area than compost filters, enabling higher gas
velocities and better gas distribution through the bed. Nutrients added
to the bed neutralize acidic degradation while providing the nitrogen
and phosphorus compounds needed for microbial growth. Synthetic support
media also provide a higher surface area for biofilm exposure to the
air contaminants.
Picture shows
a demonstration scale model available for illustration purposes of PRD
Tech's commercial bio-trickling filters for Odor/VOC control
applications.
Commercial
potential
The future of biofiltration depends on regulatory requirements placed
on industry. Some trends that will impact the market for biofiltration
technology include:
• Regulatory
concerns about nitrogen oxide emission from thermal treatment
processes. Biofilters do not create nitrogen oxide.
• Public
complaints about odors from wastewater treatment plants, manufacturing
industries and solid waste treatment facilities.
• Concerns about
air contaminant emissions, worker exposure to organics and emphasis on
environmentally friendly, low-cost treatment technologies.
Biofiltration
technology application has increased during the end of the 20th century
and could continue to grow throughout the 21st century. Although recent
studies vary, the U.S. biofiltration market for 1996 is estimated at
$10 million. Economic models speculate that by 2000, the market may top
$100 million.
Potential markets
for biofiltration include treating odors, volatile organic compounds,
hazardous air pollutants and petroleum hydrocarbons. Odor treatment
could be the largest market in wastewater treatment are that includes
industries such as tobacco, composting and sludge treatment, foundries
and pulp and paper production.
Communities have
begun to expand development toward wastewater treatment plants, which
are treating increased waste flows. Flows are pumped from greater
distances than in the past so wastewater takes longer to get to the
plant. The waste becomes more septic and produces more reduced nitrogen
and sulfur compounds.
Water conservation
has decreased water flow rates while increasing flow strength, which
produces more odor.
Wood products,
pulp and paper, and surface coating operations create volatile organic
compounds and hazardous air pollutants. In surface coating operations,
worker exposure to organic chemicals such as styrene is a concern. Some
worker exposure is inevitable but biofiltration systems on the shop
floor can reduce concentrations of organics in the air.
Refining and
transfer operations release petroleum hydrocarbons composed of
aliphatic and aromatic compounds, which are biodegradable in
biofilters. Some leaking underground storage tanks pose another
environmental hazard. The hydrocarbon contaminant released through a
leak can be separated from soil and/or ground water by using air
sparging, bioventing or vapor extraction. The volatile hydrocarbons are
transferred into the air phase where they can be treated with
biofiltration.
As biofiltration
knowledge increases and more pilot studies are conducted, the
biofiltration market is expected to increase. More industries are
learning the potential advantages of biofiltration, which include:
• Biomass is the
only byproduct of biofiltration and it can be disposed of in sewers.
The thermal processes that produce nitrogen oxide cause ozone depletion
and smog formation. Chemical oxidation processes that use hypochlorite
produce chlorine and chlorinated products.
• Biofiltration is
an ambient temperature and pressure process that produces minimal
carbon dioxide. Thermal processes increase carbon dioxide production
because they require additional natural gas to reach high temperatures.
• Biofiltration
investment and operating costs are lower than for thermal and chemical
oxidation processes. No chemical handling is needed for biofiltration.
Chemical oxidation requires handling and storing hypochlorite, hydrogen
peroxide and chlorine dioxide.
Biofiltration
could play a major role in treating organic and inorganic emissions
from a variety of industrial and wastewater treatment processes.
Compared to other available technologies, biofiltration has technical
and cost advantages. The market for biofilters could increase in the
next millennium as new applications arise.
Table
1 shows various air pollution control technologies.
|
Type
of Technology
|
Air
Flow Rate
|
Concentration
in ppmv**
|
|
Condensation
|
200-20,000
m3/h
(120-12,000 SCFM*)
|
50-200
g/m3
(2.8%-11.2% by volume)
|
|
Cryo-Condensation
|
30-600
m3/h
(20 - 400 SCFM)
|
5-90
g/m3
(0.28% - 5% by volume)
|
|
Scrubbing
|
200-20,000
m3/h
(120-12,000 SCFM)
|
10-40
g/m3
(0.56%-2.3% by volume)
|
|
Incineration
|
10,000-100,000
m3/h
(6000-60,000 SCFM)
|
8-140
g/m3
(0.5%-8% by volume)
|
|
Catalytic
Oxidation
|
10,000-100,000
m3/h
(6000-60,000 SCFM)
|
1-10
g/m3
(500 ppmv-6,000 ppmv)
|
|
Regenerative
Adsorption
|
100-10,000
m3/h
(60 - 6,000 SCFM)
|
1-10
g/m3
(500 ppmv - 6,000 ppmv)
|
|
Non-regenerative
Adsorption
|
10-60
m3/h
(6-40 SCFM)
|
0-5.0
g/m3
(< 1 ppmv-2,800 ppmv)
|
|
Compost
Biofiltration
|
60-300,000
m3/h
(40-180,000 SCFM)
|
(<
1 ppmv-25 ppmv)
|
|
Biotrickling
Filter
|
10-300,000 m3/h
(6-180,000 SCFM)
|
0-8.3
g/m3
(20 ppmv-5,000 ppmv)
|
|
*Standard
cubic feet per minute **Parts per million by volume
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Rakesh Govind is
professor of chemical engineering in the Department of Chemical
Engineering University of Cincinnati, Cincinnati, OH 45221-0171, USA; Tel:
513-556-2666, E-Mail: rgovind@alpha.che.uc.edu.
Dr. Govind also serves as
the Director of Engineering, PRD Tech Inc. located in Greater
Cincinnati / Northern Kentucky metropolitan area.
Reprinted with
permission, from Resource magazine, May 1999; published by the American
Society of Agricultural Engineers."
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