Now a day’s emission of odors is one ofthe major difficulties encountered by many waste treatment industries. Thisunavoidable emission of odorous gases into surrounding environment can cause a verysevere impact on the health of the human beings, residing close to thetreatment plant and due to which the potential may arise for closing down ofthe waste treatment plant. The odorous air from the wastetreatment plant has the typical feature of large volume with low concentrationof polluted odour compounds. So, to eliminate these odorous chemicals fromexhaust air that is emitted a Biofiltration process will be used to treatbefore releasing it into environment.
Biofilter is most effective and comparativelyinexpensive, especially in the composting industry. 7.1 Biofilter TypeIn our GreenCycle facility, a closed single bedbiofilter has been chosen with a down flow design because it is moredesirable for larger flow rates. The exhaust air that is collected fromdifferent units like storage, pretreatment, fermenter and compostingis conducted into this biofilter and treated accordingly. The inlet is placedabove the bed which allows the flow of pollutants into the filter. Due to theintroduction of the contaminated air into the filter or may be due to thegeneration of the heat by biological activities of the microorganisms there willbe a possibility for the dryness of the media that is used in the biofilter. Thisdrying impact can be regulated easily by the moisture content that is beingprovided by the humidifier which is placed prior to the biofilter and also bythe irrigation system which is placed inside the biofilter. These emissions flowsfrom the humidifiers into the biofilter and gets intact with microbes on thebed.
During this contact period, the microorganisms absorbs the pollutants andthe enzymes present in these organisms are able to change these compounds intowater, carbon-dioxide and energy. The clean air flows through the bottom outletfrom where it is liberated out into the atmosphere. 7.
2Reactor sizingIn order to determine the size of the reactor, theconditions that are given within 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 thebiofilter, the odor degradation rate of the packing material that is used inthe design of the biofilter has to be known. The sources for the packingmaterial that is going to be used are mature compost and woodchips.There are certain reasons behind choosing compostand 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 micronutrients.
Porosity is improved by using wood chips, thereby reducing pressure drop. Easily obtainable in the plant and inexpensive.Accordingly, a pilot test is performed that isspecific to a biofilter design with a particular filter media. After the testis performed, a graph can be plotted of the odor degradation rate (r) versusthe 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 2506 Odour 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 – CoutlnCinCout (Eq 7.2) Using equations 7.
1 and 7.2 the results for ´r` and´Cm´ can be calculatedas shown in the Table XX. By plotting degradation rates over logarithmic meanconcentration 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) 497 104 290 1.7 251.2467839 67041.18 558 62 290 1.7 225.
7393282 84611.76 939 22 290 1.7 244.2875458 156429.4 11925 348 290 1.
7 3275.71538 1974900 15792 608 290 1.7 4661.838798 2590212 16384 484 290 1.7 4514.511658 2712353 21720 966 290 1.7 6667.255757 3540388 32500 966 290 1.
7 8969.144197 5379329 45823 1916 290 1.7 13830.95205 7490018 74698 2246 290 1.7 20675.15598 12359459 94646 2506 290 1.
7 25372.74474 15718000 Figure xx: For the above experiment, the overalllogarithmic mean concentration is, Cm= 20000 – 500ln 20000500= 5286.16 OU/m3 (Eq7.3) Therefore, The overall degradation rate for the logarithmicmean concentration (Cm)from the above diagram can be given as, Fromfirst order kinetics, we have: r= k1.
Cm (Eq7.4)Therefore, r = 607.69 5286.16 -165673 = 30,46,673.57 (OU/m3/h) (Eq 7.5) Now, the Volume of Biofilter is calculated asfollows: V= Cin-Cout V´r (Eq 7.
6) V= 20000 – 500 850003046673.57= 544.035 m3 (Eq7.7) Biofilter surface areais calculated by the amount of flow rate to be treated times the filterarea load: The flow rate = 85 000 m3/hThe filter area load = 100 m3/m2h. Biofilter surface area, A= 85000100= 850 m2 (Eq7.8) The Packing height is: H= VA= 544.
035850= 0.64m (Eq7.9) 7.3Various Components of BiofilterThe Overall biofilter plant comprises of variouscomponents like High pressure fans that collects the exhaust air from differentprocess units(composting, pretreatment etc.,) by maintaining negative pressure,Long pipes along with holes allows the flow of malodorous air through thefilter bed, Humidifiers which eliminates the dryness by providing sufficientmoisture content , Sprinklers as irrigation system placed above the biofilterbed maintains the sufficient moisture on the filter bed medium by sprinklingthe water, Filter bed constitutes the medium and microbes where thebiodegradation process occurs and the concentration of contaminants decreasesand are converted into water and CO2.The biofilter bottom is equipped with support that contain gravels whichprovides support to the bed medium and thereby maintains the structure of thebiofilter.7.
4Important Parameters Affecting the Biofiltration Operation The various parameters such as Temperature,Moisture, Nutrients Attainability, pH, Flow direction of polluted gas effects theoperation and performance of the biofilter.In this chapter, the details about each of theseparameters are discussed below.7.
4.1Flow Direction of Odorous GasesIn biofilters two different types of flow modesnamely downflow and upflow are available. In our green cycle facility, wepreferred to use down flow closed type of biofilter. Firstly, the polluted airflows across the long pipes with holes at the sides and gets distributed evenlythroughout the entire bed. In the down flow mode the inlet for the polluted airis at the top of the biofilter and the outlet for the clean air is at thebottom. For the larger flow rates the down flow closed biofilter is desirable. 7.
4.2TemperatureThe microbial activity is a key factor for the successfulbiofilter operation.The temperature has a major effect on this microbialactivity, therefore it is important to keep the temperature at favourableconditions. The rate of the microbial activity becomes higher with increasedtemperature but on the other hand high temperature decrease the adsorption rate.Low temperatures works in the reverse manner. Generally the malodorous aircoming out from the composting plant has high temperatures and as soon as thesegases gets intermixed with the exhausted air released from different units andflows through the humidifier, where the temperature will get decreased. Thehumidifiers play a significant role in maintaining the biofilter at optimum conditions.The temperature that ranges from 20 – 40ocdesirable for the effective biofilter operation and also for the microorganismsto live and proliferate.
As a safety precaution we are incorporating a hightemperature alarm near to the air inlet and also regularly monitoring (once inevery 8 hours per day) the temperature of the filter bed. 7.4.3Moisture ContentAdequate amount of water is needed for themicrobial 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 controllingthe performance of biofilter.
The appropriate moisture percent for the mediabed ranges from 40-60%.The humidifiers that are situated prior to the biofilterprovides adequate moisture to the influent air and prevents the dryness. Besides,the sprinklers are also placed in the biofilter that distribute the waterevenly on the entire surface of filter bed, so that appropriate amount ofmoisture is supplied for the effective biofilter performance. 7.4.4Nutrients Microorganismsrequire nutrients to grow, multiply and survive.
Compost that is used as the mediain the biofilter provides sufficient nutrients for microbial activities andalso the pollutants (eg: N2)that flows into the filter bed provides some amount of energy and the nutrientsto microorganisms so that the effective biodegradation process occurs. Nitrogenis 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 nutrientsare not required.7.4.5AcidityThebiofilter to perform accurately the pH of the bed must be maintained constantin between 6-8, generally pH constant near 7 is feasible. During the biologicaldegradation process some contaminants produce acids that outturn in decreasing thepH and destroying the microorganisms.
So, it is important to remove these acidsnow and then so that constant pH can be maintained throughout the process. Washingthe medium bed or by adding appropriate biofilter medium like oyster shellswhich help in neutralizing acids are the best ways to avoid the acids