Emerging contaminants are chemicals and microorganisms not commonly monitored in the environment, yet they carry the potential to cause adverse effects on human health and ecosystems. This diverse group includes pharmaceuticals, personal care products, endocrine-disrupting compounds, pesticides, and industrial chemicals. Their increasing detection in wastewater streams has become a critical concern for water treatment facilities worldwide, as traditional treatment methods may not be adequate to remove them. Understanding how these contaminants interact with secondary wastewater treatment processes is essential for protecting water quality and public health.

Understanding Secondary Wastewater Treatment

Secondary wastewater treatment represents the biological stage of the treatment process, following primary sedimentation. Its primary goal is to remove dissolved and suspended organic matter, nutrients such as nitrogen and phosphorus, and pathogens. This stage relies on naturally occurring microorganisms that consume organic pollutants under controlled conditions. The most common systems include activated sludge, trickling filters, rotating biological contactors, and moving bed biofilm reactors (MBBRs).

Activated sludge systems aerate wastewater to promote the growth of a microbial floc that metabolizes organic material. Trickling filters use a bed of media, such as rocks or plastic, over which wastewater trickles, allowing microbial biofilms to degrade pollutants. MBBRs combine suspended growth and biofilm processes by using small plastic carriers that provide surface area for biofilms while being kept in suspension by aeration. Each system has its own operational parameters and vulnerabilities when exposed to emerging contaminants.

Secondary treatment is designed to achieve effluent quality standards that reduce biochemical oxygen demand (BOD) and total suspended solids (TSS) to levels safe for discharge into natural water bodies or for further advanced treatment. However, the presence of trace organic compounds and other emerging contaminants can disrupt this delicate biological balance.

The Nature of Emerging Contaminants

Emerging contaminants can be broadly categorized into several groups based on their origin and chemical structure:

  • Pharmaceuticals and personal care products (PPCPs): Includes antibiotics, analgesics, hormones, sunscreens, and fragrances. These are excreted or washed off and enter sewage systems.
  • Endocrine-disrupting chemicals (EDCs): Such as bisphenol A (BPA), phthalates, and nonylphenols, which interfere with hormonal systems in wildlife and humans.
  • Per- and polyfluoroalkyl substances (PFAS): Used in non-stick coatings, firefighting foams, and water-resistant textiles. They are extremely persistent and bioaccumulative.
  • Pesticides and industrial chemicals: Including herbicides, insecticides, plasticizers, and flame retardants.
  • Nanomaterials and microplastics: Tiny particles from consumer products and industrial processes that can carry other pollutants.

Many of these compounds are not regulated under current water quality standards, and their long-term effects on ecosystems and human health are still being studied. They are often present at trace concentrations (ng/L to µg/L), which complicates both detection and removal.

Impacts on Secondary Treatment Processes

Interference with Biological Activity

Microorganisms in secondary treatment systems are the workhorses of organic matter degradation. However, many emerging contaminants are biologically active by design. Antibiotics, for example, can selectively inhibit or kill sensitive bacterial species, altering the microbial community structure. This can reduce the treatment system's ability to remove BOD and nutrients, leading to poorer effluent quality.

Antimicrobial compounds such as triclosan (once common in soaps) have been shown to reduce nitrification rates in activated sludge plants. Nitrifying bacteria are particularly sensitive to toxic shocks from pharmaceutical residues. A study published in Environmental Science & Technology reported that certain non-steroidal anti-inflammatory drugs (NSAIDs) can inhibit microbial respiration, resulting in slower degradation rates and higher effluent BOD levels. Over time, microbial communities may shift toward more resistant species, but this adaptation can come at the cost of overall treatment efficiency.

Persistence and Removal Challenges

Many emerging contaminants are recalcitrant, meaning they resist biodegradation during conventional secondary treatment. PFAS compounds, for instance, have strong carbon-fluorine bonds that are not easily broken by ordinary biological processes. Studies show that activated sludge systems typically remove less than 50% of many PFAS types, with shorter-chain PFAS often more persistent than long-chain ones. Similarly, carbamazepine (an anticonvulsant) and sulfamethoxazole (an antibiotic) frequently pass through secondary treatment almost intact, being detected in effluents and receiving waters.

The persistence of these compounds raises concerns about their accumulation in the environment. Even at low concentrations, chronic exposure can cause endocrine disruption in fish and other aquatic organisms. Furthermore, the presence of antibiotics in effluents can promote the development of antibiotic-resistant bacteria and resistance genes, a growing public health threat recognized by the World Health Organization (WHO).

Some contaminants can also transform into more toxic metabolites during biological treatment. For example, certain fragrances like synthetic musks can be transformed into more persistent and bioaccumulative forms. This complicates risk assessments and demonstrates that simply measuring parent compound removal may not capture the full risk.

Effects on Sludge Handling and Disposal

Emerging contaminants do not only affect the liquid stream; they can also partition into the sludge (biosolids) generated during secondary treatment. Hydrophobic compounds such as triclosan, BPA, and some pesticides tend to adsorb onto the solid fraction. When biosolids are applied to agricultural land as fertilizer, these contaminants can enter the soil, be taken up by crops, or leach into groundwater. Regulations for biosolids often do not account for these emerging contaminants, posing a potential route for human exposure and ecological harm.

Moreover, the presence of antimicrobial agents in sludge can inhibit anaerobic digestion processes used for sludge stabilization, reducing biogas production and increasing operational costs. Operators may need to adjust retention times or implement pre-treatment steps to mitigate these effects.

Case Studies and Research Findings

Several studies have documented the real-world impacts of emerging contaminants on secondary treatment. A comprehensive review in Water Research analyzed data from over 100 wastewater treatment plants (WWTPs) in Europe and North America. It found that while conventional activated sludge could remove more than 80% of some compounds like acetaminophen and caffeine, removal rates for diclofenac and carbamazepine were below 30%. Trickling filters generally showed lower removal efficiencies for many pharmaceuticals compared to activated sludge systems.

Another study conducted at a WWTP in Sweden investigated the effect of antibiotics on nitrification. The researchers observed that spikes in ciprofloxacin concentration correlated with temporary drops in ammonia removal efficiency, requiring chemical supplementation to maintain compliance. Such events can increase operational costs and risk permit violations.

Microplastic removal during secondary treatment is also a growing area of research. A study from the University of Toronto found that activated sludge processes remove approximately 90% of microplastics from influent, but the remaining 10% (which can be billions of particles per day) are discharged into receiving waters. Furthermore, microplastics can adsorb other pollutants, acting as vectors for toxic compounds.

The persistence of PFAS has led to increasing regulatory scrutiny. The U.S. Environmental Protection Agency (EPA) has established health advisories for certain PFAS in drinking water and is considering stricter wastewater discharge limits. Recent research indicates that conventional secondary treatment achieves minimal PFAS removal, with mass balances often showing no significant degradation, only partitioning between liquid and solid phases.

Mitigation Strategies

Advanced Oxidation Processes (AOPs)

To overcome the limitations of biological treatment, many facilities are integrating advanced oxidation processes either as a tertiary step or directly within the secondary treatment train. AOPs such as ozone (O₃) combined with hydrogen peroxide (H₂O₂), ultraviolet (UV) photolysis, and Fenton reactions generate highly reactive hydroxyl radicals that can break down even the most persistent organic pollutants. Ozonation has proven effective for removing many pharmaceuticals and endocrine disruptors, but it can produce byproducts that require further treatment. Costs and energy demands remain barriers to widespread adoption, but recent advances in UV-LED technology are reducing operational expenses.

Adsorption Technologies

Activated carbon, both in powdered (PAC) and granular (GAC) forms, is widely used to adsorb organic contaminants. PAC can be dosed directly into the activated sludge basin, integrating with existing biological treatment. GAC filters, often placed after secondary clarification, provide a polishing step that captures residual trace compounds. Studies show that activated carbon can remove over 90% of many pharmaceuticals and pesticides, though saturation and regeneration costs must be managed. Alternative adsorbents like biochar and ion-exchange resins are also being explored for specific contaminant classes such as PFAS.

Membrane Bioreactors (MBRs)

Membrane bioreactors combine biological treatment with membrane filtration (microfiltration or ultrafiltration). The membrane retains biomass and particulate matter, allowing for higher biomass concentrations and longer sludge retention times—conditions that favor the growth of slow-growing bacteria capable of degrading recalcitrant compounds. MBRs have demonstrated improved removal of many pharmaceuticals compared to conventional activated sludge. However, membrane fouling and energy consumption remain challenges. Ongoing research into novel membrane materials and fouling control strategies is improving the feasibility of MBRs for emerging contaminant removal.

Process Optimization and Bioaugmentation

Adjusting operational parameters can enhance the resilience of biological processes. Increasing sludge retention time (SRT) allows microorganisms more time to adapt to and degrade slowly biodegradable compounds. Maintaining higher dissolved oxygen levels and optimizing nutrient ratios also support microbial health. Bioaugmentation—the addition of specific microbial strains with known degradation capabilities—has shown promise in lab studies for compounds like chlorinated solvents and some pesticides. However, field applications face challenges in establishing and maintaining the introduced strains in a competitive environment.

Source Control and Pretreatment

Preventing emerging contaminants from entering wastewater systems in the first place is a cost-effective strategy. This can include pharmaceutical take-back programs, restrictions on the use of certain chemicals in consumer products (e.g., banning microbeads in cosmetics), and industrial pretreatment requirements. Hospitals and industries that discharge high loads of pharmaceuticals or chemicals can be required to install on-site treatment systems. Public education campaigns about proper disposal of medications and household chemicals also play a role.

A combination of these approaches is often necessary. Treatment facilities may need to implement a multi-barrier system that includes biological treatment, AOPs, and adsorption to effectively manage the diverse array of emerging contaminants.

Regulatory and Monitoring Frameworks

Current Standards

Most countries regulate conventional pollutants in wastewater effluents—such as BOD, TSS, and nutrients—but few have specific limits for emerging contaminants. The European Union, through its Water Framework Directive, has included certain priority substances on its watch list, such as diclofenac and some hormones, requiring member states to monitor them. The United States has no federal regulations for pharmaceuticals in wastewater, though some states are beginning to address PFAS. The EPA's Unregulated Contaminant Monitoring Rule (UCMR) periodically surveys drinking water systems for emerging contaminants, but this does not directly regulate discharge.

Without regulatory drivers, utilities may lack the funding and incentive to upgrade treatment processes. However, mounting evidence of ecological harm, combined with public pressure, is pushing regulators to consider new standards. In 2021, the European Commission proposed a revision of the Urban Wastewater Treatment Directive that includes requirements to remove micropollutants and improve energy efficiency.

Future Directions

Advances in analytical chemistry, such as liquid chromatography-tandem mass spectrometry (LC-MS/MS), now enable detection of contaminants at extremely low concentrations. This allows for more comprehensive monitoring and risk assessment. However, screening for all possible contaminants is impractical. A leading approach is to use effect-based monitoring—measuring the biological activity of the effluent (e.g., estrogenicity) rather than quantifying each individual compound. This can identify toxicologically relevant mixtures and guide treatment upgrades.

Regulatory frameworks are likely to evolve toward more holistic risk-based approaches, taking into account the cumulative effects of pollutant mixtures. The concept of "chemical footprinting" and the use of environmental quality standards (EQS) tailored to specific conditions are being explored.

Looking Ahead

Emerging contaminants represent a complex and evolving challenge for secondary wastewater treatment processes. Their ability to interfere with biological treatment, persist through conventional processes, and accumulate in the environment demands a proactive response. The most effective strategies combine source control, process optimization, and advanced technologies such as AOPs, activated carbon, and membrane bioreactors. Continued research into the fate and effects of these compounds, coupled with adaptive regulations, will be essential to safeguard water quality and public health.

Utilities must also consider economic and energy trade-offs. Retrofits for advanced treatment can be costly, and energy demands of technologies like ozonation and UV may conflict with carbon reduction goals. Emerging solutions such as solar-driven AOPs, biological removal of PFAS using specific enzyme pathways, and smart monitoring systems that predict contaminant loads offer promising paths forward.

Collaboration between researchers, engineers, regulators, and the public is key. By staying informed about the latest findings and investing in resilient treatment infrastructure, the water sector can stay ahead of the emerging contaminant tide and ensure that treated wastewater is safe for the environment and for reuse applications.

For further reading on this topic, the EPA provides resources on Contaminants of Emerging Concern, and the WHO has published reports on Antimicrobial Resistance in Wastewater. Additionally, academic journals such as Water Research regularly publish peer-reviewed studies on mitigation technologies and environmental fate.