Assessing the Potential for Nitrification Inhibition at Wastewater Treatment Facilities as a Result of Zinc Orthophosphate Addition to Potable Water Distribution Systems

Abstract

Portions of the greater metropolitan District of Columbia area have water distribution system corrosion problems. This has resulted in elevated levels of heavy metals, in particular lead from some plumbing and service connections. One alternative being considered for corrosion control is the addition of either phosphoric acid (ortho-phosphate) or zinc ortho-phosphate based products at the water treatment plant to inhibit corrosion of plumbing and service connections that contain lead (Cadmus, 2004). A potential consequence of the addition of zinc orthophosphate (ZnOP) to the water supply is the impact of Zn on the operation of the wastewater treatment plants that receive and treat sanitary wastewater from treated water supplies. The objective of this study was to evaluate the impact of ZnOP, and specifically Zn 2+ , on the nitrification process at the Arlington County, Virginia Water Pollution Control Plant (WPCP). Inhibition was investigated using continuous respirometry and nitrate generation rate measurements at the Zn concentration increment expected in the influent wastewater (0.3-0.5 mg/L) as a result of corrosion inhibitor use, although much higher levels of Zn were also evaluated. Short-term batch experiments up to 5 days in length were conducted with fresh mixed liquor samples in an attempt to make 0.5 mg/L Zn as inhibitory as possible, providing conditions that would be expected to sensitize the biomass to Zn stress. Results suggest no significant inhibition at 0.5 mg/L Zn, slight inhibition at 1.0 mg/L Zn after roughly 5-10 hours of exposure, and significant inhibition at 10 mg/L Zn.