A novel approach to sustainable construction: utilising anaerobically digested sewage sludge as a partial aggregate replacement in mortars

A sustainable and innovative approach of enhancing the characteristics of sewage sludge for application in cementitious materials is described. The direct use of sewage sludge in such matrices is often limited by its high content of organic matter and its porous morphology, which adversely affect mechanical performance and durability. To overcome these limitations, anaerobic co-digestion of sewage sludge with activated sludge and agricultural residues (wheat straw and rice straw) was examined. This biological treatment significantly reduced the organic content and modified the particle size distribution and surface morphology of the sludge. Untreated sewage sludge (UTSS) and treated sludges (wheat-treated sewage sludge (WTSS), rice-treated sewage sludge (RTSS) and combined-treated sewage sludge) were used as partial replacements (5%, 10% and 15% by weight) of fine aggregates in mortar mixtures. The mortars incorporating the treated sludge exhibited superior mechanical strength and water resistance compared with those containing UTSS, with WTSS and RTSS showing the most notable improvements. This approach not only offers a cost-effective and environmentally responsible alternative to conventional fine aggregates, such as natural sand and incinerated sludge, but also promotes sustainable agricultural waste management by integrating crop residues into the treatment process. The findings demonstrate the potential of activated sludge straw co-digestion to enhance organic waste degradation while producing value-added materials for construction applications.