Conference Agenda

Wastewater Recovery Facilities (WRRFs) can leverage permitting, integrated planning, and water quality trading to stabilize biological nutrient removal (BNR) performance. Collection system industrial inputs affect BNR at WRRFs. A renewed emphasis on states’ involvement in Clean Water Act regulation has stimulated efforts to establish innovative sewershed nutrient management strategies. Water quality trading has been documented by EPA since the early 1990’s, but recent emphasis has led to consideration of pretreatment trading in the context of nutrient management at the WRRF. Integrated planning is a final piece of the puzzle, having recently been added to federal law as an amendment to the Clean Water Act.

Pretreatment allocations for discharge to a WRRF have the potential to de-stabilize otherwise robust BNR systems. The West Boise Water Renewal Facility (WBWRF) received high nitrate concentrations through a long collection main, allowing for depletion of readily biodegradable carbon. The resulting impact at WBWRF was a severe diurnal variation in raw influent volatile fatty acid (VFA) concentration, with typical levels of 23 mg/L VFA at midnight and 4 mg/L VFA at 11:00AM. This effect was verified through a robust collection system sampling effort which captured diurnal phosphorus and nitrogen species concentrations at strategic points in the collection system, showing correlation with periods of decreased VFA. The collection system nutrient study also characterized transformation of nutrient species, so that collection system kinetics could be quantified and modeled as a “plug-flow” reactor system in which nitrogen, phosphorus, and VFA species change over time. The study provided further context for implementing a successful pretreatment program.

This effort was paired with a pretreatment trading evaluation. H.R. 7279 amended the Federal Water Pollution Control Act to allow permitting authorities to incorporate integrated planning into renewed permits. The scope of the amendment highlights pretreatment trading in which the permitted WRRF administers a trading program for industrial users. The City undertook an evaluation which showed that trading of nitrogen and phosphorus credits in an exchange open to industrial users could potentially result in net economic benefit while reducing the nutrient headworks loading faced at the WBWRF.

VIrtual Speakers

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.