The Molecular Environment of Phosphorus in Sewage Sludge Ash: Implications for Bioavailability

Abstract

Producing a P fertilizer from sewage sludge ash (SSA) is a strategy to efficiently recycle P from a secondary raw material. The P speciation in four SSAs was characterized before and after the removal of heavy metals by a thermo-chemical treatment that involved CaCl addition. We chose complementary techniques to determine the direct P speciation, including X-ray powder diffraction, solid-state P direct-polarization magic-angle spinning nuclear magnetic resonance, and X-ray absorption near edge structure. Results from these techniques were compared with operational and functional speciation information obtained from a sequential P extraction and a plant biotest with Italian ryegrass grown on a soil-sand mixture with little available P. The speciation of P in untreated and thermo-chemically treated SSAs depended on their elemental composition. At a molar ratio of Ca:P ≤ 2, SSAs contained combinations of polymorphs of AlPO, β-tricalcium phosphate, and apatite-like P species. In SSAs with a molar ratio of Ca:P > 2, an apatite-like molecular environment was predominant. The thermo-chemical process induced an increase in crystalline phases and enhanced the crystallinity of the P species. The structural order of the bulk sample was the most decisive parameter in controlling the P availability of the studied SSAs to plants. We conclude that, to produce a high-quality fertilizer and despite of the successful heavy metal removal, the thermo-chemical process requires further development toward enhanced P bioavailability.

Effect of bioeffectors and recycled P-fertiliser products on the growth of spring wheat

“…This result was in line with earlier work, showing that phosphorus in sewage sludge ash is generally not readily taken up by plants [9]. On the other hand, there may be considerable variations between different sewage sludge ashes, depending on the processing of sewage sludge in the water treatment plant [7]. Sewage sludge, Thomas phosphate and sewage sludge-enriched BOF slag (LDS/SSA) all resulted in levels of available P similar to or higher than TSP.…”

“…However, the availability of P in sewage sludge ash is quite low, but is also observed to be variable depending on the type of treatment in the sewage treatment plant [7]. Techniques such as acid leaching and thermal treatment have been investigated for their potential to recover P from sewage sludge ash while separating it from the detrimental heavy metals with varying success [5,7], but there is also a possibility of combining the upgrading of sewage sludge ash with the recycling of metallurgical slags. Slags from the metallurgical industry had been recycled as fertiliser in Germany in the form of Thomas phosphate for more than 100 years, but this is no longer produced [8].…”

Potential of three microbial bio-effectors to promote maize growth and nutrient acquisition from alternative phosphorous fertilizers in contrasting soils

“…This is in line with the data on water extractable P in the sludges where the level in the Danish sludge was three times higher than in the German sludge (Table 4). Furthermore, the availability of P in sewage sludge ashes can be quite different depending on the processing of the sludge in the water treatment plant [47].The very high fertilizer efficiency recorded for the P-enriched BOF slag has been observed earlier in wheat [45] and maize [42].…”

Waste products as alternative phosphorus fertilisers part I: inorganic P species affect fertilisation effects depending on soil pH

“…Sharpley and Moyer 2000;Ylivainio et al 2008;García-Albacete et al 2012;Nanzer et al 2014). In brief, labile P, Fe-/Al-bound P and stable Ca phosphates are distinguished when P is sequentially extracted from samples using solvents with different pH and increasing extraction capacity.…”

“…X-ray powder diffraction (XRD) can be applied to waste products for the detection of crystalline phases containing inorganic P. It has been used for the identification of different Fe-/Al-bound P, Ca phosphates such as whitlockite and apatite in sludge and sludge ashes (Frossard et al 1997;Kim et al 2013;Nanzer et al 2014) and struvite in dairy manure (Güngör et al 2007;Hunger et al 2008). When X-rays interact with crystalline matter, diffraction patterns are produced, which are unique to the crystal lattice.…”