Bioactive Coating Bioreactors for Indoor Air Treatment

In this work, bioactive coating-based bioreactors have been applied to the treatment of three odorous indoor air pollutants (i.e., toluene, α -pinene, n-hexane), which are volatile organic compounds (VOC) of different hydrophobicities. The removal efficiency (RE) was evaluated at different empty bed residence times (EBRT) and inlet concentrations. The setup consisted of three bioreactors (namely BR 1, BR 2 and BR 3), packed with porous expanded clay. An enriched consortia of microorganisms was inoculated in BR 1 and BR 2, while fresh activated sludge was used in BR 3. A styrene-acrylate copolymer was employed for bioactive coating formulations in BR 2 and BR 3, while BR 1 performed as a conventional bioreactor. BR 1 and BR 2 achieved REs over 90 % for toluene and α -pinene at an EBRT as low as 30 s. On the contrary, BR 3 only achieved REs >90 % when operated at an EBRT of 60 s, toluene and α -pinene removals decreasing to 67.0 % and 49.8 % at 30 s, respectively. The poorest performance was recorded at 15 s, with REs of toluene and α -pinene of 41.5% and 55.7% in BR 1, 44.3 % and 10.5 % in BR 2, and 22.2 % and 8.0 % in BR 3. n-hexane removal was poor and non-consistent. When decreasing inlet concentrations at EBRT 15 s, toluene and α -pinene REs progressively increased to reach 87.1% and 90.9 % in BR 1, 86.5 % and 76.6 % in BR 2, and 64.2 % and 70.6 % in BR 3. Overall n-hexane removal slightly increased but was still poorly reproducible. 3 community. in any of the bioreactors. The longest acclimation period of BR 3 was attributed to the lack of adaption of the microorganism to the selected VOCs. The reduction in the EBRT to 60 s had little effect on VOC removal in BR 1 and BR 2. REs of toluene and α pinene remained over 90 %, while no increase in n-hexane removal was observed. A slight deterioration of BR 3 performance was recorded after the EBRT reduction, rapidly recovering average REs of 89.9 ± 1.6 % for toluene and 87.3 ± 4.1 % for α -pinene. No n-hexane abatement was observed. A further decrease in the EBRT to 30 s in Stage 3 resulted in an initial decrease in toluene removals in BR 1 and BR 2, with a more severe effect on α -pinene removal ( ≈ 30 % decrease). Removals of toluene and α -pinene recovered within a week to achieve steady state values of 91.3 ± 3.0 % and 97.9 ± 2.3 % in BR 1, and 92.6 ± 1.7 % and 95.2 ± 6.4 in BR 2. On the contrary, BR 3 could not recover previous REs, and after the initial decrease, steady state REs for toluene and α -pinene remained at 67.0 ± 7.8 % and 49.8 ± 12.0 %. A slight increase in n-hexane degradation was observed in the three bioreactors, although outlet concentrations showed a high variability. As previously stated, the lack of specialization of the fresh activated sludge to the VOCs was the most likely cause of the performance deterioration in BR 3. The reduction in the EBRT to 15 s entailed an overall decline in the removal values. Toluene REs decreased by ≈ 50 % in the three bioreactors. While a similar deterioration in toluene REs was observed in BR 1 and BR 2 (to 41.5 ± 11.2 % and 44.3 ± 5.4 %, respectively), a much faster deterioration was recorded in BR 3, reaching values of 22.2 ± 16.1 %. The reduction in α -pinene RE was moderated in BR 1, decreasing to 41.5 ± 11.2 %. On the other hand, BR 2 and BR 3 experienced steeper reductions to REs of 10.5 ± 10.2 % and 8.2 ± 9.3 %, respectively. No variation on n-hexane removal was observed in any of the bioreactors. This general drop in VOCs removals could be attributed to a deterioration of the bacterial activity, especially in the bioactive coatings, caused by the loss of moisture at higher inlet gas flowrates. Additionally, a higher gas flow implies a shorter contact time that might hinder VOCs mass transfer.