Two Stage Reactors with at Least One Biological Step for Waste Gas Treatment

As the name implies, two-stage reactors exploit the inherent advantages of different processes used for waste-gas treatment, by adjusting the operation strategy as reactors connected in series or in one modified reactor configuration having different pollutant removal mechanisms (Kennes and Veiga 2001; Jin et al. 2008; Rene et al. 2009b, 2010b) . Two-stage reactors are preferable for certain operating conditions and waste-gas characteristics, as given here; (i) when the waste-gas contains a mixture of inorganic and organic pollutants, and some of their degradation end-products are highly acidic, (ii) when it is desired to maintain different microbial species in the bioreactors for the removal of a mixture of pollutants, and (iii) for the treatment of exceedingly high loading rates of the pollutants, which could inhibit microbial activity in the biological reactor. Typical examples are shown in Fig. 3.12 (biotrick-ling filter followed by biofilter in one reactor configuration), Fig. 3.13 (adsorption followed by biological treatment) and Fig. 3.14 (advanced oxidation pre-treatment followed by biological treatment), respectively.

3.4.6.1 Removal of Hydrogen Sulphide/Volatile Organic Compound Mixtures in Fixed-Bed Systems

Hydrogen sulphide (H2S) and mixtures of VOCs are commonly present in waste-gases emitted from the head works and other facilities at public owned treatment works (POTWs). Cox et al. (2003) tested the removal of these pollutants in a biofilter and observed an EC of 13.8 g H2S/m3.h, while VOC removal was reportedly poor irrespective of the experimental conditions. About 25-35% of low concentrations of benzene, toluene and chlorobenzene was removed, while other chlorinated VOCs could not be removed in the biotrickling filter. The low VOC removal was attributed to the presence of inhibitory concentrations of sulphate in the recycle liquid and the possible accumulation of metabolites other than sulphate that inhibit VOC biodegradation. Under such condition and for situations like emissions from typical pulp and paper industries (as reported by Rene et al. 2009b, 2010b), a two-stage

Biofilter section

Sprinkler

Biotrickling filter section

Polluted air

Biotrickling filter section

Fig. 3.12 Schematic of a two-stage bioreactor (biotrickling filter + biofilter); used commonly for handling mixture of organic and inorganic gas-phase pollutants. Different microbial species can be independently maintained in this reactor configuration

bioreactor appears to be more promising and a practically feasible option for the co-treatment of H2S and VOCs and other gaseous-pollutant mixture, which has shown positive results in the recent past.

Chitwood et al. (1999) evaluated the feasibility of using a two-stage biofilter for the treatment of H2S, air-toxics and smog precursors. The 1st stage acid-gas biofilter (AGB) packed with lava rock contained acidophilic autotrophic bacteria to remove H2S, while the 2nd stage wood-chip biofilter removed other air-toxics that includes, methanol, acetone, methylene chloride, chloroform, toluene, xylene, ethyl benzene, methyl-tert-butyl ether (MTBE) and 2-methyl butane. However, they observed that the 1st stage AGB removed acetone and methanol completely, while other VOCs were intermittently removed depending on the concentrations, in addition to 99.6% removal of H2S at an ILR of 0.057 g/m3.h.

Ruokojarvi et al. (2001) observed EC of 47.9 and 36.6 g/m3.h of H2S and dimethyl sulphide (DMS) in a two-stage biotrickling filter connected in series and inoculated

First-stage adsorption unit

Second-stage biofilter

Treated air

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Treated air

Fig. 3.13 Schematic of a two-stage reactor (adsorption column + biofilter); used efficiently for flow equalization and resisting periodic shock loads. The first-stage adsorption unit would also release the pollutant, when required, to avoid microbial starvation in the next-stage biofilter

Nutrient supply tank

Polluted air

Leachate removal

Fig. 3.13 Schematic of a two-stage reactor (adsorption column + biofilter); used efficiently for flow equalization and resisting periodic shock loads. The first-stage adsorption unit would also release the pollutant, when required, to avoid microbial starvation in the next-stage biofilter

Nutrient addition

Cool water inlet

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