Steady-state sludge digestion model with auto-calibration

Abstract

Anaerobic digestion is a wastewater treatment technology used to treat sludge. One of the main benefits of this technology is the production of biogas that can be used as fuel for energy production. This process is extremely complex with many different steps and parallel reactions occurring simultaneously. These steps require highly specific environmental conditions that must be maintained for anaerobic digestion to occur. Modeling for anaerobic digestion began in the late 1960s to better understand and streamline the anaerobic digestion process. Many years of research were combined by the International Water Association to develop the Anaerobic Digestion Model No. 1 (ADM1), which was published in 2002 and no widely accepted updates have been published since. This model has repeatedly been proven to be a complete and accurate model for anaerobic digestion, but it is extremely complex and requires many estimated parameters. This complexity results in the model not being used by currently operating wastewater treatment plants due to the lack of data that is required for the ADM1. Creating a simple steady-state model to predict the concentration of methane in the biogas produced that does not require as many parameters would be beneficial for wastewater treatment plants to repurpose the biogas. Using full-scale solids data, a steady-state model that auto-calibrates to a specific wastewater treatment was developed by creating simple relationships between volatile solids, metabolism factors, and hydraulic retention times. This resulted in methane concentrations ranging from 55 – 71% of the biogas. These values are within expected ranges for mesophilic operations, but due to lack of measured methane data from wastewater treatment plants, it is not possible to know if this is accurate for the specific plant. This poses the need for wastewater treatment plants to routinely monitor methane concentrations to successfully develop a steady-state anaerobic digestion model. Despite its limitations, the steady-state model is successful in terms of providing a way for wastewater treatment plants to easily predict the methane concentration within the biogas