Anaerobic digestion process using membrane integrated micro aeration
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Anaerobic digestion, AD, is regarded as a key environmental technology in industrial, agricultural, and domestic sectors for integrated solid and liquid waste treatment and renewable energy production. The main objectives of this thesis are to investigate effects of micro aerating anaerobic reactors using semi permeable membranes, study biofilm effects on mass transfer and monitoring transient responses following intermittent feeding. It is hypothesized that this approach can enhance the anaerobic digestion while supplying a novel approach for process monitoring and control. A bioenergetic model is developed to study effects of micro aeration on anaerobic processes. Mass transfer characteristics of oxygen via dense polymeric membranes under varying operating conditions are determined using experimental and mathematical approaches. A new micro-aeration strategy called “Membrane Micro-aerated Anaerobic Digester” (MMAD) is developed based on this. Membrane loops with circulating deionized water with dissolved oxygen inside are used to supply oxygen to biofilms in an anaerobic bulk liquid phase in AD. Biofilm behavior and effects are investigated using in–situ oxygen transfer measurements and experiments in an external reaction chamber. Shock loading effects and transient responses are studied in lab scale batch and intermittently fed AD experiments and a full scale co-digestion plant. Anaerobic Digestion Model No.1 (ADM1) is used to simulate transient loading effects. The bioenergetic analysis of micro-aeration found that the CH4 / CO2 ratio of biogas remain stable up to an oxygen supply of 0.13 mole O2/ feed C-mole. Above this threshold limit, biogas composition deteriorates due to the dominance of the aerobic degradation pathways. It is proposed to define micro-aeration as oxygen supply below this threshold. Average overall mass transfer coefficients determined for the thick and thin silicone tubular membranes applied, are 5.9x10-5 and 8.6x10-5 m/min, respectively. Membrane resistance contributes about 5 % of the total oxygen transport resistance for both membranes. The bulk liquid side resistance contribute about 70 % of total resistance and the remaining 25 % is caused by the inside stagnant liquid layer. Biofilm oxygen utilization can reduce the liquid side resistance when the biofilm is thin (young). Thicker mature biofilms can lead to lower oxygen transfer rates (OTR) due to mass transfer limitations for the substrate from the bulk liquid into the biofilm. OTR and oxygen utilization rates, OUR, were not influenced by the aeration history of the membrane biofilms. All the biofilms grown under varying aeration levels (from zero to continuous for months) consumes oxygen at similar rates when exposed to aerobic conditions. Facultative heterotrophic micro-organisms must therefore be abundant in all these biofilms and OUR is probably transport rather than reaction limited. The shock load investigation shows that negative effect of intermittent feeding is a function of the feed load to reactor biomass ratio. At higher ratios, specific biogas production decreases, pH drop increases, ratio of CH4/CO2 decreases and the recovery time increases. The ADM1 simulates transient responses quite well at low ratios, but the correlations between experimental and simulated results deteriorate as shock load increases. This is mainly due to limitations of pH predictions in ADM1. Measuring oxygen in the membrane lumen of MMAD as a monitoring technique for these transient responses was of limited value. The oxygen level dropped fast after feeding, implying that the method can detect shock load episodes well. The oxygen drop was, however, not much influenced by the magnitude of the shock load. The reason for this is that OUR is transport rather than reaction limited under most relevant conditions. The literature survey and theoretical analysis revealed that the MMAD principle can help stabilize AD exposed to transient loads through oxygen supply and CO2 removal. Biofilm was not observed on the lumen side of membranes applied but it developed quickly on membrane‟s bulk liquid side. Oxygen was transferred to the MMAD head space when clean membranes were introduced, but after 3 days no more oxygen was found in head space. This is explained by complete oxygen consumption in the membrane-attached biofilm. The MMAD principle is therefore suitable for oxygen supply to AD without diluting the biogas with oxygen. It is generally concluded that the MMAD principle can be used to enhance AD performance by controlling micro-aeration, improving acidogenesis, maintaining biogas quality and detecting process changes.