Microplastic pollution has emerged as a pervasive environmental contaminant, infiltrating aquatic ecosystems, soil, and even the atmosphere. Wastewater treatment plants (WWTPs) serve as both a major pathway for microplastics entering the environment and a potential point of intervention. Among the various treatment technologies, trickling filters—a classic biofilm-based system—rely heavily on robust and diverse microbial communities to degrade organic pollutants. The introduction of microplastics into these systems poses a novel threat, potentially disrupting the delicate microbial ecology and compromising treatment performance. Understanding the mechanisms by which microplastics affect trickling filter biofilms is critical for optimizing plant operations and protecting downstream water quality.

Sources and Characteristics of Microplastics in Wastewater

Microplastics are defined as plastic particles smaller than 5 mm in their longest dimension. They enter wastewater from a wide range of sources, including synthetic textile fibers from laundry, microbeads from personal care products (now banned in many jurisdictions but still present in legacy systems), fragments from plastic packaging degradation, and tire wear particles from road runoff. A typical WWTP influent can contain thousands of microplastic particles per cubic meter. The particles vary in shape (fibers, fragments, films, spheres), polymer type (polyethylene, polypropylene, polystyrene, polyester), and density—properties that influence their transport and interaction with biological treatment processes. Many microplastics also carry chemical additives such as phthalates, bisphenol A, and flame retardants, or sorb hydrophobic organic pollutants and heavy metals from the wastewater, acting as vectors for toxic compounds.

The Microbial Community of Trickling Filters

Trickling filters consist of a fixed bed of media (rock, plastic, or slag) over which wastewater is distributed. A complex biofilm develops on the media surface, composed of bacteria, archaea, fungi, protozoa, and occasionally small metazoa. The biofilm is stratified, with aerobic layers near the surface and anoxic or anaerobic zones deeper within. Key functional groups include heterotrophs that oxidize organic carbon (BOD), nitrifiers (ammonia-oxidizing bacteria and nitrite-oxidizing bacteria), and denitrifiers in deeper layers. The structure of the biofilm—its thickness, porosity, and extracellular polymeric substance (EPS) matrix—is critical for mass transfer and resistance to shear. The microbial diversity and redundancy within the community provide resilience against environmental stresses and loading fluctuations.

Mechanisms of Microplastic Impact on Biofilm Communities

Physical Interference and Biofilm Architecture

Microplastic particles can physically embed within the biofilm matrix, altering its porosity and reducing the diffusion of oxygen and nutrients to deeper layers. Fibers, in particular, may entangle with EPS, leading to clogging and sloughing. The presence of hard, non-degradable particles can also increase shear stress, causing premature detachment of biofilm—especially during hydraulic surges. Studies have shown that microplastic accumulation can reduce biofilm thickness and total biomass, diminishing the overall metabolic capacity of the filter.

Chemical Toxicity and Leachate Effects

Many microplastic particles leach additive chemicals that are toxic to microorganisms. Phthalates and bisphenol A, for example, have been shown to inhibit the growth and activity of nitrifying bacteria, which are particularly sensitive to environmental contaminants. Moreover, microplastics sorb and concentrate other pollutants from the wastewater, potentially delivering high local doses of heavy metals or pesticides to biofilm cells. This chemical stress can lead to reduced enzymatic activity and shifts in community composition, favoring stress-tolerant but less efficient species.

Alteration of Microbial Community Structure and Function

Exposure to microplastics often results in a decline in microbial diversity and a shift toward biofilm-forming or plastic-degrading taxa. For instance, some studies have observed an increase in the relative abundance of Pseudomonas and Bacillus species, which are known to produce EPS and can degrade certain plastics, at the expense of sensitive nitrifiers and specialized degraders. This functional restructuring can impair key treatment processes such as nitrification and the removal of recalcitrant organic compounds. Additionally, the selective pressure exerted by microplastics may reduce the functional redundancy of the biofilm, making the system more vulnerable to further perturbations.

Consequences for Treatment Efficiency

Decreased Organic Matter Removal

The loss of heterotrophic activity due to biofilm disruption directly affects the removal of biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Pilot-scale studies have reported a 10–30% reduction in BOD removal efficiency in trickling filters receiving microplastic-contaminated influent compared to controls. This can lead to higher effluent organic loads, increasing the burden on subsequent treatment stages or the receiving water body.

Impaired Nitrification and Ammonia Removal

Nitrifying bacteria are slow-growing and sensitive to toxicants, making them prime targets for microplastic-associated stress. Reduced nitrification rates have been observed in both laboratory-scale biofilm reactors and full-scale trickling filters, with ammonia concentrations in the effluent rising by up to 40% in some cases. This compromises the plant’s ability to meet discharge permits for nitrogen, forcing operators to increase aeration or add chemicals to compensate.

Increased Sludge Production and Operational Costs

The physical accumulation of microplastics in the filter bed can contribute to clogging, increasing backwashing frequency and energy consumption. Biofilm destabilization also leads to more frequent sloughing events, resulting in higher solids loading to secondary clarifiers. The combined effect is increased operational costs for maintenance, energy, and sludge handling. Furthermore, the presence of microplastics in biosolids limits their beneficial reuse in agriculture, posing a disposal challenge.

Vector for Pathogens and Antibiotic Resistance

Microplastics can act as substrates for biofilm formation by pathogenic bacteria and serve as a platform for horizontal gene transfer, including antibiotic resistance genes. This may elevate the risk of effluent carrying antibiotic-resistant organisms, a growing public health concern that wastewater treatment is expected to mitigate.

Mitigation and Management Strategies

Source Control and Pre-Treatment

The most effective strategy is to reduce microplastic inputs at the source. Regulations banning microbeads in rinse-off products, promoting natural fiber clothing, and improving stormwater management for tire wear particles are essential. Within WWTPs, installing fine screens (≤1 mm) or dynamic filtration systems at the headworks can remove a significant fraction of larger microplastics before they reach biological treatment. An emerging technology is the use of coagulation and flocculation followed by dissolved air flotation (DAF) to remove microplastics physically, though these processes add chemical and energy costs.

Optimizing Trickling Filter Operation

Operators can modify trickling filter parameters to mitigate microplastic impacts. Increasing the hydraulic loading rate may help flush out accumulated particles, but must be balanced against biofilm retention. Improving pre-treatment to reduce solids loading can minimize particle entrapment. Additionally, ensuring adequate dissolved oxygen levels and nutrient balance can support healthier biofilm resilience.

Bioaugmentation and Microbial Management

Research is exploring the use of plastic-degrading microbial strains to bioaugment trickling filters, potentially breaking down microplastics into less harmful byproducts. However, this approach is still in early stages and raises questions about byproduct toxicity and ecological impacts. Another avenue is the selection of microplastic-tolerant microbial communities through controlled acclimation, though this may not restore full functional diversity.

Monitoring and Early Warning Systems

Regular monitoring of microplastic concentrations in influent and effluent, combined with biofilm health indicators (such as ATP content, respirometry, or metagenomic profiling), can help plant operators detect problems early. Real-time sensors for turbidity or particle counts can indirectly signal microplastic accumulation. Developing predictive models that correlate microplastic loads with performance declines would enable proactive management.

Future Directions and Research Needs

Despite growing awareness, significant knowledge gaps remain. Most studies have been conducted at lab scale with high microplastic concentrations that may not reflect real-world scenarios. Long-term field studies across different climates and wastewater compositions are needed. The role of nanoplastics (particles <1 µm) in trickling filter performance is almost entirely unexplored. Additionally, the interactions between microplastics, biofilm, and protozoan grazers—which control bacterial populations in filters—warrant investigation. Finally, the economic analysis of implementing microplastic removal technologies vs. the cost of downstream environmental damage should be conducted to guide investment decisions.

Conclusion

Microplastic pollution represents a significant and under-recognized threat to the biological processes that underpin wastewater treatment in trickling filters. By physically disrupting biofilms, delivering toxic chemicals, and altering microbial community structure, microplastics can impair BOD and ammonia removal, increase operational costs, and potentially facilitate the spread of antibiotic resistance. A multifaceted approach combining source reduction, improved pre-treatment, operational adjustments, and ongoing research is essential to protect the efficiency and sustainability of these vital treatment systems. As plastic production continues to rise, the wastewater sector must adapt to safeguard water quality and public health.

For further reading, see the EPA's microplastics research, a comprehensive review on microplastics in wastewater treatment, and a study on biofilm response to microplastics.