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

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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 Zn2+, 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.

INTRODUCTION
Lead levels within the DC metropolitan area, have recently exceeded the EPA’s maximum contaminant level. Elevated levels of heavy metals have been directly associated with water distribution system corrosion. Both ortho-phosphate and zinc ortho-phosphate based products have been evaluated as corrosion control additives. The benefits of corrosion control in the water supply distribution system are generally understood (reduction in the potential for lead corrosion) and will provide a significant public health benefit. One consequence of the addition of zinc ortho-phosphate (ZnOP) to the water supply is the potential impacts on the operation of the wastewater treatment plants that receive and treat sanitary wastewater from treated water supplies. The objective of this project was to evaluate the impact of zinc ortho-phosphate and specifically Zn on the inhibition of nitrification and biological nutrient removal at the Arlington, Virginia Water Pollution Control Plant (WPCP).

Zinc can have a toxic affect on the biomass responsible for biological treatment of wastewater. In particular, the bacteria responsible for the biological nitrogen removal (BNR) via nitrification and denitrification are particularly sensitive to metals at low concentrations (part per million range). The presence of zinc at low concentrations is known to inhibit BNR and cause upset and interference, typically as a result of nitrification inhibition. The nitrification process involves the biological oxidation of ammonia-N to nitrite-N and subsequently to nitrate-N by two genera of aerobic autotrophic bacteria. This process is accomplished in the aeration basin of the biological wastewater treatment process (e.g. activated sludge) simultaneously with the removal of biodegradable organic compounds (i.e. biochemical or chemical oxygen demand BOD/COD) by heterotrophic bacteria. Nitrifying bacteria have relatively slow specific growth rates in the activated sludge process and are especially sensitive to toxic compounds including both heavy metals and xenobiotic organic chemicals (Barth et al., 1965; Blum and Speece, 1991; Juliastuti et al., 2003a) – much more so than the aerobic heterotrophic bacteria responsible for BOD removal or the heterotrophic/facultative bacteria responsible for denitrification and BOD removal under anoxic conditions (anoxic conversion of nitrate-N to nitrogen gas). Thus, the nitrification process is generally the “weakest link” in terms of chemical inhibition and nitrogen removal, and the point of concern for this project. Since the product of nitrification (nitrate) is the substrate for denitrification, inhibition of nitrifying bacteria affects both nitrification and denitrification, thus limiting the ability for a wastewater treatment facility to meet its ammonia-N and total nitrogen (TN) permit limits. The VPDES discharge limits for the Arlington WPCP for ammonia-N and the Virginia Water Quality Improvement Fund effluent treatment objective for TN are shown in Table 1.

Previous laboratory and pilot plant studies on the impacts of zinc on nitrification have been documented, and are summarized in Tables 2 and 3 below. The results suggest that the threshold of inhibition to nitrification varies from about 0.1 – 50 mg/L total influent zinc. There is considerable discrepancy in this data however, and there is an indication that the impact of zinc on BNR, and specifically nitrification, is site-specific. The primary objective of this project is to determine whether the maximum expected increase in influent total zinc to the Arlington WPCP based on the anticipated zinc ortho-phosphate dosing scheme, will have an effect on nitrification process performance.

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