Aqueous Ammonia Soaking as a Pretreatment of Lignocellulosic Biomasses for Improving Manure-based Anaerobic Digestion

Abstract

Liquid manure is one of the most important sources of environmental pollution, contributing significantly to the anthropogenic Greenhouse Gas (GHG) emissions. The anaerobic digestion process of liquid manure is a mature technology that allows capturing emissions in the form of biogas, while simultaneously improving the characteristics of manure for soil application. Nevertheless, the anaerobic digestion process in biogas plants treating solely swine manure, is usually economically non-feasible due to the low conversion rate of the solid lignocellulosic fraction (manure fibers), and due to the high water and ammonia content. The pretreatment of manure fibers, or of other lignocellulosic biomasses, with Aqueous Ammonia Soaking (AAS) coupled to an ammonia recovery step, could potentially overcome these limitations when added to manure. However, the efficiency of AAS on increasing the methane yield of lignocellulosic biomasses may vary significantly depending on the conditions of the pretreatment applied. The main objective of this thesis was to evaluate the efficiency of AAS on improving the methane yield of swine manure fibers when applied under different conditions. The importance of different AAS parameters was tested and the most influencing factors identified were the NH3 concentration of the reagent, the duration of AAS and the solid-to-liquid (S:L) ratio. Heating up to 50°C during AAS did not produce any significant effect on the methane yield of pretreated fibers, allowing thus for a less energy intensive pretreatment process at ambient temperature (20°C). The AAS of swine manure fibers, depending on the conditions applied resulted in a variation of the methane yield from 90 to 214 ml/g TS after 17 days of batch anaerobic digestion. The optimal conditions for maximizing the methane yield corresponded to 7% w/w NH3, 4 days of AAS and 0.16 kg fibers/l reagent, resulting in average to a 244% increase of the methane yield after only 17 days of digestion. Nevertheless, a strong interaction effect among the NH3 concentration and the duration of AAS on the resulting methane yield was found, providing some flexibility to the process configuration. Empirical models were constructed for predicting the methane yield of manure fibers as a function of the levels of the AAS parameters. Based on these models, 5-11% w/w NH3 and 3.8-6 days of AAS duration can be applied in order to obtain 95% of the maximum increase of methane yield. The influence of the same AAS parameters as for manure fibers were also investigated on the methane yield of wheat straw, as this is an abundant lignocellulosic residue that could be considered for boosting the methane production of manure-based anaerobic digestion. The AAS of wheat straw at ambient temperature (20°C) resulted in a variation of the methane yield between 223 to 325 ml/g TS after 17 days of batch digestion, depending on the conditions of AAS applied. The NH3 concentration was found to be the most influencing factor for the efficiency of the AAS pretreatment of wheat straw, and the optimal conditions corresponded to 18% w/w NH3, 7 days of duration and 50 g/l reagent, resulting in a 43% increase of the short-term methane yield (after 17 days of digestion). Strong interactions were identified among the NH3 concentration and the duration of AAS, permitting a higher flexibility on the process configuration for increasing the short-term methane yield of pretreated wheat straw, as compared to swine manure fibers. According to the results obtained, a 95% of the maximum increase of methane yield can be obtained by pretreating wheat straw with 7.3-29% w/w NH3 concentration for 3.5-7 days. In an attempt to better understand how the biomasses investigated in this Thesis were affected by AAS under optimal conditions, an evaluation of the compositional changes that occurred due to the pretreatment was carried out. The results obtained, showed that no lignin removal took place on swine manure fibers, in contrast to wheat straw where limited removal was observed (9%). The hemicellulose faction of both biomasses was significantly solubilized (37% for swine manure fibers and 62% for wheat straw), an effect that is hypothesized to promote a better access of enzymes to carbohydrates improving thus the hydrolysis rate of the biomasses and their conversion to methane. The performance of the optimally AAS-treated manure fibers on continuous manure-based anaerobic digestion was evaluated, as compared to the digestion of manure enriched with untreated manure fibers. A significantly improved performance of the digester running on manure enriched with optimally AAS-treated fibers was observed in all aspects. Based on the experiments run, an 18% and 38% increase of the biogas productivity and methane yield respectively can be achieved by pretreating manure fibers under optimal conditions. Additionally, a higher reduction of all organic components (carbohydrates, lipids, proteins, and lignin) can be achieved as compared to untreated fibers, being cellulose the fraction most significantly affected (42% increased reduction efficiency). Overall, the results obtained in this Thesis, contribute to a better understanding of the potential and flexibility of the AAS pretreatment for ensuring high methane yields of the lignocellulosic biomasses tested. A systematic experimental procedure was followed for evaluating the effects of different AAS parameters on the methane yield of manure fibers, including the exploration, screening and finally optimization of the most influencing AAS parameters for ensuring high methane yields through anaerobic digestion. Wheat straw was considered as an alternative biomass for improving manure-based anaerobic digestion, and the effects of the AAS parameters on the methane yield of wheat straw were also extensively studied. Empirical models were produced for facilitating a techno-economic analysis of the AAS process of swine manure fibers as well as of wheat straw and for providing valuable information on the process flexibility and limitations prior to scaling up. Additionally, the compositional analyses of the optimally pretreated biomasses contributed to a better understanding of the mechanism of AAS under optimal conditions. Finally, the continuous anaerobic digestion experiments demonstrated that a higher reduction efficiency of the organic compounds is possible when swine manure is enriched with optimally AAS-treated manure fibers as compared to untreated manure fibers.