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. 2020 Feb;43(2):283-291.
doi: 10.1007/s00449-019-02224-4. Epub 2019 Oct 10.

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

Eva-Maria Ekstrand  1 , Bo H Svensson  2 , Luka Šafarič  2 , Annika Björn  2
Affiliations

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

Eva-Maria Ekstrand et al. Bioprocess Biosyst Eng. 2020 Feb.

Abstract

The production processes of the pulp and paper industry often run in campaigns, leading to large variations in the composition of wastewaters and waste sludges. During anaerobic digestion (AD) of these wastes, the viscosity or the production of extracellular polymeric substances (EPS) and soluble microbial products (SMP) may be affected, with the risk of foam formation, inefficient digester mixing or poor sludge dewaterability. The aim of this study was to investigate how viscosity and production of EPS and SMP during long-term AD of pulp and paper mill sludge is affected by changes in organic loading rate (OLR) and hydraulic retention time (HRT). Two mesophilic lab-scale continuous stirred tank reactors (CSTRs) were operated for 800 days (R1 and R2), initially digesting only fibre sludge, then co-digesting fibre sludge and activated sludge. The HRT was lowered, followed by an increase in the OLR. Reactor fluids were sampled once a month for rheological characterization and analysis of EPS and SMP. The production of the protein fraction of SMP was positively correlated to the OLR, implicating reduced effluent qualities at high OLR. EPS formation correlated with the magnesium content, and during sulphate deficiency, the production of EPS and SMP increased. At high levels of EPS and SMP, there was an increase in viscosity of the anaerobic sludges, and dewatering efficiency was reduced. In addition, increased viscosity and/or the production of EPS and SMP were important factors in sludge bulking and foam formation in the CSTRs. Sludge bulking was avoided by more frequent stirring.

Keywords: Extracellular polymeric substances; OLR; Rheology; Sludge bulking; Soluble microbial products.

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Conflict of interest statement

The authors declare that they have no conflict of interest.

Figures

Fig. 1
Fig. 1
Scatterplots for a, b the protein fraction of soluble microbial products (SMPp) to the organic loading rate (OLR), measured as volatile solids (VS) and c, d the protein fraction of extracellular polymeric substances (EPSp) to the amount of suspended solids (SS) in the reject water after centrifugation of the digestate, with reactor 1 (R1) in the left panel and reactor 2 (R2) in the right panel
Fig. 2
Fig. 2
Graph showing a the organic loading rate (OLR), b, c the protein fraction of the extracellular polymeric substances (EPSp) and the protein fraction of the soluble microbial products (SMPp), and d the hydraulic retention time (HRT) for reactor R1 and R2 over time (days). The vertical lines marks where the production of EPS and/or SMP increased after the OLR was increased. Conc. concentration
Fig. 3
Fig. 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
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