Use of biochar in food waste anaerobic digestion: is there a dichotomy between process stability and hygienization capacity ?

International audience

English. Introduction: In the frame of regulation CE 1069/1009, the European Union requires food waste (FW) to be pasteurized before anaerobic digestion (AD) to eliminate potential pathogens. AD of FW produces high amounts of total ammoniacal nitrogen (TAN) and volatile fatty acids (VFA) which provide a pathogens hygienization capacity. These chemical condition are however hazardous regarding process stability and adjuvants such as biochar can be used to mitigate them. Methodology: One-week successive batches were performed at 37°C in 12 bottles with a 250 mL working volume. The inoculum originated from a FW-processing AD plant and the substrate consisted of household FW fed at a rate of 5 gVS/L.d. Biochar at the concentrations of 0, 1 or 2% (w:w) was added to the bottles. After 10 successive batches, the TAN concentration was increased in half of the 2% biochar bottles using (NH4)HCO3. At the start of the 15 th batch, the bottles were spiked with a solution of digestate-isolated Escherichia coli at a final concentration of 10 6 CFU/mL. Samples were collected regularly during a 14-day batch to assess E. coli survival through qPCR and to evaluate the effect of TAN and VFA. Results: A higher VFA accumulation (mainly butyrate and isovalerate) was observed in the bottles without biochar. TAN level was around 5.0 and 3.0 gN/L in the TAN-supplemented and non-supplemented conditions, respectively. Such levels are commonly encountered during AD of FW. Regarding E. coli concentration, the low-TAN conditions with biochar showed a growth phase followed by a plateau during the first 3 days before an inactivation kinetic began. The low-TAN condition without biochar also showed a growth phase but followed by a quicker decimal reduction compared to the conditions with biochar. However, the high-TAN condition with 2% biochar showed the best hygienization kinetics without any growth and with decimal reduction of 2.8 log after 14 days. Conclusions: While biochar had a VFA accumulation reduction effect, it did not seem to impact the TAN content. Hence, biochar addition could interfere with AD hygienization capacity by reducing the VFA accumulation, but this could be offset by a sufficient TAN content that is easily achievable due to the high protein content of FW. Acknowledgements: Theses results originate from the Biogaz RIO project which is jointly financed by the Occitanie Region and the European Regional Development Fund (funding n°24001371). The experiments were conducted at the SUEZ BioRessourceLab and at the LBE Bio2E platform