Composting potential of the solid fraction of digested pulp produced by a biogas plant

Agricultural biogas plants produce biogas and residues referred to as digested pulp. In Europe, digestate is mostly used as fertilizer for agricultural soils although it can be converted to solid fertilizer and also used for biofuel production. The aim of this study was to evaluate if solid fraction of the digestate (DSF) is a suitable material for composting. Composting of DSF was performed for 51 d in a specially designed and constructed system of composting bioreactors (each with volume of 165 l). The process was controlled by daily measurements of temperature and gaseous emissions and samples of composted material tested for dry and organic matter, pH and conductivity. The maximum temperature of the composted mixture was 70.2 °C. High temperature (over 60 °C) for at least two days will allow to annihilate most pathogens. Dynamic temperature rise was related to significant decrease in organic matter (by 39.6%) - from an initial 8.46 kg to 5.12 kg at the end of the process. Decomposition of organic matter during the thermophilic phase of composting was also related to intensive emissions of CO2. The maximum values of carbon dioxide occurred at the highest compost temperature on the third day of the experiment and amounted to 13.3% in the chamber. The results of the analysed parameters allowed us to conclude that the composting process was operating properly and that composting of DSF can a solution for the management of biogas effluent.