The evaluation of metham sodium and dichlobenil impacts on activated sludge nitrification

Abstract

Sanafoam Vaporooter II is a trademark name for a process which uses metham sodium and dichlobenil to remove tree roots from municipal sewer and storm drain lines. One or more of the chemicals in this process, or their degradation products, have been suspected of disrupting the nitrification process in waste treatment plants downstream of the points of application. This work was undertaken to identify the chemical responsible for the disruption, and to recommend means for assuaging the problem.

The impacts of the herbicides used in the Vaporooter II process were separately tested in bench scale reactors. Metham sodium was found to be the herbicide responsible for disrupting nitrification. It did so at concentrations of 4 mg/L or higher in systems which have a mixed liquor volatile suspended solids concentrations of 1800 mg/L. This corresponds to a herbicide to biomass ratio of 0.017 moles/gram. The stability of metham sodium was also tested in bench scale experiments. The compound was found to be fairly stable at the pH values, temperatures and oxygen concentrations typically found in sewage collection systems and so undergoes little degradation after application.

Powdered activated carbon (PAC) is effective in absorbing metham sodium from raw sewage. In bench scale reactors, application rates of 10 mg PAC per 1 mg metham sodium reduced metham sodium concentrations to levels which did not disrupt the nitrification process.