DEVELOPMENT OF A BIOFILM ON-SITE PROCESS FOR THE AEROBIC COMETABOLIC BIOREMEDIATION OF A GROUNDWATER CONTAMINATED BY TRICHLOROETHYLENE AND 1,1,2,2-TETRACHLOROETHANE

Chlorinated aliphatic hydrocarbons (CAHs) are widespread groundwater contaminants. Aerobic cometabolism, that requires the supply of a suitable growth substrate, represents an interesting option for the remediation of CAH-contaminated aquifers, thanks to its capability to lead to the complete mineralization of a very wide range of CAHs. The aim of this study was to develop and validate a procedure relative to the lab-scale tests required to obtain the essential information for the design of a process of CAH aerobic cometabolism in a packed bed reactor (PBR). To validate the procedure, the latter was applied to the development of a PBR process for the on-site aerobic cometabolism of an aquifer contaminated by trichloroethylene (TCE) and 1,1,2,2-tetrachloroethane (TeCA). The specific goals of this study, corresponding to the main steps of the above-mentioned procedure, were: (i) to select the best growth substrate for the aerobic cometabolic process, and to develop and characterize an effective CAH-degrading microbial consortium, obtained from the site’s indigenous biomass by exposition to the selected substrate; (ii) to select the best carrier for the PBR process, and to evaluate the effect of bacterial adhesion on the developed suspended-cell consortium; and (iii) to identify and test suitable chemical-physical remediation alternatives in the case of presence of CAHs poorly biodegradable through AC. With regard to the growth substrate selection, the comparison of the CAH degradation performances obtained with 5 candidate substrates (methane, propane, butane, pentane and phenol) led to the selection of butane and to the development from the site’s indigenous biomass of a suspended-cell consortium capable to degrade TCE (TCE first-order constant (k1,TCE) = 96 L gprotein-1 d-1 at 30 °C and 4.3 L gprotein-1 d-1 at 15 °C) with a 90% mineralization of the organic Cl. Based on PCR-DGGE analysis of the 16S rRNA genes followed by band excision and sequencing, the microbial consortium enriched was mainly composed of Bacteroidetes and Alpha- and Beta-Proteobacteria that were distantly related to known CAH-cometabolizing bacteria. With regard to the selection of the best-performing biofilm carrier, a preliminary screening based on the previous experience of the research group led to the pre-selection of four candidate biofilm carriers (porous materials specifically designed for biofilm processes): Biomax, Biomech, Biopearl and Cerambios. The choice of the best-performing carrier was made by means of a 2-level procedure. The 1st level consisted of batch tests, operated both at 30 and 15 °C, whereas the 2nd level consisted of continuous-flow tests, operated at 30 °C. The 30 °C continuous-flow tests were conducted in four 1 L packed columns, connected to a feeding system designed so as to attain a pulsed feed of both oxygen and the selected growth substrate (butane). The four columns were operated in continuous mode for about 100 days. The results of the attached-cell tests were compared on the basis of the TCE normalized degradation rate ( and ) and of the attached cell concentration attained, at the two temperatures, at the end of the biofilm development process. On the basis of both the batch and the continuous-flow tests, Biomax resulted the best-performing biofilm carrier. Biomass immobilization on the carrier changed remarkably the structure of the microbial consortium. The effect on k1,TCE of biomass attachment depended on temperature: at 15 °C the attached consortium performed slightly better than the suspended one, whereas at 30 °C an opposite trend was noticed. On the basis of a 1st-order simulation, a 99.9% TCE conversion can be attained, at the site’s temperature, with a 9-hour HRT. Lastly, the low TeCA degradation rate by the developed consortium suggested the introduction of a chemical pre-treatment based on the TeCA to TCE conversion via -elimination, a very fast reaction at alkaline pH. On the basis of the overall results, the procedure for the development of a PBR AC process appears to be correctly designed and generally applicable to CAH contaminated sites.