Microbiological Evaluation of the Effectiveness of Sewage Sludge Sanitization with Solar Drying Technology

The aim of study was to estimate the sanitization effectiveness of the sludge solar drying process carried out on technical scale in Poland based on the inactivation of bacteria and parasite eggs. Sewage sludge samples inoculated with Escherichia coli, Salmonella Senftenberg W-775 and enterococci and perlon bags containing Ascaris suum eggs were placed inside the carriers fixed in the dried sludge pile and on the shovels and frame of the sludge turner. The number of reisolated microorganisms was determined with MPN method and the percentage of invasive A. suum eggs-with the microscope counting. On the basis of regression equations, the theoretical survivability and elimination rate of bacteria and parasite eggs were calculated. Experiment showed low hygienization efficiency of solar drying method. The theoretical survival time was 46-104 days in summer and 90-98 days in winter for S. Senftenberg W-775 and, respectively 42-55 and 71-148 days for E. coli, depending on the carriers location. Enterococci were able to survive for 52-168 days in summer and in winter its number increased. The decrease in the percentage of invasive A. suum eggs was almost not observed. Results indicated that solar drying is a technology, which does not guarantee biosafety of product.