Toluene biodegradation in an algal-bacterial airlift photobioreactor: Influence of the biomass concentration and of the presence of an organic phase

•Photosynthetic oxygenation was tested in a toluene removal algal-bacterial airlift.•Toluene elimination capacities of 320 g m−3 h−1 were achieved.•Optimum suspended solids of ∼5 g L−1 and dilution rates of 0.45 d−1 were obtained.•Silicone-oil enhanced toluene removal and O2 transfer, buffering toxic metabolites.

The potential of algal-bacterial symbiosis for off-gas abatement was investigated for the first time by comparatively evaluating the performance of a bacterial (CB) and an algal-bacterial (PB) airlift bioreactors during the treatment of a 6 g m−3 toluene laden air emission. The influence of biomass concentration and of the addition of a non-aqueous phase was also investigated. A poor and fluctuating performance was recorded during the initial stages of the experiment, which was attributed to the low biomass concentration present in both reactors and to the accumulation of toxic metabolites. In this sense, an increase in the dilution rate from 0.23 to 0.45 d−1 and in biomass concentration from ∼1 to ∼5 g L−1 resulted in elimination capacities (ECs) of 300 g m−3 h−1 (corresponding to removal efficiencies ∼ 90%). Microalgae activity allowed for a reduction in the emitted CO2 and an increase in dissolved O2 concentration in the PB. However, excess biomass growth over 11 g L−1 hindered light penetration and severely decreased photosynthetic activity. The addition of silicone oil at 20% (on a volume basis) stabilized system performance, leading to dissolved O2 concentrations of 7 mg L−1 and steady ECs of 320 g m−3 h−1 in the PB. The ECs here recorded were considerably higher than those previously reported in toluene-degrading bioreactors. Finally, microbial population analysis by DGGE-sequencing demonstrated the differential specialization of the microbial community in both reactors, likely resulting in different toluene degradation pathways and metabolites production.