Using temperature and time criteria to control the effectiveness of continuous thermal sanitation of piggery effluent in terms of set microbial indicators

Abstract

To determine the minimal conditions (temperature-time), necessary to achieve set sanitation targets for selected microbial indicators during the continuous thermal treatment of pig slurry. The effectiveness of thermal treatment between 55 and 96°C was studied using Escherichia coli, enterococci, sulfite-reducing Clostridia (SRC), mesophilic culturable bacteria (MCB), F+-specific and somatic phages. Identification of SRC and MCB was performed using 16S rRNA gene analysis. Ten minutes at 70°C or 1 h at 60°C was sufficient to reduce the vegetative bacteria by 4-5 log(10), but it had little effect on somatic phages nor on spore formers, dominated by Clostridium sp. At 96°C, somatic phages were still detected, but there was a reduction of 3.1 log(10) for SRC and of 1.4 log(10) for MCB. At 96°C, Clostridium botulinum was identified among the thermotolerant MCB. Only those hygienic risks relating to mesophilic vegetative bacteria can be totally eliminated from pig slurry treated at 60°C (60 min) or 70°C (<10 min). Hygiene standards based on the removal of the indicators E. coli and enterococci can easily be met by treatment as low as 60°C (enabling, a low-cost treatment using heat recovery). However, even at 96°C, certain pathogens may persist.