INTRODUCTION research shows that biofilters can be used


are many physical and chemical methods to treat air pollution. Bioprocesses
have been very efficient for the treatment of air pollution. The most popular
bioreactor configuration is the conventional biofilter. Biofilters were
initially designed for odour control at wastewater treatment plants, but now
they are becoming renowned for the treatment of volatile organic compounds
(VOCs) and other organic compounds. These gases have bad odour and are
dangerous for humans and animals.


process is preferred over other air pollution control techniques and has gained
popularity because it has economic advantages,
as the energy requirement is significantly low and avoids cross media transfer
of pollutants. This technology can be mainly applied to waste gases to control
air pollution, which have low concentration and readily biodegradable
pollutants. Biofilters utilizes microorganisms to reduce air pollution. Air
pollutants come in contact with microorganisms inside the biofilter, where
microorganisms demean pollutants to produce energy and by-products (C02 and
H20). Biofilter includes a filter material, where microorganisms breed. Biofilm
is a very important part of the biofilter. Biofilm is a thin layer of moisture
where the microorganisms reside. In the process of biofiltration polluted air
is pushed inside the biofilter slowly thereby, the pollutants in the air get
absorbed in the filter media, then as a result the polluted gas is dispersed in
the biofilter and absorbed onto the biofilm.


the escalation of the conventional biofilter, new configurations were developed
for air pollution control which is known as biotrickling filters and bioscrubbers.
A new set of technologies were also introduced for air pollution control such
as membrane bioreactor and rotating biological contractor. A lot of industries
use this biofilter technology for example the food industry, pharmaceuticals,
wood products manufacturing, paint and coatings application, petroleum
industry, sewage treatment etc. Polluted air stream comprises VOCs and many
sulfur compounds. Biofilters require a large area while treating large
airflows, which can be as big as a basketball court. Recent research shows that
biofilters can be used to remove a variety of airborne contaminants which
includes aliphatic and aromatic hydrocarbons, alcohols, aldehydes, organic
acids, acrylate, carbolic acids, amines and ammonia.




The main three bio reactor configurations used for
air pollution control are biofilters, biotrickling filters and bioscrubbers.
The core mechanism is same for all but they make use of microbes in different
phases which may be suspended, fixed or in a liquid state. Biofilters are not
exactly filtration units but a combination of different unit operations such as
absorption, adsorption, degradation and desorption of gas phase contaminants.

Water is added into the biofilter system to control
the moisture content and add essential nutrients. Humidification of gas streams
is generally carried out before feeding it into the system.






The two categories of biofilters are open bed or
closed bed.

Open bed biofilters are the
ones that are used mostly and its depth varies from about 10 to 18 inches. They
are larger than the closed-bed biofilters. Open bed biofilters are made in such
a way that they are exposed to the weather outside. Closed bed biofilters are
usually surrounded with a small exhaust port to let out the air which is clean.
Closed bed biofilters are commonly used to treat small airflows and tend to have
deeper media approximately (2-3 feet), in order to reduce the space which is
required to attain odour control also it is  more expensive as compared to open bed


In short,
the conventional biofilter can be referred to a fixed film or packed bed
bioreactor which consists of a natural filter bed, it depicts that inert
packing materials are also successful in a closed bed biofilter.  In this biofilter polluted air is let into in
down flow or an up flow manner through the reactor where contaminants of the
waste air are decomposed with the help of biocatalyst which is already present
in the packed bed. Therefore, to treat hydrophobic and water soluble compounds,
this reactor is used. Conventional biofilters are not considered for the
treatment of the pollutants like chlorinated compounds which lead to
acidification because there is no moving liquid present in it. Key elements of
a biofilter are Filter beds, Biocatalysts, Conditions, water content and flow
rate and inlet concentration.





1.     Filter bed

The filter
beds which were initially used in conventional biofilters were made up of
natural materials. Common materials used were soil, compost or peat. Natural
filter beds are still used in some areas. The benefit of using natural filter
beds is the availability of microorganisms and nutrients. Major disadvantage in
using these filters is depletion of essential nutrients over time and increase
in pressure drop due to gradual degradation and compaction. This also causes
decrease in efficiencies over time. New materials have been introduced to
overcome such problems. Inert and synthetic materials are combined with natural
filter beds to regulate the pressure drop across the filter bed. It is
essential to humidify waste gas while using inert materials as, microorganisms
need a significant level of relative humidity for proper functioning. Generally,
in conventional biofilters, continuous liquid phase is not added. Nowadays, filter
beds are fully made up of inert materials in which feeding liquid solution is
necessary to avoid drying of the filter bed which can result in decrease in
performance and reactor failure.

One of the major
issues in a biofilter is to maintain a favourable water content. Regulations
and moisture levels can be smoothly maintained these days with the help sensors
that are cost effective. When the level of loading rates reach a peak point,
the occurrence of adding the liquid phase automatically increases. The
temperature of the filter bed rises with high loading rates. In open biofilters
sprinklers are especially placed while using natural filter beds for maintaining
favourable conditions in accordance to the content of water. This usually takes
place in summer season where the top layer of the bed happens to dry out often.


2.     Biocatalyst

Biocatalyst is an important element
for smooth functioning of a biofilter. Microorganisms contains enzymes which
are responsible for biodegradation of the pollutants. Different types of biocatalyst
are used to tackle different types of pollutants. In natural filters some
amount of microorganisms are naturally present. Pollutants which are readily
biodegradable like hydrogen sulphide, the growth of microorganisms inside the natural
filter will be significant. Pollutants which are not easily degradable such as
MTBE or some halogenated compounds, addition of special biocatalyst becomes
crucial. When inert or synthetic filter medium is used in the filter bed,
inoculation of such special biocatalyst becomes a precondition. Again, if
the pollutant is not highly recalcitrant, almost any type of sludge or mixed
culture can be inoculated in the biofilter; otherwise, for more recalcitrant
pollutants, seeding specialized microorganisms may be required






The most favourable values of
maintaining a sufficient water contents in biofilters varies from 40 to 60%.
Very low water content tends to affect the activity of microorganisms whereas
excess water increases the pressure drop, and composition of anaerobic areas in
the reactor, therefore adequate water content is necessary to maintain an
efficient functioning of the biofilter. The biofilters in which fungus is
prevalent are said to be more unaffected to low water contents.























The use of biofiltration for air pollution control was
started in 1950s in Germany and USA. Since then, biofilter technology for air
pollution control has evolved greatly and gained popularity. Initially this
technology was developed for waste water treatment. Biofilters are pocket
friendly and require less energy than other APC systems. New systems have been
developed after biofilters such as biotrickling and bioscrubbers specifically
for air pollution control. However, this technology has few limitations related
to the inlet concentrations but biofilters are also very flexible in terms of
changing the operation conditions to handle different pollutants. This technology
has a very wide range of application in various industries. The use of
bioreactors which include conventional biofilters and other similar technologies
which make use of microorganisms is growing worldwide.