Hydrolysis inhibition of complex biowaste

The increasing demand of renewable energy sources and reuse of wastes, challenges our society for better technological solutions for energy production. Co-digestion of agricultural biowaste, such as animal manure and plant residues, offers an interesting contribution to the renewable energy strategies. The biogas plants, where the complex substrates, such as agricultural biowaste, get converted into biogas, are then able to produce electricity and heat, which can be used in the farm and delivered to the main electricity grid. Moreover, due to its decentralised nature, the implementation of small-scale biogas plants can supply renewable energy to people without the need for large-scale infrastructural networks such as electricity grids, thereby solving part of the populations’ energy demands. The production of biogas from complex biowaste is rate-limited by the hydrolysis step of the anaerobic digestion process. However the hydrolysis step has been poorly described and not very well understood, resulting in non-optimized anaerobic digester volumes. Due to that, a review on the anaerobic hydrolysis step is in this thesis presented, together with ways to accelerate the hydrolysis, either by mitigating the revealed inhibiting compounds, by pre-treating difficultly hydrolysable substrates, or as is nowadays also applied, by adding hydrolytic enzymes to full scale biogas co-digestion plants. In this thesis two compounds were studied in terms of its inhibiting effect on hydrolysis: ammonia nitrogen and Humic Matter (HM). Ammonia nitrogen did not show an inhibiting effect on anaerobic hydrolysis. On the other hand Humic acids-like (HAL) and Fulvic acids-like (FAL) extracted from fesh cow manure and silage maize, and in this thesis extensively described in terms of its chemical characteristics, showed a strong inhibiting effect on the hydrolysis step. Plant matter is high in lignocellulosic biomass. Lignocellulosic biomass consists of lignin, which is resistant to anaerobic degradation, cellulose and hemicelluloses. Pre-treatment of plant material, is particularly important in order to increase biogas production during co-digestion of manure. Calcium hydroxide pre-treatment was shown, in this thesis, to improve the biodegradability of lignocellulosic biomass, especially for high lignin content substrates. Maleic acid generated the highest percentage of dissolved COD during pre-treatment, however its high market price makes it not so attractive as calcium hydroxyde. Enzyme addition has recently gained the attention of biogas plants’ operators in order to accelerate hydrolysis, however further research is needed.