The effect of liming on the microbiological quality of urban sludge

Treatments applied to sludge in order to stabilise and dehydrate them may give notable inactivation of microorganisms. This is observed when sludge is exposed either to high temperature or drastic pH when residual sludge is limed. The control of virological, parasitological and bacteriological sludge quality by detecting pathogenic microorganisms is slow and too expensive to be commonly practised. Thus, it is possible to replace pathogenic microorganisms detection by that of contamination indicators. The aim of this study was to determine the influence of liming on the behaviour of pathogenic microorganisms detected in urban sludge. The detection of Salmonella and helminth eggs was carried out in liquid sludge (2-3% dryness) and solid sludge (23% dryness) with added lime (0-45% weight/dry weight) and stored for 24 h-46 weeks. The results showed that liming modified some characteristics such as temperature, dryness and pH of the sludge. It appeared that, whatever the percentage of added lime, the temperature of liquid sludge did not change while it increased by about 9 degrees C when 30-45% lime was added to solid sludge. In the same way, the dryness of liquid sludge did not change during the liming, whereas the dryness of 45% limed solid sludge increased from 23% to 31%. Finally, 15%, 30% and 45% liming gave a pH of at least 10, 11.5 and 12, respectively, although the pH increase depended on the sludge type. The efficiency of liming was considered to be related to the pH and not to the percentage of added lime. Three factors determined the efficiency of pathogen elimination: (a) the pH reached by the sludge, (b) the period of liming activity and (c) the dryness of the sludge. Salmonella were eliminated from liquid sludge in 24 h at pH 10.7 and from solid sludge in 24 h at pH 10.0. Viable helminth egg concentration decreased to 3 eggs/10 g DM in liquid sludge in 14 d at pH 11.9 and 60 d at pH 11.6. In solid sludge, egg reduction was achieved in 24 h at pH 12.5, 7 d at pH 12.0 and 14 d at pH 11.5. From this study, it appeared that liming resulted in a much better microbiological quality of liquid sludge if its pH was maintained at 11.6 over 60 d or at pH 11.9 for 14 d. Solid sludge needed to be maintained at pH 11.5 for 14 d, pH 12.0 for 7 d or pH 12.5 for 24 h to achieve similar results.