Abstract
Phosphorus has been identified as a critical element by the European Union and recycling efforts are increasingly common. An important phosphorus-containing waste stream for recycling is municipal sewage sludge (MSS), which is used directly as fertilizer to farmland. However, it contains pollutants such as heavy metals, pharmaceutical residues, polychlorinated bi-phenyls (PCBs) and nano-plastics. The interest in combustion of MSS is continuously growing, as it both reduces the volume as well as destroys the organic materials and could separate certain heavy metals from the produced ashes. This results in ashes with a potential for either direct use as fertilizer or as a suitable feedstock for upgrading processes. The aim of this study was to investigate co-combustion of MSS and biomass to create a phosphorus-rich bottom ash with a low heavy metal content. A laboratory-scale fixed-bed reactor in addition to an 8 MWth grate-boiler was used for the experimental work. The concentration of phosphorus and selected heavy metals in the bottom ashes were compared to European Union regulation on fertilizers, ash application to Swedish forests and Swedish regulations on sewage sludge application to farmland. Element concentrations were determined by ICP-AES complemented by analysis of spatial distribution with SEM-EDS and XRD analysis to determine crystalline compounds. The results show that most of the phosphorus was retained in the bottom ash, corresponding to 9–16 wt.% P2O5, while the concentration of cadmium, mercury, lead and zinc was below the limits of the regulations. However, copper, chromium and nickel concentrations exceeded these standards.
Highlights
Phosphorus (P) is one of the elements that is listed as a critical raw material by the European Union (EU) [1] for its high economic importance and high supply risk
Co-combustion strategies were evaluated at laboratory scale based on elemental concentration of phosphorus and heavy metals
Sweden currently has a ban on the application of sewage sludge ash in farmland, and there is no regulation in place with respect to the heavy metal concentrations in such ashes
Summary
Phosphorus (P) is one of the elements that is listed as a critical raw material by the European Union (EU) [1] for its high economic importance and high supply risk. Only a small amount of the phosphates used in food production and other industries are recycled, and most of them instead end up in the oceans, causing algae bloom and eutrophication. Energies 2020, 13, 1708 and elements the sludge contains, i.e., heavy metals, pharmaceutical residues [2,3], persistent organic pollutants (POPs) like polychlorinated bi-phenyls (PCBs) [4,5,6,7] and Nano-plastics [8,9]. Several European countries, like Belgium, Germany, the Netherlands, and Switzerland, have chosen combustion as the primary method of disposal of this complex material [11]
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