 |
Biotreatment of gaseous streams
|
|
|
|
Gaseous emissions consisting of organic compounds have consequences on air quality, as well as on the different trophic levels of the biosphere.
A large part of the problems associated with this type of compounds is related to the generation of offensive odors and with which, in some cases, they lead to respiratory diseases. Additionally, they generate negative economic effects for activities such as recreation and tourism, also affecting the value of real estate within the affected areas.
Control of this kind of emissions is becoming increasingly important, due to the growing environmental awareness, which translates into greater pressure from residential communities on neighboring industries and a tendency to establish increasingly stringent environmental legislation.
Over the twentieth century, human activities such as the use of fossil fuel, agricultural practices and deforestation have been affecting the climate by altering the composition of Earth’s atmosphere. These activities have resulted in increasing concentrations of many gasses such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), tropospheric ozone (O3), and aerosols in the atmosphere. These gases have the ability to trap outgoing longwave in the atmosphere resulting in an increase in the atmospheric temperature. This enhanced greenhouse effect has the potential to significantly modify climatic conditions. Due to the growing concern that human activities are changing the atmosphere and climate, several global efforts to reduce greenhouse gas emissions have been made.
Globally, the problem of odors has been attacked with the use of conventional air purification systems, which generally include physicochemical treatments. These technologies, although they are effective, are generally expensive and generate by-products of lower assimilation for later dispositions.
Although there are various techniques for the treatment of gaseous effluents, biological systems, such as biofilters and biowashers, are preferred for their multiple advantages, including lower initial investment, as well as lower operating costs and low generation of additional waste.
The biofiltration technology provides a versatile, economical, simple and effective method of purification for a wide variety of malodorous or toxic compounds, especially in those cases where there are high flows and low concentrations.
A variety of substances can be treated, including volatile organic compounds like alcohols, ketones or aldehydes and odorous substances like ammonia and hydrogen sulphide (H2S). While biotechnology is often thought of as something of a new science, the history of its application to air-borne contamination is relatively long. The removal of H2S by biological means was first discussed as long ago as 1920 and the first patent for a truly biotech-based method of odour control was applied for in 1934. It was not until the 1960s that the real modern upsurge began, with the use of mineral soil filter media and the first true biofilters were developed in the succeeding decade. This technology, though refined, remains in current use.
|
|
|
A number of general features characterize the various approaches applied to air contamination. Typically, systems run at an operational temperature within a range of 15–30 0C, in conditions of abundant moisture, at a pH between 6-9 and with high oxygen and nutrient availability. In addition, most of the substances, which are commonly treated by these systems, are water-soluble.
The available technologies fall naturally into three main types, namely biofilters, biotrickling filters and bioscrubbers. To understand these approaches, it is probably most convenient to adopt a view of them as biological systems for the purification of waste or exhaust gases. All three can treat a wide range of flow rates, ranging from 1000–100 000 m3/h, hence the selection of the most appropriate technology for a given situation is based on other criteria. The concentration of the contaminant, its solubility, the ease of process control and the land requirement are, then, principal factors.
Biofilters
Biofilters are one of the main biological systems used, which work at normal operating conditions of temperature and pressure. Therefore, they are relatively cheap, with high efficiencies when the waste gas is characterized by high flow and low pollutant concentration.
The system is shown schematically in Figure 10. In a biofilter, the to-be-cleaned gas stream is passed upwards through a filter bed, which has been constructed of biological material, for example, compost, tree bark or peat. The filter material carries a thin film of water, which is home to microorganisms. The pollutants in the gas stream are retained in the filter material via adsorption and absorption, and are then decomposed by the present microorganisms (bacteria and others). The filter material serves as a supplier of necessary nutrients. The degradation products for conversion are carbon dioxide, sulphate, nitrate etc.
Fig. 10. Biofilter.
The dry matter content in the filter typically varies between 40 to 60%. To prevent the bed from drying-out, the gas stream must be fairly well saturated with water. This is why to-be-treated air is normally moistened in advance. The gas must have a relative humidity of 95%. In practice, it is always better to implement a humidifier in advance, in order to prevent the filter from drying out.
The filter material naturally contains enough different types of microorganisms to deal with easily-degradable substances. In case of substances that are more difficult to degrade, the filter can be injected with special cultures in order to realize faster filter start-up, though close follow-up is essential to safeguard correct working parameters. The addition of minerals and nutrients may be desired if there is insufficient nutrient-release in the filter material.
In biofiltration, it is important for the filter material to have a pH between 7 and 8 for the break-down of organic components. The break-down speed reduces quickly at pHs less than 6, 5.
The residence time for the gas in the filter must be at least 30–45 seconds in order to effectively realize odor and solvent removal.
Bioscrubber
A scrubber or scrubber system is a system that is used to remove harmful materials from industrial exhaust gases before they are released into the environment. There are two main ways to scrub pollutants out of exhaust, and they are:
|
|
|
- Wet scrubbing. The removal of harmful components of exhausted flue gases by spraying a liquid substance through the gas.
- Dry scrubbing. The removal of harmful components of exhausted flue gases by introducing a solid substance to the gas, generally in powdered form.
Both of these methods work similarly and perform the same process of removing pollutants. The main difference is the materials they use to filter the gases. By removing acidic gases from the exhaust before it is released into the sky, scrubbers help prevent the formation of acid rain.
Although it is normally included in the same group, the bioscrubber (Fig. 11) is not itself truly a biological treatment system, but rather a highly efficient method of removing odor components by dissolving them. Unsurprisingly, then, it is most appropriate for hydrophilic compounds like acetone or methanol. The contaminant solution is then removed to a secondary bioreactor where the process of biodegradation takes place. In practice, activated sludge systems are often used in this role.
Summarizing, a bioscrubber consists of a gas scrubber and a biological reactor. In the gas scrubber, to-be-removed components are absorbed from the gas stream by the wash water. In the biological reactor, the pollutants that have been absorbed by the wash water are biologically degraded. The purified scrubbing liquid is circulated to the scrubber, where it is able to reabsorb pollutants.
The biologically degradable hydrocarbons are converted into H2O and CO2. The non-degradable hydrocarbons remain in the wash water. Components such as H2S and NH3 are converted into sulphate and nitrate respectively. Regular draining needs to take place in order to keep down the salt content and the level of non-degradable hydrocarbons. This can take place based on conductivity or via fixed discharge. The level of discharge is determined by the flue gas composition. It has been established that stable biological degradation can still be realized with salt content equivalent to a conductivity of 5 mS/cm. A hydraulic residence time for wash water of 20-40 (maximum) days, produces good results.
The gas scrubber must be designed to ensure that the residence time of gases in the scrubber amounts to approximately 1 second. This may be slightly more or slightly less, depending on the solubility of the components. The scrubber must have a special open packing and special spray nozzles to prevent blockage by biosludge.
Besides a carbon source (hydrocarbons), the biological system also needs nutrients in order to survive. For this purpose, a mix of nutrients is added to bioscrubbers. This mix of nutrients contains nitrogen, phosphorus and trace elements.
Fig. 11. Bioscrubber diagram.
The biological reactor contains an aeration device to supply bacteria with enough oxygen to break down the components. In the event of poorly soluble and difficult to degrade components, there is a real risk of components being stripped into the air. In order to prevent pollutants being emitted via stripping, it would be best to send the air from the aeration device back through the bioscrubber.
The biological reactor can be set up as an active sludge system or a biofilm system on a carrying material. Systems with a carrying material normally have a lower sludge production. When the bioscrubber is started, the biology is injected with sludge from a biological water purification installation or another bioscrubber. This sludge must adjust to the specific component-composition of flue gases. The adjustment to difficult-to-degrade components can take a few weeks to a month before the pre-determined efficiency is realized.
To degrade specific sulphur and chlorine components, bacteria cultures are sometimes injected that have been specifically grown in laboratory settings.
|
|
|
Biotrickling filters
A biotrickling filter is a combination of a biofilter and a bioscrubber (Fig. 12). The bacteria responsible for decomposition are immobilized on a carrier or filter material. The filter material consists of synthetic foam, lava or a structured plastic packing. The surface must have a structure that allows biomass to bond to it effectively.
The carrier material is constantly covered with water. This mean that water must be uniformly sprayed over the packing. The polluted components are absorbed in the liquid film and are decomposed by the bacteria.
In order to feed the biomass, the required nutrients are added to the water. This water also carries away excess sludge/biofilm, as well as decomposition products, which may hinder the biomass.
The scrubbing liquid which is circulated over the packing must be checked for pH, nutrients and salt concentration. The pH can be continuously measured and corrected. The nutrients are constantly dosed and dosage is periodically checked via analyses. In order to keep salt concentrations within acceptable limits, one section must be discharged and filled with fresh water. This can take place on the basis of conductivity.
To safeguard the process, a temperature measurement can be taken for flue gases, if there is a likelihood of temperatures exceeding required limits. This will help to spare the biotrickling filter.
In the biotrickling filter, the packing may become blocked due to strong growth in the biofilm. This will lead to prefered flows, which will in-turn reduce efficiency and increase pressure drop. If the blockage is too severe, the packing will have to be replaced. During dimensioning it is important to set the load at a modest level in order to avoid such blockages.
Fig. 12. Biotrickling filter. Diagram.
In summary, the selection of the most appropriate system depends on the characteristics of the gas stream being treated, the expected removal efficiency and the costs involved. The main parameters to consider when designing a biofiltration system are:
- The characteristics the gas contaminate (concentration, flow, particulate matter, temperature)
- Selection of filter material
- Moisture content of the filter material
- Microorganisms
|
|
|
