Anaerobic Co-Digestion of Municipal Wastewater Sludge and High Strength Waste: An Evaluation of Waste Degradation Potentials

Abstract

Research has demonstrated the abundant treatment and economic opportunities presented by co-digestion of high strength wastewater, but these wastes also have the potential to cause digester failure. Indeed, based on what is known about the disruption of digesters, if a municipality is considering co-digestion, it needs to know the characteristics of high strength wastewaters that have a predictable potential for disruption. The Metropolitan Council-Environmental Services (MCES) is one such municipality interested in pursuing co-digestion. MCES is interested in expanding current digester use to include co-digestion of high strength waste streams to reduce their energy and sludge disposal costs. The objective of this research was to develop a protocol for MCES to be used in evaluating the feasibility of co-digesting municipal wastewater sludge with other high strength wastes. The protocol involves anaerobic biochemical methane potential (BMP) assays inoculated by source anaerobic digesters and amended with PBS, sludge (TPS/TWAS mixture), waste, or waste combined with sludge. Co-digestion experiments examined well-characterized synthetic wastes (starch, gelatin, tween 80, and cysteine) and wastes of interest to MCES with unknown impacts (dimethylformamide, dairy processing waste, propylene glycol, ethylene glycol, thickened primary sludge, thickened waste activated sludge, and scum collected from the primary clarifier at an MCES wastewater treatment facility). These wastes were characterized as degradable, non-degradable, toxic or degradable after an adaptation period. Gelatin, tween 80, dairy processing waste, ethylene glycol, and propylene glycol were degradable, while dimethylformamide was non-degradable. Cysteine, a sulfur-rich waste, was found to be toxic at concentrations greater than 200 mg/L. Scum, ethylene glycol, and propylene glycol required an adaptation period before being degraded. Additionally, the microbial analysis via qPCR indicated that the abundance of Bacteria and methanogens in the co-digestion experiments were not statistically different from the sludge-only experiments, even when different types of wastes were tested (i.e., degradable, non-degradable, toxic, etc.). The developed method and subsequent guidelines will be useful for MCES and other wastewater treatment facilities when making informed decisions about whether or not to accept potential high strength wastes for co-digestion with sludge.