Non-ideal continuously stirred bioreactor model for multi-media denitrifying processes

Denitrifying bioreactors are used to treat excess nutrients in agricultural runoff. Efforts to optimise bioreactor design focus on increasing rates of denitrification while reducing hydraulic residence times (HRTs). However, the internal hydraulics that dictate bioreactor efficiency are difficult to characterise. A general bioreactor model is presented that includes non-ideal flow using combined plug and continuously stirred reactors (CSTRs) with dead space and short-circuiting based on zero and first order reaction coefficients. Reactor characteristics of a novel media mixture of walnut shell biochar, Brotex, and woodchips were defined using a conservative tracer (bromide) for two HRTs (12 and 4 h). The fraction of dead space, non-short-circuiting fraction, and the number of reactors were obtained by matching the model's response to measured tracer outflow curves. Secondary parameters calculated include hydraulic efficiency, effective volume, volumetric efficiency, and the Morrill Dispersion Index. The model characterised the reactors as having 1.1 % plug flow with no short-circuiting or dead space and four CSTRs (98.9 % of flow) with 3.7 % short-circuiting and 1.1 % dead space. The closeness between the theoretical and actual HRTs (−6 % for the 12 h design and −4 % for the 4 h design) demonstrates the high level of hydraulic efficiency of this novel bioreactor design as well as the model's ability to properly characterise complicated flow regimes. Additionally, this model can help provide insight into bioreactor performance under a wide range of design criteria, leading to computational supported advancements in bioreactor science.