Valorization of swine manure for a circular approach through hydrothermal carbonization

In recent years, swine farming has changed from small to concentrated farms, posing environmental and economic problems for manure management and disposal. Considering this background, this work evaluates innovative scenarios where swine manure (SM) is treated through hydrothermal carbonization (HTC) in view of utilizing the liquid streams for on-site fertigation and the hydrochar for off-site biogas production through anaerobic digestion (AD). The scheme proposed (on-site HTC and off-site AD) allows to avoid nitrogen and phosphorus leaching risks associated with the direct use on land of the raw manure, and in addition allows for energy recovery through biogas production. HTC was performed at 190 ◦C for 1 h at three different SM dry matter concentrations, expressed as total solids (TS). After HTC, the dewaterability of SM strongly improved, above all for thickened SM samples (10% and 15% TS). The soluble chemical oxygen demand greatly increased confirming the hydrolysis speed up of the HTC. Phosphorus contained in the SM accumulated in the hydrochar while total Kjeldahl nitrogen decreased in HTC liquor, resulting suited for fertigation. AD tests revealed a significant biogas potential of the hydrochar. The scenario with 15% TS SM fed to the HTC systemresulted the most energy efficient, when considering both the energy input to the HTC process and the energy output from AD; in the case when HTC and AD were both carriedout on-site, the proposed scheme would allow covering 42% of the energy required for the coupled processes.