Which of the following pollutants is not considered compatible for municipal treatment plants?

Heavy metals are not considered compatible for municipal treatment plants because they can be highly toxic and persistent in the environment. Municipal treatment plants are generally designed to handle organic materials and certain inorganic compounds, but heavy metals do not break down biologically and can accumulate in sludge and effluent, leading to contamination of waterways and adverse effects on aquatic life.

When heavy metals enter the treatment system, they can interfere with biological processes that are crucial for treating wastewater, causing damage to the microorganisms used in treatment. Furthermore, due to their density and solubility characteristics, heavy metals can create challenges in sedimentation and filtration processes, making their removal more complex and potentially requiring specialized treatment methods.

In contrast, organic contaminants are typically biodegradable, which means municipal treatment plants can effectively process them. Plastic materials, while problematic in a different way, can sometimes be filtered out, and sand and gravel, being inert, generally settle out easily. Thus, heavy metals represent a significant challenge for municipal wastewater treatment operations.