Anaerobic digestion of excess sludge by cascade digesters

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The management and disposal of excess sludge is one of the main challenges for wastewater treatment facilities across the world. Anaerobic digestion (AD) is a widely accepted treatment method for stabilizing excess sludge due to its robustness, ability to reduce pathogens, and capacity to convert the biochemical energy enclosed in organic compounds into biogas. However, the efficiency of converting excess sludge organics into biogas using conventional continuous stirred tank reactors (CSTR) is relatively low, primarily due to the slow hydrolysis rate. Various enhancement technologies, including thermal, chemical, and enzymatic methods, have been developed to accelerate the hydrolysis rate. Among these, enzymatically enhanced hydrolysis has attracted attention for its advantages, such as the absence of toxic byproduct formation and the ability to operate under moderate conditions. However, the scaling-up of these methods to industrial scale presents ongoing challenges. The research in this dissertation explored the feasibility of an innovative cascade anaerobic digestion (CAD) technology, consisting of differently sized CSTR digesters in series. The overall objective of the CAD technology is to achieve enzymatically enhanced hydrolysis of excess sludge in the first reactor stages.