Biological Systems

There are three basic types of biological reactor systems used to treat waste gases: bioscrubbers, biotrickling filters, and biofilters. These can be grouped into those employing microorganisms dispersed freely throughout the liquid phase (bioscrubbers), and those using microorganisms immobilized on a packing or carrier material (biotrickling filters and biofilters). Moreover, in bioscrubbers and biotrick-ling filters the water phase is continuously moving, whereas in biofilters it is stationary.

A bioscrubber consists of a scrubber unit and a regeneration unit. In the scrubber (absorption column), water-soluble gaseous pollutants are absorbed and partially oxidized in the liquid phase (the culture medium containing the microorganisms), which is distributed from the top of the unit. The contaminated water is subsequently transferred into an aerated stirred-tank reactor (regeneration unit), similar to an activated-sludge unit, where the contaminants are fully biodegraded. The regenerated suspension is continuously recirculated to the top of the scrubber section, thereby enhancing process efficiency (Fig. 1).

Biotrickling filters (Fig. 2) and biofilters (Fig. 3) are different from bioscrubbers in that gaseous pollutant absorption and biological degradation occur simultaneously in the same compartment, resulting in more compact systems. The polluted air flows through a biologically active bed, where microorganisms are attached in the form of a biofilm. As the gas diffuses through the packed bed, the pollutants are transferred to the biolayer and degraded.

As illustrated in Figure 3, in order to ensure optimal operation of a biofilter, the inlet gas usually requires pre-treatment processes such as: (1) particulate removal in order to prevent possible clogging and/or sludge build up, (2) load equalization in case the waste-gas concentration is subject to strong fluctuations, (3) temperature control, and (4) humidification.

In biological trickling filters the packed bed consists only of inert materials (glass, ceramics, and plastics), while the liquid phase, containing inorganic nutrients, flows downward over the packaging material in countercurrent with the contaminated gaseous stream and is continuously recirculated through the bioreactor.

Bioscrubbers and biotrickling filters are applicable mainly to the treatment of waste gases containing good or moderately water-soluble compounds, whereas biofilters, due to the large surface area available for mass transfer, are also suited to treat poorly water-soluble compounds. Moreover, due to their high reaction selectivity, biofilters are particularly suitable for treating large volumes of air containing easily biodegradable pollutants with relatively low concentrations, typically below 1,000 ppm.

Compared with the other biological systems, biofilters have the widest application because they are easy to operate, simply structured, and imply low installation and operating/maintenance costs. Also, the reliability of biofilter operation is higher than that of bioscrubbers, where the risk exists of washing away the active microorganisms. Moreover, the presence of a large amount of packing material with a buffering capacity diminishes the sensitivity of biofilters to different kinds of fluctuations (15). Because the major disadvantage is the difficult control of parameters such as pH, temperature, and nutrient supply, biofilters may be unsuitable for degrading halogenated compounds (as acid metabolites are produced) and treating gas streams containing high concentrations of VOCs, unless long residence times or large bed volumes are applied (3). Biotrickling filters and biofilters are currently utilized mainly in compost-production plants, sewage-treatment plants, and agriculture, whereas biofilters and bioscrub-bers are preferred in industrial applications.

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