Nitrification of High-Ammonia Reject Water (Centrate) for Improved Efficiency in the Main-Stream Treatment Train
Abstract:Recycling raw reject water (centrate) to the activated sludge tanks, from the dewatering of anaerobically digested biosolids, contributes significantly to the ammonia (NH3) load entering a wastewater treatment plant. Some plants have found it more economical to remove this NH3 from centrate in a side-stream prior to recycling. The feasibility of side-stream nitrification of centrate to produce a nitrifying biomass that could augment the main-stream nitrifier population was explored. Through seeding, nitrifiers would always be present in the activated sludge tank thus theoretically decreasing the system SRT required to maintain nitrification; i.e. short-SRT nitrification.
Modeling showed that a nitrifying system SRT necessary to achieve an effluent NH3 concentration of 2.3 mg/L could be decreased by approximately 20 to 30% by seeding 1.0 and 1.6 mg/L of nitrifiers with the primary effluent that is entering the activated sludge tank. Various other centrate flow management practices were also investigated but the highest concentration of nitrifiers and the lowest effluent NH3 concentrations were achieved with seeding.
In addition, the issue of sudden temperature change on nitrifier performance was investigated in the laboratory. The purpose was to simulate the large difference in temperature between the sidestream (20 to 30°C) and the main-stream in winter (10°C). Three chemostat reactors were operated at 20, 25 and 30°C with SRT 5 d. Over an 8-month period the reactors were fed centrate as an NH3 source. Upon exposure to 10°C, nitrification commenced immediately but nitrification rates were decreased by an average of 53%, 66% and 82% for biomass acclimated to 20, 25 and 30°C respectively. This large difference in nitrification rate due to temperature change has significant implications for modeling short-SRT nitrification. The BioWin (version 4.46) currently assumes that supplemental nitrifiers have the same potential for nitrification as the main-stream indigenous population. The rate of recovery or acclimation of the seeded nitrifiers is unknown, and remains to be determined, before the full benefits of seeding can be realized.
Document Type: Research Article
Publication date: January 1, 2000
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