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Bench Scale Evaluation of two Enhanced Digestion Processes: Enhanced Enzymic Hydrolysis and Acid-Gas Phased Digestion Open Access

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The overall objective of this research was to make a comparative evaluation of two enhanced digestion processes for digestion of thickened municipal sludge from Blue Plains Advanced Wastewater Treatment plant. The digestion processes studied were Enhanced Enzymic Hydrolysis (Process I) and Acid-Gas Phased digestion (Process II). Both of these processes are two-phase digestion processes. A Conventional Mesophilic digestion process was maintained as the control process. The performances of the processes were studied in terms of volatile solids reduction, pathogen destruction and biogas production. Process I achieved 53.2% volatile solids reduction, compared to 51.7% for the Process II and 49.6% for the Control process. The final effluent from Process I and Process II met the 40 CFR Part 503 standards for fecal coliform reduction requirement for Class A Biosolids, while the effluent of Control process failed to do so, when standard culturing method was used. The specific gas production (ft3/lb VS destroyed) was 16.34 for Process I, 17.48 for Process II and 17.30 for the Control and these values are well within the typical range of 12-18 (ft3/lb VS destroyed) for the anaerobic digestion processes. The average methane content of Process I and Process II mesophilic reactors, and Control reactor were 69.3%, 69.4% and 60%, respectively.The average values of COD reduction were 53.0% for Process I, 51.3% for Process II and 53.4% for Control Process. During the study period, no ammonia inhibition was observed in any of the processes and average free ammonia concentrations were 73 mg/L and 70 mg/L for the final steps of Process I and Process II, respectively. In Control Process, average free ammonia concentration was 77 mg/L. The monthly average VFA levels as acetic acid (HAc) were 9000 mg/L and 3096 mg/L for the pretreatment steps of Process I and Process II, respectively. The mesophilic stages acted as the polishing steps where VFA levels were reduced to 121 mg/L for Process I and 603 mg/L for process II. The average VFA level in the Control was 217.8 mg/L.

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