Medium-scale Plant Experiment of Sewage Sludge-based Phosphorus Fertilizers from Large-scale Thermal Processing

ABSTRACT

Phosphorus (P) recycling from sewage sludge for agricultural needs has to meet requirements for agricultural implementation, such as short and long-term P-plant-availability under field conditions. Field experiments often bring no evaluable results, because agricultural soils got a high potential of P-supply even if they are classified as low in P-supply according to the CAL extraction method. The present study presents a possible way to investigate the P-plant-availability of P-recycling-fertilizers under field-like conditions. The plant experiments are firstly performed in small Mitscherlich pots in growth chambers and subsequently in containers with a high soil volume of 170 kg under greenhouse conditions, in which plants can grow until ripening. The tested P-recycling-fertilizers were produced from sewage sludge in a large-scale thermal process. It was a two-step treatment process performed with a pyrolysis of sewage sludge at 550°C (SSC-550) and a subsequent thermochemical post-treatment at 950°C with Na₂SO₄ (SSA-Na) and HCl + Na₂SO₄ (SSA-HCl/Na) as additives. The results show, that the P-recycling-products from pyrolysis got an adequate long-term but a 65% lower short-term P-plant-availability compared to triple-superphosphate. SSA-Na and SSA-HCl/Na show both a high short and long-term P-plant-availability comparable to triple-superphosphate. This can be explained by their highly plant-available P-compound CaNaPO₄.

KEYWORDS:

• Large-scale P-recycling
• pyrolysis
• thermochemical treatment
• P-plant-availability
• sewage sludge
• plant-experiment

Acknowledgments

We wish to thank the entire Institute of Plant Nutrition and the Institute of Soil Science and Conservation (Justus-Liebig-University of Giessen) for facilitating our work. We also want thank Corinna Alles-Becker and Lutz Wilming for taking care of our plants during the pot experiments.

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Funding

This work was supported by the German Federal Office for Agriculture and Food on basis of a resolution of the German Bundestag in the context of the Federal Program on Organic Farming (Bundesprogramm Ökologischer Landbau und andere Formen nachhaltiger Landwirtschaft, BÖLN) (2811NA022/2811NA023).