Biofilters and biotricklings: odour abatement in air
Biofilters and biotricklings exploit the property of a natural deodourization process oldest than life. In order to keep themselves alive bacteria shall find an energy source (food) and water (humidity). The subject of this section is just the way air pollutants are removed from air streams.
What is a biological reaction?
A bioogical reraction is just the explotation of bacteria in order to consume organic air pollutants from waste air streams. Almost all the organic substances – at riight environmental conditions and with the help of proper specific bacteria – may be decomposed.
As far as the last statement particoularly for organic substances, some bacteria may decompose inorganic substances as Hydrogen Sulphide and Nitrogen Oxides.
Why biological reaction is such important?
It is cost effective! Biofilters and biotrickling capital costs is lower than a traditional chemical equipment (wet or dry). Also running costs are sensibly lower if compared to other technologies. Thermal combustors (catalytics o recuperatives) employ a big amount of expensive fuel. Biofilters use a small amount of electricity for fans.
During normal run biofilters don’t need the full-time presence of technicians and the only consumable products are small amounts of micronutrients.
Biofilters are the most widespread type of bioreactors and usually use biomass beds (organic media within bacteria lives) composed by natural materials (wood chips, heather, bark, peat or vulcanic stone/lava rock).
Biomass bed changeout may take place within 2 – 5 years from first installation, depending on the type of employed natural material.
Biological reaction is a “green” slow-process, while traditional approacches are based on faster chemical reactions. Burning any fossil fuel is going to produce NOx, particulate matter, SO2 and CO2.
Normally bioreactors won’t generate none of the above mentioned air pollutants. A certain amount of leacheate shall be considered as a by-product of bioreaction. A bioreaction produces water and CO2.
Bioreactors will function well, but bacteria are very selective with respect to their favourite food. Apart from the right concentration of pollutants, bacteria need the right temperature, the right water amount and right environmental (neutral) pH
There are so many aspects to be take in consideration and which may take in error during biofilters design: it is mandatory to consider bacteria as living beings.
How does it work a biological reactor (biofilters, biotircklings and bioscrubbers)?
Bacteria have populated the earth since the planet got as cool to permit the existance of a life form. Bacteria has a very simple life cycle: they born, they eat and grow, they reproduces and then they die. Their diet is mostly based on Carbon, Water and Oxigen based compund and macronutrients. Biological reactors exploit the action of bacteria to remove air pollutant from air emissions by consuming them. The idea is quite simple, but the execution may be a little bit more complicated.
Biological reactors were used for hundreds years to puriy vents from sewers and other smelly compounds. About ninety years ago in Europe they started to use biological reactors to abat foul air particoularly from wastewater treatment plants and rendering plants for carcasses.
Biofilter: the most natural and simply bioreactor
The first deodorization equipment to be empolyed was a biofilter. A biofilter is a big rectangular box composed by a bottom air plenum – for foul air distribution along the biomass bed – and a grid to support a layer of biomass on the upper part.
Peat, heather, bark, gravel, lava rock or plastic filling bodies are only few of the materials used as media bed. In some cases it is recommended to add oyster shells in order to neutralize acids and fertilizer to add macronutrients.
The speration grid between air plenum and filtering media permits the free passage of foul air from the bottom to the upper part and – as a consequence – the free formation of leacheate in the other sense.
The air is distributed inside the biofilter through an air fan. If the air is too hot, too cool, too dry or too dusty it may affect the proper filter functioning. A pre-treatment of the foul air may be needed.
The pre-treatment indeed permits to obtain the optmium conditions for the correct functioning before the entering of foul air inside the plenum.
As the air passes through the filtering media bed, pollutants are absorbed through the humidity on the bed and put in intimater contact with bacteria. Therefore enzimes inside bacteria – during the digestion process – convert compounds in energy, CO2 and water. Residual undigestible material passes through the bed.
Although this brief explenation makes the proper functioning of a biofilter incredibly simple, in the reality things may be more complicated.
Variables that may influence the proper fucntioning of a biofilter are: temperature, pH, humidity, pollutant composition, macronutrients feeding, contact time and bed compaction with channeling of foul air phenomena.
These are crucial variables for which optimal operating conditions must be determined, controlled and maintained.
Is a biofiler/biotrickling indicated in your air pollution control scenario?
This is not a simple and foregone question. The intent of this page is to provide you with the tools to determine if a specific flow of contaminated air is suitable for treatment with a biofilter.
Why should I take care? Biological reactor are less expensive than traditional technologies for air pollutant.
Factors influencing biofilters and biotricklings performances.
As bioological reactors use living bacterical coltures, they suffer from so many variables occurring in their developing environment. We report some variables and limitations influencing the performances of every biological reactor beyond the type of process.
Although all the variables described are really important, the most important is temperature above all. A shot of hot air may kill the biomass more fastly than other accident.
The most part of bacteria may survive and grow in a temperature range between 30 – 40 °C Daily monitoring of temperature of the biomedia bed is very important. Also an high-temperature alarm in filter inlet may be a good safety precaution.
When a process emission is too hot, it would be better to pre-treat it with an air scrubber as humidifier. Beside refreshment effect this process enreaches relative humidity in foul air: this side effect may be a desiderabel effect.
Although a shot of very hot air may be more letal than other variables for bacteria, also fresh air slow down their work. Cold air may reduce indeed the bacterical activity to a state of suspende animation. Even freezing won’t kill bacteria: after defreezing they may re-acclimatize in a very short loss of time. In order to optimize efficiency during the winter period it may be useful to heat the inlet foul air with a thermal insulation of the filter.
So – just to optimize running costs – it may be used thermal waste, if they are free from pollutant that may kill bacteria.
The second variable by importance ranking is bed humidity. Bacteria needs to develop in a humid environment and humidity creates a biofilm which absorbs pollutant from the foul air stream.
Therefore problems of low humidity may be correct through the use of a pre-humidifier. Problem of humidity may be solved by taking the air humidity near to 100% HR. The humidification unit is a simple single stage air scrubber (horizontale or vertical)
Usually biofilters operate without the formation of backwater. A low level of humidity for short period of time wan’t kill bacteria, but it will soon reduce biofilter efficiency.
Ecciciency will be low after the re-acclimatization of bacteria.
On the other side, flooding biofilter with water will cause an increase in pressure losses through the bed and a loss of efficiency by channelling phenomena. Channelling on the long run will cause the collapse of the biomedia. Those are two conditions to avoid to run properly a biofilter.
Just keep in mind that bioreactions by-product is water. If emission is saturated with water on the outlet, over the biomedia you will encounter condensation phenomena. In the bottom part of plenum you have to consider some extra space to collect leacheate.
The optimum humidity value is 40 – 60% of HR.
Maintenance and feed for biological filter
In order to live and grow bacteria needs water, temeprature and a balanced diet. Air pollutants are the main source of food and energy, but bacteria needs also macronutrients. Biomedia degradation may give the right amount of macronutrients.
If a biomedia bed misses some nutrients, bacteria would stop to grow and then theu become to die.
Nitrogen is essential food to the bacterical growing. Bacteria uses Nitrogen to build cellular walls and it is the main protein constituent (also for RNA and DNA). Bacteria can use all the soluble forms of Nitrogen, but not all the Nitrogen is ready for use. Som nitrogen compounds from digestion are in gaseous form (Ammonia and NOx) and low quantity will exit from biofiltration process with emissions. Anyway the most part of Nitrogne inside vapours is re-absorbed in liquid phase and consumed by bacteria. Furthermore some nitrogen compounds form soluble compounds in water and they are evacuated with leacheate.
Other essential macronutreints are Phosphorous, Potassium, Sulphur, Magnesium, Calcium, Sodium and Iron. Nitrogen, Phosphorous and Potassium (reported as NPK on fertilizer) may be added through fertilizer. While the less soluble macronutrients such as Magnesium, Calcium, Sodium and Iron are always found as amendments for agriculture. The macronutrient content should also be checked frequently.
Most biological reactors work best at neutral pH (close to 7). Since some pollutants form Acids when they decompose, they could lower the pH of the bed and – in the long run – destroy bacteria. The substances that decomposing produce acids are hydrogen sulphide, organic sulfur compounds and halogens (chlorine, fluorine, bromine and iodine).
Especially in processes that emit these pollutants, the neutralization of acids must be considered.
There are a variety of acid neutralization techniques. Some techniques may be incorporated by specification into the bed type chosen for the biofilter. One of the simplest techniques involves mixing oyster shells with the filter media. Eventually the shells will dissolve and will need to be replaced – how long the shells last depends on the amount of acids produced. Therefore it depends on how many pollutants there are in inlet foul air.
Another simple technique consists in installing irrigation pipes for gardening through the filtering media bed. Periodically, a diluted solution of Sodium Carbonate will be inserted when the pH of the bed begins to decrease.
Or you can spray the same diluted solution of Sodium Carbonate over the bed, although this last practice may be less efficient than humidifying the internal layers with an irrigation hose.
Based on experience and similar applications, it is possible to inoculate the filter media with starter bacterial loads containing the “right” bacteria for the application.
More simply, nature will take its course starting from a filter media that contains the widest spectrum of bacteria such as compost, peat or activated sludge. The bacterial strains that will take root on the media will dominate the environment of the filter media.
The natural method will need more time to acclimate to optimal efficiency, but thanks to the diversity of bacterial strains it will be more able to adapt in the long term.
Laboratory-selected bacterial strains are much more susceptible to changes than naturally occurring strains.
Periods of time of non running conditions of the biological reactor may take to imbalance in bacterical population.