King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia
In order to optimize sludge depth in sand drying beds under the climatic conditions of Saudi Arabia, the effect of initial sludge depth on the fate of pathogens was investigated.The investigation was carried out for one year in Al-Khobar Wastewater Treatment Plant, where initial sludge depths of 10, 15, 20, 25, 30 and 35 cm were implemented. Sludge samples were collected on 0, 1, 2, 4, 7, 14, and 30 days and were analyzed for various types of bacterial species and protozoan and helminthic pathogens.The study showed that the effect of the initial sludge depth in drying beds on the dye-off rate of pathogens under investigation was apparent.Total coliform, streptococci, shigella, salmonella and clostridium were found to
survive longer as the sludge initial depth was increased. As an example, the streptococci count reached values of 25, 37, 49, 59, 71, and 90 organisms per gram dry weight for sludge initial depths of 10, 15, 20, 25, 30 and 35 cm, respectively.The same trend was also observed for protozoan and helminthic pathogens. For example, the number of ascaris lumbricoides after 2 days of drying was 3, 4, 6, 7, 7, and 10 in sludge samples collected from drying beds with initial depths of 10, 15, 20, 25, 30 and 35 cm, respectively.A mathematical representation was formed to describe the pathogens dye-off with respect to time that included the effect of the sludge initial depth. The results indicated that the mathematical representation of the drying beds for individual species was dramatically improved when average values of constants for individual species were used in the model.