INACTIVATION OF VIRUS DURING ANAEROBIC-DIGESTION OF MANURE IN LABORATORY-SCALE BIOGAS REACTORS

Reduction of porcine parvovirus, bovine enterovirus and faecal enteroc occi were measured in biogas reactors continuously run on manure and m anure supplemented with household waste at 35 degrees C and 55 degrees C and in batch test run at 70 degrees C. The aim of the experiments w as to study the sanitation effect of anaerobic digestion and to evalua te the use of faecal enterococci as an indicator of sanitation. Parall el studies on the reduction of virus and faecal enterococci were done in physiological saline solution. Heat inactivation was found to be an important factor in thermophilic biogas plants and the overall domina nt factor at 70 degrees C. However, other environmental factors with a substantial virucidal and bactericidal effect were involved in inacti vation. The death rates for faecal enterococci were generally higher t han for porcine parvovirus and lower than for bovine enterovirus. For faecal enterococci, a logarithmic reduction of 4 (corresponding to the recommended minimum guaranteed retention time) was obtained after 300 hours at 35 degrees C and after 1-2 hours at 55 degrees C. In continu ously-fed reactors, a high reduction rate was initially seen for the v irus tested, followed by a reduction in the rate. For porcine parvovir us, a minimum guaranteed retention time of 11-12 hours is necessary at 55 degrees C in the initial phase (0-4 hours) and 54 hours hereafter (4-48 h). Correspondingly, for bovine enterovirus a MGRT of 23 hours w as necessary at 35 degrees C and < 0.5 hours at 55 degrees C. The data indicate that faecal enterococci measurements give a good indication of inactivation of enterovirus and other more heat sensitive virus, es pecially under thermophilic conditions. Parvovirus is very suitable fo r comparative investigations on inactivation in the temperature range of 50-80 degrees C, due to the extreme thermal resistance of this viru s. However, in stipulating sanitation demands for biogas reactors it s eems more reasonable to use less resistant virus, such as a reovirus o r picornavirus, which better represents the pathogenic animal virus.