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Dual wastewater treatment processes that include a trickling filter (TF) bioreactor were originally developed during the late 1970s. Early accounts were designed to bring existing TF facilities into compliance with the United States Environmental Protection Agency (US EPA) definition of
secondary treatment standards. However, numerous research efforts extending from the early 1980s until present have identified that dual processes such as the trickling filer/solids contact (TF/SC) process are capable of meeting moderate to advanced ammonia-nitrogen (NH3-N) and total nitrogen (TN) effluent permit limitations. In certain cases, dual process facilities have been equipped with enhanced biological phosphorus removal (EBPR) capabilities. Additionally, such research has resulted in existing activated sludge wastewater treatment facilities being retro fitted with TFs in order to utilize advantages imparted by dual processes. The present study describes a brief history of dual treatment processes, and surveys state-of-the art research efforts
supporting their ability to achieve advanced nutrient control. The research is presented in order to develop optimal empirical hydraulic and organic loading envelopes for systems containing both rock-media and high-rate modular plastic systems, and communicate the viability of using dual processes as an expansion alternative. Further, a discussion of removal
mechanisms and operational control features designed to impart a mechanistic understanding for WWTP operations personnel is detailed. Finally, an analysis of 5 WWTPs that implement dual processes is presented. Effluent quality, power consumption, operational issues, and reliability are discussed for the Salmon Arm Water Pollution Control Centre, Salmon Arm, British Columbia; Corvallis Wastewater Reclamation Plant, Corvallis, Oregon; Yakima Regional Wastewater Treatment Facility, Yakima, Washington; Duck Creek Wastewater Treatment Plant, Garland, Texas; and Littleton/Englewood Wastewater Treatment Plant, Englewood, Colorado. In each case, the use of existing assets was maximized, and the WWTPs evaluated in the present study are recognized as award winning treatment facilities.
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