Anaerobic Digestion Of Different Feedstock Used At A Refurbished Gold Processing Mill For The Production Of Biogas

Abstract

Anaerobic digestion is a technology that is commonly used to treat organic waste for biogas production. Biogas can be used to replace fossil fuel as a source of energy for electricity production. However, despite the benefits of anaerobic digestion, the degradation of organic material via this process is complicated and requires proper operational stability and applications to optimise methane (CH4) production to promote its commercialisation. This bioenergy project rehabilitated disused mine structures and land and aimed to produce biogas via anaerobic digestion of energy crops planted on contaminated soil. This study investigated and characterised the factors that needed understanding and had to be overcome for the proper start-up processes and operational procedures of a biogas digester. The study was divided into three parts: (a) A case study was conducted for which industrial digesters were commissioned. During Phase I, anaerobic digester 2 (AD2) was seeded with rumen solid contents (RSC) in ambient temperature. To improve AD2, municipal wastewater treatment sludge (MWWTS) was added as the second seed. AD2 was soon considered dormant and remedial methods were applied during Phase II. These methods included pH amendment by addition of lime and sodium hydroxide (NaOH), reducing the total solids (TS) by transferring the contents into an empty anaerobic digester (AD1), and the addition of inoculums obtained from an operational digester (DFOD) and prepared from long-standing cow manure (used as cow bedding) (LSM). During Phase III, process advancement was observed without any additional parameter adjustments. (b) Laboratory revitalisation of dormant digesters was performed in 5 L digesters. These trials were performed after Phase I to determine the methods that could be applied to remediate AD2. Two methods for reviving the dormant digesters in batch laboratory tests were investigated, which included the addition of seeding material and increasing the retention time and temperature. The batch testing were divided into two trials: the first trial operated in psychrophilic and the second trial operated in mesophilic conditions (31.5±1.5ºC). (c) Biogas was produced from sugar beet roots (SBR), sugar beet leaves (SBL) and sorghum (SOR) used for phytoremediation of mine-impacted land. The co-digestion potential of RSC with SBR and SOR at ratios of 25:75 and 50:50 was investigated. Rumen fluid was used as inoculum in both mono- and co-digestion tests and the trials were performed in mesophilic conditions. The inoculation was declared a failure after Phase I because AD2 was overloaded. This resulted in a low CH4 yield, high volatile fatty acids (VFA), unstable alkalinity, and a pH of 4. AD2 inoculation failed because RSC as an inoculum was inapt to provide consortia of facultative and anaerobic microorganisms and the method chosen for seeding was unsuitable. Moreover, the incorrect treatment of RSC as inoculant led to regular congestion of the system. To achieve successful inoculation of AD2, a reduction of TS and the addition of LSM were accomplished whilst AD1 was successfully seeded by mixing DFOD with AD2 digestate. AD1 and AD2 performances improved after the introduction of inoculums because the CH4, VFA, pH and alkalinity in both digesters were at operational levels. During the laboratory revitalisation of dormant digesters, the digesters that operated in psychrophilic temperature took longer to produce biogas. This process was characterised by low CH4%, pH, gas production and higher VFA. The mesophilic digesters were characterised by lower VFA, higher CH4% and gas production and a stable pH compared with the psychrophilic digesters. Temperature affected the quantity of VS (volatile solids) that were degraded because a higher VS removal was achieved in digesters operated in mesophilic temperature. The introduction of LSM and DFOD in the mesophilic digesters improved gas production and reduced the lag phase of microbial growth. During mono-digestion of RSC and energy crops, a profuse amount of biogas was produced from each feed: SBL produced a mean CH4 concentration of 53% with a CH4 yield of 282.60 m3 t-1 VS. This was economical and thus offers an alternative to simply dumping this feedstock. RSC mono-digestion had a high CH4 yield of 399.66±1.47 m3 t-1 VS whilst co-digestion using RSC affected the biogas production positively, because the RSC:SOR (25:75) ratio mix had the highest CH4 at 65% and the highest CH4 yield of 515.45±4.91 m3 t-1 VS. It was concluded that a refurbished gold plant can be repurposed for biogas production, given that the correct inoculum is used and that proper process monitoring is practised. LSM and DFOD inoculums at different concentrations can be utilised to revitalise dormant digesters and are proficient in improving gas production, VS degradation, and reducing the lag phase of microbial growth of dormant digesters. This study proposes that the co-digestion of energy crops exposed to contaminated land with abattoir waste presents the possibility of rehabilitating soil and circumventing landfilling by organic waste, while at the same time producing bioenergy.