Retrofitting a Trickling Filter Plant for Biological Nutrient Removal
Aging wastewater treatment plants often have valuable infrastructure that remains structurally sound long after its original process objectives have changed. Retrofitting that infrastructure for biological nutrient removal (BNR) can expand treatment capacity, reduce chemical use, and meet tighter discharge limits without building an entirely new facility.
This case study follows a representative Los Angeles Basin water resource recovery facility that converted a conventional trickling filter plant into a process capable of nitrification, denitrification, and biological phosphorus uptake. The project required careful hydraulic analysis, new aeration and mixing systems, operational retraining, and a phased startup strategy.
The experience illustrates how engineers and operators can combine fixed-film treatment assets with suspended-growth biology. It also shows why successful BNR projects depend as much on process control and workforce preparation as on concrete, pumps, and instrumentation.
Defining the Plant’s Treatment Gap
The facility treated approximately 18 million gallons per day using primary clarification, trickling filters, secondary clarification, and chlorine disinfection. The original process reliably removed carbonaceous pollutants and produced partial nitrification during warm weather. However, seasonal ammonia exceedances and a new total nitrogen limit created the need for more consistent nitrogen removal.
The plant had several advantages. Its influent pump station had spare wet-weather capacity, the primary clarifiers were in good condition, and the secondary clarifiers had sufficient surface area for a moderate increase in solids loading. The main constraints were limited basin volume, low dissolved oxygen control capability, and a process flow scheme designed before nutrient limits became common.
Rather than abandon the trickling filters, the design team treated them as an upstream biological polishing step. The filters could continue reducing biochemical oxygen demand and converting part of the ammonia load, while newly created suspended-growth zones would provide reliable denitrification and enhanced biological phosphorus removal.
Converting Existing Structures
The retrofit divided two underused aeration basins into anaerobic, anoxic, and aerobic zones. Internal baffles created the required contact sequence, while submersible mixers maintained solids suspension in zones without aeration. Fine-bubble diffusers were installed in the aerobic compartments, supported by variable-frequency blowers and automated dissolved oxygen control.
A portion of the trickling filter return flow was redirected to improve oxygen transfer and distribute nitrified liquor. More importantly, a dedicated internal mixed-liquor recycle moved nitrate-rich flow from the aerobic zone back to the anoxic zone. A separate return activated sludge system brought settled biomass from the secondary clarifiers to the anaerobic selector, where readily biodegradable carbon could support phosphorus-accumulating organisms.
The plant retained the trickling filters as roughing and nitrifying units, but the retrofit introduced a more controllable biological process downstream. Operators could adjust recycle rates, aeration intensity, wasting, and zone volumes in response to loading conditions. This flexibility was essential because the influent carbon-to-nitrogen ratio was lower than ideal for denitrification during some winter and wet-weather periods.
Designing for Carbon, Solids, and Flow
The process model used several years of influent records rather than relying on average-day conditions. Peak ammonia, minimum temperature, primary effluent soluble carbon, return sludge concentration, and wet-weather dilution all influenced the sizing and operating strategy. The design target was an effluent total nitrogen concentration below 10 milligrams per liter, with a phosphorus target of 1 milligram per liter or less.
The analysis showed that the plant could achieve the nitrogen target with approximately 10 hours of total biological solids retention time, provided the aerobic zone maintained adequate nitrifying biomass. A mixed-liquor suspended solids concentration near 2,500 milligrams per liter provided a practical balance between treatment capacity, clarifier loading, oxygen demand, and pumping costs.
Several modifications helped preserve performance during low-carbon conditions. Primary clarifier operation was adjusted to avoid excessive removal of soluble biodegradable material. Fermentation of primary sludge was added as a future-ready feature, allowing the plant to generate volatile fatty acids for biological phosphorus removal. Chemical phosphorus polishing remained available as a backup during startup and unusual loading events.
| Retrofit element | Existing condition | Modification | Operating benefit |
|---|---|---|---|
| Trickling filters | Carbon removal and seasonal nitrification | Retained as upstream biological treatment | Reduced load on new suspended-growth zones |
| Aeration basins | Limited process zoning | Added anaerobic, anoxic, and aerobic compartments | Enabled nitrogen and phosphorus removal |
| Mixing | Minimal basin circulation | Installed submersible mixers in anoxic zones | Prevented settling without adding oxygen |
| Internal recycle | No dedicated nitrate return | Added variable-speed recycle pumps | Supplied nitrate to denitrification |
| Aeration | Constant-speed blowers | Added fine-bubble diffusers and DO control | Lowered energy use and stabilized nitrification |
| Secondary clarification | Existing solids separation | Optimized sludge blanket and return rates | Protected effluent quality during higher MLSS |
| Instrumentation | Periodic manual testing | Added online ammonia, nitrate, DO, and ORP sensors | Improved real-time process control |
The retrofit also required hydraulic checks that were easy to overlook. New baffles, diffusers, mixer supports, and recycle channels reduced available volume and introduced additional headloss. Computational fluid dynamics and field tracer testing helped confirm that short-circuiting would not undermine the intended anaerobic-to-aerobic sequence.
Commissioning the BNR Process
Startup began with clean-water testing of blowers, mixers, recycle pumps, valves, analyzers, and supervisory control functions. The plant then seeded the new suspended-growth zones with return activated sludge from a neighboring nitrifying facility. Operators increased solids inventory gradually while monitoring ammonia, nitrite, nitrate, alkalinity, and settleability.
The first operational challenge was unstable dissolved oxygen distribution. Excessive air in the first aerobic compartment caused nitrate production before the biomass had adequate opportunity to remove phosphorus under anaerobic conditions. Rebalancing diffuser headers and narrowing the DO setpoint restored the intended biological sequence.
A second challenge involved clarifier blanket rise during periods of filamentous growth. The team responded by tightening wasting control, correcting low-oxygen pockets, and adjusting return activated sludge rates. These changes demonstrate why BNR commissioning cannot be reduced to equipment acceptance testing; the biological community needs time, observation, and disciplined adjustments.
Within six months, average effluent ammonia fell below 1 milligram per liter, while total nitrogen stabilized between 6 and 9 milligrams per liter under normal conditions. Total phosphorus generally remained below 1 milligram per liter, with supplemental chemical dosing used during high-flow events and low-carbon periods.
Controlling Energy and Reliability
The new aeration system became the largest electrical load in the treatment process. Automatic dissolved oxygen control reduced blower output during low-load periods, while ammonia-based aeration trim prevented unnecessary oxygen delivery. Operators also used nitrate trends to avoid over-aerating the final aerobic zone when downstream denitrification capacity was available.
The facility’s cogeneration system supplied part of the electrical demand from anaerobic digester gas. Heat recovery supported digester temperature control and selected building loads. Because BNR increased the importance of stable power and instrumentation, the maintenance team incorporated generator testing, gas conditioning checks, and blower sequencing into a common reliability program. Guidance on cogeneration system troubleshooting helped frame these assets as interconnected parts of treatment resilience rather than isolated maintenance specialties.
Energy performance improved after the first year of tuning. Although the retrofit increased total aeration demand compared with the former process, lower blower discharge pressure, better diffuser efficiency, and automated control kept the increase manageable. The facility also gained operational visibility through trend displays that linked air flow, ammonia removal, nitrate recycle, and effluent compliance.
Preparing People for Process Change
BNR altered daily work for every operating shift. Staff had to understand why an anaerobic zone should contain no measurable dissolved oxygen, why anoxic mixers must run continuously, and how wasting decisions affect nitrification several days later. Laboratory personnel also adopted more frequent sampling during the transition from conventional trickling filter operation.
Cross-training reduced dependence on a small group of process specialists. Operators learned basic instrumentation checks, electricians became familiar with analyzer fault modes, and maintenance staff received practical instruction on mixer inspection and diffuser fouling. The plant used a structured cross-training program to document competencies and assign backup coverage for critical tasks.
Professional networks can reinforce this internal preparation. Technical workshops, facility tours, and automation courses give plant teams access to comparable operating experiences, while recognition programs encourage staff to share improvements. Leadership continuity also matters during multiyear upgrades; the experience of past presidents reflects how sustained professional stewardship can support water-sector innovation across changing technologies and regulations.
Practices That Sustained Performance
After commissioning, the facility formalized a short list of operating disciplines:
- Trend ammonia, nitrate, dissolved oxygen, oxidation-reduction potential, and alkalinity together rather than reviewing isolated readings.
- Maintain a wasting strategy that protects nitrifying solids retention time during cold weather.
- Calibrate online analyzers on a defined schedule and compare readings with laboratory results.
- Inspect mixers, diffusers, recycle pumps, and control valves before seasonal peak loads.
- Keep chemical phosphorus removal available as a controlled contingency, not as a substitute for biological process management.
The plant also created alarm response guides that connected each instrument signal to a likely process cause. For example, rising effluent ammonia with falling aerobic-zone DO prompted an air delivery check, while rising nitrate in the final anoxic zone triggered review of carbon availability and internal recycle distribution.
The retrofit succeeded because the facility used existing assets selectively instead of preserving every legacy practice. Trickling filters continued to provide useful treatment, while new BNR zones supplied the process control required for modern nutrient limits. The resulting system was more adaptable, more measurable, and better aligned with the capabilities of its workforce.
Water and wastewater professionals can apply these lessons when evaluating their own facilities. Share this case study with design, operations, maintenance, and laboratory teams, then use a local workshop, technical presentation, or facility tour to turn the concepts into a practical retrofit strategy.