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To meet stringent nitrogen discharge limits, the Los Angeles County Sanitation Districts (Districts) needed to upgrade the Valencia Water Reclamation Plant (VWRP) from conventional activated sludge to nitrification/denitrification (NDN) mode. The discharge limits that have to be met are approximately 2 mg NH3-N/L and 10 mg NO3 -N + NO2 -N/L by 2003 (interim limits). The NO3 -N + NO2 -N limit will be further reduced to 6.8 mg N/L by 2011. The secondary process needed to treat the filtrate from dewatering of anaerobically-digested biosolids. The ammonia in the filtrate added up to 50% of the nitrogen load to the secondary influent. Achieving compliance with the stringent discharge limits with this high nitrogen load presented a challenge to the Districts.

The Modified Ludzack-Ettinger (MLE) was selected as the NDN process. Expanding the existing aeration tanks to handle the high nitrogen load was not a viable alternative because space limitations at the VWRP. Consequently, the Districts pursued an innovative approach using a separate side-stream treatment system to nitrify the filtrate. The side-stream treatment system utilizes the nitrifying bacteria present in the return activated sludge. The partially nitrified filtrate is then returned to the anoxic zone in the MLE process for denitrification. To provide the basis for designing this integrated biological nitrogen removal system and to optimize the nitrogen performance following process start-up, extended research has been conducted since 1995. This paper highlights the challenges that the Districts faced, and how these challenges were handled with emphases on the start-up period. Following several modifications to the process and operations including extension of the anoxic zone volume in the MLE process, equalization of filtrate flow, and elimination of polymer addition in the primary clarifiers, the Districts have successfully met not only the interim, but also future discharge limits for various nitrogen species.
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Document Type: Research Article

Publication date: 2004-01-01

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