Evidence for tin inhibition of enhanced biological phosphorus removal at a municipal wastewater treatment plant.

Concentrations of 34 metals were determined in the concentrated waste activated sludge from a dissolved air flotation unit at an advanced municipal wastewater treatment plant (WWTP) with biological nutrient removal. Reduction in enhanced biological phosphorus removal (EBPR) efficiency was observed at total tin concentrations greater than 4 microg L(-1) in the solids fraction of the mixed liquor suspended solids. No influence on carbon or nitrogen removal efficiency was found by elevated tin concentrations. Other process control variables and metal concentrations were not correlated with reduced EBPR efficiency on dates with elevated tin levels. The known high contributions of organotin species toward total tin in activated sludges from other municipal WWTPs, and the high toxicity of these compounds, suggests elevated organotin levels may be responsible for the observed reduction in EBPR efficiency.