The potential for bioaugmentation of sand filter materials from waterworks using bacterial cultures degrading 4-chloro-2-methylphenoxyacetic acid

Background: The herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) is found frequently in Danish groundwater in concentrations exceeding the EU threshold limit of 0.1μgL ^-1. Groundwater is used for drinking water, and one potential remediation strategy is bioaugmentation using inoculation of sand filters at affected waterworks with degrader bacteria. Numerous bacteria degrading phenoxyacetic acid herbicide have previously been isolated, and they may be candidates for bioaugmentation processes. Designing the optimum inoculum, however, requires knowledge of the capacity for degrading realistically low herbicide concentrations and the robustness of the bacteria when inoculated into sand filter materials.

Results: Testing a range of different MCPA-mineralising bacterial combinations, using a high-throughput microplate radiorespirometric mineralisation assay, highlighted three efficient cocultures for mineralising low MCPA concentrations. Cocultures demonstrating a shorter time delay before initiation of ¹⁴C-ring-labelled MCPA mineralisation to ¹⁴CO ₂, and a more extensive mineralisation of MCPA, compared with those of single strains, were found. When inoculated into different sand filter materials, the coculture effect was diminished, but several single strains enhanced MCPA mineralisation significantly at low MCPA concentrations.

Conclusion: This study shows that an increase in the potential for mineralisation of low herbicide concentrations in sand filter materials can be achieved by inoculating with bacterial degrader cultures.