The agricultural sector requires large amounts of phosphorus (P) for food production (in the EU consumption of fertilizer-P exceeds 1.2 million tons of P per year). However, P is a non-renewable resource, becoming scarce in the near future, and should be saved by applying recycling technologies to P-bearing waste streams.

One of the major carriers of P is sewage sludge. In the EU more than 10 million tons (dry mass) of sewage sludge are produced annually, which contain about 0.3 million tons phosphorous. However, sewage sludge is not only a carrier of P but it is contaminated with organic pollutants and heavy metals. Thus, the direct agricultural utilization of sewage sludge is a controversial issue.

The European project SUSAN (Sustainable and Safe Re-use of Municipal Sewage Sludge for Nutrient Recovery) bundles the research efforts of seven partners from Austria, Finland, Germany and The Netherlands, aiming at the development of a sustainable and safe strategy for phosphorus recovery from sewage sludge using a two-step thermal treatment including mono-incineration of sewage sludge and subsequent thermochemical treatment of these ashes.

One essential aspect of the thermochemical treatment is the addition of a chlorine-donor to the ash, leading to the conversion of the heavy metal oxide impurities into chlorides and their subsequent sublimation / evaporation. A second essential aspect is the transformation of all phosphorus-bearing components into such mineral phases that are bio-available (available for plants).

To gain insight into the chemical processes accompanying the thermochemical treatment of sewage sludge ashes X-ray powder diffraction (XRD) was applied to a large number of samples produced either in a laboratory-scale equipment or in a pilot plant. Making use of the procedure described in [1] it was shown that the thermochemical treatment is accompanied by a sequence of chemical reactions and transformations of the phosphorus-bearing mineral phases.

The present paper is focused on a detailed XRD-study of the reaction sequence in the thermochemical treatment of an iron-bearing sewage sludge ash using two alternative chlorine-donors.

[1] Peplinski B. et al., Proceedings EPDIC-9, Z. Kristallogr. Suppl. 23 (2006) p. 29

Some of the mineral phases detected by XRD in an iron-bearing sewage sludge ash before and after being thermochemically treated with magnesium chloride as a chlorine-donor at temperatures between 450 and 1050°C.
black circles = phase detected by XRD, white circles = phase not detected by XRD, grey circles = phase just (dis)appearing.

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