Optimization of the volume fraction of the NAPL, silicon oil, and biodegradation kinetics of toluene and DMDS in a TPPB
Abstract
The volume ratio between Non-Aqueous Phase Liquid (NAPL) and water was optimised in order to remove the two hydrophobic volatile organic compounds (VOCs), toluene and dimethyldisulfide (DMDS). Biological oxygen demand after 5 days (BOD5) measurements showed that ratios ranging from 20 to 30% of silicone oil in water enabled optimal removal of both selected substrates, certainly due to a high adhesion of microorganisms at the interface and a high O2 consumption to metabolize pollutants. The removal of these compounds was then efficiently carried out in batch cultures in a Two-Phase Partitioning Bioreactor (TPPB) containing 25% silicone oil. If compared to the results obtained in the absence of NAPL, its presence led to 62 and 107% improvement of the biodegradation rate (global removal rate) for toluene and DMDS respectively. In addition silicone oil was a more efficient NAPL than di-ethylhexyladipate (DEHA), since biodegradation rates were 0.17 and 0.95 g m3 h−1 for toluene in 25% DEHA and silicone oil respectively. The use of silicone oil as NAPL led to removal yields close to 90 and 75% for toluene and DMDS respectively, and hence to the complete removal of residual VOC if losses by stripping were taken into account, showing the efficiency of TPPB containing silicone oil for the removal of these compounds.