Rheological characteristics of reactor liquid from 12 full-scale biogas reactors

• Rheological properties of reactor liquids are important for the construction and intensity mixing systems in biogas reactors. Most frequently the total solids content (TS) is used as a proxy to guide in these matters. In order to establish a more comprehensive basis the rheology of twelve full-scale continuously stirred tank biogas reactors was characterized and related to differences in substrate composition and operational conditions. Reactor material from eight mesophilic (36−38°C) and four thermophilic (52−55°C) reactors were sampled at two occasions. The feedstocks of nine of these reactors were included in the analysis. Two of the mesophilic and one of the thermophilic digesters were fed sewage sludge (SS), while the others digested mixtures of organic matter including slaughterhouse waste (SHW), food industry waste (FIW), fat, manure, fodder residues and the organic fraction of municipal solid waste (OFMSW). The organic loading rates ranged 2.4−3.6 kg VS m-3 d-1 and the TS of the reactor materials were at 1.8−5.3% . The rheological characteristics of the reactor liquids were interpreted from flow- and viscosity curves as well as from determination of dynamic viscosity, limit viscosity, yield stress, flow behavior and consistency index.The fluid dynamic- and limit viscosities of the fluids ranged 5−600 mPa*s and 4−40 mPa*s, respectively. All reactor fluids except one from a thermohilic CD-reactor showed pseudoplastic behavior, since they became thinner with increasing shear stress until the viscosity reached a plateau of limit viscosity. In addition the mesophilic CD reactors were strongly thixothropic, i.e. they exhibited partial structure recovery. The results from the analysis of the thermophilic CD-reactors indicated a weak dilatant behavior, i.e. shear thickening behavior.The results showed differences in viscosity despite similar TS-content for several reactor liquids. From this survey it is clear that the TS content of biogas reactor fluids is not a good estimator of the fluid viscosity and that the fluid characteristics vary as a result of substrate composition and process operation conditions.

Nyckelord

Biogas

continuous stirred tank reactors (CSTRs)

flow behavior

rheology

viscosity

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