INTRODUCTION Thereare many physical and chemical methods to treat air pollution. Bioprocesseshave been very efficient for the treatment of air pollution. The most popularbioreactor configuration is the conventional biofilter. Biofilters wereinitially designed for odour control at wastewater treatment plants, but nowthey are becoming renowned for the treatment of volatile organic compounds(VOCs) and other organic compounds.

These gases have bad odour and aredangerous for humans and animals. Biofiltrationprocess is preferred over other air pollution control techniques and has gainedpopularity because it has economic advantages,as the energy requirement is significantly low and avoids cross media transferof pollutants. This technology can be mainly applied to waste gases to controlair pollution, which have low concentration and readily biodegradablepollutants. Biofilters utilizes microorganisms to reduce air pollution.

Airpollutants come in contact with microorganisms inside the biofilter, wheremicroorganisms demean pollutants to produce energy and by-products (C02 andH20). Biofilter includes a filter material, where microorganisms breed. Biofilmis a very important part of the biofilter. Biofilm is a thin layer of moisturewhere the microorganisms reside. In the process of biofiltration polluted airis pushed inside the biofilter slowly thereby, the pollutants in the air getabsorbed in the filter media, then as a result the polluted gas is dispersed inthe biofilter and absorbed onto the biofilm.  Afterthe escalation of the conventional biofilter, new configurations were developedfor air pollution control which is known as biotrickling filters and bioscrubbers.A new set of technologies were also introduced for air pollution control suchas membrane bioreactor and rotating biological contractor. A lot of industriesuse this biofilter technology for example the food industry, pharmaceuticals,wood products manufacturing, paint and coatings application, petroleumindustry, sewage treatment etc.

Polluted air stream comprises VOCs and manysulfur compounds. Biofilters require a large area while treating largeairflows, which can be as big as a basketball court. Recent research shows thatbiofilters can be used to remove a variety of airborne contaminants whichincludes aliphatic and aromatic hydrocarbons, alcohols, aldehydes, organicacids, acrylate, carbolic acids, amines and ammonia.   The main three bio reactor configurations used forair pollution control are biofilters, biotrickling filters and bioscrubbers.The core mechanism is same for all but they make use of microbes in differentphases which may be suspended, fixed or in a liquid state. Biofilters are notexactly filtration units but a combination of different unit operations such asabsorption, adsorption, degradation and desorption of gas phase contaminants.Water is added into the biofilter system to controlthe moisture content and add essential nutrients. Humidification of gas streamsis generally carried out before feeding it into the system.

  BIOFILTER CONFIGURATION AND ELEMENTS CONVENTIONAL BIOFILTERSThe two categories of biofilters are open bed orclosed bed.Open bed biofilters are theones that are used mostly and its depth varies from about 10 to 18 inches. Theyare larger than the closed-bed biofilters.

Open bed biofilters are made in sucha way that they are exposed to the weather outside. Closed bed biofilters areusually 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 havedeeper media approximately (2-3 feet), in order to reduce the space which isrequired to attain odour control also it is  more expensive as compared to open bedbiofilters.  In short,the conventional biofilter can be referred to a fixed film or packed bedbioreactor which consists of a natural filter bed, it depicts that inertpacking materials are also successful in a closed bed biofilter.  In this biofilter polluted air is let into indown flow or an up flow manner through the reactor where contaminants of thewaste air are decomposed with the help of biocatalyst which is already presentin the packed bed. Therefore, to treat hydrophobic and water soluble compounds,this reactor is used. Conventional biofilters are not considered for thetreatment of the pollutants like chlorinated compounds which lead toacidification because there is no moving liquid present in it. Key elements ofa biofilter are Filter beds, Biocatalysts, Conditions, water content and flowrate and inlet concentration.

   IMPORTANT ELEMENTS1.     Filter bedThe filterbeds which were initially used in conventional biofilters were made up ofnatural materials. Common materials used were soil, compost or peat. Naturalfilter beds are still used in some areas.

The benefit of using natural filterbeds is the availability of microorganisms and nutrients. Major disadvantage inusing these filters is depletion of essential nutrients over time and increasein pressure drop due to gradual degradation and compaction. This also causesdecrease in efficiencies over time. New materials have been introduced toovercome such problems. Inert and synthetic materials are combined with naturalfilter beds to regulate the pressure drop across the filter bed. It isessential to humidify waste gas while using inert materials as, microorganismsneed a significant level of relative humidity for proper functioning. Generally,in conventional biofilters, continuous liquid phase is not added.

Nowadays, filterbeds are fully made up of inert materials in which feeding liquid solution isnecessary to avoid drying of the filter bed which can result in decrease inperformance and reactor failure.One of the majorissues in a biofilter is to maintain a favourable water content. Regulationsand moisture levels can be smoothly maintained these days with the help sensorsthat are cost effective. When the level of loading rates reach a peak point,the occurrence of adding the liquid phase automatically increases. Thetemperature of the filter bed rises with high loading rates. In open biofilterssprinklers are especially placed while using natural filter beds for maintainingfavourable conditions in accordance to the content of water. This usually takesplace in summer season where the top layer of the bed happens to dry out often. 2.

     BiocatalystBiocatalyst is an important elementfor smooth functioning of a biofilter. Microorganisms contains enzymes whichare responsible for biodegradation of the pollutants. Different types of biocatalystare used to tackle different types of pollutants. In natural filters someamount of microorganisms are naturally present.

Pollutants which are readilybiodegradable like hydrogen sulphide, the growth of microorganisms inside the naturalfilter will be significant. Pollutants which are not easily degradable such asMTBE or some halogenated compounds, addition of special biocatalyst becomescrucial. When inert or synthetic filter medium is used in the filter bed,inoculation of such special biocatalyst becomes a precondition. Again, ifthe pollutant is not highly recalcitrant, almost any type of sludge or mixedculture can be inoculated in the biofilter; otherwise, for more recalcitrantpollutants, seeding specialized microorganisms may be required   3.

    Watercontent The most favourable values ofmaintaining a sufficient water contents in biofilters varies from 40 to 60%.Very low water content tends to affect the activity of microorganisms whereasexcess water increases the pressure drop, and composition of anaerobic areas inthe reactor, therefore adequate water content is necessary to maintain anefficient functioning of the biofilter. The biofilters in which fungus isprevalent are said to be more unaffected to low water contents.                      CONCLUSIONThe use of biofiltration for air pollution control wasstarted in 1950s in Germany and USA. Since then, biofilter technology for airpollution control has evolved greatly and gained popularity.

Initially thistechnology was developed for waste water treatment. Biofilters are pocketfriendly and require less energy than other APC systems. New systems have beendeveloped after biofilters such as biotrickling and bioscrubbers specificallyfor air pollution control. However, this technology has few limitations relatedto the inlet concentrations but biofilters are also very flexible in terms ofchanging the operation conditions to handle different pollutants. This technologyhas a very wide range of application in various industries. The use ofbioreactors which include conventional biofilters and other similar technologieswhich make use of microorganisms is growing worldwide.

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