Evaluation of Biomonitoring Strategies to Assess Performance of a Bioremediation Bioprocess

Abstract

This study was conducted to propose a suitable set of methods to evaluate the efficiency of two biotreatments. For this purpose, two sets of four 7.5 L bioreactors were followed over 90 days, containing natural sediments from the Bizerte Lagoon (Tunisia) contaminated with 35 mg·kg−1 benzo(a)pyrene (BaP) and 28 mg·kg−1 dichlorodiphenyltrichloroethane (DDT). One set was biostimulated with N/P and bioaugmented with the indigenous Pseudomonas stutzeri, Cupriavidus metallidurans and Rhodococcus equi, and the other set was only biostimulated. In the effluent, organic carbon decreased from 42 gC·L−1 to 0.2 gC·L−1 for the bioaugmented treatment compared to 15 gC·L−1 for biostimulation. Statistical analyses confirmed a significant difference in BaP concentration after bioaugmention from 35 mg·kg−1 to 21 mg·kg−1 sediment, whereas no difference was found with biostimulation. Considering DDT, biostimulation was more efficient (8.5 mg·kg−1 sediment final concentration) than bioaugmentation (15 mg·kg−1 final concentration). Native organotin and metals were also monitored using bioluminescent bioreporter strains. The bioaugmented treatment brought about a significant decrease in TBT content, to below 0.01 µM, whereas its concentration remained significant after biostimulation. The biostimulation did not alter As3+, Cu2+, Cd2+, and Hg2+ concentrations, whereas bioaugmentation induced a decrease of 1 to 2 log for each metal. At the end of the experimental period, toxicity decreased to 90% in the effluent of the bioaugmented reactors compared with a drop of only 48% for biostimulation, and a significant decrease in mutagenicity appeared for bioaugmention only. Interestingly, not all the strains used in the treatments were maintained, as P. stutzeri and R. equi increased up to densities of 8.3 × 1013 and 5.2 × 1012 DNA·g−1 sediment, respectively, while in both treatments, C. metallidurans decreased down to the detection threshold. Among the different methods used, a restricted monitoring panel of analyses appears essential to follow the change occurring over the bioremediation process: (i) organic carbon measurement reporting all biodegradation events, as well as a specific method to monitor the main compounds; (ii) dissolved N, P, O2 and pH measurements, (iii) a qPCR method to track the degraders; and (iv) measurements of the acute toxicity and the mutagenicity.