Influence of pH on hydrothermal treatment of swine manure: Impact on extraction of nitrogen and phosphorus in process water

•Investigates the extraction of N and P from swine manure.•Hydrothermal treatment is performed using water, alkali, mineral and organic acids.•Extraction of P highly pH dependent but N less so.•Maximum P extracted at 170 °C in the presence of H2SO4.

This study investigates the influence of pH on extraction of nitrogen and phosphorus from swine manure following hydrothermal treatment. Conditions include thermal hydrolysis (TH) at 120 °C and 170 °C, and hydrothermal carbonisation (HTC) at 200 °C and 250 °C in either water alone or in the presence of 0.1 M NaOH, H2SO4, CH3COOH or HCOOH. Phosphorus extraction is pH and temperature dependent and is enhanced under acidic conditions. The highest level of phosphorus is extracted using H2SO4 reaching 94% at 170 °C. The phosphorus is largely retained in the residue for all other conditions. The extraction of nitrogen is not as significantly influenced by pH, although the maximum N extraction is achieved using H2SO4. A significant level of organic-N is extracted into the process waters following hydrothermal treatment. The results indicate that operating hydrothermal treatment in the presence of acidic additives has benefits in terms of improving the extraction of phosphorus and nitrogen.