Converting a Trickling Filter to an Integrated Fixed-Film System
Aging wastewater treatment plants often have valuable structures that no longer provide enough capacity for current permit limits. Replacing an entire biological process can require new land, major construction, and long shutdowns. Converting an existing trickling filter into an integrated fixed-film activated sludge system offers another path: retain useful infrastructure while adding the aeration, biomass, and process control needed for stronger treatment.
This case study follows a representative Southern California retrofit at a municipal wastewater facility. The project illustrates how a trickling filter conversion can improve nitrification, increase organic loading capacity, and support more consistent effluent quality without building a completely new treatment train.
The project also demonstrates why biological upgrades require coordination among design engineers, operators, laboratory staff, and regulators. Equipment selection matters, but so do hydraulic profiles, oxygen transfer, instrumentation, process startup, and the operating team’s ability to respond to changing loads.
The Need For A Retrofit
The facility treated an average daily flow of approximately 8 million gallons per day, with peak wet-weather flows reaching 14 million gallons per day. Its existing trickling filter had served reliably for years, but influent ammonia loads had increased as the service area expanded. The plant also faced tighter nitrogen expectations associated with its coastal discharge permit.
The original process removed carbonaceous biochemical oxygen demand effectively under normal conditions. Nitrification, however, was inconsistent during colder weather, high-flow events, and periods of elevated ammonia loading. The downstream clarifiers also experienced occasional solids carryover when operators increased return activated sludge rates to compensate for weak biological performance.
A conventional expansion would have required additional aeration basins and clarifier capacity. Available land was limited, and the utility wanted to avoid extended construction inside an operating plant. The retrofit objective became clear: use the existing biological footprint, increase attached-growth capacity, and preserve as much hydraulic flexibility as possible.
Establishing The Design Basis
Before selecting media or blowers, the project team completed a condition assessment. The review covered the trickling filter shell, underdrains, walls, access structures, electrical service, downstream clarifiers, pumping systems, and odor-control equipment. Existing rock media was evaluated for structural condition and biological usefulness. Because the original media geometry was not designed for submerged aeration, the final concept replaced it with engineered fixed-film carriers.
The conversion design placed the former trickling filter within an integrated process that combined attached growth and suspended growth. The retrofit included carrier-retention screens, fine-bubble diffusers, new air headers, automated dissolved oxygen control, improved mixing, and modifications to inlet and outlet channels. The existing structure became a biological reactor rather than a conventional pass-through trickling filter.
The design basis targeted an average effluent ammonia concentration below 2 milligrams per liter, while maintaining reliable carbon removal. Engineers modeled peak hydraulic conditions, oxygen demand, alkalinity consumption, media fill fraction, solids separation, and minimum dissolved oxygen. These calculations were paired with bench testing and a staged field evaluation to confirm that the selected carriers could remain in place under the plant’s hydraulic conditions.
Building The Integrated Process
The key process change was submerging the fixed-film media and supplying oxygen through a dedicated aeration grid. Biofilm grew on the carrier surfaces, while suspended microorganisms remained active in the surrounding mixed liquor. This combination gave the system two treatment populations: attached nitrifiers with long effective solids retention time and suspended organisms that responded quickly to soluble organic loading.
The retrofit did not treat the carrier media as a substitute for activated sludge. Instead, the fixed film supplemented the suspended-growth process. Operators retained control over return activated sludge, wasting, dissolved oxygen, and mixed-liquor concentration. This allowed the plant to adjust the balance between attached and suspended biomass as influent conditions changed.
| Design or operating feature | Existing trickling filter | Integrated fixed-film retrofit |
|---|---|---|
| Primary biological mechanism | Unsaturated attached growth | Submerged attached growth plus suspended growth |
| Oxygen delivery | Natural draft and passive airflow | Fine-bubble aeration with blower control |
| Nitrification capacity | Variable under peak loading | Increased through retained nitrifying biofilm |
| Process control | Limited online adjustment | Dissolved oxygen, airflow, level, and ammonia monitoring |
| Major construction approach | Existing reactor structure | Reused structure with new media, diffusers, and controls |
| Solids management | Secondary clarification dependent on filter operation | Integrated control of suspended solids and return sludge |
The aeration system became one of the most important design elements. Too little air would limit nitrification and create septic zones; excessive air would waste energy and could disturb solids separation. Variable-frequency drives, zone-level airflow control, and dissolved oxygen probes allowed the plant to match oxygen delivery with real-time demand.
Results From Startup And Operation
Startup began with clean-water testing, diffuser verification, and carrier-retention inspections. The team then introduced wastewater gradually, maintaining conservative loading while biofilm developed. Ammonia performance improved in stages rather than immediately. During the first several weeks, operators tracked alkalinity, pH, dissolved oxygen, temperature, ammonia, nitrate, and suspended solids at multiple points through the process.
After approximately four months, the retrofit consistently produced effluent ammonia below the project target during normal flows. At the same time, the plant maintained stable carbon removal and reduced the frequency of clarifier upsets. The most significant gains appeared during periods when the suspended-growth inventory alone would previously have been insufficient for nitrification.
Energy use increased because the former trickling filter now required mechanical aeration. However, the plant avoided the capital and operational burden of constructing a separate biological expansion. Energy optimization later focused on reducing excess dissolved oxygen, correcting diffuser airflow distribution, and using ammonia-based aeration control during lower-load periods.
The project also improved operational resilience. Attached biomass remained available after short periods of elevated flow or unusual loading, giving the plant a biological buffer while suspended solids and wasting rates were rebalanced. Operators still needed to monitor media fouling, screen condition, oxygen transfer, and clarifier performance; fixed-film technology reduced risk but did not eliminate routine process management.
Compliance And Long-Term Resilience
A retrofit must be evaluated against the plant’s complete regulatory setting, not only its ammonia target. For a facility discharging to a sensitive coastal receiving water, permit changes can affect monitoring, toxicity controls, nitrogen expectations, reporting, and bypass documentation. The project team used coastal permit guidance to keep the design review connected to current NPDES obligations.
Water conservation and changing influent patterns also influenced the operating strategy. Lower indoor water use can increase wastewater strength even when average flow declines. That shift affects oxygen demand, alkalinity, solids production, and hydraulic loading. Planning resources on drought conservation helped the utility consider how conservation measures could alter future process conditions.
The upgraded process supported resilience in several ways. It used existing concrete structures, reduced dependence on new land, and provided additional nitrification capacity within the current footprint. The design also included bypass and isolation provisions so individual zones could be inspected or serviced without taking the entire biological process offline.
Implementation Priorities
A successful conversion depends on disciplined planning well before construction begins. Utilities considering a similar project should focus on the following priorities:
- Verify the structural condition, hydraulics, and usable volume of the existing trickling filter.
- Characterize seasonal ammonia, organic loading, temperature, alkalinity, and peak-flow conditions.
- Select media, retention screens, diffusers, and mixing equipment as one integrated system.
- Provide reliable online monitoring for dissolved oxygen, ammonia, airflow, level, and pH.
- Build a staged commissioning plan that gives nitrifying organisms time to establish.
- Train operators on attached-growth behavior, carrier inspection, aeration control, and solids management.
Procurement should also account for long-term maintenance. Carrier-retention screens need access for cleaning, diffusers need inspection and replacement provisions, and instrumentation must be installed where readings represent actual process conditions. A low-cost retrofit can become expensive if operators cannot reach critical equipment or if sensors foul without a practical maintenance plan.
The human side of implementation is equally important. Operators who understand the biological process can identify whether a performance change comes from oxygen limitation, insufficient alkalinity, hydraulic short-circuiting, biomass loss, or clarifier stress. Professional organizations such as LABS of CWEA provide technical programs, facility connections, workshops, and peer learning that can strengthen this knowledge before and after commissioning.
A trickling filter conversion is most effective when it is treated as a complete process upgrade rather than a media replacement. Begin with a site-specific feasibility study, confirm the regulatory design basis, and involve operations staff in every major decision. Connect with LABS of CWEA to access water-sector education and professional networks that can help turn an innovative retrofit into dependable treatment performance.