Recycling of Nitrogen and Phosphorus with Bone Biochar and Biogas Digestates from Abattoir Residues

The by-products of abattoirs may become valuable resources for nutrient recycling and energy generation by including pyrolysis and biogas production in the value creation chain. This study investigated the potential of bone chars as sorbents for ammonium in order to produce a soil amendment useful for fertilizing purposes. Ammonium enriched from the digestate by membrane distillation or from pure ammonium sulphate solutions accommodated the nitrogen sorption to the bone chars. The plant availability of the sorbed nitrogen was studied by a standardized short-term plant test with rye (Secale cereale L.).

The results showed that ammonium, both from biogas digestate of the abattoir and from pure salt solutions, could be sorbed successfully to the bone chars post-pyrolysis and increased the nitrogen concentration of the chars (1.6±0.3 %) by 0.2-0.4 %. This additional nitrogen was desorbed easily and supported plant growth (+17 to +37 %) and plant nitrogen uptake (+19 to 74 %). The sorption of ammonium to the bone chars had a positive effect on the reversal of pure bone char phytotoxicity and on nitrogen availability. In summary, this study showed that abattoir wastes are useful pyrolysis input materials to produce bone chars and to use biogas digestates as ammonium source for nitrogen sorption to the chars. This innovation offers the possibility to produce nitrogen-enriched bone chars as a new type of fertilizer that upgrades the known value of bone char as phosphorus fertilizer by an additional nitrogen fertilizer effect. The study also shows that abattoirs, too, may become contributors to circular economy by facilitating the recovery of nitrogen and phosphorus.