What is it?

Biotrickling works under similar conditions to bioscrubbing but, in contrast to bioscrubbing, the microbes are fixed on supporting elements.

In a biological trickle-bed reactor, an aqueous phase is continuously circulated through a bed of inert material. This packing can consist of irregular bulk material, such as rings, saddles, etc. or of structured packing. When selecting the packing material, it is necessary in every single case to ensure that, even in the event of an anticipated excess sludge formation, the reactor will not become choked in the long term.

The surface properties should be such that the biofilm adheres to it firmly. The pollutants in the waste gas and the oxygen are absorbed by the water phase and transported to the biofilm, where the biological transformation takes place. The quality of the mass transfer from the gas to the liquid phase and the elimination performance of the reactor essentially depend on the wetted surface area of the packing. In order to achieve optimum elimination results, i.e. to maximise the wetted surface area, the liquid phase should be distributed uniformly over the surface of the biofilm.

The immobilisation of the biomass and the formation of the biofilm are generally a naturally controlled process which starts after inoculation of the water phase. The continuously circulating liquid phase takes on the function of supplying the microbe population with the necessary nutrients. At the same time, excess activated sludge and reaction products which can also be inhibitors, e.g. hydrogen chloride during the degradation of dichloromethane, are washed out of the reactor. In the liquid phase, the essential conditions such as pH, nutrients and salt accumulation need to be controlled.

Design, maintenance and efficiency

The handling of the biological film layer (biofilm) of the packing is essential: too much growth can lead to (local) clogging that finally results in preferential streams, causing the size of the exchange surface and thus the performance of the biotrickling filter to worsen. The growth and thickness of the biofilm can be controlled by adjusting the thickness with mechanics (like varying the humidification) or by adjusting the growth of the microorganisms by varying the degree of acidity and/or the salt content.


The application of biotrickling is comparable to that of bioscrubbing. Slight differences are found in the pollutant compounds for which both treatment techniques are suitable. Biotrickling filters are primarily used to remove gases with acidic components.

Table 1 shows application limits and restrictions associated with biotrickling (adapted from EIPPCB, 2016, Table 3.191).

Table 1. Application limits and restrictions associated with biotrickling.
Issue Limits/ Restrictions
Gas flow (Nm3/h) 1000-500000
Temperature (°C) 15-40
30-35 (optimum)
Pressure (MPa) Atmospheric
Pressure drop (mbar) 1-10
Concentration of microorganisms > 15 g/l dry matter
Odour concentration (ouE/m3) >10000


EIPPCB (2016). Best Available Techniques (BAT) Reference Document for Common Waste Water and Waste Gas Treatment/Management Systems in the Chemical Sector. JRC Science for Policy Report.
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