Economic Benefits of Enhanced Anaerobic Digestion: A Survey

This paper summarizes anaerobic digestion performance for more than 30 wastewater treatment facilities around the world. The data set may be unique in that it captures many details of plant performance and costs from a large number of diverse facilities. In addition, economic models for the expected benefits of adding sludge pretreatment technology are developed for each facility. Given the range of starting conditions, the expected benefits from adding pretreatment varies widely. This paper summarizes the data and draws cost/benefit conclusions regarding pretreatment and beneficial biogas use. The key finding from this work is that anaerobic digestion, as is currently practiced, is only somewhat successful at completely capturing the energy value in wastewater solids. Based on conservative assumptions, none of the plants in this study are currently achieving more than 40% VSR for the WAS fraction. Using an energy-efficient pretreatment technology offers substantial operational and financial benefits and is critical to change anaerobic digestion performance. Although the expected performance benefits are site specific, the weighted-average payback for all the plants studied was less than 3 years. The primary benefit from adding pretreatment technology is reducing the mass of biosolids that require handling, storage, hauling, and disposal. Thus, biosolids hauling and disposal costs are a key driver for achieving economic benefits from a pretreatment technology. Beneficial use of additional biogas produced by enhanced digester performance is the second main benefit for achieving economic benefits. In certain geographic areas where electrical costs are more than 3 times equivalent natural gas costs, combining pretreatment technology with cogeneration equipment could improve the economic benefits considerably.