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March 2013 Nitrogen Removal prepared 2012 Emerging Technologies Technology Summary Nitritation and Denitritation (Sidestream) Objective: State of Development: Biological ammonia removal from high-strength Innovative. streams (e.g., sludge liquors, landfill leachate). Description: This process involves the oxidation of ammonia to nitrite (nitritation) in an aerobic environment; however, unlike nitrification, the nitritation process stops the oxidation at nitrite and does not proceed from nitrite to nitrate (nitratation). To accomplish nitritation without nitratation, reactor environmental conditions are controlled to promote the growth of ammonia-oxidizing bacteria (AOB), such as nitrosomonas, while inhibiting the growth of nitrite-oxidizing bacteria (NOB), such as nitrobactor and nitrospira. The high temperature of the sludge liquors favor NOB washout because the aerobic NOB grow faster than NOB at temperatures above 20 °C (Hellinga et al. 1998). Nitritation is desirable because it consumes approximately 25 percent less oxygen than complete nitrification. To provide complete nitrogen removal, nitritation is often coupled with denitritation. Similar to the more common denitrification process for reducing nitrate, the process of denitritation involves reducing nitrite to nitrogen gas by heterotrophic bacteria using carbon as an electron donor in an anoxic environment. The reactor is likely carbon limited requiring a supplemental carbon source. The denitritation process requires 40 percent less carbon than the denitrification process. The nitritation-denitritation process (the nitrite shunt) results in a reduction in sludge production of approximately 30 to 40 percent compared to a conventional nitrification-denitrification process. The nitritation process is also used to produce nitrite as an electron acceptor for the deammonification process (i.e., DEMON ), which uses specialized autotrophic microorganisms (ANAMMOX) to oxidize ammonium and generate nitrogen gas (from ammonia and nitrate) without the carbon consumption of denitrification or denitritation. Example processes – Single-Reactor High-activity Ammonia Removal Over Nitrite (SHARON), which is a chemostat process without biomass retention; and Strass Sequencing Batch Reactor (SBR), often with a high solids retention time (SRT), which increases the internal carbon source for denitritation. Where is it applied – The nitritation and denitritation process has been successfully implemented as a sidestream process for treating centrate and filtrate recycle streams from dewatering anaerobically digested biosolids. The relatively high temperature and high ammonia concentrations typically found in these recycle flows make them ideal candidates for this process. Nitritation-denitritation is currently being tested in the main liquid stream process—where temperature and ammonia concentration is lower than sidestreams—to investigate design and operational parameters, the difficulty in inhibiting NOB growth, the risk of poor mixed liquor settling, and the increased risk of discharging highly toxic nitrite to the receiving stream. Mainstream nitritation/denitritation will be included in a future update of this report. Process controls – The main process controls include the water temperature, SRT, pH, dissolved oxygen concentration, and the nitrite concentration. At temperatures above 20 °C, AOB have a faster growth rate than NOB. Operating at an SRT that is long enough to promote AOB growth but too short for NOB growth (i.e., 1 day) allows for proper control to stop the ammonia oxidation process at nitrite. The SHARON process operates as a chemostat without solids recycle as a process control but with a small volume to give a short HRT and SRT. This prevents an NOB population from developing but also limits the mass of heterotrophs and, therefore, the denitritation capacity. The Strass process includes solids retention control through the use of an SBR and is operated to provide a longer SRT (i.e., 20 days) to allow good denitritation. NOB inhibition is achieved through control of pH and nitrite concentration in the SBR using cyclical aeration. During the aeration interval, the pH drops because of acidification from the nitritation process. When the low pH setpoint is achieved, aeration stops so that denitritation can occur, which adds alkalinity, resulting in an increase in pH. The pH operating band is relatively narrow but can be kept below the optimal growth range for NOB. In addition, a low dissolved oxygen concentration in conjunction with a high nitrite concentration can be used during the aeration cycle to inhibit NOB growth. Wastewater Treatment and In-Plant Wet Weather Management 3-19PDF Image | Emerging Tech for Wastewater Treatment
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