Abstract
The necessity of recycling anthropogenically used phosphorus to prevent aquatic eutrophication and decrease the economic dependency on mined phosphate ores encouraged recent research to identify potential alternative resource pools. One of these resource pools is the ash derived from the thermochemical conversion of sewage sludge. This ash is rich in phosphorus, although most of it is chemically associated in a way where it is not plant available. The aim of this work was to identify the P recovery potential of ashes from sewage sludge co-conversion processes with two types of agricultural residues, namely wheat straw (rich in K and Si) and sunflower husks (rich in K), employing thermodynamic equilibrium calculations. The results indicate that both the melting behavior and the formation of plant available phosphates can be enhanced by using these fuel blends in comparison with pure sewage sludge. This enhanced bioavailability of phosphates was mostly due to the predicted formation of K-bearing phosphates in the mixtures instead of Ca/Fe/Al phosphates in the pure sewage sludge ash. According to the calculations, gasification conditions could increase the degree of slag formation and enhance the volatilization of K in comparison with combustion conditions. Furthermore, the possibility of precipitating phosphates from ash melts could be shown. It is emphasized that the results of this theoretical study represent an idealized system since in practice, non-equilibrium influences such as kinetic limitations and formation of amorphous structures may be significant. However, applicability of thermodynamic calculations in the prediction of molten and solid phases may still guide experimental research to investigate the actual phosphate formation in the future.
Highlights
Depleting natural resources and the ongoing population growth require major changes in the concepts of resourceSewage sludge represents a suitable source for P recovery because of its high P content
This observation is substantiated by the fact that the ash of a 50/50 wt% fuel blend for both sludge + sunflower husks (SSH) and WSS respectively contains about 90 mol.% elements derived by the sewage sludge fuel
The main difference between SSH mixtures and WSS mixtures is the share of Si in the ash, which is increasing in WSS mixtures and decreasing in SSH mixtures
Summary
Depleting natural resources and the ongoing population growth require major changes in the concepts of resourceSewage sludge represents a suitable source for P recovery because of its high P content. Through the thermochemical conversion of sewage sludge, harmful components can be destroyed (e.g., hormones or pathogens) or separated (e.g., heavy metals) from the P-rich coarse ash fraction [5,6,7]. The speciation of P in the sewage sludge ash tends to display mainly low plant available Ca phosphates and requires additional preparation before being used as fertilizer [10, 11]. Potential methods to create plant available P compounds from sewage sludge ash are subsequent thermochemical treatment with alkali salts [12] and acidic or basic extraction [10]. Through the addition of alkali salts, the compounds whitlockite (Ca3(PO4)2) and apatite (Ca5(PO4)3OH) in sewage sludge ash from mono-combustion plants, which are poorly plant available, may be transferred into alkali phosphates such as CaNaPO4, which are more readily taken up by plants [12]. A positive effect of K amendments on the solubility and plant availability of P in the ashes was found [15]
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