Evaluation of phosphorus in thermally converted sewage sludge: P pools and availability to wheat

Aims: Dried sewage sludge (SS) and the by-products of four SS thermal conversion processes (pyrolysis, incineration and two types of gasification) were investigated for phosphorus (P) availability. 

Methods: A sequential extraction was used to determine the distribution of P among different P pools. After mixing materials with soil, availability of the P was determined with soil P extractions and in a growth experiment with wheat.

Results: Thermally converted SS contained a greater proportion of P within recalcitrant pools than dried SS. Despite having very different P pool distributions, the incinerated and dried SS provided similar amounts of P to plants. Plant P supply from dried and incinerated SS was lower than the comparable soluble P treatment (50 mg P kg⁻¹), but higher than a soluble treatment at a lower rate (20 mg P kg⁻¹). Plant P uptake in gasified and pyrolysed treatments was only marginally greater than uptake in a control (no P) treatment. Plant P uptake correlated most closely with diffusive gradients in thin films (DGT) P analysis of soil-material mixes. Phosphorus availability in the dried and incinerated SS treatments increased over time. 

Conclusions: We propose that the dried and incinerated SS have potential as slow release P fertilisers in low pH soils.