tabilisation of biomass and its bio

tabilisation of biomass and its biochemical conversion into energy-rich biogas are commonly performed in anaerobic digesters, in which good mass transfer and heat transfer rely on a proper mixing. In digesters designed as a continuous stirred tank reactor (CSTR), mixing is commonly achieved by biogas recirculation (Lindmark et al., 2014). However, the mixing is often insufficient in full-scale gas-mixed digesters, due to unaccounted scale-up effects (Bello-Mendoza and Sharratt, 1998; Capela et al., 2009; Terashima et al., 2009). Prevailing short-circuiting or dead zones in practice result in treatment performances below the theoretical potential (Capela et al., 2009; Samstag et al., 2016). In order to optimise mixing, the flow behaviour and mixing performance should be well characterised, and the actual contribution to mixing from the biogas recirculation should be evaluated. Any additional mixing brought forward by the produced biogas resulting from sludge digestion is not taken into account in the design process and the exact impact of evolving biogas has not yet been evaluated.For research purposes it is very challenging to obtain a clear insight of the mixing behaviour in an opaque and gas tight system when using only experimental approaches. A general mixing pattern can be determined using tracer tests (Capela et al., 2009; Smith et al., 1993; Terashima et al., 2009), but local flow fields responsible for any poor mixing cannot be determined from the results of tracer residence time distribution (RTD). Advanced non-invasive techniques, such as computer automated radioactive particle tracking (CARPT) and computed tomography (CT) can provide detailed information of flow fields (Karim et al., 2004; Varma and Al-Dahhan, 2007), but are yet not applicable to full scale facilities.