Evaluation of Intensification of Thermal Hydrolysis Pretreatment for Anaerobic Digestion
Open AccessThe Thermal Hydrolysis Pretreatment (THP) of biosolids has gained practical interest because of its ability to achieve high quality biosolids (e.g., Class A in the United States) after Anaerobic Digestion (AD), reduced digester volume, and high cake dewaterability. To further intensify the THP-AD process, this study evaluated Recuperative Thickening (RT) as a potential combined solids pretreatment strategy. This study also investigated the free ammonia inhibition level in the typical THP-AD systems and optimized the current process model for more accurate high-rate AD process modeling. This study evaluated the overall impact of dynamic sludge characteristics and THP conditions on THP-AD system performance, aimed to gain better understanding of the process and optimize the full-scale THP operation.Bench-scale studies were performed to evaluate the application of recuperative thickening to further enhance THP-AD system. It was observed that due to the low SRT (solids retention time) requirements for THP-AD system, RT application for SRT enhancements from 15 to 30 days did not improve AD performance or kinetics. Only when one would operate close to or lower than washout SRT (estimated at 6-7 days), RT application for SRT enhancement made sense. However, RT application to maintain SRT while increasing throughput rates showed great potential for further intensification of AD system while maintaining biogas yield, VSR (volatile solids reduction) and COD removal efficiencies. Based on a calibrated process model, RT application was estimated to allow for at least 100% increase of throughput rates without sacrificing AD performance. In order to understand the ammonia inhibition capacity of the typical THP-AD system treating municipal biosolids, a bench-scale study was conducted in which fifteen different steady state TAN (total ammonia nitrogen) and free ammonia conditions were created. A decrease in volatile solids reduction from 54 ± 5% (at <554 mg NH3-N/L) to 35 ± 6% at the maximum free ammonia concentration of 966 mg NH3-N/L was observed at steady-state conditions. No impact of free ammonia on final dewaterability was detected. Free ammonia mostly limited methanogenesis. A free ammonia Monod inhibition constant of 847 ± 222 mg NH3-N/L for methanogens was estimated based on the digester steady-state conditions. This study showed that current THP-AD digesters (typically 110-260 mg NH3-N/L) operate under 12%-18% ammonia inhibition for methanogenesis. Operation at SRT of 15 days, about 2 times more than needed to retain methanogens, can compensate for lower methanogenesis rates and avoid performance impacts. The latter showed good potential to operate under higher free and total ammonia concentration without jeopardizing performance. The ammonia inhibition bench-scale study generated extensive data that was used for the first time, to calibrate and validate the Sumo2ACOX model. A set of kinetic parameters for methanogenesis was proposed based on the calibration. The logistic half-inhibition constant of NH3 for ACOX and AMETO was calibrated to be 0.024 mol N/L and 0.018 mol N/L, respectively. The model simulation results showed that the calibrated Sumo2ACOX model could properly simulate the kinetic activities of methanogens and digester overall performance for increased loading, while Sumo2 model overestimated the digester performance when volatile solids loading was higher than 0.5 g VSS/L/d. A detailed analysis was performed on a 400-day sampling data at the THP-AD facility to evaluate the overall impact of dynamic sludge characteristics and THP conditions on THP-AD system performance. It was observed that increased THP reaction time, especially above a thermal dose of 6000 ˚C*min, significantly increased the gelation behavior of sludge. The gelation factor increased from 0.68 to 0.89. This was mainly driven by increased gelation of primary sludge at increased THP thermal doses and indicated the need for shorter THP times when higher primary sludge over waste activated sludge portions (PS/WAS> 50%) were present. The gelation factor described the overcooking behavior within the THP system well, despite sensitivity differences between primary and waste activated sludge. Full-scale data revealed that the AD system has a probability of 88% to achieve above 60% volatile solids reduction when gelation factor of THP sludge was maintained below 0.7, while the probability reduced to only 18% when THP sludge gelation factor was above 0.8. This was explained by avoiding overcooking of highly biodegradable primary sludge. No statistical correlation between THP gelation factor and digestate dewaterability was found. However, increased gelation behavior of digestate led to lower dewatered cake sludge. Overall, this study showed the value of the gelation factor parameter for optimization of THP conditions, and suggested directions for full-scale facilities to maximize the benefit of THP pretreatment for biogas production and final dewatering.
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