Controlling industrial discharges to a Publicly Owned Treatment Work (POTW) is an integral part of the NPDES permit program. POTWs conduct a local limits analysis to establish limits for industrial users. Factors that are considered in this process include water quality standards, water quality-based NPDES permit effluent limits, biosolids use and disposal, and the potential for inhibition of biological processes within the treatment plant. Nitrifying bacteria are often highly sensitive to environmental stressors and pollutants, and local limits are frequently driven by the need to protect the biological nitrification process.
The U.S. EPA has published nitrification inhibition threshold values for a variety of substances in its Local Limits Development Guidance (EPA, 2004). For many substances, a wide range of possible inhibition levels have been identified. However, because these data are decades old and studies are not well documented, there is limited confidence that even the most conservative thresholds would be protective of biological treatment processes. Additionally, use of the most conservative values can result in overly stringent local limits that place a significant burden on industrial users.
Clean Water Services (CWS) is in the process of revising local limits for its four wastewater treatment facilities (WWTFs). Three CWS facilities have ammonia limits and rely on nitrification to meet these limits. Copper and zinc were of particular interest due to prior operational data. Preliminary analysis suggested that local limits for these metals would likely be driven by the nitrification inhibition threshold. At times, CWS WWTFs have experienced copper and zinc levels higher than the minimum threshold concentrations in the EPA Guidance with no discernable impact to the biological processes.
In order to establish site-specific inhibition levels, CWS conducted nitrification rate testing using two methods: a modified version of ISO 9509:2006 and a simplified respirometry protocol. Biomass from the three nitrifying WWTFs was spiked with varying concentrations of zinc and copper. Results were analyzed to determine nitrification inhibition thresholds. The resulting facility-specific nitrification inhibition levels were higher than the EPA-published minimum thresholds for both metals while providing confidence that local limits would be protective of biological nitrogen removal processes at each WWTF.