Potential of off-gas analyses for sequentially operated reactors demonstrated on full-scale aerobic granular sludge technology

Abstract

This work shows how more variables can be monitored with a single off-gas sampler on sequentially operated than on continuously fed and aerated reactors and applies the methods to data from a full-scale aerobic granular sludge reactor as a demonstration and to obtain insight in this technology. First, liquid-gas transfer rates were calculated. Oxygen (O2) absorption and carbon dioxide (CO2) emission rates showed comparable cyclic trends due to the coupling of O2 consumption and CO2 production. Methane (CH4) emissions showed a stripping profile and nitrous oxide (N2O) emissions showed two peaks each cycle, which were attributed to different production pathways. Secondly, aeration characteristics were calculated, of which the gradual improvement within cycles was explained by surfactants degradation. Thirdly, liquid phase concentrations were estimated from off-gas measurements via a novel calculation procedure. As such, an average influent CH4 concentration of 0.7 g·m−3 was found. Fourthly, reaction rates could be estimated from off-gas data because no feeding or discharge occurred during reaction phases. The O2 consumption rate increased with increasing dissolved oxygen and decreased once nitrification was complete. Fifthly, greenhouse gas emissions could be derived, indicating a 0.06% N2O emission factor. Sixthly, off-gas gave an indication of influent characteristics. The CO2 emitted per kg COD catabolized corresponded with the TOC/COD ratio of typical wastewater organics in cycles with balanced nitrification and denitrification. High nitrogen removal efficiencies were associated with high catabolized COD/N ratios as estimated from the O2 absorption. Finally, mass balances could be closed using off-gas O2 data. As such, an observed yield of 0.27 g COD/g COD was found. All these variables could be estimated with a single sampler because aeration without feeding creates a more homogeneous off-gas composition and simplifies liquid-phase mass balances. Therefore, off-gas analyzers may have a broader application potential for sequentially operated reactors than currently acknowledged.