Using a previously developed filter adsorption technique, the inactivation of a human rotavirus, a coxsackievirus B5, and a bovine parvovirus was monitored during sludge treatment processes. During conventional anaerobic mesophilic digestion at 35 to 36°C, only minor inactivation of all three viruses occurred. The k' values measured were 0.314 log10 unit/day for rotavirus, 0.475 lglo unit/day for coxsackievirus B5, and 0.944 log10 unit/day for parvovirus. However, anaerobic thermophilic digestion at 54 to 56°C led to rapid inactivation of rotavirus (k' > 8.5 log10 units/h) and of coxsackievirus B5 (k' > 0.93 loglo unit/min). Similarly, aerobic thermophilic fermentation at 60 to 61°C rapidly inactivated rotavirus (k' = 0.75 loglo unit/min) and coxsackievirus B5 (k' > 1.67 log1o units/min). Infectivity of parvovirus, however, was only reduced by 0.213 loglo unit/h during anaerobic thermophilic digestion and by 0.353 lglo unit/h during aerobic thermophilic fermentation. Furthermore, pasteurization at 70°C for 30 min inactivated the parvovirus by 0.72 log10 unit/30 min. In all experiments the contribution of temperature to the total inactivation was determined separately and was found to be predominant at process temperatures above 54°C. In conclusion, the most favorable treatment to render sludge hygienically safe from the virological point of view would be a thermal treatment (60°C) to inactivate thermolabile viruses, followed by an anaerobic mesophilic digestion to eliminate thermostable viruses that are more sensitive to chemical and microbial inactivations.

A simple and reliable method is described which allows determination of virus inactivation rates during sludge treatment processes in situ. Bacteriophage f2 was adsorbed onto an electropositive membrane filter which was then sandwiched between two polycarbonate membranes with pores smaller than the virus diameter. The resulting sandwich was fixed in an open filter holder, and several such devices were connected before being exposed in sludge-digesting tanks. The device described prevented uncontrolled virus escape, but allowed direct contact of the various inactivating or stabilizing substances present in the environment tested with the virus adsorbed to the carrier membrane. After exposure to an environment, the surviving fraction of virus was eluted from the inner filter and determined by plaque counting. By using polycarbonate membranes without pores for sandwiching, the influence of temperature alone on virus inactivation could be measured. Thermophilic fermentation at 60 degrees C and at 65 kPa pressure led to a bacteriophage f2 titer reduction of 3.5 log10 units per h, whereas during thermophilic digestion at 54.5 degrees C titers decreased 1.2 log10 units per h. During mesophilic digestion an inactivation rate of only 0.04 log10 units per h was observed. Under these latter conditions, temperature had only a minor effect (19%) on virus inactivation, whereas at 54.5 degrees C during thermophilic digestion heat accounted for 32% of the total inactivation, and during thermophilic fermentation at 60 degrees C temperature and pressure were 100% responsible for virus denaturation.

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