Optimization of Nutrient and Carbon Recovery from Anaerobic Digestate via Hydrothermal Carbonization and Investigation of the Influence of the Process Parameters

Abstract

Within this paper the influence of the process parameters time, temperature and pH on properties and composition of hydrochars from anaerobic digestate is investigated using statistical methods. Response surface method was applied to assess the optimal process conditions for nutrient recovery. Additionally, the chemical structure of several hydrochar samples was analysed via solid state NMR to make statements about the process parameters influence on the built up of the char. It was shown that optimal conditions for carbonization and for nutrient recovery require conditions that are divergent in temperature and pH. Carbonization is maximized at a temperature of 230 °C and pH of around 3. Phosphorus recovery shows its highest values at a pH of 8 and a temperature of 165 °C. Nitrogen recovery is maximized at a pH of 3 and a temperature of 165 °C. Maximized nitrogen recovery at a low pH is opposite to the trend expected. This result is explained by the pH and temperature dependent equilibrium of solved ammonium and free ammonia. Changes in the chemical structure of the hydrochar start at carbonization-temperature of 220 °C. The results show that cellulose could be completely degraded and anaerobic digestate could be fully carbonized at severe reaction conditions.