Comparison of anaerobic digestion approaches using selected fibrous and non-fibrous organic waste

Abstract

While biogas production through anaerobic digestion (AD) is becoming more auspicious as a sustainable approach for energy production and waste management, finding a suitable feedstock and the right anaerobic digestion approach that results in maximum biogas production is still a challenge. Generally, any feedstock that contains carbon and hydrogen can be used in anaerobic digestion. However, some feedstock such as fibrous materials (lignocellulose materials) poses a challenge due to their complex structure that offers recalcitrance during hydrolysis, even though they have the potential to yield higher biogas yields. On the other hand, non-fibrous materials that have been explored such as abattoir waste also pose a challenge because they are oftentimes high in nutrient content which introduces an imbalance in the C:N ratio and thereby hinders the process. Several anaerobic digestion approaches have been suggested to deal with these hurdles, such as the pre-treatment of feedstock prior to anaerobic digestion and the co-digestion of fibrous and non-fibrous feedstock. The aim of this study was to compare different anaerobic digestion approaches using Napier grass as the fibrous feedstock and abattoir waste as the non-fibrous feedstock. To achieve this aim, biomethane potential tests were carried out to compare the biogas yields between non-pre-treated Napier grass and thermally pre-treated Napier grass (TPN) that was treated with heat in an autoclave at 151℃ for 15 minutes. Biomethane potential tests were further used to compare the biogas yield when thermally pre-treated Napier grass was co-digested with abattoir waste at ratios of 1:1, 1:2 and 2:1 AW:TPN. Central composite design was used to find the optimum conditions for maximum biogas yields using a three-factor level design (operation temperature, co-digestion ratio, inoculum substrate ratio) which gave rise to 20 experimental runs and was conducted using BMP for 30 days. The obtained experimental results were then fitted into the model and a second order polymonial equation was obtained. The conditions that resulted in maximum biogas yield from the optimisation were further tested for their feasibility when the process was scaled up in a 5L single-stage batch reactor that was allowed to take place for 30 days. Furthermore, the bacterial community present in the 5L single-stage batch reactor was studied. Inoculum samples were collected on day 1 and day 30 of the experiment and sent to Inqaba Biotech laboratory for 16s ribosomal ribonucleic acid (rRNA) analysis to compare the different bacterial communities present.

At 38℃ mono-digested thermally pre-treated Napier grass yielded the highest biogas yield of 70.3 Nml/g•VSadded while mono- digested raw Napier grass accumulated the least biogas of 46 Nml/g•VSadded. Moreover, pre-treated Napier grass accumulated a total of 72% of methane while raw Napier grass only accumulated 61% of methane. The co-digestion ratio that proved to be the most effective was 50:50 which resulted in a total of 117 Nml/g•VSadded biogas yield. The optimum range was determined to be 35℃ with a 50:50 co-digestion ratio and an F/M of 5 with predicted biogas yields of 188 NmL/g•VSadded. This optimum range was still feasible even after the conditions were scaled up in a 5L single-stage reactor. Finally, microbial analysis showed that the phyla present in the process confirmed consistency on both day 1 and day 30 even though there were discrepancies in read count, with day 1 having a higher read count than day 30. This study highlighted some of the possible options that can result in optimum biogas yield when using Napier grass and abattoir waste; moreover, it highlighted the most effective options which led to the creation of templates that could be used in future studies when researching biogas using Napier grass and abattoir waste as substrate. Even though these optimums may be applicable when using other substrate other than Napier grass and abattoir waste, they may not necessarily be applicable to all potential biogas substrate.