ENHANCED NITROGEN REMOVAL WITH A HYBRID MLE/STEP-FEED ACTIVATED SLUDGE PROCESS

The Sanitation Districts of Los Angeles County (Districts) operate the Valencia Water Reclamation Plant (VWRP) to provide wastewater treatment and disposal services for approximately 170,000 residents in southern California's Santa Clarita Valley. In 2003, the plant was expanded and upgraded for biological nitrogen removal (BNR). Two existing treatment trains were converted from step-feed systems designed for BOD removal to modified Ludzack-Ettinger (MLE) operation. At the same time, three new MLE treatment units were added to accommodate a projected increase in average dry weather flow from 14 to 21.6 MGD. The current NPDES permit allows the VWRP to discharge an effluent containing, on a monthly average basis, 10 mg/L of nitrate plus nitrite nitrogen (NO3-N plus NO2-N). This limit has interim status and may be lowered to 6.8 mg/L in 2009 (also monthly average basis). Achieving the more stringent standard presents significant challenges. At the design flow of 21.6 MGD, the hydraulic retention time in the biological reactors will be a relatively short 6.7 hours. Yet, the plant must be able to treat the nitrogen loading in the influent wastewater in addition to the ammonia nitrogen (NH3-N) in a filtrate stream generated from dewatering anaerobically digested biosolids produced by the VWRP, and by a 6.5-MGD satellite plant. The filtrate NH3-N comprises approximately 20% of the overall nitrogen loading.In anticipation of a lower effluent NO3-N limit, the Districts initiated studies to identify opportunities for maximizing nitrogen removal within the existing VWRP facilities. The VWRP provides a unique environment for such studies. The original treatment units could be operated in either a MLE or a stepfeed nitrification/denitrification (NDN) mode. In addition, features unique to MLE and step-feed NDN configurations could be combined to create a new hybrid mode of operation. During 2005–06, the Districts tested the nitrogen removal capacity of the MLE, step-feed NDN and hybrid configurations in treatment units that operated side-by-side. In full-scale testing, the hybrid produced an effluent that contained less NO3-N plus NO2-N than the step-feed and MLE configurations. Subsequently online nitrate nitrogen analyzers were installed to characterize and compare hybrid to MLE performance in greater detail. The effluent produced by the hybrid was typically lower in NO3-N by 2 mg/L throughout the day. BioWin was used to model nitrogen removal performance at the VWRP. The model was calibrated against the online nitrate nitrogen analyzer data. The model was then run to project nitrogen removal performance at wintertime wastewater temperatures and at the future 21.6 MGD flow. The effluent NO3-N levels were predicted to be slightly greater than the 6.8 mg NO3-N plus NO2-N future limit. To meet the future limit, the three existing MLE treatment units could be converted to hybrid operation, if necessary, to improve nitrogen removal performance.