Introduction: The Growing Challenge of Emerging Contaminants in Wastewater

Wastewater treatment plants worldwide face an increasingly complex challenge: the presence of trace organic compounds known as emerging contaminants (ECs) or contaminants of emerging concern (CECs). This broad category includes pharmaceuticals, personal care products, endocrine-disrupting chemicals (EDCs), pesticides, industrial additives, and microplastics. Unlike traditional pollutants measured by biochemical oxygen demand (BOD) or total suspended solids (TSS), these compounds can persist through conventional treatment processes and enter receiving waters at nanogram to microgram per liter concentrations.

Research over the past two decades has documented a wide range of adverse effects from ECs in aquatic ecosystems, including reproductive disruption in fish, antibiotic resistance propagation, and unknown long-term human health risks via drinking water sources. While advanced oxidation, reverse osmosis, and activated carbon can achieve high removal, these technologies carry substantial energy and capital costs. This economic reality has driven interest in optimizing biological treatment systems that can provide cost-effective removal of multiple ECs. Among these, trickling filters have re-emerged as a promising technology capable of supporting diverse microbial communities that can degrade recalcitrant compounds.

Fundamentals of Trickling Filters: Design and Biology

An Aerobic Fixed-Film Process

A trickling filter is a biological reactor that uses a fixed bed of media to support a biofilm of microorganisms. Wastewater is distributed over the top of the bed through rotating arms or fixed nozzles, then trickles downward through the media. Air typically circulates through the bed either by natural convection (encouraged by the temperature difference between the liquid and ambient air) or by forced ventilation, providing oxygen for aerobic metabolism. The treated liquid is collected at the underdrain, with a portion recirculated to maintain hydraulic loading and dilute incoming load.

Media Types and Surface Area

Traditional trickling filter media were stones or rocks, typically 2–4 inches in diameter. Modern plastic media, such as cross-flow or vertical-flow modules, offer significantly greater surface area per unit volume (100–300 m²/m³) while reducing weight and clogging risk. The choice of media directly influences biofilm thickness, hydraulic retention time, and contaminant mass transfer. For emerging contaminant removal, higher surface area allows for a larger, more diverse biofilm community capable of degrading a wider range of compounds.

The Biofilm: A Complex Microbial Ecosystem

Biofilms in trickling filters are stratified, with aerobic bacteria (e.g., Pseudomonas, Nitrosomonas) dominating the outer layer exposed to oxygen, while facultative and anaerobic organisms inhabit deeper zones. This gradient creates microenvironments that support different metabolic pathways, including co-metabolism—a key mechanism for breaking many emerging contaminants. Protozoa and higher organisms such as nematodes graze on the biofilm, controlling thickness and maintaining a young, active culture. The biodiversity of trickling filter biofilms has been shown to exceed that of activated sludge, which may be a critical advantage for degrading complex organic molecules.

Mechanisms of Emerging Contaminant Removal in Trickling Filters

Removal of ECs in trickling filters occurs through a combination of physical, chemical, and biological processes. Understanding these mechanisms is essential for designing filters that achieve consistent, high removal efficiencies.

Biodegradation

The primary route for mineralization of many ECs is aerobic or anoxic biodegradation by biofilm microorganisms. Compounds such as ibuprofen, naproxen, and some estrogens are readily degraded as primary substrates. However, many pharmaceuticals and personal care products are recalcitrant and require co-metabolic transformation: a non-specific enzyme produced during degradation of a growth substrate (e.g., organic carbon in wastewater) coincidentally transforms the target contaminant. The ammonia-oxidizing bacteria in trickling filters, for example, produce the enzyme ammonia monooxygenase, which has been documented to oxidize compounds like triclosan and 17α-ethinylestradiol.

Sorption to Biofilm and Media

Hydrophobic compounds (e.g., nonylphenol, some polycyclic musks) partition into the organic matrix of the biofilm. This reversible sorption can temporarily remove contaminants from the aqueous phase, but if not subsequently biodegraded, desorption may occur. Trickling filters with a high biomass concentration and appropriate solids retention time (SRT) promote true degradation after sorption. Additionally, extracellular polymeric substances (EPS) in the biofilm can bind certain metals and polar organics.

Photodegradation and Hydrolysis

In open trickling filters exposed to sunlight, photodegradation can contribute to removal of light-sensitive compounds such as dicloflenac and sulfonamide antibiotics. The presence of humic acids in wastewater can act as photosensitizers, enhancing direct photolysis. Hydrolysis, particularly for some beta-blockers and antibiotics, may also occur under the slightly alkaline conditions typical of well-nitrifying trickling filters.

Predation and Sloughing

The periodic sloughing of excess biofilm removes accumulated contaminants and microbial cells, including those that may have sorbed or transformed compounds into secondary metabolites. Adequate sloughing is important to prevent the re-release of sorbed contaminants and to maintain a young biofilm with high metabolic activity.

Performance for Key Classes of Emerging Contaminants

Numerous studies have evaluated trickling filter performance for EC removal. While results vary with filter design, media type, loading rate, temperature, and wastewater composition, some generalizations emerge.

Pharmaceuticals

Analgesics and anti-inflammatories: Ibuprofen and naproxen typically achieve >80% removal in well-operated trickling filters due to rapid aerobic biodegradation. Diclofenac is more variable, with removals ranging from 20% to 70%, influenced by redox conditions and light exposure.

Antibiotics: Compounds such as sulfamethoxazole, trimethoprim, and ciprofloxacin show moderate removal (40–70%). Biodegradation is often the dominant route, but adsorption to biofilm can be significant for some quinolones. Importantly, antibiotics also exert selective pressure on the biofilm community, potentially enriching resistant genes—a factor that requires careful monitoring.

Hormones and endocrine disruptors: The natural estrogen 17β-estradiol is nearly completely removed (>95%). The synthetic estrogen 17α-ethinylestradiol, used in oral contraceptives, is more resistant but still achieves 60–85% removal in well-nitrifying trickling filters due to co-metabolic oxidation.

Personal Care Products

UV filters (e.g., oxybenzone): Removal is often limited (20–50%) due to low biodegradability, but can be enhanced with plastic media that retain biomass at higher concentrations.

Preservatives (parabens): Short-chain parabens undergo rapid primary biodegradation, although the ultimate fate of byproducts requires further study.

Fragrances (musk compounds): Polycyclic musks such as galaxolide and tonalide are moderately removed (50–85%), primarily by sorption and subsequent biodegradation.

Industrial Chemicals

Nonylphenol ethoxylates (surfactants) degrade through sequential removal of ethoxy groups, yielding nonylphenol—a persistent endocrine disruptor. Trickling filters with high SRT and diverse biofilms can achieve >90% reduction of the parent compounds, but careful management is required to prevent accumulation of recalcitrant metabolites.

Advantages of Trickling Filters for Emerging Contaminant Removal

  • Low energy consumption: Natural aeration eliminates the need for diffused aeration, reducing operational electricity demands by 50–75% compared to activated sludge systems.
  • Resilience to shock loads: The fixed biofilm buffers against toxic spikes and hydraulic fluctuations, which is beneficial for industrial influents containing intermittent pharmaceutical loads.
  • Consistent nitrification: Nitrifying trickling filters achieve stable ammonia oxidation, which contributes to removal of ECs through co-metabolism by ammonia-oxidizing bacteria.
  • Lower sludge production: The film-based growth produces less waste biomass per unit treatment, reducing costs and environmental impact of sludge handling.
  • Simplicity and reliability: Minimal mechanical components and operator attention make trickling filters suitable for decentralized or resource-limited settings.

Challenges and Design Considerations

Biofilm Thickness and Clogging

Excessive biofilm growth can lead to clogging (ponding), especially with stone media. This reduces ventilation and creates anaerobic zones, decreasing EC removal efficiency. Regular media flushing, recirculation rate control, and using high-void-volume plastic media mitigate this issue.

Temperature Sensitivity

Biological activity and mass transfer decrease at low winter temperatures. For many ECs, removal significantly declines below 10°C. Thermal insulation, deeper filter beds, and increased hydraulic loading (to maintain biomass contact) can partially compensate.

Inconsistent Removal for Recalcitrant Compounds

Certain ECs—such as some X-ray contrast media (iopromide), artificial sweeteners (acesulfame), and per- and polyfluoroalkyl substances (PFAS)—are poorly removed by conventional trickling filters. These compounds require polishing with activated carbon, advanced oxidation, or membrane filtration.

Metabolite Formation

Biodegradation can produce transformation products with equal or greater toxicity than the parent compound. For example, trimethoprim may produce hydroxylated forms that are more estrogenic. Monitoring the effluent for known metabolites and applying toxicity assays is recommended for systems treating EC-rich wastewater.

Integrating Trickling Filters with Other Technologies

No single technology can remove all ECs economically. Trickling filters are increasingly used as the primary biological stage in hybrid systems:

  • Trickling filter + activated sludge (TF/AS): Combining the robust biofilm of a trickling filter with the polishing capabilities of suspended growth enhances removal of biodegradable ECs and provides stability.
  • Trickling filter + ozonation: Ozone decomposes recalcitrant compounds and also controls biofilm thickness, improving overall performance.
  • Trickling filter + constructed wetland: A low-energy tertiary wetland can capture metabolites, pharmaceuticals, and trace metals, while providing wildlife habitat.
  • Trickling filter + granular activated carbon (GAC): GAC adsorbs ECs that escape biological treatment, with the biofilm on the carbon also enabling biodegradation (biological activated carbon).

Future Directions and Research Needs

Bioaugmentation with Specific Degraders

Scientists have isolated bacteria capable of degrading difficult ECs such as diclofenac and carbamazepine. Introducing these as biofilms on trickling filter media may speed up adaptation, provided they can compete with indigenous communities.

Optimization of Media Design

3D-printed biomimetic media with controlled surface roughness and porosity can maximize biofilm surface area and oxygen transfer. Research at pilot scale is evaluating media coated with nanomaterials that catalyze oxidation reactions (e.g., titanium dioxide under UV).

Real-Time Monitoring and Control

Online sensors for ammonia, dissolved oxygen, and UV absorbance (as a proxy for organic ECs) can guide recirculation rates and dosing, ensuring consistent removal while minimizing energy use.

Understanding the Role of Fungi

Filamentous fungi in trickling filter biofilms produce lignin-modifying enzymes that can break down a wide variety of organic pollutants, including certain pharmaceuticals and dyes. Encouraging fungal colonization without causing severe clogging remains a challenge.

Conclusion

Trickling filters represent a viable, low-energy biological technology for reducing the load of many emerging contaminants in wastewater effluents. Through mechanisms of biodegradation, sorption, and co-metabolism, these systems can achieve significant removal for diverse classes of pharmaceuticals, personal care products, and endocrine disruptors. While not a universal panacea—difficult compounds such as PFAS, iodinated contrast media, and some metabolites remain poorly removed—trickling filters offer an attractive base upon which to build more advanced treatment trains.

As regulatory limits tighten and public awareness grows, wastewater utilities will increasingly turn to tailored biological solutions that balance cost, energy, and performance. Investing in research on biofilm ecology, media innovation, and hybrid integration will further unlock the potential of trickling filters in protecting aquatic ecosystems and public health. For water professionals seeking sustainable, scalable approaches, the trickling filter deserves a renewed place in the toolbox for emerging contaminant management.

External resources for further reading:
US EPA – Contaminants of Emerging Concern
Removal of emerging contaminants by trickling filters (IWA Publishing)
Nature: Biofilm-based wastewater treatment and emerging contaminants