Now a days emission of odors is one of the major difficulties faced by many waste treatment industries. This unavoidable emission of odorous gases into the surrounding environment can cause a 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 low concentration of contaminated odor compounds. In order to eliminate or control these odorous chemicals from exhaust air that is being emitted from different process units a Biofiltration process will be used to treat before being released in environment. Biofilters are one of the most effective and relatively inexpensive treatment process, especially in the composting industry.7.1 Biofilter TypeIn 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.To this biofilter, the exhaust air that is collected from different units like storage, pretreatment, fermenter and composting is conducted and treated accordingly. The inlet is placed above the bed which allows the pollutants to go inside the filter. Irrigation system that provides moisturising ability and the biodegradability of the compounds takes place on the top of the bed. Due to the introduction of the contaminated air into the filter or may be due to the generation of the heat by biological activities of the microorganisms there is a possibility for the occurrence of the dryness of the media that is being used in the biofilter. In down flow closed biofilter this drying impact can be regulated easily by the moisture content that is being provided by the humidifier which is placed prior to  the biofilter and also by the surface water irrigation system. From the humidifiers these emissions flow through the biofilter and gets intact with the 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 the bottom outlet from where it is liberated out into the atmosphere.7.2 Reactor sizingIn order to determine the size of the reactor, the conditions that are given within GreenCycle are as follows:Flow rate to be treated: 85 000 m3/hOdor concentration in raw gas: 20 000 OU/m3Odor concentration in clean gas: 500 OU/m3Firstly, 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. The sources for the packing material that is going to be used are mature compost and woodchips.There are certain reasons behind choosing compost and woodchips as packing materials and they are as follows:These materials meet the necessary requirements (like moisture holding capacity, porosity, microorganisms, nutrient content etc.) as the packing material in the biofilters.Compost is a source of microorganisms and micronutrients.Porosity can be improved by using wood chips, thereby reducing the pressure drop.Easily available in the plant and inexpensive.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). Cin (OU/M3) Cout (OU/M3)                 497               104                 558                62                 939                22               11925              348               15792              608               16384              484               21720              966               32500              966               45823            1916               74698            2246               94646            2506Odour Degradation Rate:                         r =  (Cin -Cout) V´V(OU/m3/h) (Eq 7.1)Where,r = Degradation rateCin   = Concentration of raw gas Cout = Concentration of clean gas V*  = flow rate (m3/h)V   = Volume of filter (m3)Logarithmic Mean Concentration:                             Cm = 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 bedrawn as shown in Figure XX.    Table xx: Cin (OU/M3) Cout (OU/M3) V’    (M3/h) V         (M3)       Cm            (OU/M3)         r (OU/M3)4971042901.7251.246783967041.18558622901.7225.739328284611.76939222901.7244.2875458156429.4119253482901.73275.715381974900157926082901.74661.8387982590212163844842901.74514.5116582712353217209662901.76667.2557573540388325009662901.78969.14419753793294582319162901.713830.9520574900187469822462901.720675.15598123594599464625062901.725372.7447415718000Figure xx:For the above experiment, the overall logarithmic mean concentration is,      Cm = 20000 – 500ln 20000500= 5286.16 OU/m3 (Eq 7.3)Therefore,The  overall degradation rate for the logarithmic mean concentration (Cm) from the above diagram can be given as,  From first order kinetics, we have:                                                                                          r = k1 . Cm (Eq 7.4)Therefore,r = 607.69 5286.16 -165673 = 30,46,673.57 (OU/m3/h) (Eq 7.5)Now, the Volume of Biofilter is calculated as follows:                              V = Cin -Cout V´r (Eq 7.6)                              V = 20000 – 500 850003046673.57= 544.035 m3 (Eq 7.7)Biofilter surface area is calculated by the amount of flow rate to be treated times the filter area load:The flow rate = 85 000 m3/hThe filter area load = 100 m3/m2h.Biofilter surface area,  A = 85000100= 850 m2 (Eq 7.8)The Packing height is:                                           H = VA= 544.035850= 0.64m (Eq 7.9)7.3 Various Components of BiofilterThe Overall biofilter plant comprises of various components such as High pressure fans with negative pressure to collect the exhaust air from different process units(composting, pretreatment etc.,), Long pipes with holes to allow the polluted gas to flow through the filter bed, Humidifiers that eliminates the drying effect by providing sufficient moisture content , Sprinklers are used as irrigation system that are placed above the filter bed sprinkles necessary water to maintain the required moisture on the filter bed medium, Filter bed constitutes the medium and microbes where the biodegradation process takes place due to which the concentration of contaminants decreases and gets converted into water and CO2. The bottom of the biofilter is equipped with support that contain gravels which provides support to the bed medium and thereby maintains the structure of the biofilter.7.4 Important Parameters Affecting the Biofiltration Operation There are various parameters such as Temperature, Moisture, Nutrients Attainability, pH, Flow direction of polluted gas that affect the functioning and performance of the biofilter.In this chapter, the details about each of these parameters are discussed below.7.4.1 Flow Direction of Odorous GasesIn biofilters two different types of flow modes namely downflow and upflow are available. In our green cycle facility, we preferred to use down flow mode to the closed type of biofilter. Firstly, the polluted gases flows across the long pipes with many holes at the sides so that the air gets distributed evenly throughout the entire bed. In the down flow mode the inlet for the pollutants will be at the top of the biofilter and the outlet for the cleansed air will be at the bottom. For the larger flow rates the down flow mode for the closed biofilter is desirable.7.4.2 TemperatureThe activity of the microorganisms is an important factor for the effective operation of the biofilter.The temperature has a major effect on this microbial activity, therefore it is essential to maintain the temperature at favourable conditions. Increased temperature fast up the rate of the microbial activity but decrease the rate of adsorption and lower temperatures works in the reverse manner. Usually the gases coming out from the composting unit has high temperatures and as soon as these gases are intermixed with the exhausted air that are released from other different units and flows through the humidifier, where the temperature will get decreased. The humidifiers play a significant role in operating the biofilter at the desired conditions. The temperature that ranges from 20 – 40oc is generally esteemed as the optimal temperature conditions for the effective biofilter operation and as well as for the microorganisms to survive and proliferate.It is a good safety measure to incorporate a high temperature alarm near the air inlet and also regular monitoring (once in every 8 hours pre day is preferable)of the temperature of the filter bed is necessarily important.                                                7.4.3 Moisture ContentMicroorganisms need adequate amount of water for their microbial activities and also for the microbial population to propagate.It is one of the most crucial variable and has a greater impact on regulating the performance of the biofilter. The optimal moisture content for the media bed of the biofilter ranges from 40-60%.The humidifiers are placed prior to the biofilter so that it provides sufficient moisture to the influent air and prevents it from being dry. Additionally the sprinklers are used in the biofilter to distribute the water evenly on the entire filter bed surface, so that sufficient amount of moisture is supplied for the effective biofilter performance.                                             7.4.4 NutrientsNutrients are essential for the microorganisms to grow, multiply and survive. As Compost is used as one of the packing material in the biofilter, it provides enough nutrients for the microbial activity and also the contaminants(eg: N2) that flow through the filter bed provides some amount of energy and essential nutrients to microbes so that the biodegradation takes place effectively. Nitrogen is used as a source for the growth and also to build cell wall ( contains 15% of nitrogen) of the microorganisms. From the above information it is clearly seen that no excess nutrients are required.7.4.5 AcidityFor the biofilter to perform accurately the pH of the bed must be maintained constant in between 6-8, generally pH constant near 7 is considered to be feasible. During the biodegradation process some contaminants produce acids that results in decreasing the pH and thereby destroying the microorganisms. Therefore it is important to remove these acids now and then so that constant pH can be maintained throughout the process. One of the best way to eliminate the acids is by washing the medium bed or by adding appropriate biofilter medium like oyster shells which help in neutralising acids.