What is the examples of sewage treatment residue?

Sewage treatment residue, often referred to as sludge, includes various byproducts from the wastewater treatment process. Examples include primary sludge, which is the solid material settled out during the primary treatment phase, and secondary sludge, produced during the biological treatment of wastewater. Additionally, digested sludge, which has undergone anaerobic digestion to reduce pathogens and organic material, is another common type of sewage treatment residue. These residues can be further processed for use as fertilizers or soil amendments.