Hygienization of mixed animal by-product using pulsed electric field in a continuous treatment system: Synergistic effect with ohmic heating on the inactivation of indicator bacteria
Graphical abstract
Introduction
Animal by-products (ABP) mainly derive from the slaughter of animals (e.g. animal organs and blood), the production of animal-originated products (e.g. dairy products, animal manure of feedstock) and the disposal of dead animal bodies (e.g. carcasses of sick animals). Many studies have revealed that ABP might be conveyors of pathogenic agents that are dangerous to the environment. High occurrence and abundance of various pathogens were seen in animal waste, such as Listeria spp., Campylobacter spp., Escherichia coli O157:H7, Salmonella spp., Cryptosporidium parvum, Giardia lamblia (Hutchison et al., 2004, Sui et al., 2019, Ziemer et al., 2010). He et al. (2019) confirm the potential sanitary risk of introducing antibiotic resistance genes from food waste into the environment despite biological treatment. Public health and the environment may therefore be threatened by improper waste ABP management.
Many valorization methods (e.g. incineration, composting and anaerobic digestion) have been proposed to transform ABP into valuable resources (Moukazis et al., 2018). In the European Union (EU), the transformation and the disposal of ABP are strictly regulated by the relative legislations like EC No. 1069/2009 and EU No. 142/2011 (European Union, 2011, European Union, 2009). According to their biological risk level, the regulations divide ABP into three categories to which different processing and transformation methods could apply. One of the promising ABP valorization approaches is anaerobic digestion, a waste-to-energy technology that transforms organic matter into biogas and digestate. Biogas is used as renewable energy source or as primary material for other chemical processes. Digestate can be returned to soil as fertilizer or soil amendment. This creates a circular economy for sustainable development (Tsui and Wong, 2019).
Certain ABP classified as Categories II and III are authorized by the EU to be anaerobically digested for biogas production if they have been subject to the thermal pasteurization operated at 70 °C for 1 h and the maximum particle size is reduced to 12 mm (Ware and Power, 2016). It is considered prudent of the EU to set such operational parameters (high temperature and long treatment time) (Liu et al., 2019a). Recent studies conclude that this thermal pasteurization serving as the pre-hygienization process of ABP accounts for about 6–25% of the primary energy produced by biogas plants in the EU (Liu et al., 2019a). As authorized by the European Commission, the thermal pasteurization can be replaced by other alternative physico-chemical hygienization processes if they are able to reduce Enterococcus faecalis or Salmonella Senftenberg by a factor of 105 (i.e. 5 log10) and to achieve 3-log10 reduction of thermo-resistant viruses (European Union, 2011).
Electro-technologies have been widely studied for the alternative inactivation of microorganisms present in food and water, including electric discharge (Singh et al., 2019) and pulsed electric field (PEF) (Brennan and Grandison, 2012). The advantages of electrical treatment compared with conventional thermal pasteurization rely on short process time and high bacteria inactivation performance. PEF is a promising electrical technology developed during last three decades. It involves the delivery of discontinued electric field (pulses) to target products where the microorganisms can be inactivated due to the breakdown of microbial cell membrane (Brennan and Grandison, 2012). The application of PEF induced the membrane charging and polarization of microbial cell. The frequent attraction of opposed electric charges leads to thinning of cell membrane, modifying its mechanic modulus. Once electric field strength exceeds certain threshold, the additional transmembrane potential brought by PEF results in breakdown of cell membrane (Vorobiev and Lebovka, 2020). The passage of electric current in a conductor, as described by Joule's law, can generate ohmic heat (Sakr and Liu, 2014). PEF treatment inevitably causes ohmic heating (OH) in a resistive conductor (Lindgren et al., 2002), which may give rise to a supplementary thermal effect on pathogens inactivation performance.
The application of electro-technology on hygienization of ABP is rare. A large amount of research effort was put in the microbial control of sewage sludge (Astals et al., 2012, López et al., 2019). Many studies payed attention to the effect of electrical and thermal pretreatments on the intensification of biogas yield from organic waste like biosolids and pig manure (Salerno et al., 2009), slaughterhouse waste (Edström et al., 2003, Liu et al., 2018, Luste and Luostarinen, 2011) and other ABP waste (Chamaa, 2017, Safavi and Unnthorsson, 2017). Keles et al. (2010) contributed to the removal of Salmonella spp. in sewage sludge by PEF. Recently, Zhang et al. (2019) studied the joint effect of microwave pretreatment on antibiotic resistance genes removal and methane yield enhancement in chicken manure. A recent research primarily proved that PEF, by removing the effect of ohmic heat, was able to give the bacterial hygienization (i.e. 5-log10 reduction of Ent. faecalis) of ABP at an electric field strength of 25 kV⋅cm−1 for an effective treatment time of PEF over 30 ms during batch treatment (Liu et al., 2019b). The mentioned PEF experimental system remains in small laboratory scale. It consumes more energy than conventional thermal hygienization (Liu et al., 2019a). More studies are therefore needed to investigate alternative pretreatment of ABP for sanitary purpose.
The objectives of this paper are to upgrade the work of Liu et al. (2019b). An innovative coupling of PEF treatment and ohmic heating was realized to study its synergistic effect on ABP hygienization. In this paper, the bacterial hygienization means a reduction of the inoculated Enterococcus faecalis by 5 log10 in animal by-products. The treatment system was scaled up from batch treatment in a small electroporation cuvette to the continuous treatment of ABP suspension in several liters. A newly designed treatment chamber and a more powerful high voltage generator were used.
The paper mainly deals with (1) the synergistic effect of PEF and ohmic heating on the inactivation kinetics of the indicator bacteria inoculated in ABP suspension at four electric field strengths, (2) the comparison with the single effect of PEF by removing ohmic heat, (3) the modeling of the bacterial inactivation curves and (4) the effect of the PEF energy input on the survival behaviors of two indicator bacteria.
Section snippets
Animal by-products
The same mixed animal by-product as Liu et al. (2019b) was used. It was the feedstock for the local biogas plant SEM LIGER in Locminé (France), composed of fish industry waste and pig slurry. In the biogas plant, the substrates had been crushed into small particles (less than 10 mm) and then pasteurized in an industrial thermal hygienizer at 70 °C for 60 min, conforming to the EU ABP regulations mentioned above. The hygienized ABP was directly collected in hot condition at the outlet of the
Synergistic effect of PEF and ohmic heating on bacterial inactivation
The reduction kinetics of two indicator bacteria treated by PEF with the effect of ohmic heating (Tave = 41 °C) were realized at four electric field strengths (E = 15, 20, 25 and 30 kV⋅cm−1). Fig. 3 illustrates the survival fractions of the given indicator bacteria against the effective PEF treatment time (tPEF).
Fig. 3A presents the kinetics of the studied Ent. faecalis inactivation. The final reduction ratios at tPEF = 1 ms were of 0.8 ± 0.0, 3.1 ± 0.1, 5.0 ± 0.1 and 5.2 ± 0.1 log10 for
Conclusions
The continuous treatment of PEF at over 25 kV⋅cm−1 for an effective treatment time (tPEF) of 1 ms, combined with the effect of ohmic heating (Tave = 41 °C and Tmax < 55 °C), could safely reduce 5 log10 of Ent. faecalis and E. coli inoculated in the ABP suspension. This pasteurization efficiency complies with the EU regulations to validate an alternative treatment that replaces the thermal pasteurization of ABP at 70 °C for 60 min. The synergistic effect of PEF and ohmic heating could
Declaration of Competing Interest
None.
Acknowledgement
This work was jointly financed by the French Regional Council of Brittany, France and the French Departmental Council of Morbihan, France [grant reference: ARED-HYDATE]. Thanks are extended to the mixed economy company LIGER (Locminé, France) for their financial and technical contribution [grant reference: 2017_00212]. The authors would like to thank the French government and the European Regional Development Fund (ERDF), the European Union for the investment in the high voltage PEF generator
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