Phosphorus speciation in sewage sludge and their ashes after incineration as a function of treatment processes

Abstract

Phosphorus (P) is a key component in agricultural fertilizers, but it is also a scarce resource, why its recycling has been thoroughly investigated and one promising resources is sewage sludge. Because of stricter regulations in terms of sludge disposal, thermal treatment (e.g. incineration) has become an attractive option. The incineration process alters the chemical speciation of P in favour to calcium-associated (apatite, apatite phosphorus (AP)) species, which is preferred for P recovery. In order to achieve qualitatively transformation, it is important to identify limiting or promoting factors. This study reports on the impact of iron, aluminium and calcium on the transformation of iron- and aluminium-phosphate (NAIP) to AP species, assessed by studying sludge and ash from 10 municipal wastewater treatment plants in Sweden. The effect of iron and aluminium added in the treatment processes was also evaluated. The obtained results show that high calcium concentration favours formation of AP species in both sludge and ashes, whereas high concentration of iron and aluminium favours formation of NAIP species in the sludge. The transformation from NAIP to AP species is hampered by aluminium, irrespectively of its origin, whereas no such correlations could be seen for iron. Therefore, in order to enable efficient P recovery from sewage sludge ash, the amount of aluminium added in the treatment process, as well as its concentration in influent streams to the treatment plants, must be limited.

Keywords: Sewage sludge; aluminium; apatite; incineration; phosphorus; recirculation; speciation; sustainability.

Figure 1.

Workflow of the SMT protocol for phosphorus speciation. The figure is redrawn from Ruban et al. (2001) and has been previously published (Nilsson et al., 2022). Samples (S) are extracted in the fractions total phosphorus (TP), IP, OP, NAIP and AP. All extractions were performed at 20°C.

Figure 2.

Non-apatite (NAIP) and apatite (AP) phosphate fractions (mg g⁻¹) in sewage sludge (S) and ashes (A) following the SMT protocol, concentrations in the ash samples are also shown as mg g⁻¹ to facilitate for comparison. All values are given as mean values ± standard deviation (n = 3). Data from chemical speciation of phosphorus in digested samples of sludge and ashes from MWWTPs 1, 2 and 3 have been previously reported in Nilsson et al (2022).

Figure 3.

Percentage increase in apatite (AP) and decrease in non-apatite (NAIP) phosphate fractions following incineration of sewage sludge from the 10 facilities.

Figure 4.

AP/NAIP ratio as a function of total concentration of aluminium in sewage sludge and ash.