(619ai) Biomethane Potential Determinants of Different Agro Industrial Substrates

Anaerobic digestion is a promising technology offering the opportunity for conversion of biomass into biogas, a renewable fuel that can be used for energy generation in internal combustion engines, microturbines and boilers. Throughout the years this technology has been applied for the valorization of waste streams including but not limited to manure, olive mill waste and municipal sludge. While the method is offering a promising pathway for generation of biogas, it may suffer by low efficiency, inhibition by product or substrate accumulation and lack of available enzymes.
The literature is rich with studies describing the biomethane potential of comparable (according to the presented analysis) substrates where the efficiency and the recovered yields showed significant variation. For example Pitk et al. (2013) studied the biomethane potential of different slaughterhouse wastes. The sterilized mass (i.e. bone and meat meal) used in this study achieved a production of 721 mLCH4gVSadded. In contrast Wu et al. (2009) reported a yield of only 381 mLCH4gVSadded while assessing bone and meat meal. Furthermore, Ebner et al. (studied the digestion of cattle manure and reported a biomethane potential of 238 mLCH4gVSadded which is at least 30% higher to this described by Amon et al. (2007). Bayr et al. (2012) assessed the biomethane production of fat and reported yields of upto 403 mLCH4gVS FM (FM = 935g/Kg, 100%VS) corresponding to approximately 430 mLCH4/gVSadded. In contrast to this, Martínez EJ et al. (2016) reported yields higher than 800 mLCH4/gVSadded. Despite the fact that all these values are well within the theoretical limits, the reason of the variability is not easily distinguished. While the variations can be the outcome of differences in the influent substrate and the presence of inhibitors, other technical issues and lack of standardization cannot be excluded.
In this work we investigated the anaerobic digestion of four substrates (animal fat, wheat straw, sterilized slaughterhouse wastes and glucose). The different substrates selected were representative of the most common feedstock used in industrial digesters.
The substrates were assessed in batch vials with the reaction time set to 30 days under mesophilic conditions. The loading rates applied were controlled to range between 10 and 20 kgVS/m3, while the inoculum to substrate ratio between 2 and 0.5. Furthermore, the experiments were divided into two stages with two complete sets of vials inoculated with different inoculums sourced from reactor operating under different loading rates and feed composition regimes. The last experiment targeted the recovery of data from where information related to the importance of the initial inoculum can be extracted.
According to the results the source of inoculum appears as the most important factor for the AD process. The inoculum from the reactors treating the agro industrial waste (Inoc A), was able to offer a robust process when fat was used as substrate with only the 0.5 inoculum to substrate ratio failing. On the other hand, when the inoculum recovered from the reactors treating manure (Inoc B) was used only digestion where the lowest loading rate applied was successful. In this case the ultimate yield was at least 25% lower compared to this achieved by Inoc A. The same is true for the case of the sterilized mass where the daily productivities of the batches with Inoc A were up to 100% higher than these offered by Inoc B. On the other hand when sugars were digested Inoc B trsulted in higher yields and productivities compared to Inoc A. The second important parameter is the initial inoculum to substrate (I/S) ratio. A low I/S ratio can delay the biomethane production and in some cases inhibit the overall process which can be easily described by the limited number of available cells in the inoculum and the detrimental effect that an inhibitor can have in an unbalanced biological process. The least important factor among the ones assessed is the initial organic load per batch. Loadings up to the 20 kgVS/m3 can be sustained by AD systems treating slowly hydrolyzed substrates. On the other hand, addition of 20kgVS/m3 of simple sugars into AD systems can inhibit the process through intermediate product accumulation, manifesting the inherent problem of the process and the different rates of the biochemical reactions involved.

Our analysis underlined the critical role that inoculum (both in terms of its source and in terms of its relative concentration in the reactor) plays on the determination of the overall methane production efficiency in anaerobic digestion systems. Further work will investigate the microbial and biological profile of the system to determine optimal operational regimes as a function of the performance determinants analyzed above.

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