Biochar characteristics and its roles in optimizing anaerobic digestion

Abstract

Over decades anaerobic digestion (AD) has been successfully established as technology to treat organic wastes. The perspective of turning organic wastes into biogas, a source of renewable energy and profit, through a low-cost process, has gained increased interest around this technology. The AD as biological process is strongly dependent on the environmental conditions such as temperature, pH, presence of inhibitors, and nutrients content, that, in particularly unfavourable situations, can attribute to undesirable drop in performance and even for detrimental failures. Therefore, several strategies such as feedstock pre-treatment and process optimization have been developed to secure AD performance. However, these approaches do not remove inhibitor from the process, which may result in accumulation of the inhibitor and further destabilization of the AD system. From this point of view, it is beneficial to develop methods that remove and/or reduce the mobility and bioavailability the inhibitor within the digestion process. In this study, char derived from lignocellulosic wastes was added to anaerobic digestion to elucidate their roles on process performance and efficiency. The results of this analysis suggest that changes of AD performance (i.e. biogas production and intermediates concentrations) are strongly correlated to biochemical characteristics of char materials.

Furthermore, the impact of changes in inhibitor concentrations on the digester’s performance was observed, and found that inhibition tolerance of AD was improved by the presence of selected char materials. However, contrary to the initial hypothesis based on previous studies in related hydrochar application, the addition of RH hydrochar led to reduction in biogas production but an enhanced methane content in biogas was observed.

Except from batch AD tests, a larger-scale AD system (semi-continuous stirring tank reactor) was used to determine the possibility of biochar on enhancing digestate structure and reduce the sensitivities towards environmental factors such as pH and other inhibitors. The further outcome from scale-up study revealed that biochar had improved ammonium tolerance (1.59 mg NH4-N/kg) by recovering 12-16.4% biogas production. This study also investigated effects of iron impregnated-thermal activation on physiochemical properties of the resultant biochar. Effects of these modified biochars on gas production and volatile fatty acids concentration was also measured and significant changes was observed. Overall, char material had a positive effect on AD system stability and promoted bio-methanation during the long run operations.