Solar drying technology of wastewater sludge and its impact on Morocco sludge management

Sludge management is a major problem in wastewater treatment plants (WWTPs), its cost can reach up to 60% of the global operating cost. The objective of this work is to review the solar drying technology of wastewater sludge worldwide and then analyze its impact on sludge management in Morocco. The sludge solar drying is done in greenhouses that can be open, closed or heated. The performance of a greenhouse is evaluated by its drying rate (DR). The DR varies from 2.2 to 12 kg of evaporated water per m² per day depending on the climate of the installation site, the season of the year and the type of technology installed. The DR is also a major parameter in the design of a sludge solar drying plant. The use of solar energy reduces the operating costs compared to conventional drying. The investment cost varies from 250 to 500 €/m² and the operating cost from 20 to 250 €/tonne of dry matter (DM) per year, depending on the technology installed. The use of dried sludge as a biofuel is increasingly being adopted. In Morocco, by 2030, with the completion of the national sanitation programme (PNA), about 900 million m3 of wastewater will be treated per year. This treatment will generate about 2 million tonnes of sludge per year with 25% dryness degree. In terms of sludge management, the technology used consists of installing solar drying greenhouses, as in the cities of Marrakech, Laayoune, Ben Guerir and Youssoufia. Assuming that half of the sludge production by 2030 will be treated by solar drying, the estimated costs are 2 billion Dirhams (197 M€) for the installation and 750 million Dirhams (68 M€) for a 20-year operation. Drying is only one step in sludge management, one or more disposal and/or recovery processes still need to be put in place. The co-composting of sludge with green waste is an application that is being studied and tested in Morocco. On the regulatory side, no specific laws for WWTPs sludge management are formally in place but other environmental and water protection laws can already be applied.

Keywords: Solar drying of sludge, WWTP, greenhouse, energy recovery, sludge management, Morocco