Sewage sludge thermal treatment technologies with a focus on phosphorus recovery: a review

Phosphorus presents a limited, irreplaceable and essential nutrient necessary for the growth of organisms. There is an increasing effort to recover phosphorus from production waste streams. Sewage sludge presents an important source of phosphorus but also contains organic pollutants and heavy metals. Thermal treatment technologies seem to be a promising option to treat sewage sludge and obtain ash/char from which high recovery rate of phosphorus can be reached. In this review, sewage sludge management options in compliance with EU legal requirements are first reviewed. Follows, an overview of sewage sludge thermal treatment technologies including incineration, hydrothermal carbonisation, pyrolysis and gasification, for the purpose of phosphorus recapture. We summarize recent advances in thermal treatment processes of sewage sludge and phosphorus recovery, identify challenges and knowledge gaps. Thermochemical methods proved to have many advantages over pure wet chemical methods for phosphorus recovery. The review provides the foundation for future research aimed at achieving efficient, economic and environmental sustainable recapture of phosphorus from sludge thermal treatment products.