Ανάπτυξη διβαθμίου συστήματος παραγωγής βιοαερίου από στερεά απόβλητα και βιομάζα

Abstract

In our days anaerobic digestion has received an increasing interest, as it is an effective method for the biological treatment of a variety of organic wastes, by degrading the organic matter and converting it into energy in the form of biogas (a mixture of methane and carbon dioxide). The use of biomass, especially energetic plants, as substrate has been proved that could yield a high biogas production. In this research work, the anaerobic treatment of the solid/liquid wastes from alcohol fermentation of sweet sorghum, but also the use of the cultivated sweet sorghum as substrate was investigated, in order to study the hydrolysis and degradation rates of organic matter and the ability of anaerobic systems to produce energy in the form of biogas using such solids substrates. The main aim of this work was the development of an innovative two-stage anaerobic digestion system for solid wastes and biomass, in which hydrolysis and methanogenesis was taking place in two different bioreactors (a hydrolyser and a methanizer) respectively. Hence, it was possible to investigate for each separate stage the optimal operating conditions and parameters that affect the anaerobic digestion process with the intention to maximize the biogas production. The sweet sorghum residues stream, originating from the alcoholic fermentation of sweet sorghum and the subsequent distillation step, contained high concentration of solid matter (9% TS) and thus could be characterized as a semi-solid, not easily biodegradable wastewater with high COD (115 g/l). At first, the possibility of direct hydrolysis and digestion of bioethanol process sludge (sweet sorghum residues) in a single-stage system was examined. Optimal conditions for the anaerobic digestion of this particular waste were determined using different organic loadings, concluding that solids hydrolysis was the process limiting step. Thus, in order to optimize the process performance, it was suggested to separate the solid and liquid phases of the wastewater and to treat the two streams under different operating conditions. Hence, a novel two-stage anaerobic bioreactor system consisted of a thermophilic hydrolyser and a mesophilic high-rate methaniser was made. The application of the proposed two-stage configuration achieved a methane production of 16 l/l wastewater under a hydraulic retention time of 19 days. Energetic plants such as sweet sorghum are a promising renewable energy resource. The energy contained in the chemical bonds of carbohydrates could be converted to fuels such as methane through anaerobic digestion. The anaerobic conversion of sweet sorghum to biogas was studied using the novel two-stage bioreactor system. Since a large portion of carbohydrates in sorghum were easily extractable, a water extraction step was preceded. The extracted liquid portion of sweet sorghum, rich in COD (14-34 g/l) and the remaining solid portion with 20% total solids and high COD (~1,20 g/g VS), were treated successfully in a two-stage anaerobic digestion system achieving a solids hydrolysis of 70-80% with a high simultaneous methane production on the order of 0,63 l/l reactor/d under a hydraulic retention time of 22 days. It could be concluded that using a two-stage anaerobic digestion system in treatment of organic materials with high solids concentration, performs efficiently in hydrolysis of solids and production of biogas and could be employed for energy production from biomass (such as energetic plants). Finally, a study over the use of other microbial biomasses, such as cow manure, which seems that has particular properties that improve the anaerobic digestion yields during processing of lingocellulosic materials, is proposed.