Improving biogas production by sugar beet silage co-fermentation: an approach for on-demand biogas energy

Biogas has become a well-established energy resource, especially through the use of renewable biomass i.e. energy crops. The advantages of using energy crops in biogas production are high biogas yield (Weiland, 2010), reducing greenhouse gas emission (Meyer-Aurich et al., 2016), increasing soil nutrients by crop rotations (Björnsson et al., 2013), mitigating disposal problems of agricultural residues (Zhang and Zhang, 1999). Unsolicited problems, however, were realized during mono-digestion of energy crops such as low buffering capacity (Braun et al., 2010), foam formation as well as various physical damages (Bachmann, 2015). Animal manure as co-feedstock is beneficial as it balances the missing nutrients, and therefore gas production rate increases (Comparetti et al., 2013). Nevertheless, the availability of manure can be locally limited due to increasing demand. Demirel and Scherer (2009) reported that about 15% of German biogas plants were operated without manure addition because of logistic problems. Manure limitation emphasizes to focus on another feedstocks and sugar beet silage is considered as an option, since sugar beet contains high alkalinity within the range of 2.5-6 g CaCO3-equivalents L-1 (Scherer et al., 2009). This thesis describes the effects of sugar beet silage in co-fermentation with fibrous substrates (Manuscript 1) and the potential role of beet silage in demand-based energy production (Manuscript 2). Different structural growth of microbial communities as well as their activities in anaerobic digestion using sugar beet as co-feedstock will be illustrated finally in the last part of this thesis (Manuscript 3). In Manuscript 1, the study investigated the effects of sugar beet silage in co-digestion with maize silage as well as grass silage, respectively. Grass silage is known as low degradable feedstock due to its low water-soluble carbohydrates and high proportion of proteins. Furthermore, low buffering capacity during mono-digestion of grass silage is asking for additional alkalinity to stabilize the process (Koch et al., 2009). Adding sugar beet silage showed a positive effect as it increased biogas yield with increasing amount of beet silage. The reason was attributed to the easily degradable components present in beet silage that released the energy while decomposing organic matter. Such energy would enhance the microbial metabolism and, as a consequence, the degradation of grass silage components is improved and leading to higher biogas yield. Such phenomenon, as mentioned above, is mostly implied in soil sciences where readily available carbohydrates decompose soil organic matter as they spur the overall microbial activity and is regarded as “priming effect” (Fontaine et al., 2003). It was found from this study that even small share of sugar beet silage (14% of volatile solids) increased the biogas yield by 44% in comparison to fermentation of grass silage alone. On the other hand, the effect was barely found during co-digestion of sugar beet silage and maize silage. In the future, biogas can be utilized as an attractive solution for demand-based energy generation. Demand-based energy is defined as the variable energy compensating the volatile production by solar or wind. Both solar and wind energies vary due to seasonal and meteorological scarcity, that has an impact on electricity production from these sources. Unlike other renewables, biogas production does not depend on weather conditions and can be easily stored, thus it opens a new aspect in terms of demand based energy production. In this context, different approaches have been tested in lab scale (Mauky et al., 2015; Persson et al., 2014). Among all, just-in-time biogas production with flexible feeding scheme seemed to be very effective as it minimized the requirement for additional biogas storage capacity (Mauky et al., 2015). Flexible feeding scheme aims to produce biogas in times of its demand and such effort can be met by introducing easily degradable substrates. In this context, sugar beet silage may have positive influence as it contains easily degradable compounds. In Manuscript 2, demand-based energy production was evaluated by testing sugar beet silage in co-fermentation with grass silage. The experiments were conducted in continuous mode (8hrs feeding interval) at two different organic loading rates (OLRs) of 1.5 kgVS m-3 day-1 (referred as low OLR) and 2.5 kgVS m-3 day-1 (high OLR), each with mixtures of grass silage and sugar beet silage at the ratios of 1:0, 3:1, 1:3 (based on volatile solids). The results showed that maximum biogas production rates were reached within short time after feeding in the reactors with sugar beet silage, but were noticeable only at high OLR. Also, these findings elucidated similar lag time required to reach maximum biogas or methane production after feeding, irrespective of OLRs. It was obvious that adding sugar beet silage to low degradable substrates (e.g. grass silage) stimulated degradability of the latter and increased overall biogas production. So, it can be concluded from the study presented in Manuscript 2 that such feedstocks can meet demand-based energy requirements under preferential use of high OLR. Manuscript 3 illustrated the structural changes of microbial communities in anaerobic digestion supplied with the feedstocks, maize silage and sugar beet silage, at different ratios (volatile solids based) of 1:0, 6:1, 3:1 and 1:3 at OLR 1.25 kgVS m-3 day-1. Both bacterial and archaeal communities shifted with increasing amount of beet silage. In respect of bacterial communities, family of Clostridiales was mainly dominating in the reactors with sugar beet silage and was escalating while sugar beet silage increased. It was demonstrated by Klang et al. (2015) that family of Clostridiales bacteria was mainly involved in the degradation of sugars during anaerobic digestion. Similar phenomenon can be realized in this study as the abundance of family Clostridiales was increasing in the reactors while increasing sugar, one of the main components of sugar beet. In respect to archaeal community, different members of acetoclastic archaea were observed in the relation to beet silage. However, Methanosaeta species were dominating in the reactors fed with maize silage mainly, whereas Methanosarcina species were controlling in the addition of higher amount of beet silage, i.e. at 3:1 and 1:3, respectively. Similar findings were also found by Klang et al. (2015) who mentioned that Methanosaeta and Methanosarcina were prevailing during anaerobic digestion of maize silage and sugar beet silage, respectively. Most surprisingly, irrespective of diverse microbial communities, similar biogas production rates were realized in all reactors. It was shown that microbial communities stabilized the digestion process by optimizing their metabolism under different feedstocks compositions. This work was a cooperative study between the Institute of Microbiology and Biotechnology and the Institute of Systematic Botany and Ecology, whereas this thesis concentrated mainly on process parameters and biogas yield. In summary, this thesis elucidated, as explicated in the above mentioned manuscripts, different positive aspects of sugar beet silage as a co-feedstock with fibrous substrates (such as grass silage). Because of easy-to-degrade characteristics, sugar beet silage stimulates the hydrolysis process in co-fermentation with the grass silage. Most precisely, adding beet silage increases the cellulolytic activity of microorganisms that may enhance the hydrolysis of lignocellulose components, mainly cellulose, in grass silage. Indeed, sugar beet silage shows a better option in comparison to animal manure to enhance the biogas production of grass silage in co-digestion process. It has also the advantage for the production of demand-based energy. In the case of co-digestion of sugar beet silage with the grass silage, easy-to-digest compounds from beet silage could promote the anaerobic degradation of grass silage. Thus, the surface area of grass silage would be increased permitting microorganism to access. Thereafter, the mixtures of beet silage and grass silage in anaerobic digestion will boost the biogas production within short time. Such co-digestion can be applied to produce biogas for generating electricity when it is required. Furthermore, the analysis of microbial function at different feedstocks showed that microbial communities adapted towards optimal biogas production. Even though, different structural growth of microorganisms, irrespectively bacteria or archaea, are evolved during anaerobic digestion based on feedstock compositions