Integrating Fixed-Film Biofiltration into Primary Treatment Processes

As wastewater treatment facilities face increasing constraints on footprint, energy consumption, and capital expansion, there is growing interest in treatment technologies that intensify performance within existing process boundaries. Conventional primary clarification is largely limited to gravity-driven particulate solids removal and provides minimal attenuation of soluble organic matter, thereby placing the full burden of biological oxidation on downstream secondary treatment processes. This conventional separation of physical and biological treatment functions results in large reactor volumes, high aeration demand, and limited operational flexibility. This study evaluates an integrated primary biofiltration approach that introduces fixed-film biological activity into a primary treatment configuration, effectively redefining primary treatment as an intensified, multifunctional unit process capable of combining physical filtration and biological oxidation. An up-flow floating media biofiltration pilot system was designed and operated over a seven-month period (August 2024-March 2025) at the Linda County Water District municipal wastewater treatment plant in Northern California. The reactor consisted of two vertically stacked floating media zones: a lower, unaerated filtration zone providing physical capture of suspended solids, and an upper, aerated zone supporting attached biofilm growth and biological oxidation. This configuration enabled concurrent solids removal and biological treatment within a single vessel, while maintaining short hydraulic retention times compatible with primary treatment applications. Pilot testing was conducted at empty bed contact times (EBCTs) of 20, 30, 60, and 120 minutes to evaluate system performance across a range of hydraulic and organic loading conditions representative of both high-rate and extended-contact operation. Treatment performance was assessed using total suspended solids (TSS), five-day biochemical oxygen demand (BOD), and soluble chemical oxygen demand (sCOD) as key indicators of physical and biological treatment efficacy. Results demonstrate that integrating fixed-film biological activity into a primary treatment configuration can substantially intensify organic removal at hydraulic retention times far shorter than those required for conventional secondary treatment. At EBCTs of 20-30 minutes, the system consistently achieved TSS removals on the order of 80%, exceeding values typically reported for conventional primary clarification, while simultaneously achieving measurable reductions in soluble organics (50-59% sCOD removal). These results indicate that the system provided meaningful attenuation of both particulate and soluble organic loading under short-contact conditions relevant to high-rate treatment. At an EBCT of 60 minutes, average BOD removal exceeded 80% and sCOD removal exceeded 70%, demonstrating that a significant fraction of biological oxidation typically assigned to secondary treatment could be achieved upstream within a compact primary-scale reactor. Analysis of effluent sCOD:BOD ratios revealed a progressive shift toward more stabilized organic fractions with increasing EBCT, confirming that soluble organic removal was driven by biological oxidation within the attached biofilm rather than physical filtration alone. Importantly, solids removal performance remained stable across all tested EBCTs, indicating that biological activity did not compromise filtration performance and that the integrated system was resilient to hydraulic variability. These findings suggest that the combined physical-biological configuration can decouple solids capture from biological kinetics while maintaining consistent treatment performance. High oxygen transfer performance was observed within the aerated biofiltration zone, sustaining biofilm growth and enabling biological oxidation at elevated areal and volumetric rates. Measured oxygen transfer efficiencies per unit depth ranged from 1.10 to 1.69% OTE·ft⁻¹ exceeding values commonly reported for conventional activated sludge systems, including those equipped with fine-pore diffusers. These elevated oxygen transfer efficiencies are notable given the relatively shallow reactor depth and primary-scale configuration, and they suggest favorable gas-liquid mass transfer conditions associated with the floating media architecture and internal hydraulics. At extended EBCTs, partial nitrification was observed, further confirming the biological functionality of the integrated system and indicating that the reactor environment supported both heterotrophic and autotrophic microbial activity. Similar performance trends were independently documented at a separate facility in Ohio in 2025, supporting the broader applicability of the observed mechanisms. Collectively, these results demonstrate that biologically active primary biofiltration can shift a meaningful portion of organic oxidation upstream of secondary treatment, thereby reducing the biological load imposed on downstream processes. From a treatment-train perspective, this redistribution of treatment functions has implications for secondary reactor sizing, aeration demand, and solids production dynamics. By consolidating physical filtration and biological oxidation within a single intensified unit process, integrated primary biofiltration offers a pathway toward more compact, energy-efficient, and adaptable wastewater treatment configurations. This work reframes primary treatment not as a passive, gravity-driven step, but as an active and biologically relevant process with the potential to simplify treatment trains and enhance overall system performance.