Evaluation and Novel Application of 45ºC Anaerobic Digestion Open Access
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This study was aimed to evaluate the performance of anaerobic digestion system at 45°C condition. Both mesophilic 35°C and thermophilic 55°C temperature anaerobic digestion have been widely studied. However, very limited research has been conducted on 45°C anaerobic digestion. It is due to the notion that there is limited microbial activity in the temperature transition zone between mesophilic and thermopilic condition. The individual anaerobic digesters were operated with identical solids retention time (SRT) of 10d at 35°C, 45°C and 55°C. The application of 45°C digestion in a Temperature Phased Anaerobic Digestion (TPAD) systems were evaluated for this study. For TPAD systems, both 45°C-35°C TPAD and 55°C-35°C TPAD were operated. The performance of each system was based on methane yield and production, biogas composition, total solids (TS) and volatile solids (VS) destruction, soluble and total COD reductions, volatile fatty acids (VFAs) concentrations, ammonia content, buffering capacity and pathogen deactivation. Another objective was to evaluate the systems for producion of Class A biosolids.The 45°C individual anaerobic digestion (AD) was observed to be a viable system that was capable of producing Class A biosolids. Also, 45°C AD was capable of producing comparable amount of methane gas as the 35°C AD. In addition to that, the expected high VFA accumulation observed in the 45°C AD was overcome by sufficient buffering capacity. No inhibition occured within the year of 45°C AD operation. Furthermore, 45°C AD was able to achieve comparable pathogen deactivation as 55°C AD. When pathogen deactivation to produce Class A biosolids is the treatment objective, it would result in significant savings in energy cost to achieve that in a 45°C AD instead of a 55°C AD.Four different sequences of TPAD were operated. They were TPAD Run 1 (45°C-2.5days + 35°C-10days), TPAD Run2 (45°C-7.5days + 35°C-10days), TPAD Run3 (55°C-2.5days + 35°C-10days) and TPAD Run4 (55°C-7.5days + 35°C-10days); The days after the temperature indicate SRT. The best operating condition was observed in the TPAD Run 2 with total SRT of 17.5 days. This was supported by its high methane production, VS reduction, low total ammonia nitrogen (TAN) and free NH3 level, less VFA accumulation, well buffered ability, sufficient pathogen deactivation and stable operation condition as well. Other than TPAD Run 3, all other three TPAD systems remained as viable options for Class-A biosolids producing purposes. Except for COD reduction, all environmental parameters associated with 45°C-35°C TPAD were observed with better performance than 55°C-35°C TPAD, suggesting the option of replacing a conventional TPAD with a 45°C-35°C TPAD that would result in lower energy requirement.