Model-based optimization biofilm based systems performing autotrophic nitrogen removal using the comprehensive NDHA model
Abstract
Completely autotrophic nitrogen removal (CANR) can be obtained in single stage biofilm-based bioreactors. However, their environmental footprint is compromised due to elevated N2O emissions. We developed novel spatially explicit biochemical process model of biofilm based CANR systems that predicts N2O dynamics and stripping, using the biological NDHA model coupled with a simple and roubust pH calculator. In this work we present two case studies: i) membrane aerated biofilm reactor (MABR) with focus on model calibration; and ii) granular system with focus on process optimization
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* METlab, Department of Environmental Engineering, Technical University of Denmark Building 115, 2800 Kongens Lyngby, Denmark ** Department of Chemical Engineering, Universidade de Santiago de Compostela, 15782, Santiago de Compostela, Spain 1 corresponding author: [email protected]; 1. INTRODUCTION Model-based optimization of biofilm based systems performing autotrophic nitrogen removal using the comprehensive NDHA model Borja Valverde-Pérez*,1, Yunjie Ma*, Martin Morset *, Carlos Domingo-Félez*, Miguel Mauricio-Iglesias**, Barth F. Smets* References: [1] Volcke et al. Water Science and Technology 56.9 (2007): 117-125. [2] Mulder et al. FEMS microbiology ecology 16.3 (1995): 177-183. [3] Domingo-Félez et al. Environmental Science: Water Research & Technology 2.6 (2016): 923-930. [4] Blum, J.M., et al., 2017. The pH dependency of N-converting enzymatic processes, pathways and microbes: effect on net-N2O production. In press, Environmental Microbiology. [4] Vangsgaard et al. Water Science and Technology 67.11 (2013): 2608-2615. 2. MODEL DEVELOPMENT 1D model combining NDHA [3], Advection-Diffusion approximation and pH solver pH modeling: •Brent-Dekker method •Guaranteed convergence •pH affects microbial activity by •Impact on enzymatic activity and basic cellular functions [4] •Speciation of substrates and inhibitors PN/A based treatment has many benefits: •Lower aeration costs (63% [1]) •Lower sludge production (90% [2]) Certain operational conditions cause N2O emissions which could offset the carbon footprint of PN/A! PDE-system discretized in space Two-scale model: •CSTR with biomass grown as biofilm •Bulk and biofilm connected via mass transfer resistance (boundary layer) Technical University of Denmark ACKNOWLEDGEMENTS 4. RESULTS Objectives: 1) Develop a model to predict N2O emissions from biofilm Anammox reactors 2) Define optimal operational conditions for maximum nitrogen removal and minimum N2O emissions The work has been partially funded by The Danish Council for Independent Research Technology and Production Sciences (FTP) (Project N2Oman, File No. 1335-00100B). @Metlab_DTU Granular Based Reactor •Large granules support Anammox growth and lead to NOB suppression •Lower N2O emisions in big granules. N-removal is reduced compared to S1 •At high oxygen load Anammox are outcompeted and all ammonia is nitrified and N-removal is low •Low pH caused increased N2O production through the ND pathway due to more HNO2 accumulation compared to S1 (not shown) •High pH increases free ammonia inhibition by AOB, decreasing the overall Nremoval •High pH decreases AnAOB activity, leading to nitrite accumulation •Nitrite accumulation leads to higher N2O emissions at high pH 3. SCENARIO ANALYSIS Evaluation of pH and granule size (constant biomass concentration in the reactor) impact on N2O emissions Nitrogen loading (mg/Ld) R=500µm pH=8 R=1000µm pH=7.5 R=500µm pH=7.5 Nitrogen loading (mg/Ld) Cell growth and metabolism Stratified granular biofilm Granules Aeration Bulk liquid Boundary layer pH calculation Gas stripping Loading and discharge S1 S3 S2 •pH 7.5 promotes a good balance between AOB and AnAOB enzimatic activity •500 µm radius promotes AnAOB and gives enough oxyge exposure for AOB •N2O production is low at low ammonia oxidation rates •Process performance is stable with high N-removal and low N2O emissions under the optimal LO2/LNH and ammonia loading LO2/LNH vs N-removal proving insight into the optimal loading ratio (in terms of nitrogen removal) and the associated N2O emissions