A study for CEN and the German Federal Environment Ministry compares analysis methods for recycled nutrient products including struvites, sewage sludges and sludge incineration ashes.

The report concludes (as already known) that drying of struvites at standard fertiliser analysis temperatures (105°C) results in 30-50% mass loss, not due to water evaporation but to thermal molecular decay. The report notes that milling of struvite can result in local temperatures of this order. Milled struvite and milled triple super phosphate both showed lower NAC phosphorus availability (neutral ammonium citrate). The report further notes that NAC P-availability results vary with temperature, higher at 70°C than 65°C. Solubility results also vary with filtration method: paper filtration resulted in higher results (and also higher variation) than membrane filtration, probably because of removal of fine particles by the membrane. Six different chemical digestion methods are compared for analysis of content of main nutrient elements and of contaminant heavy metals and the report concludes that results were strongly different between these, depending on the recycled product and the element being investigated. In particular, spiking experiments with chromium showed that Cr (VI) can reduce to Cr (III) during the extraction process (the latter is considered less toxic), in the presence of organic matter, so giving incorrect results for Cr (VI). This could be resolved either by reducing the Cr (VI) extraction temperature from 92.5°C to 80°C. ESPP comment: this study confirms the need to develop specific EU standards for testing recycled nutrient fertilisers to support the EU Fertilisers Regulation.

“Untersuchung der Anwendbarkeit der im Rahmen des CEN-Projekts HORIZONTAL entwickelten Analyseverfahren auf Düngemittel und Klärschlamm/ -aschen”, Umwelt Bundesamt, Germany, Texte 03/2018 (UBA-FB 002575) You can read the full report in our Library