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A Study To Better Understand The Factors Affecting Eutrophication In The Tualatin River and Update the TMDL
Scott Mansell, Bob Baumgartner, Raj Kapur, Ken Williamson, Clinton Cheney
Clean Water Services, United States of America;
The Tualatin River in Oregon was one of the first watersheds in the country to receive a TMDL in 1988. Among other things, this TMDL addressed phosphorus loading from sewage discharges from several WWTPs discharging to the river. As a result of improved WWTP processes and additional data collection throughout the 1990s, a new TMDL was issued in 2001 which set phosphorus limits at measured background concentrations without additional modeling. While continuous improvements have been made to biological phosphorus removal at the AWWTPs since that time, alum is still required during the dry season to meet stringent phosphorus limits. However, conditions in the lower river have changed dramatically since the 2001 TMDL. Increased flows from WWTPs as a result of population growth, operational changes at the diversion dam near the mouth of the river, and enhanced flow augmentation have greatly decreased the residence time of the lower river, and nuisance algae has been virtually eliminated. The river may, therefore, be much less sensitive to phosphorus loading from the treatment plants than it was when the TMDL was written. A recent USGS study suggested that the Tualatin River may actually be sensitive to algal ‘seeding’, to the type of algae discharged, and to the zooplankton discharged upstream. During 2019, CWS has been seeking to understand the complex dynamics affecting algae blooms and DO in the river through extensive data collection in partnership with USGS, experimentation, and complex water quality modeling using a CE-QUAL-W2 model of the river. Data collected include algae, zooplankton, nutrient, DO, and others related to eutrophication. The results of this study are intended to be used to update the phosphorus TMDL of the Tualatin River, aid in decision making about phosphorus removal at the treatment plants, and help optimize operation of the Natural Treatment System (NTS). While the project is ongoing, preliminary findings suggest that the river is insensitive to phosphorus loading from the AWWTPs but is sensitive to the algae loading from the NTS.
2:00pm - 2:45pm
Copper Removal In Full-Scale Constructed Surface Wetlands And Pilot Subsurface Reactors
Leila Barker, Kenneth Williamson
Clean Water Services;
The Forest Grove Wastewater Treatment Facility (WWTF) is a secondary treatment facility that discharges to the Tualatin River. The facility includes a post-secondary Natural Treatment System (NTS) designed for dry-season operation, consisting of 54 acres of surface-flow wetlands that provide additional treatment prior to discharge. The combination of conventional secondary treatment followed by the NTS at the Forest Grove facility is very effective at reducing nutrients (phosphorus, ammonia, nitrates) and temperature. Copper is one of the parameters of concern in the discharge from the Forest Grove facility. Target discharge levels for copper vary seasonally and may require the WWTF to treat to concentrations as low as 4.5 µg/L dissolved copper. Early analysis suggested that an additional treatment process might be necessary to meet target discharge copper concentrations. Since the NTS began operation in May 2017, monitoring has found that the wetlands are also effective at reducing copper concentrations. Clean Water Services (CWS) has conducted regular sampling to evaluate the degree of copper removal achieved with existing WWTF and NTS infrastructure. Concurrently, CWS has conducted a number of pilot studies to explore the copper removal capabilities of horizontal-flow subsurface reactors, demonstrating high percent removals at relatively short hydraulic retention times. This presentation will summarize results of WWTF and NTS monitoring and pilot testing with various media at a range of flowrates and effluent conditions.