Improving our Understanding of the Differences between Fixed and Moving Bed Media IFAS Systems for Design, Operations and for Real Time Control of Plants (in Aquifas+) to Simultaneously Enhance Nutrient Removal and Minimize GHG Emissions
Evaluation of full scale plants show that both fixed bed media and moving bed media can work in IFAS systems, provided that the design and operating conditions account for the differences between the two types of media. With respect to MLSS, the operating conditions of fixed bed and moving bed media systems showed that while fixed bed systems tend to operate at MLSS levels of 3000 to 6000 mg/L, moving bed systems are typically operated at less than 3000 mg/L to reduce the formation of foam in the basin and entrapment at the screens used to retain the media. The two types of media have very different biofilm thickness and specific surface areas. The equations to compute biofilm thickness were modified to account for the effect of type of aeration and the hydrodynamic forces generated, the type of media surface, and the shape of the media. Once these corrections were made, the computations showed that the thicker biofilm in fixed bed media denitrified 25 to 50% of ammonia nitrified in the biofilm in the aerobic zone, while the moving bed showed denitrified less than 10%; this confirmed the findings in full scale systems with each type of media. The process models for the activated sludge and the biofilm were enhanced to understand how the media would change the production of nitrous oxide and nitric oxide. The Aquifas model was upgraded to enable a three step reaction by ammonia oxidizing bacteria (AOB) to convert ammonia to nitrous oxide, nitrous oxide to nitric oxide, and nitric oxide to nitrite. The denitrification by polyphosphate accumulating bacteria (PAO) and ordinary heterotrophs (OHO) were each broken up into four steps. Additional analysis is being conducted to determine how the maximum rate constants and half saturation constants for each step in the AOB reaction may change with DO and MCRT to reflect the changes in genetic makeup of AOB observed at plants which do not have sufficient oxygen or MCRT for parts of the day. The research is also studying how the genetic makeup may be altered when such a plant is upgraded to an IFAS system with additional aeration for mixing the media and provide the shear forces for the biofilm. The mathematical procedures to solve non-linear partial differential equations were enhanced to enable accurate and rapid solutions which are necessary for developing an IFAS and biofilm operations control software that works inside a plant and optimizes the process in real time.
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Document Type: Research Article
Publication date: 2010-01-01
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