Assessment of alternative herbicides for residential sewer root treatment and their effects on downstream treatment plant nitrification

The conveyance of wastewater in sewer pipes can be severely limited by the growth of plant roots, which can be controlled with herbicides. However, adding herbicides in sewer lines may affect downstream biological wastewater treatment processes. The effects of three herbicides (Dithiopyr, Penoxsulam, and Triclopyr) on the mortality of cottonwood tree roots and on downstream biological nitrification were determined. The results showed that Triclopyr achieved the highest root mortality (96%) followed by Penoxsulam (77%) and Dithiopyr (75%). At concentrations used at the point of application in sewer pipes, all herbicides caused nitrification inhibition and reduction in organic carbon removal in activated sludge. However, no inhibition was observed at the more diluted concentrations approximately equal to levels that may reach the wastewater treatment facility. Overall, Triclopyr appears to be the best performing herbicide with the highest root kill. •Effect of three herbicides on root intrusion in sewer lines was investigated.•Triclopyr showed high shoot damage to the upper portion of the plants.•Root regrowth was observed in plants treated with Dithiopyr.•Penoxsulam achieved acceptable root kill with no upper plant damage.•Herbicides did not inhibit nitrification at diluted concentrations.