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

Thermal hygienization of waste animal by-products (ABP) before anaerobic digestion is imposed by EU regulations in order to minimize its sanitary risk during digestate land application. This process is energy and time consuming. The present paper deals with the hygienization of ABP using pulsed electric field (PEF) in a continuous system. Enterococcus faecalis ATCC 19433 and Escherichia coli ATCC 25922 were tested as indicator bacteria characterizing the microbial inactivation efficiency. Four electric field strengths (15, 20, 25 and 30 kV∙cm^-1) were applied to the continuous treatment chamber where circulated the ABP suspension. Synergistic effect of PEF and ohmic heating (T_ave = 41 °C) and single effect of PEF (T_ave = 28 °C) on bacterial inactivation were investigated. With the effect of ohmic heating, PEF treatment at 25 and 30 kV∙cm^-1 for 0.9 ms could obtain 5-log10 reduction of Ent. faecalis. This efficiency complies with the EU criteria to validate an alternative hygienization process. The time estimated by Weibull model for 5-log10 reduction of both indicator bacteria (5-D value) was significantly reduced by 2-24.5 times when the synergistic effect of PEF and ohmic heating was present. The increase in electric field strength from 25 kV∙cm^-1 to 30 kV∙cm^-1 did not amount to a further inactivation. PEF process coupling ohmic heating at 25 kV∙cm^-1 was the most efficient for ABP hygienization among the four electric field strengths studied.