Biological trickle-bed reactor for gas purification

The technology for VOCs biodegradation in trickle-bed bioreactor was developed at the Institute of Chemical Engineering of the Polish Academy of Sciences in Gliwice and jointly patented with ICHEMAD-Profarb sp. z o.o. in Gliwice.

Air pollution caused by human activity is an international problem since harmful substances emitted into the atmosphere are transmitted across the state borders both in the ground zone, in the rhythm of meteorological changes, and through the stratosphere. Air pollution, first of all, with volatile organic compounds (VOCs) is becoming a more and more serious problem since they are mostly toxic substances, aromatically troublesome, and they are also tropospheric ozone precursors. There are several methods of purifying a gas from VOCs but their limitation is the condition of high concentration of contamination.

The presented trickle-bed bioreactor is a very good equipment for the treatment of large gas streams with low and variable concentration values. It is easy to control and adjust the operational parameters. The applied solutions enable to avoid several common problems such as bed clogging, uneven bed spraying, decreased activity of microorganisms. Importantly, the biodegradation process is safe and environmentally friendly, it does not move pollutants from one environmental phase to another, but completely breaks them down. In the trickle-bed bioreactor, no by-products or secondary pollutions that need to be deposited are generated.

The principle of biopurification:

In simplified terms, the process of VOC biodegradation can be presented as follows:

VOC + O2 -> biomass + CO2 + H2O

The pollution removed from the air is the only source of carbon and energy for the microorganisms; substrates are biodegraded in aerobic conditions to carbon dioxide, water and biomass.

The biofilm acts as a biocatalyst that is constantly renewing itself. The biofilm consists of microorganisms adapted to the biodegradation process of a given VOC or a mixture of organic compounds. The use of microorganisms adapted to a given contamination significantly reduces the time of biofilm formation and guarantees high efficiency of eliminating contamination for a long period of bioreactor operation.

The principle of operation of the bioreactor:

A trickle-bed bioreactor with a flow of air is a tank with at least one layer of inert packing in the form of polypropylene Ralu rings with a free volume of 0.95 and a specific surface of 110 m2/m3. Each of the 0.5 – 0.6 m high inert bed layers rests on a grate. By regulating the number of layers of packing bed,  the right residence time in the bed can be ensured for the contaminant.

The sprinkler, which is placed in the upper part of the tank between the gas inlet and the bed, contains 150-200 sprinkling points/m2 of column cross-section. The polluted air is evenly distributed around each spray point. During the co-current flow of liquid and gas, the pollutant is transported from the gas phase, through the liquid phase, to the active layer of biofilm. The packing bed is being continuously sprinkled with mineral salts solution what ensurs the availability of ingredients necessary for their proper functioning to the microorganisms. At daily intervals, 10% of the volume of the mineral salt solution circulating in the installation is replaced with a fresh solution containing ammonium salts, phosphates and microelements.

The clean air outlet is located in the lower part of the tank sidewall. The valves of the circulating liquid level control system are located in the conical bottom, which is filled with the solution to such a height that the cleaned air is directed entirely to the outlet.

The main advantage of the invention is such a design of a trickle bioreactor which, with downstream gas-liquid flow, enables the purification of large gas streams without the risk of clogging.

Benefits :

  • The major advantage is high ecological efficiency, no additional emissions are generated and substrates are biodegraded in aerobic conditions to carbon dioxide, water and biomass.
  • Biological methods are a very good solution for the treatment of large gas streams with low and variable concentration values; the concentration of the contaminants cannot exceed 10 g/m3. If the inlet pollutant load does not exceses ~40 g/m3h then the pollutant conversion is close to 100%. The efficiency coefficient value decreases slightly for higher loads of bed with pollutant.
  • VOCs biodegradation in trickle-bed bioreactor is a process conducted at low temperature (about 30oC) and at atmospheric pressure which results in low energy consumption and long bioreactor life.
  • The presented trickle bioreactor is easy to control and adjust the operational parameters. It is also easy to maintain the proper thickness of the biofilm layer. To remove excess biomass, it is enough to wash the bed with a liquid stream 50% larger than that used in the process.
  • Other Benefits: safety of the process, the possibility of full process automation, effective odor removal, energy saving, minimized operating costs.

The experimental work was carried out in a laboratory-scale and a pilot-scale units.

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