Thermal inactivation of plant seeds in sewage sludge

Abstract

Seeds of Tomato (Lycopersicon lycopersicum) and Millet (Digitaria sanguinalis) were exposed to temperatures from 60°C to 70°C for 20 to 75 minutes. The influence of aeration as well as heat protection by sludge solids was investigated in laboratory scale and in full scale sludge hygienisation. Thermal inactivation of Millet was achieved at temperatures of 65°C where 1 - 5% of the seeds survived an exposure of 60 minutes. Lower temperatures showed little effect on Millet. At 60°C only 1 - 10% inactivation was found after 70 minutes. Complete inactivation could be achieved at 70°C and above with exposition periods of 20 minutes. Tomato seeds showed a higher resistance to heat inactivation at low temperatures. Temperatures of 60 - 65°C were not sufficient to inactivate the seeds. An exposition of tomato seeds to 65°C for 60 minutes still resulted in a survival of > 20% of the initial seed concentration. Complete inactivation was found at 70°C and higher with exposition periods of 20 minutes. No difference of heat inactivation was found between static and aerated sludge. The protective action of sludge solids was found to be present but negligible for both species. Today's sludge hygienisation technology will significantly reduce Millet and Tomato seeds in sludge and eliminate or reduce the risk of contamination of agricultural areas. Thermal pasteurisation processes show a higher seed destruction potential for Tomato and Millet than aerobic - thermophilic pretreatment.