Operational impacts of heavy metals on activated sludge systems: the need for improved monitoring

Biological nutrient removal is highly reliant on maintaining a heterogeneous, balanced, and metabolically active microbial community that can adapt to the fluctuating composition of influent wastewater and encompassing environmental conditions. Maintaining this balance can be challenging in municipal wastewater systems that sporadically receive wastewater from industrial facilities due to the impact of heavy metals and other contaminants on the microbial ecology of the activated sludge. A thorough understanding of the impacts of heavy metals on activated sludge and of practical monitoring options is needed to support decision-making at the wastewater utility level. This paper is divided into two parts. In the first part, the review explains what happens when heavy metals interact with activated sludge systems by highlighting biosorption and bioaccumulation processes, and when an activated sludge system switches from bioaccumulation to toxic shock. Here, it also summarizes the impacts of heavy metal exposure on plant performance. In the second part, the review summarizes practical approaches that can be used at the plant outside the realm of traditional toxicological bioassays testing to determine the possible impacts of influent heavy metal concentrations on the BNR process. These approaches include the following: monitoring operational parameters for major shifts; respirometry; microscopy; ATP; chemical analyses of heavy metals with a focus on synergistic impacts and inhibitory limits; and other novel approaches, such as EPS chemical analyses, molecular techniques, and quorum sensing.