Enhancing the biological reductive dechlorination of trichloroethylene with PHA from mixed microbial cultures (MMC)

Abstract

The biological reductive dechlorination (BRD) process is often limited by the lack of suitable electron donors, requiring the supply of long-lasting substrates to enhance the biological metabolism. Among the others, polyhydroxyalkanoates (PHA) are a particularly interesting slow-release source of electron-donors, being entirely biodegradable polyesters. Generally, industrial PHA production processes are based on pure culture fermentation with high related costs. In recent years, innovative and cost-effective PHA production from mixed microbial cultures (MMC) and waste feedstocks is attracting considerable attention. This research, for the first time, investigated the effect of distinctive types of PHA on the BRD process of trichloroethylene (TCE), in a continuous-flow lab-scale system using two PHA materials with different purity grade produced from MMC at pilot scale in comparison with a commercial PHA produced from pure culture. Promising results have been obtained with non-extracted MMC-PHA, a material consisting of both PHA (56%, w/w) and microbial cells, with constant production of acids over 110 days, which stimulated a nearly complete TCE dechlorination with the non-toxic ethene as the main byproduct (approximately 92%). Dehalococcoides mccartyi was the main microorganism responsible for TCE dechlorination process, representing ≥ 54.05% of the bacterial population, mainly carrying the reductive dehalogenase genes tceA and vcrA (≥9.17E+08 and ≥2.01E+07 gene copies/g of PHA or sand, respectively). This finding suggests the possibility to directly use PHA-rich biomass deriving from MMC production process, which does not require any polluting and expensive extraction procedures, as a novel material in the field of groundwater remediation with noticeable economic and environmental advantages.