Substrate and operational conditions as regulators of fluid properties in full-scale continuous stirred-tank biogas reactors – implications for rheology-driven power requirements

Abstract

Understanding fluid rheology is important for optimal design and operation of continuous stirred-tank biogas reactors (CSTBRs) and is the basis for power requirement estimates. Conflicting results have been reported regarding the applicability of total solid (TS) and/or total volatile solid (TVS) contents of CSTBR fluids as proxies for rheological properties. Thus, the present study investigates relationships between rheological properties of 12 full-scale CSTBR fluids, their substrate profiles, and major operational conditions, including pH, TS and TVS contents, organic loading rate, hydraulic retention time, and temperature. Rheology-driven power requirements based on various fluid characteristics were evaluated for a general biogas reactor setup. The results revealed a significant correlation only between the rheological fluid properties and TS or TVS contents for sewage sludge digesters and thermophilic co-digesters (CD), but not for mesophilic CD. Furthermore, the calculated power requirements for pumping and mixing, based on the various fluid characteristics of the studied CSTBRs, varied broadly irrespective of TS and TVS contents. Thus, this study shows that the TS and/or TVS contents of digester fluid are not reliable estimators of the rheological properties in CSTBRs digesting substrates other than sewage sludge.

Viscosity dynamics and the production of extracellular polymeric substances and soluble microbial products during anaerobic digestion of pulp and paper mill wastewater sludges

“…3e, f). The values were lower than reported apparent viscosities for reactor sludge receiving other types of substrate, e.g., food waste and slaughterhouse waste [9,37], indicating a lower power consumption for achieving complete mixing compared to the full-scale processes studied by Björn et al, [9]. The flow curves of the two reactors were similar, showing close to Newtonian behaviour with no or low yield stress (not shown).…”

“…3 The concentration of extracellular polymeric substances (EPS) and soluble microbial products (SMP) (a-d) and the corresponding apparent viscosities (e, f) for reactors R1 and R2, over time (days). The lowercase letters p and s of the EPS and SMP denote the protein and polysaccharide fractions, respectively, and the apparent viscosity was estimated at a shear rate of e 100/s and f 300/s f activated sludge cannot be used to predict the viscosity of all types of AD sludges [9]. Thus, other parameters should be looked for to predict viscosities for AD of sludges from pulp and paper wastewater treatment.…”

“…Most research on the factors affecting sludge rheology so far addressed activated sludge, primarily showing a positive correlation between viscosity and the amount of total solids (TS) [4][5][6][7]. For AD sludges, however, additional regulators for sludge viscosity have been identified, such as temperature [8], substrate type [9] and hydraulic retention time (HRT) [10]. At pulp and paper mills, the production processes are often run in campaigns to meet product demands, i.e., switching between bleached and unbleached pulp production and using different raw materials.…”

A Comparative Study of Biogas Reactor Fluid Rheology—Implications for Mixing Profile and Power Demand

“…The latter is, however, not always straightforward to evaluate, since different authors report different relationships between solids and rheological parameters. Previous research reported by Björn et al (2018) [14] on rheology in biogas reactors digesting a selection of substrates revealed tendencies toward different rheological behaviour of the digester sludge for different substrate types. Furthermore, it was demonstrated that the type of substrates used in AD processes determines the importance of different process parameters for rheology in CSTBRs.…”

“…Furthermore, it was demonstrated that the type of substrates used in AD processes determines the importance of different process parameters for rheology in CSTBRs. For example, application of sewage sludge in AD processes was reported to lead to reactor fluid rheology that correlates well with their total solid (TS) and total volatile solid (TVS) contents, while co-digestion of combinations of organic wastes (e.g., food, slaughterhouse, or agricultural wastes) may result in an establishment of complex fluid behaviours, which cannot be simply associated with the solids content of the sludge [14]. Similarly, Battistoni (1997) [15], who compared the rheology of anaerobic sludges from reactors treating either the organic fraction of municipal solid waste (OFMSW), primary, or mixed sewage sludge, reported varying behaviour, depending on the substrate.…”