Biochar as an additive in biogas production from food waste

The long-term anaerobic digestion (AD) of food waste is characterized by poor stability due to high accumulation of volatile fatty acids (VFA), which is caused by the deficiency of essential trace elements. The use of biochar as an additive in AD of food waste is an emerging trend for enhancing the biogas production. The biochar can provide a surface area for colonization by the microbial flora in the AD, adsorb metabolic by-products like ammonia, act as a source of trace elements and provide buffering against VFA accumulation, which would otherwise inhibit the performance of the AD. The aim of this study was to use biochar as a trace elements slow releasing agent to improve the bioavailability of trace elements and stability of biogas production from food waste, in batch and continuous (UASB) reactors. Leaching experiments were carried out to evaluate the ability of several biochars to leach trace elements at 30 °C. Three sets of batch AD tests were then carried out at an inoculum: substrate ratio of 1:1, at 30 °C for 6 days. The effect of different doses of the brewery residue hydrochar and the treated waste wood pyrochar were evaluated, where daily pressure increments were recorded and used to calculate the biogas volume. Two identical UASB reactors, control (R1) and (R2) amended with biochar, were operated simultaneously at 30 °C for 40 days. The organic loading rate was increased from 3.4 to 7.8 g COD/L.day. Fe, Co, Ni, and Mn were leached in very small quantities from the five biochars (less than 10 mg/kg). The spruce wood pyrochar leached 13.2 mg/kg of Zn. High amount of K (1510 and 1969 mg/kg) was leached from the treated waste wood and the willow tree pyrochar, respectively. The 1.2 g and 0.2 g hydrochar dose treatment produced 438 and 435 mL/g COD of biogas, respectively. The 0.3 g and 1.2 g treated waste wood pyrochar dose produced 447 and 405 mL/g COD, respectively. The control without biochar addition produced 538 mL/gCOD, higher than all the biochar treatments. The food waste with 1.5 mL trace el ements supplement produced the highest biogas of 588 mL/g COD and 48% CH4 composition. The pH in all the treatments with the biochar reduced to levels that inhibited the methanogenesis step due high accumulation of VFA in the reactors of up to 9.3 g/L acetic acid. The average COD removal efficiency of the control reactor (R1) was 47% and 77% for the reactor with biochar amendment (R2) at an OLR of 6.9 to 7.8 g COD/L.day. The CH4% composition in the biogas was 39% for R1 and 65% for R2. From the batch experiments, the brewery residue hydrochar and the treated waste wood pyrochar did not improve the biogas production, and CH4% composition when compared to the control without the biochar addition even after the pH buffering with 4M NaHCO3. However, the high COD removal efficiency and CH4% composition observed in the continuous AD showed that the UASB reactors when amended with the treated waste wood pyrochar can be used for biogas production from food waste