Feasibility of Anaerobic Digestion at 45C Open Access
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Anaerobic digestion at mesophilic and thermophilic temperatures have been widely studied and evaluated by numerous researchers. Limited research has been conducted on anaerobic digestion in the intermediate zone of 45°C, mainly due to the notion that limited microbial activity occurs within this zone. The objective of this research was to evaluate the performance of anaerobic digestion at 45°C, in comparison to a mesophilic digestion system at 35°C and thermophilic digestion at 55°C. The performance was evaluated based on methane gas production, methane gas composition, specific methane yield, TAN concentration, free ammonia concentration, VFA concentration, soluble and total COD reductions, volatile and total solids reductions, pH, alkalinity, buffering capacity, and pathogen destruction. In general, 45°C anaerobic digestion systems were not able to achieve comparable methane yield and high effluent quality (in term of COD and ammonia content) compared to the mesophilic system, though the systems produced biogas with about 62 - 67% methane. However, despite of that, 45°C systems were capable in producing comparable amount of methane provided that, higher OLR was introduced into the sufficiently matured colonies of involved microorganisms. Additionally, 45°C digesters suffered from high VFA accumulation, but sufficient buffering capacity was observed as the pH, alkalinity and VFA-to-alkalinity ratio were within recommended values. As a result, complete inhibition or failure was not observed even after two years of operation. Furthermore, at 45°C, the digesters were able to achieve VS reduction of 48%, and the pathogen counts were less than 1,000 MPN/g total solids, comparable to those achieved in thermophilic system, hence producing class A biosolids. The performance of 45°C digesters were also assessed at different SRT of 15, 10 and 7.5 days. Conclusively, the optimum SRT was observed to be at 7.5 days, attributed to its highest methane production, highest VS destruction, best effluent quality, minimal VFA accumulation, well-buffered system and sufficient pathogen deactivation. In addition to experimental evaluation, the performance of 45°C anaerobic digester systems was numerically investigated through the determination of kinetic parameters Ks and rmax using Monod model. The parameters were estimated for varying experimental conditions namely, temperature, SRT, free NH3 concentration, background acetate concentration and type of acetate presents in the system. Ks values ranged from 0.125 to 0.190 mg/L as unionized HAc for uninhibited condition and 0.026 to 0.060 mg/L as unionized HAc for inhibited condition. While for systems with higher substrate concentration, Ks values ranged from 0.236 to 0.278 mg/L as unionized HAc. As for rmax, the highest value was observed in 35°C system at 0.167 mg/L/day, higher than those observed in all 45°C system regardless the experimental condition. In overall, the outcomes of this model suggested that the accumulation of acetate observed in 45°C systems were presumably due to its high temperature which contributed to high hydrolysis rate. Consequently, it produced large amount of toxic salts, that combined with the substrate and making them not readily available to be consumed by methanogens. In addition to that, no ammonia inhibition was observed within 45°C systems as the average free NH3 concentration was well below 110 mg/L. Other than Ks and rmax, inhibition parameter KI was also determined for inhibitory condition caused by high free NH3 content. Using Michaelis-Menten model, the calculated KI value was 0.072 mg/L as unionized HAc and the inhibition type was uncompetitive. Corresponding to the inability of 45°C systems to produce good quality effluent, the system was combined with a mesophilic digester into a TPAD system. It was evidenced that TPAD system was able to offer the advantages of each of the digestion process while avoiding the disadvantages, particularly the poor effluent quality associated with 45°C digestion and insufficient pathogen destruction associated with mesophilic digestion. TPAD system was operated at 3 different SRT, namely TPAD Run 1 (7.5 days + 7.5 days), TPAD Run 2 (7.5 days + 10 days) and TPAD Run 3 (2.5 days + 10 days). Conclusively, the best operating condition of our TPAD system was observed in TPAD Run 2 (7.5 days + 10 days) with total SRT of 17.5 days. It was attributed to its high methane production, sufficient VS destruction, low TAN and free NH3 content, less acetate and VFA accumulation, good effluent quality, well buffered system and most importantly, sufficient pathogen deactivation. However, the other two options, TPAD Run 1 (7.5 days + 7.5 days) and TPAD Run 3 (2.5 days + 10 days), were remained as viable options, as even at shorter retention times, the TPAD systems still achieved excellent removals of VS, fecal coliform and VFA concentration.
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