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Magnesium ammonium phosphate (MAP) precipitation generally requires an external phosphorus source to increase the recovery of NH 4  + -N from biogas slurry. However, P-rich piggery biogas residue has been ignored as a phosphorus source. In this study, biogas residue was carbonized into biogas residue biochar (BRC), followed by acid leaching to synthesize functionalized BRC and release PO 4  3− -P from its ash as the phosphorus source. The effects of different acids on the leaching efficiency and morphological changes of P in BRC were investigated, and NH 4  + -N and PO 4  3− -P in the biogas slurry were recovered with functionalized BRC and MAP precipitation. The results showed that oxalic acid-hydrochloric acid mixed acid could leach more than 96% of P in BRC, while weakening the inhibitory effect of Ca 2+  on MAP precipitation. The BRC was mainly composed of inorganic P, and most nonapatite IP and apatite P (Ca 3 (PO 4 ) 2 ) were leached during acid leaching, with the latter more easily leached. Under optimal recovery conditions, the method had a significant recovery effect on NH 4  + -N (96.4%) and PO 4  3− -P (99.3%) in biogas slurry. The recovery of NH 4  + -N and PO 4  3− -P by functionalized BRC was mainly through chemical precipitation (forming NH 4 MgPO 4 ·H 2 O precipitate) while bonding with -OH, C = O and C-H functional groups. The final recovery product was also a BRC-based slow-release N-P fertilizer rich in struvite. This study solved the disposal problems of P-rich biogas residue and N-rich biogas slurry while providing an innovative technology for the resource utilization of faecal sewage at pig farms. Graphical Abstract

Biogas residue biochar integrated with phosphate from its ash for the effective recovery of nutrients from piggery biogas slurry