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

Anna Karlsson,Annika Björn,Bo H Svensson,Jörgen Ejlertsson,Luka Šafarič,Sepehr Shakeri Yekta,Åsa Danielsson

doi:10.2166/wst.2018.352

Anna Karlsson, Annika Björn + Show 5 more

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https://doi.org/10.2166/wst.2018.352

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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.

Highlights

Application of anaerobic digestion (AD) of organic wastes and production of biogas, containing methane as a renewable energy carrier, is widely recognized (Kampman et al )
The total volatile solid (TVS) content showed the best correlation with η20 and ηlim of SS reactors, while in the case of thermophilic CD reactors, total solid (TS) revealed the strongest correlation with ηlim
The results showed that using TS or TVS as estimators of Continuous stirred-tank biogas reactors (CSTBRs) fluid rheology may lead to erroneous predictions of fluid behaviour and power demand

Summary

Introduction

Application of anaerobic digestion (AD) of organic wastes and production of biogas, containing methane as a renewable energy carrier, is widely recognized (Kampman et al ). A substantial share of the energy required for CSTBR operation is spent on mixing, contributing to a considerable part of the biogas production costs (Lindmark et al ). The mixing needs to be efficient, implying that it should use as little energy as possible, while allowing a homogeneous distribution of the substrate and heat as well as a complete utilization of CSTBR operational volume (Lindmark et al ). The amount of energy required to reach this level of efficient mixing is linked to rheological properties of the reactor fluid and the scale of the system (Nienow ), in addition to energy needs associated with, for example, friction or inherent inefficiencies of the electric motors. Inadequate mixing, damaged mixing equipment, foaming, and sludge bulking may occur when fluid behaviour in a reactor changes (Nordberg & Edström ; Lindorfer & Demmig )

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