The use of biochar as strategy to mitigate the NH3 emissions during the digestate storage

Introduction The environmental impact of manure storage in terms of ammonia (NH3) emissions has recently become a subject of greater interest. In this context, a common mitigation strategy is to cover the surface of the storage tank with a variety of floating materials, including straw, sawdust, clay, and oil. A common issue associated with the use of covers is the financial cost and the necessity for maintenance over time. Biochar has recently been investigated as an alternative to floating covers due to its durability and hydrophobicity. This work examines the potential of biochar as a floating cover to reduce NH3 emissions, providing an overview of different biochar applications, in terms of dosage and biochar characteristics. Material and Methods The simulation of digestate storage was conducted in a climate-controlled room, utilising 5 L glass vessels. Buffalo digestate was collected from an anaerobic digestion plant in Southern Italy and analysed according to Apha (2005). First and second tests were performed with a commercial biochar, produced via pyrolysis (30 minutes) at 550 °C, using as feedstock mixed wood chips from Piedmont. Three different biochar applications methods were compared: 140 g (2 cm layer) and 70 g of biochar applied on the surface, and 140 g of biochar mixed with the digestate. In the third test, biochar was produced in the laboratory, using two different pyrolysis temperatures: 285 and 450 °C. Wood chips of Poplar tree (Populus nigra) were used as feedstock. A 2 cm layer was adopted. NH3 emissions were monitored by means of the dynamic chamber technique, as described by Scotto di Perta et al. (2022). After tests, N adsorbed during the manure storage tests by the biochar was evaluated, as described by Scotto di Perta et al. (2024). Results and Discussion Biochar dosage and application method When the biochar layer is floating and compact, it reduces 78% NH3 released into the atmosphere with 2 cm layer thickness, since it introduces an additional resistance to the gas transfer. This aspect is even more effective than the N-NH4 + adsorption. Moreover, the results show that a 43% reduction in NH3 emissions can be achieved by using biochar as a floating cover of 2 cm rather than its mixing with digestate (Scotto di Perta et al., 2024). Pyrolysis temperature Biochar obtained at 450°C was characterized by the higher specific surface of the biochar, this can explain its higher N-NH4 + adsorption (2.86 ± 0.75 mg/gchar), since the possibility of interaction of the biochar with the ions present in the solution increases. On the other hand, the greater adsorption capacity of biochar at 450°C does not seem to have played a relevant role in the reduction of NH3 emissions, confirming that the "lid" action is prevailing in limiting the gaseous exchange. Biochar produced at 285 °C is more hydrophobic and durable, thus it emitted 42% less than the other one (Scotto di Perta et al., 2022). Conclusions In conclusion, the results demonstrate that the application of biochar on the manure surface is an effective method for reducing NH3 emission by up to 78%, with a 2 cm layer thickness. Furthermore, biochar functions as an adsorbent material. Biochar produced at a lower temperature resulted in a greater degree of hydrophilicity thus, a more durable product when applied as a floating cover.