Critical evaluation of biochar effects on methane production and process stability in anaerobic digestion

Abstract

Biochar is an emerging biomaterial for managing residual biomass while simultaneously sequestering carbon. To extend the biochar value chain, applying biochar to enhance anaerobic digestion (AD) processes is gaining attention in the context of a circular economy and cascading use of biomass. However, the comparative effects of various biochar dosages under normal and severe AD conditions are still unclear. To further our understanding of its potential application, this work investigated the impact of adding various biochar dosages on AD processes under normal and high substrate loadings. Three inoculum-to-substrate ratios (ISRs): one representing normal substrate loading (ISR 2) and two representing substrate overloading (ISR 1 and 0.5) were investigated. Each substrate loading rate was tested with a biochar dosage of 0% (control), 10%, and 25% based on substrate volatile solids. The results revealed that under the severe condition of high substrate overload (ISR 0.5), a high biochar dosage of 25% significantly increased cumulative methane production by 5.6% (p = 0.06) when compared to the control. Under the same condition (ISR 0.5, 25%), the time required to achieve a particular extent of ultimate methane potential was significantly reduced (p = 0.04), indicating that the methane production rate was increased. At ISR 0.5, the increase of process stability was also significant with 25% biochar addition, while the control (0%) and 10% biochar addition exhibited high variance among replicates. However, biochar did not affect AD processes under normal substrate loading (ISR 2) and mild substrate overload (ISR 1). Thus, a positive effect of biochar on the AD process was only observed under severe conditions with the highest biochar dosage. Future works should consider optimising substrate loadings and biochar dosages under real conditions when testing the practical application of biochar addition in AD processes.

Characterization of biochars of different origin and application to the anaerobic digestion of source-selected organic fraction of municipal solid waste under batch conditions and at different dosages

“…Frontiers in Chemical Engineering frontiersin.org more similar experiments, Hu et al (2023) found that BC addition to AD only had significative positive effects on methane production when the process operated under inhibitory conditions and at high BC doses (25%). In that scenario, BC addition to AD would not be economically profitable per se, as the improvement in methane yield would not compensate the BC addition cost and BC would occupy an important portion of the digesters working volume.…”

Improving acid-stressed anaerobic digestion processes with biochar - towards a combined biomass and carbon management system

“…A blank test with inoculum only and a positive test with cellulose as substrate were also conducted but without replicates in the remaining two bottles. To simulate a process condition of organic overloading, the inoculum-to-substrate ratio (ISR) based on VS was chosen to be 0.5, while a non-stressed condition is usually with ISR 2 (Holliger et al 2016, Hu et al 2023. The biochar was added at a ratio of 25% according to the previous study (Hu et al 2023).…”

“…To simulate a process condition of organic overloading, the inoculum-to-substrate ratio (ISR) based on VS was chosen to be 0.5, while a non-stressed condition is usually with ISR 2 (Holliger et al 2016, Hu et al 2023. The biochar was added at a ratio of 25% according to the previous study (Hu et al 2023). This ratio represents the relationship between biochar total solids and substrate volatile solids and was equal to 4.1 g TS /L.…”

“…With the embedded real-time data acquisition system of the Gas Endeavour, both the biogas and methane volume were recorded and normalised to the standard temperature (0 °C) and pressure (1 atm) automatically. A more detailed description of the experimental setup and procedure can be found in the previous study (Hu et al 2023).…”

“…The earlier study conducted by Hu et al (2023) has identified the conditions for positive BC effects on AD, i.e. at a high biochar addition rate and under substrate overloading.…”