The traditional wastewater treatment process, involving primary, secondary, and sometimes tertiary stages, was designed to handle bulk physical solids and biodegradable organic matter. Primary treatment removes large debris and settleable solids, while secondary treatment uses biological processes to consume dissolved organic compounds. Modern wastewater treatment plants must now process a complex mixture of materials that were not a concern when these systems were first engineered. As a result, synthetic chemicals, microscopic particles, and stable elements often pass through the system largely untouched, eventually entering the environment.
Synthetic Organic Micropollutants
These human-made chemicals enter the water supply at extremely low concentrations, often measured in parts per billion or trillion. Many Pharmaceuticals and Personal Care Products (PPCPs), such as certain antibiotics and endocrine-disrupting compounds, fall into this category. Their complex molecular structures are designed to resist biological breakdown in the human body, which translates into resistance to the microbial communities in the secondary treatment stage.
Micropollutants also include industrial solvents and specific pesticides or herbicides, which are often highly soluble in water. High solubility prevents these compounds from easily adsorbing onto the biological sludge or solid particles removed during treatment. They remain dissolved in the water phase, flowing directly into the final effluent. Their low concentration makes them difficult to detect and economically unviable to treat using conventional methods.
Persistent Inorganic Contaminants
Inorganic contaminants cannot be destroyed through biological or chemical means; they can only be changed in form or separated physically. Heavy metals like lead, mercury, and cadmium are discharged from industrial sources, including manufacturing and electronics. These elements are non-biodegradable, highly toxic, and pose a risk due to their tendency to bioaccumulate up the food chain.
Conventional treatment removes heavy metals using coagulation and flocculation, where chemicals clump the metals into larger particles for sedimentation. While this removes a significant portion, residual amounts persist in the treated water. Excess nutrients, particularly nitrogen and phosphorus, are also major inorganic contaminants. High inputs from agricultural runoff and human waste often overload secondary and tertiary treatment stages, resulting in the discharge of these nutrients, which can cause harmful algal blooms.
Microplastics and Nanomaterials
These contaminants are defined by their physical size, allowing them to bypass conventional physical separation methods. Microplastics are plastic particles less than five millimeters in size, originating from sources like the wear of synthetic clothing or the fragmentation of larger debris. A significant fraction, especially the smaller particles, is not captured by screens and sedimentation tanks.
Even more challenging are nanomaterials, which are particles with at least one dimension smaller than 100 nanometers. Their minute size prevents effective capture by many filtration or membrane processes. While treatment plants remove a large percentage of microplastics, the sheer volume of water processed means billions of particles are still released daily into aquatic environments. Furthermore, removed microplastics are transferred into sewage sludge, reintroducing the plastic into the terrestrial environment when used as fertilizer.
Recalcitrant Contaminants: The Case of PFAS
Per- and Polyfluoroalkyl Substances (PFAS) represent a group of contaminants that are exceptionally difficult to remove due to their unique chemical stability. Often called “forever chemicals,” their persistence stems from the exceptionally strong carbon-fluorine bond, which is one of the strongest single bonds in organic chemistry. This bond structure renders PFAS resistant to almost all forms of degradation, including thermal, chemical, and biological processes.
This chemical robustness allows PFAS to completely bypass the biological processes of secondary treatment and resist the chemical oxidation methods that work for many other organic micropollutants. Even advanced methods like granular activated carbon filtration merely concentrate the PFAS without destroying them. Complete destruction requires highly specialized, energy-intensive techniques not standard in municipal treatment, such as Supercritical Water Oxidation (SCWO) or electrochemical oxidation.