Journal of Applied Microbiology 1999, 87, 387–395 Effect of process temperature, pH and suspended solids content upon pasteurization of a model agricultural waste during thermophilic aerobic digestion J.O. Ugwuanyi, L.M. Harvey and B. McNeil Strathclyde Fermentation Centre, Department of Bioscience and Biotechnology, University of Strathclyde, Glasgow, UK 7136/04/99: received 1 April 1999, revised 30 April 1999 and accepted 13 May 1999 J.O. UGWUANYI, L.M. HARVEY AND B. McNEIL. 1999. Thermophilic aerobic digestion (TAD), or liquid composting, is a versatile new process for the treatment and stabilization of high strength wastes of liquid or, perhaps more importantly, slurry consistency. The pattern of inactivation of various pathogenic and indicator organisms was studied using batch digestions under conditions that may be expected to be found in full- scale TAD processes. Rapid inactivation of test populations occurred within the first 10 min from the start of digestion. The inactivation rate was slightly lower when digestions were conducted below 60 °C. In some instances, a ‘tail’ was apparent, possibly indicating the survival of relatively resistant sub-populations particularly in the case of Serratia marcescens and Enterococcus faecalis, or of clumping or attachment of cells to particulate materials. The effect of pH on the inactivation of the test populations depended on the temperature of digestion, but varied with the test population. At 55 °C Escherichia coli was more sensitive to temperature effects at pH 7 than at pH 8, but was more sensitive at pH 8, 60 °C. The reverse was the case at 60 °C for Ent. faecalis. An increase in the solid content of the digesting waste caused a progressive increase in the protection of test organisms from thermal inactivation. Challenging a TAD process with test strains allows (via estimation of D-values) a quantification of the cidal effects of such processes, with a view to manipulating process variables to enhance such effects. INTRODUCTION Wastes can no longer be considered as that which society needs to eliminate, as the drive for recycling of the abundant organic matter in wastes increases. However, the recycling of various waste biomasses, particularly those resulting from agriculture and animal production practices, as well as sewage, is strictly tied to their microbiological and sanitary quality. This is necessary to safeguard against the spread of disease-bearing pathogens (De Bertoldi et al. 1988). Con- ventional waste treatment processes, including aerobic and anaerobic digestion, have been found to be inefficient from a hygienic and epidemiological point of view, being generally Correspondence to: B. McNeil, Strathclyde Fermentation Centre, Department of Bioscience and Biotechnology, University of Strathclyde, 204 George Street, Glasgow G1 1XW, UK (e-mail: b.mcneil@strath.ac.uk). © 1999 The Society for Applied Microbiology incapable of achieving acceptable levels of pathogen reduction and vector attraction limit in treated wastes, namely the class A sludge quality according to the US Environmental Pro- tection Agency (EPA) classification (De Bertoldi et al. 1988; EPA 1990, 1992; Juris et al. 1992, 1993; Plachy et al. 1993; Pagilla et al. 1996). Limitation of pathogen levels, particularly those of human or animal origin, in wastes intended for land or sea disposal is the subject of legislation in Europe and North America (EPA 1992; Droffner and Brinton 1995) and is a major driving force for the development of aerobic thermophilic digestion. Auto-thermal thermophilic aerobic digestion (ATAD or TAD) utilizes the heat of microbial aerobic metabolism to raise the temperature of waste undergoing treatment in an insulated system to thermophilic levels (−45 °C). Waste pas- teurization results from the thermal inactivation of meso-