Building A New Digester or Intensifying the Existing Anaerobic Digesters? A Deep Dive into Advanced AD Intensification Technologies in North America

Introduction and Objectives
Anaerobic digestion (AD) has been widely adopted by municipalities for solids management in wastewater resource recovery facilities (WRRF). AD enhances resource recovery, adding value to wastewater treatment. However, its kinetics pose challenges, with slow hydrolysis and methanogenesis necessitating a solid residence time (SRT) of 15-20 days, leading to larger footprints and higher construction costs. Faster growing acidogenic microorganisms produce inhibitory substances that destabilize the process, especially when organic loading rates increase or feed material resists hydrolysis. Furthermore, population growth demands a capacity increase in AD tanks to handle the higher wastewater sludge flowing into the plants. A traditional method to increase the capacity of the digesters for a utility is building new digesters which requires relatively high capital and operation costs. Tackling such challenges necessitates adopting advanced AD intensification technologies that alleviate the biological limitations, provide smaller footprint, and increase capacity at lower costs.

Intensification technologies are a general umbrella term to include any technology that improves the performance of AD systems. The benefits of intensification technologies include, but are not limited to, enhanced production of valuable products, reduced process footprints, higher organic loading, and improved process stability. In practice, municipalities make ad hoc decisions to select intensification technologies available commercially that meet their regulatory and financial needs. To the best of our knowledge, there are no comprehensive analyses and references available in our industry that cover the practitioners to select appropriate intensification technology for their municipalities and compare the existing advanced AD technologies in North America.

The objective of this study is to discuss and analyze the benefits of intensification technologies for AD in WRRF. The goal is to provide the municipalities with a comprehensive framework to select and implement appropriate intensification technologies, considering their specific needs and conditions, to enhance the performance and efficiency of AD processes.

Methodology
Six municipalities in North America were interviewed that each have implemented one of the full-scale intensification technologies, and further compared and analyzed the benefits of intensification technologies. First, a literature survey was conducted to identify and compare currently used intensification technologies by municipalities in North America. Six major intensification technologies were identified including Acid-Phase Digestion (APD), Temperature-Phased Digestion (TPAD), Recuperative Thickening (RT), Thermal Hydrolysis (THP), Post-Aerobic Digestion (PAD), and High Solid Digestion. Then, for each intensification technology, an offline survey and an hour-long interview were conducted with a municipality that has adopted the technology to understand their experience. Finally, a framework for evaluating the advantages and disadvantages of each intensification technology was developed.

Results
Figures 1 and 2 show approximately sixty plants in North America have adopted advanced AD technologies with TPAD, THP, and APD being the top 3 technologies implemented today. While for RT, high solid digestion, and PAD only one facility for each has adopted the technology to date. The interviews of municipalities sought many common features of intensification technologies. These attributes included higher organic loading rate, decoupling SRT from HRT, improved process stability, ease of operation, and enhanced recovery of valued products reported in Table 1. Interestingly, while seeking similar attributes from an intensification technology, the choice of intensification technology rarely depended on the technological advantages alone. Rather, each municipality's choice of intensification technology reflected their unique concerns reflecting the history of their facility, relationship with their local constituents, influent characteristics, and other economic opportunities. For example, Madison Metropolitan Sewerage District (MMSD) adopted a two-phase AD process to retrofit an existing AD facility to cope with persistent foaming issues they had been dealing with historically. The City of Montpelier Water Resource Recovery Facility constructed a new AD with a recuperative thickener (RT) to be able to maintain high SRT in response to incidental high organic loading events. DC Water adopted Thermal Hydrolysis to increase capacity.

In conclusion, intensification technologies significantly enhance anaerobic digestion (AD) performance by boosting stability, increasing organic loading rates, and reducing costs and footprints. This study provides a detailed framework for municipalities to choose the best technology tailored to their unique needs, emphasizing the importance of a customized approach over a one-size-fits-all solution.