The effect of pre-treatment and anaerobic digestion for pathogens reduction in agricultural utilization of sewage sludge

The aim of the research work was to explain the possibilities of application of waste activated sludge (WAS) pretreatment processes prior to anaerobic digestion (mesophilic fermentation). Hydrodynamic disintegration and freezing/thawing disintegration methods were used. Based on the microbiological and parasitological analyses, a significant decrease in pathogenic bacteria, coliphages, and parasite eggs was observed. The number of bacteria analyzed (Salmonella sp., Escherichia coli, Clostridium perfringens) and coliphages were reduced from 19.3to 42.3% after hydrodynamic cavitation. A similar effect was achieved for destruction by freezing/thawing with dry ice between 7.8 and 14.9%. The effectiveness of parasite eggs reduction (Ascaris sp., Trichuris sp., Toxocara sp.) for these disintegration methods ranged from 10.7 to 29.3%. The highest results were observed for the hybrid disintegration method (hydrodynamic cavitation + dry ice disintegration) caused by a synergistic effect. Salmonella sp. in 1 g_d.w. decrease about 69.7%, E. coli by 70.0%, Clostridium perfringens by 38.4%, and coliphages by 48.2%. Disruption of WAS by a hybrid method led to a reduction in the number of helminth eggs Ascaris sp. (63.8%), Trichuris sp. (64.3%), and Toxocara sp. (66.4%). After anaerobic digestion under mesophilic conditions, an additional reduction of analyzed bacterial pathogens and helminth eggs were observed. The introduction of hybrid disintegrated WAS to the fermentation chamber resulted in higher efficiency in decrease (from 1 to 23%) in comparison to the control sample (70%WAS + 30%DS (inoculum-digested sludge)).

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.

Effect of ultrasound, low-temperature thermal and alkali pre-treatments on waste activated sludge rheology, hygienization and methane potential

Waste activated sludge is slower to biodegrade under anaerobic conditions than is primary sludge due to the glycan strands present in microbial cell walls. The use of pre-treatments may help to disrupt cell membranes and improve waste activated sludge biodegradability. In the present study, the effect of ultrasound, low-temperature thermal and alkali pre-treatments on the rheology, hygienization and biodegradability of waste activated sludge was evaluated. The optimum condition of each pre-treatment was selected based on rheological criteria (reduction of steady state viscosity) and hygienization levels (reduction of Escherichia coli, somatic coliphages and spores of sulfite-reducing clostridia). The three pre-treatments were able to reduce the viscosity of the sludge, and this reduction was greater with increasing treatment intensity. However, only the alkali and thermal conditioning allowed the hygienization of the sludge, whereas the ultrasonication did not exhibit any notorious effect on microbial indicators populations. The selected optimum conditions were as follows: 27,000 kJ/kg TS for the ultrasound, 80 °C during 15 min for the thermal and 157 g NaOH/kg TS for the alkali. Afterward, the specific methane production was evaluated through biomethane potential tests at the specified optimum conditions. The alkali pre-treatment exhibited the greatest methane production increase (34%) followed by the ultrasonication (13%), whereas the thermal pre-treatment presented a methane potential similar to the untreated sludge. Finally, an assessment of the different treatment scenarios was conducted considering the results together with an energy balance, which revealed that the ultrasound and alkali treatments entailed higher costs.