Wastewater sludges were dried in an open greenhouse pilot in order to provide experimental data about solar greenhouse drying under an arid climate during hot and cold periods. Climatic conditions, solids contents and geometric dimensions were constantly measured. The shape change of samples was constantly recorded with digital scan. The results indicated that solar greenhouse drying reduced the moisture content from about 4 kg water/kg DS to 0.08 kg water/kg DS in hot period and to 0.2 kg water/kg DS in cold period in only 72 h. The higher values of drying rate were obtained in hot season. The maximum ranges obtained were 0.2 kg water/kg DS.h and 0.13 kg water/kg DS.h in hot and cold campaigns, respectively. Furthermore, a large volume reduction representing at least 80 % of initial volume was observed in both periods after 72 h of drying process. Also, shrinkage and cracks phenomena took place inside sludges. An improvement of the microbial condition of treated sludges was noted in hot campaign with a decrease by one order of magnitude after only 24 h for total and fecal coliforms and after 72 h for Clostridium perfringens, Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus contents. These pathogenic bacteria were almost constant for fresh and dry sludges in cold period. The paper recommends the solar drying in hot as in cold periods under arid climate considering its average drying rate outside the rainy periods.

This paper presents a site-specific study of solar drying of wastewater sludge in the region of Marrakesh in Morocco. During the course of the experiments, drying pans (DP) and solar greenhouse drying processes were studied in summer and winter seasons. To simulate the DP process, representative samples were dried in plastic cylindrical plates in ambient conditions, near an Open Greenhouse Sludge Dryer (OGSD) designed and constructed as a tunnel plant. Results showed that the dry solids' (DS) concentration of 80% was reached in only 32 h in summer and 57 h in winter by DP process. The OGSD reached the same concentration in 45 and 65 h in summer and winter, respectively. The important influence of wind speed had a greater effect than the temperature on drying rate. The sludge water evaporation caused a large volume reduction during DP and OGSD processes in both seasons. The removal of total and fecal coliforms during summer experiment was noticeably better in the OGSD, even for Clostridium perfringens. Based on the obtained results, the solar greenhouse drying was practical and beneficial to wastewater sludge management and can be recommended as an alternative to DP in arid climate zones generally, and in Marrakesh particularly.

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