Now a day’s emission of odors is one of
the major difficulties encountered by many waste treatment industries. This
unavoidable emission of odorous gases into surrounding environment can cause a very
severe impact on the health of the human beings, residing close to the
treatment plant and due to which the potential may arise for closing down of
the waste treatment plant.
The odorous air from the waste
treatment plant has the typical feature of large volume with minimum concentration
of polluted compounds. So, to eliminate these odorous chemicals from exhaust
air that is emitted a Biofiltration process will be used to treat before releasing
it into environment. Biofilter is most effective and comparatively inexpensive,
especially in the composting industry.
7.1 Biofilter Type
In our GreenCycle facility, a closed single bed
biofilter has been chosen with a down flow design because it is more
desirable for larger flow rates. The exhaust air that is collected from
different units like storage, pretreatment, fermenter and composting
is conducted into this biofilter and treated appropriately. The inlet is placed
above the bed which allows the flow of pollutants into the filter. Due to the
introduction of the contaminated air into the filter or may be due to the
generation of the heat by the microbial activities there is possibility for the
dryness of the biofilter media. This drying effect can be regulated easily by
the moisture content that is provided by the humidifier which is placed before the
biofilter and also by the irrigation system which is placed inside the
biofilter. These emissions flows from the humidifiers into the biofilter and
gets intact with microbes on the bed. During this contact period, the
microorganisms absorbs the pollutants and the enzymes present in these
organisms are able to change these compounds into water, carbon-dioxide and
energy. The clean air flows through outlet at the bottom and is liberated out
into the atmosphere.
In order to determine the size of the reactor, the
conditions that are specified within our GreenCycle are as follows:
Flow rate to be treated: 85 000 m3/h
Odor concentration in raw gas: 20 000 OU/m3
Odor concentration in clean gas: 500 OU/m3
Firstly, to calculate the required volume of the
biofilter, the odor degradation rate of the packing material that is used in
the design of the biofilter has to be known: As the packing materials mature
compost and woodchips are chosen to use in the biofilter.
There are certain reasons to choose compost and
woodchips as packing materials and they are as follows:
These materials meet the necessary
requirements (like capacity to hold the moisture, porosity, nutrients
etc.) as the packing material in the biofilters.
Compost is a source of microorganisms and
Porosity is improved by using wood chips,
thereby reducing pressure drop.
Easily obtainable in the plant and
Accordingly, a pilot test is performed that is
specific to a biofilter design with a particular filter media. After the test
is performed, a graph can be plotted of the odor degradation rate (r) versus
the average logarithmic mean concentration (Cm).
Odour Degradation Rate:
= (Cin -Cout) V´V(OU/m3/h) (Eq 7.1)
r = Degradation rate
Cin = Concentration of raw gas
Concentration of clean gas
= flow rate (m3/h)
V = Volume of filter (m3)
Logarithmic Mean Concentration:
= Cin – Coutln
CinCout (Eq 7.2)
Using equations 7.1 and 7.2 the results for ´r` and
´Cm´ can be calculated
as shown in the Table XX. By plotting degradation rates over logarithmic mean
concentration a graph can be
drawn as shown in Figure XX.
For the above experiment, the overall
logarithmic mean concentration is,
= 20000 – 500ln 20000500= 5286.16 OU/m3 (Eq
The overall degradation rate for the logarithmic
mean concentration (Cm)
from the above diagram can be given as,
first order kinetics, we have:
. Cm (Eq
r = 607.69 5286.16 -165673 = 30,46,673.57 (OU/m3/h)
Now, the Volume of Biofilter is calculated as
-Cout V´r (Eq 7.6)
= 20000 – 500 850003046673.57= 544.035 m3 (Eq
Biofilter surface area
is calculated by the amount of flow rate to be treated times the filter
The flow rate = 85 000 m3/h
The filter area load = 100 m3/m2h.
Biofilter surface area,
= 85000100= 850 m2 (Eq
The Packing height is:
= VA= 544.035850= 0.64m (Eq
Various Components of Biofilter
The Overall biofilter plant comprises of various
components like High pressure fans that collects the exhaust air from different
process units(composting, pretreatment etc.,) by maintaining negative pressure,
Long pipes along with holes allows the flow of malodorous air through the
filter bed, Humidifiers which eliminates the dryness by providing sufficient
moisture content , Sprinklers as irrigation system placed above the biofilter
bed maintains the sufficient moisture on the filter bed medium by sprinkling
the water, Filter bed constitutes the medium and microbes where the
biodegradation process occurs and the concentration of contaminants decreases
and are converted into water and CO2.
The biofilter bottom is equipped with support that contain gravels which
provides support to the bed medium and thereby maintains the structure of the
Important Parameters Affecting the Biofiltration Operation
The various parameters such as Temperature,
Moisture, Nutrients Attainability, pH, Flow direction of polluted gas effects the
operation and performance of the biofilter.
In this chapter, the details about each of these
parameters are discussed below.
Flow Direction of Odorous Gases
In biofilters two different types of flow modes
namely downflow and upflow are available. In our green cycle facility, we
preferred to use down flow closed type of biofilter. Firstly, the polluted air
flows across the long pipes with holes at the sides and gets distributed evenly
throughout the entire bed. In the down flow mode the inlet for the polluted air
is at the top of the biofilter and the outlet for the clean air is at the
bottom. For the larger flow rates the down flow closed biofilter is desirable.
The microbial activity is a key factor for the successful
biofilter operation.The temperature has a major effect on this microbial
activity, therefore it is important to keep the temperature at favourable
conditions. The rate of the microbial activity becomes higher with increased
temperature but on the other hand high temperature decrease the adsorption rate.
Low temperatures works in the reverse manner. Generally the malodorous air
coming out from the composting plant has high temperatures and as soon as these
gases gets intermixed with the exhausted air released from different units and
flows through the humidifier, where the temperature will get decreased. The
humidifiers play a significant role in maintaining the biofilter at optimum conditions.
The temperature that ranges from 20 – 40oc
desirable for the effective biofilter operation and also for the microorganisms
to live and proliferate.
As a safety precaution we are incorporating a high
temperature alarm near to the air inlet and also regularly monitoring (once in
every 8 hours per day) the temperature of the filter bed.
Adequate amount of water is needed for the
microbial activities inside the biofilter and also for the microorganisms to proliferate.
This is one of the main crucial variable and has a greater impact on controlling
the performance of biofilter. The appropriate moisture percent for the media
bed ranges from 40-60%.The humidifiers that are situated prior to the biofilter
provides adequate moisture to the influent air and prevents the dryness. Besides,
the sprinklers are also placed in the biofilter that distribute the water
evenly on the entire surface of filter bed, so that appropriate amount of
moisture is supplied for the effective biofilter performance.
require nutrients to grow, multiply and survive. Compost that is used as the media
in the biofilter provides sufficient nutrients for microbial activities and
also the pollutants (eg: N2)
that flows into the filter bed provides some amount of energy and the nutrients
to microorganisms so that the effective biodegradation process occurs. Nitrogen
is used for the growth and as well as to build cell wall ( contains 15% N2)
of microorganisms. From the above information it is clear that excess nutrients
are not required.
biofilter to perform accurately the pH of the bed must be maintained constant
in between 6-8, generally pH constant near 7 is feasible. During the biological
degradation process some contaminants produce acids that outturn in decreasing the
pH and destroying the microorganisms. So, it is important to remove these acids
now and then so that constant pH can be maintained throughout the process. Washing
the medium bed or by adding appropriate biofilter medium like oyster shells
which help in neutralising acids are the best ways to avoid the acids.