The rapid growth of the world population, which is expected to reach 8.5 billion by 2030, has led to an increase in water consumption and consequently an increase in the production of wastewater. Due to stricter legislation that imposes rigid parameters both for water intended for human consumption and for wastewater already subjected to treatment and destined to be discharged into water bodies, the need for drinking water treatment plants (DWTPs) and for wastewater treatment plants (WWTPs) is therefore expected to increase in the coming years. Generally, water treatment plants have many residues both in liquid (e.g. WWTPs effluents, oil) or solid (e.g. biological and/or chemical sludge, sands, exhausted activated carbon) phases. These types of products will therefore also likely increase and will have to be managed appropriately.

In recent years, a lot of efforts have been made to study the conversion of water treatment plants into recovery facilities, adding the objective of resource recovery to the objectives of protecting human health and the environment. From a circular economy perspective, the residues produced by DWTPs and WWTPs could therefore be subject to subsequent treatments and recovered. However, several aspects should be better investigated, such as a complete characterization of residues and their chemical properties, possible organic and inorganic substances released, and environmental compatibility of products made with recycled materials.

This Special Issue aims to bring together original research articles covering any aspect of residue characterization and reuse from DWTP and WWTP by providing new insights, and review articles that describe the state of the art and highlight new findings in this area.

Potential topics include but are not limited to the following:

• Characterization of water treatment plant residues
• The contribution of biological and chemical sewage sludge reuse in the circular economy perspective
• The use of biological sewage sludge spreading to tackle soil nutrient depletion
• The influence of wastewater type on the chemical composition of biological sewage sludge
• Environmental compatibility of products made with recycled sand
• Reuse of wastewater treatment plant effluent: chemical and microbiological quality
• Organic and inorganic substances released from residues
• Reuse of sands in building sector: characterization and chemical properties
• Backwashing water: characteristics and possible uses
• Management of residues from membrane processes (concentrate) and ionic exchange resins (brine)
• Management of exhausted materials used for water treatment (e.g. activated carbon, iron oxides/hydroxides, exhausted ionic exchange resins, etc.)