Influences of temperature and reaction time on nutrient conversion and metal interactions during hydrothermal treatment of pig manure

Pig manure is a renewable source of nutrients, such as phosphorus and nitrogen, that can be used to produce fertilizers. Hydrothermal treatment experiments using real pig manure were conducted to investigate the effect of temperature (107-200 °C) and reaction time (25-95 min) on nutrient conversion and distribution into hydrochar and process water. Sequential extractions were also performed to determine the phosphorus speciation in raw pig manure and hydrochars. The results showed that phosphorus and nitrogen recovery in the hydrochar was consistently above 94 % and 56 %, respectively, for all the experiments. Phosphorus content in the hydrochar increased from 15.4 to 24.6 mg P/g by increasing temperature and/or reaction time. Sequential extractions showed the increase in temperature led to a decrease in non-apatite inorganic phosphorus and organic phosphorus fractions in the hydrochars, while the apatite inorganic phosphorus fraction (Ca phosphates) increased. The concentration of total dissolved nitrogen in process water increased to values higher than 6000 mg N/L, while the ammonium concentration decreased to values lower than 2000 mg N/L by increasing temperature and/or reaction time. These results appear to indicate that the increase in hydrothermal temperature from 107 to 200 °C promoted the extraction and dissolution of organic nitrogen compounds from solid matrices to process water. This study provides valuable insights into P and N transformation during hydrothermal treatment of pig manure, which represent crucial information for developing sustainable treatment processes that aim to minimize waste disposal while enhancing the closure of anthropogenic P and N cycles.