Formatted Title
Coupling Biological Reductive Dechlorination and Adsorption for TCE Removal Exploiting Raw Polyhydroxyalkanoates (PHA) from Organic Waste as Electron Donor and Pine Wood Biochar (PWB)
Background/Objectives
In anaerobic conditions, an organic fermentable substrate can be oxidized, providing the eligible electron donor (H2) for the biological reductive dechlorination (BRD) of highly chlorinated compounds. Recent field application studies in Italy have shown the effectiveness of combining the immobilization of chlorinated solvents and the biostimulation, allowing the quick reduction of dissolved contaminant levels and promoting the RD kinetics. In compliance with sustainability and circular economy principles, the current research interest is focused on alternative materials such as long-lasting electron donors and possible growth support for biofilm as adsorbents. Previous studies showed the potentialities of bio-based materials for bioremediation purposes, including polyhydroxybutyrate (PHB), a biodegradable microbial polyester tested as a fermentable source of slow-release electron donor. On the other hand, a low-cost biobased material, biochar (BC), also used as sorbent, has recently been proposed to accelerate reductive microbial dehalogenation. Here we proposed a coupled adsorption and biodegradation (CAB) process for trichloroethylene (TCE) removal in a mini-pilot-scale reactor filled with a raw PHA produced from mixed microbial cultures (MMC) and fermented organic waste (as feedstock) and pinewood BC. This work aimed to evaluate the performance of the CAB process with particular regard to the effectiveness of the BC in sustaining the biofilm, mostly enriched by Dehalococcoides mccartyi (Dhc).
Approach/Activities
The reactor was carried out in a column of 150 x 10 cm, in which the dechlorinating biofilm has grown on pinewood biochar (4% wt) mixed with sand for the entire length of the reactor. In the lower half of the column a dry raw PHA-rich biomass (35 %wt of PHA) in powder form was added. The reactor was equipped with 13 gates for side sampling. The startup was carried out with an active TCE to ethylene consortium, cultivated in our lab. After the fluid dynamics characterization, the flow rate was maintained at 2.8 L/d on average, with 35 hours as hydraulic retention time (HRT) in the first part of the experimentation (until day 220). Then, the flow rate was increased up to 7.3 L/d with a 10 h HRT. The feed solution consisted of contaminated tap water, resulting in a final TCE concentration of 100 µM. The monitoring of volatile fatty acids (VFA) and chlorinated compounds was carried out through regular sampling of the side doors of the column. Samples were stored for microbiological analysis.
Results/Lessons Learned
During the first two months of operation, the reactor has treated 180 Liter of contaminated water (10.5 ± 1.7 mg L-1 was the average of TCE IN) removing 2.6 g of TCE. The PHA compartment yielded a very high concentration of total VFA at the beginning of the experiment (1.3 g/L of VFA during the first week, at the outlet), decreasing progressively until a constant concentration of 6 mg/L from day 118 to 160. After 160 days of operation, a complete conversion of TCE to cis-dichloroethylene and the following daughter product vinyl chloride was observed in the lower part of the column. A slow and constant release of acetate from the PHA compartment and the high flow rate used are interesting conditions for field applications. This configuration allowed the treatment of high contaminant load at high solution feed rate (30.6 mg TCE/d; 2.8 L/d) and resulted stable and feasible also at higher flow rate (430 mg TCE/d; 7.3 L/d). Indeed, after almost two years of working activities around 4 m3 of contaminated water were successfully treated, exploiting sustainable materials derived from waste and making the technology usable at larger scale and by different possible configuration (e.g., P&T or PRB).