Evaluating the effect of enzymatic pretreatment on the anaerobic digestibility of pulp and paper biosludge

Abstract

Anaerobic digestion of biosludge has not yet been implemented in pulp mills due to low biogas yields. Enzymatic pretreatment of biosludge has shown improvements in biogas yields but results are varied. A key limitation of previous studies is that they fail to consider the COD contribution from the enzyme solutions. The aim of this study was to systematically investigate the potential for enzymatic pretreatment on the anaerobic digestibility of pulp mill biosludge. Out of the six enzymes tested, four enhanced the anaerobic digestibility of biosludge. At the end of the BMP, a maximum improvement of 26% in biogas yield was observed with protease from B. licheniformis. There was no correlation between enzymatic activities on standard substrates and/or on biosludge and the effect of enzymes on biogas yields. Enzymes have potential for improving biosludge anaerobic digestibility but more research on optimal conditions and potential synergies with other pretreatment is needed.

Keywords: Anaerobic digestion; Biogas; Biosludge; Enzymes; Pretreatment.

Graphical abstract

Fig. 1

General approach for investigating the effect of enzymatic pretreatment on biosludge anaerobic digestibility.

Fig. 2

Specific biogas production, SBP, of biosludge pretreated with enzymes over the 62 days of anaerobic digestion. a) protease from A. oryzae; b) lysozyme; c) protease from B. licheniformis; d) glycosidase SCO6604; e) protease BCE_2078 and f) CTec 2. Untreated (control) for all samples had phosphate buffer added to biosludge instead of enzyme solution. Range differences between BMP 1 (a, c, e) and BMP 2 (b, d, f) are mainly due to differences in biosludge and granules used in each BMP, inoculum to substrate ratios and, soluble chemical oxygen demand (sCOD) variations.

Fig. 3

Soluble chemical oxygen demand (COD) content during enzymatic pretreatment of gamma irradiated biosludge for 24 h. a) Proteases and b) glycosidases. The control samples are biosludge with only phosphate buffer. Error bars (not always visible) represent the standard deviation of triplicates.

Fig. 4

Biogas production from enzyme solutions. Total biogas productions (TBP) are presented for BMP 3, samples that contained enzyme solutions and inoculum. a) protease from A. oryzae; b) lysozyme; c) protease from B. licheniformis; d) glycosidase SCO6604. Inoculum only is the control, i.e. no enzyme added. Error bars show standard deviation of triplicates.

Fig. 5

Enzymatic assays. a) protease activity assays for enzymes studied in BMP 1. Casein was used as the standard substrate. B) cellulose activity assays for enzymes studied in BMP 2 (except lysozyme). Casein was used as the standard substrate for proteases and CMC was used as the standard substrate for cellulases. Enzymatic activity was tested on standard substrates, biosludge and a combination. Active and inactive enzymes were included. Note the two vertical axis on b) are the same units but ranges are different. Error bars (not always visible) show standard deviation of triplicates.