Nitritation performance and biofilm development of co- and counter-diffusion biofilm reactors: modeling and experimental comparison.
ABSTRACT A comparative study was conducted on the start-up performance and biofilm development in two different biofilm reactors with aim of obtaining partial nitritation. The reactors were both operated under oxygen limited conditions, but differed in geometry. While substrates (O2, NH3) co-diffused in one geometry, they counter-diffused in the other. Mathematical simulations of these two geometries were implemented in two 1-D multispecies biofilm models using the AQUASIM software. Sensitivity analysis results showed that the oxygen mass transfer coefficient (Ki) and maximum specific growth rate of ammonia-oxidizing (AOB) and nitrite-oxidizing bacteria (NOB) were the determinant parameters in nitrogen conversion simulations. The modeling simulations demonstrated that Ki had stronger effects on nitrogen conversion at lower (0-10 m d(-1)) than at the higher values (>10 m d(-1)). The experimental results showed that the counter-diffusion biofilms developed faster and attained a larger maximum biofilm thickness than the co-diffusion biofilms. Under oxygen limited condition (DO<0.1 mg L(-1)) and high pH (8.0-8.3), nitrite accumulation was triggered more significantly in co-diffusion than counter-diffusion biofilms by increasing the applied ammonia loading from 0.21 to 0.78 g NH4+-NL(-1) d(-1). The co- and counter-diffusion biofilms displayed very different spatial structures and population distributions after 120 days of operation. AOB were dominant throughout the biofilm depth in co-diffusion biofilms, while the counter-diffusion biofilms presented a stratified structure with an abundance of AOB and NOB at the base and putative heterotrophs at the surface of the biofilm, respectively.
SourceAvailable from: Ming Zeng11/2013, Degree: Docter of chemical engineering, Supervisor: Nicolas ROCHE; Audrey SORIC
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ABSTRACT: A nitrifying biofilm was grown in a laboratory-scale membrane aerated bioreactor (MABR) to calibrate and test a one-dimensional biofilm model incorporating chemical equilibria to calculate local pH values. A previously developed model (Shanahan and Semmens, 2004) based upon AQUASIM was modified to incorporate the impact of local pH changes within the biofilm on the kinetics of nitrification. Shielded microelectrodes were used to measure the concentration profiles of dissolved oxygen, ammonium, nitrate, and pH within the biofilm and the overlying boundary layer under actual operating conditions. Operating conditions were varied to assess the impact of bicarbonate loading (alkalinity), ammonium loading, and intra-membrane oxygen partial pressure on biofilm performance. Nitrification performance improved with increased ammonium and bicarbonate loadings over the range of operating conditions tested, but declined when the intra-membrane oxygen partial pressure was increased. Minor discrepancies between the measured and predicted concentration profiles within the biofilm were attributed to changes in biofilm density and vertical heterogeneities in biofilm structure not accounted for by the model. Nevertheless, predicted concentration profiles within the biofilm agreed well with experimental results over the range of conditions studied and highlight the fact that pH changes in the biofilm are significant especially in low alkalinity waters. The influent pH and buffer capacity of a wastewater may therefore have a significant impact on the performance of a membrane-aerated bioreactor with respect to nitrification, and nitrogen removal. Copyright © 2015 Elsevier Ltd. All rights reserved.Water Research 02/2015; 74. DOI:10.1016/j.watres.2014.12.055 · 5.32 Impact Factor