Anaerobic biodegradation of DDT in contaminated soil by biostimulation: laboratory and pilot-scale studies

Bioremediation of 1,1,1-trichloro-2,2-bis(4-chlorophenyl) ethane (DDT) by biostimulation of native microbial populations in soil was investigated in a lab-scale and pilot scale under anaerobic conditions. To evaluate the role of molasses (co-substrate) and potassium nitrate (electron acceptor) in the reductive dechlorination of DDT, experiments were conducted in a microcosm using five treatments: (T1) control, (T2) natural attenuation, (T3) molasses, (T4) molasses + potassium nitrate and (T5) potassium nitrate. Results showed that after 30 days of incubation, DDT concentration was reduced by 30.3% in the control, 32.85% for natural attenuation, 72.3% with addition of molasses, 92.5% for amendments with molasses + potassium nitrate, and 70.2% for biostimulation only with potassium nitrate. An upscaling of the microcosm to a larger fixed-bed reactor was conducted for treatment T4. After one month of incubation, DDT concentrations in the reactor decreased by 91.54% of the initial quantities. The DDT biodegradation rate fit a pseudo-first-order kinetic decay function and declined to 0.077 d−1, with half-life of 8.9 days in the absence of oxygen. Predominant microbial strains were isolated and identified through biochemical and molecular tests before and after the bioremediation process. The microorganisms isolated were identified as Bacillus circulans and Bacillus megaterium before and after the treatment application, respectively. This study provides evidence that the combination of a donor electron substance (molasses) and acceptor electron (KNO3) can enhance the DDT biodegradation rates under anaerobic conditions.