Evaluating phosphorus availability from sewage sludge derived pyrochar and hydrochar in European agriculture

Currently, phosphorus (P) mine reserves are monopolised by several countries and its market price variations represents a challenge for modern agriculture systems. Consequently relying on alternative renewable sources of P such as sewage sludge (SS) is timely as its supply is expected to increase with population worldwide. However, this has to be carefully managed to ensure potential pollutant transport when applied to soils.

However, alternative treatment options can reduce this risk and create greater value from SS as a P-fertiliser. By carbonizing the residues through Pyrolysis or Hydrothermal Carbonization (HTC), organic pollutants can be significantly decomposed, and its volume reduced, which ease enormously its management (R. Huang & Tang, 2015). Different characteristics will be obtained depending on the thermal process and the conditions to which the sample is subjected, differentiating the potential applications of the pyrochar/hydrochar obtained respectively. Nevertheless, the data gathered for yield crop responses from sewage sludge thermal derivatives is still very scarce and hence more information needs to be produced.

The aim of this research is to evaluate interactions in phosphorus availability of spring wheat from SS and its thermally treated derivatives, when added on its own and in combination with raw SS as soil amendment. Two pyrochars were produced at the Polytechnic University of Madrid though pyrolysis at 400⁰C-1h and 300⁰C-1h using pre-oven dried (105⁰C-48h) sewage sludge from Spain.  Two hydrochars were obtained through Hydrothermal Carbonization in another reactor at 180⁰C-4h and 240⁰C-4h using raw sludge adjusted to 15% d.m. All samples were analyzed for physical-chemical changes and applied to the soil in a glasshouse experiment.

Results confirmed different degrees of carbonization through the selected treatments, gaining similar characteristics to sub-bituminous coals after pyrolysis and midpoint after HTC. A germination test indicated that the phytotoxicity of the raw material was reduced after all thermal treatments, with the best effect being through pyrolysis. However, P availability was reduced in all derivatives, 65.6% in Pyrochars and 41.5% HTC from the original SS.

 A 136 pots study with amended soils at different rates showed that despite P availability on initial conditions, after 3 months P became more available, being at least twice the amount found in the original soil, higher if the treatments were combined with additional wet SS (1:1). It also revealed a reduction of pH_initial=[7-8] to pH_final=[6-7] after harvesting and a slight increment on the Electrical Conductivity [0.15-6.7]µS/cm (max value 16.6µS/cm) probably due the different mineralization of the derivatives amendments and the washing of the materials through the soil profiles.

The data gathered with this research to date suggests that, the addition of the sewage sludge derivatives on their own indeed decreases the production of grain. However, with the combination of a commercialized sludge (SS2) at the highest rate, no negative effects have been reported after the first crop season. Derived pyrochar and hydrochar offer an alternative source of available phosphorus to mitigate the growing demand of mineral phosphorus reserves whilst providing at the same time a good base of organic matter for low fertile soils.