Integrating renewable energy in sewage sludge treatment through greenhouse solar drying: A review

Sewage sludge is the main by-product of wastewater treatment plants, requiring significant environmental and economic burdens for its management and disposal. Recently, solar drying processes, often performed through solar greenhouses, received interest due to their limited energy requirement and renewable energy exploitation. The dried sludge shows significant volume and mass reductions, reducing transportation and disposal costs. However, its physicochemical and microbiological characteristics must be properly assessed, especially if agricultural reuse is the final sludge destination, due to the possible accumulation of (micro)pollutants in the soil. This review depicts the state-of-the-art solar drying processes of sewage sludge, with a focus on the technological aspects and the sludge quality. The review discusses greenhouse-specific features, sludge composition (organic matter, pathogens, heavy metals and emerging pollutants) and drying conditions (seasonality, ventilation, sludge mixing and thickness, and drying speed). The economic aspects connected to sludge solar drying are presented. The limitations of this technology are discussed as well, including the limited applicability to wet sludge and the environmental issues connected to greenhouse structure degradation. A wider application of sludge solar drying is recommended to increase the sustainability of small and medium wastewater treatment plants, especially in areas with high amounts of solar radiation and dry weather conditions, while thermal drying still appears preferable for large plants. More agronomic studies must be conducted to assess the possible pollutant accumulation in crops, and alternative uses of the dried sludge (e.g., energy recovery through incineration, pyrolysis or gasification; utilization in construction materials) should be explored, also using life cycle assessment.