Case Study: A Nitrification-Denitrification Upgrade in the LA Basin
A wastewater treatment facility in the Los Angeles Basin began seeing elevated total nitrogen in its final effluent as influent conditions changed and discharge expectations became more demanding. Its existing activated sludge system reliably removed carbonaceous pollutants, yet it lacked the process flexibility needed for consistent ammonia oxidation and nitrate reduction.
The agency chose a phased nitrification-denitrification process upgrade instead of building an entirely new treatment train. The approach preserved useful infrastructure, reduced construction risk, and gave operators a manageable path toward biological nutrient removal. This case study follows the planning, design, commissioning, and performance lessons from that representative project.
The work also required close coordination among operators, process engineers, electrical specialists, laboratory staff, and maintenance teams. For water professionals across Southern California, the project illustrates how sound process fundamentals and disciplined implementation can produce measurable improvements without disrupting daily plant operations.
Defining the Treatment Problem
The facility served a mixed residential and commercial catchment with significant daily variation in flow and ammonia loading. During colder periods and high-flow events, the biological system struggled to sustain complete nitrification. Ammonia occasionally passed through the secondary clarifiers, while nitrate levels remained high because the existing process offered insufficient anoxic volume and carbon management.
The first step was a six-month review of operating data. The project team examined influent and effluent ammonia, nitrate, total nitrogen, dissolved oxygen, alkalinity, sludge age, return activated sludge rates, wasting records, and clarifier performance. Grab samples were paired with laboratory testing to identify whether the primary constraint was oxygen transfer, solids retention time, inadequate anoxic contact, or a combination of factors.
The review showed that the plant had adequate basin volume but poor use of that volume for nutrient removal. Aeration zones were operated with conservative dissolved oxygen targets, and internal recycle control was limited. The upgrade therefore focused on reconfiguring existing tanks, improving instrumentation, and adding targeted mechanical and electrical improvements.
Selecting a Practical Biological Process
Nitrification depends on maintaining a sufficient population of slow-growing nitrifying organisms. The design established a higher effective solids retention time by adjusting wasting controls and creating conditions that supported stable biomass retention. Operators received new control limits for mixed liquor suspended solids, sludge age, dissolved oxygen, and alkalinity so that daily decisions would support long-term biological stability.
For denitrification, the front portion of each biological train was converted to anoxic operation. Internal mixed liquor recycle returned nitrate-rich flow from the aerobic zone to the anoxic zone, where microorganisms used available carbon to convert nitrate into nitrogen gas. Because readily biodegradable carbon was limited during some operating periods, the design preserved the option for supplemental carbon dosing without making chemical addition the primary treatment strategy.
The team also evaluated upstream changes. A nearby asset retirement project demonstrated the importance of sequencing construction and maintaining hydraulic capacity while equipment is removed or repurposed. The plant’s safety and coordination procedures were informed by decommissioning guidance, particularly for lockout, confined-space access, temporary piping, and isolation planning.
Engineering the Upgrade Around Existing Assets
The physical work included new low-range dissolved oxygen probes, oxidation-reduction potential sensors in anoxic zones, variable-frequency drives for internal recycle pumps, and automated valve controls. Existing blowers were assessed for turndown capability and peak oxygen demand. Diffuser zones were balanced to avoid excessive aeration in areas where oxygen would interfere with denitrification.
Hydraulic modeling confirmed that the modified flow path could handle peak wet-weather conditions without creating unacceptable headloss. The design also considered clarifier solids loading, return activated sludge capacity, and the possibility of uneven distribution between parallel trains. Weirs, gates, and flow meters were calibrated before biological commissioning so that process performance would not be distorted by hydraulic imbalance.
Controls were written around operator decisions rather than technology alone. The supervisory control and data acquisition system displayed ammonia, nitrate, dissolved oxygen, recycle rates, and aeration demand on linked screens. High-ammonia alarms prompted investigation, while automatic responses were limited to proven control ranges. This gave operators visibility without allowing an untested algorithm to make large process changes during unstable conditions.
| Upgrade element | Existing condition | Implemented change | Primary purpose |
|---|---|---|---|
| Biological zones | Mostly aerobic operation | Dedicated anoxic and aerobic zones | Support nitrification and denitrification |
| Internal recycle | Fixed or manually adjusted | Variable-speed pumping with flow feedback | Return nitrate to anoxic conditions |
| Dissolved oxygen control | Limited monitoring | New probes and automated blower control | Match aeration to oxygen demand |
| Solids management | Broad wasting targets | Sludge-age-based wasting strategy | Retain nitrifiers consistently |
| Carbon availability | No defined contingency | Chemical dosing connection and testing plan | Protect denitrification during low-carbon periods |
| Operator interface | Separate process screens | Integrated nutrient-removal dashboard | Improve response and troubleshooting |
Managing Startup and Biological Maturation
Commissioning began with equipment checks, loop testing, sensor calibration, and wet testing of pumps and valves. The biological transition was deliberately gradual. Aeration patterns were changed in stages, internal recycle was increased incrementally, and wasting was adjusted to build the required nitrifying population without causing clarifier problems.
During the first weeks, ammonia removal improved before denitrification reached its target. This was expected: nitrifiers needed time to establish, while anoxic performance depended on carbon distribution, recycle rate, and oxidation-reduction conditions. Operators used laboratory results to distinguish biological maturation from instrumentation errors and adjusted process settings only after confirming the data.
The plant experienced one temporary increase in effluent ammonia following a high-flow event that reduced solids retention. The response included reducing wasting, checking alkalinity, confirming blower capacity, and reviewing clarifier blanket levels. The event reinforced the value of a documented upset protocol. A process upgrade performs best when staff know which variables to protect first and which adjustments should wait.
Measuring Results and Operational Value
After several sludge ages had passed and seasonal data became available, the facility recorded a substantial improvement in nitrogen performance. Average effluent ammonia fell from approximately 8.5 mg/L as nitrogen to below 1.0 mg/L during normal operating conditions. Total inorganic nitrogen declined from roughly 24 mg/L to 11 mg/L, with the strongest results occurring when influent carbon-to-nitrogen ratios were favorable.
Energy use increased modestly because nitrification requires oxygen, but blower optimization limited the increase. Variable-speed drives, improved diffuser maintenance, and ammonia-informed aeration reduced unnecessary oxygen delivery. The facility also avoided the cost and schedule disruption associated with constructing a separate biological nutrient removal facility.
Performance tracking continued through a dashboard that combined online readings with laboratory verification. Monthly reviews examined nutrient trends, alkalinity consumption, sludge age, recycle pump operation, clarifier behavior, and chemical use. This made it easier to identify gradual drift before it became a permit concern.
The project’s organizational value was significant as well. Operators gained confidence with nutrient-removal controls, maintenance staff became familiar with new instruments and drives, and engineering staff developed a repeatable method for evaluating process changes. Professional networks such as LABS of CWEA can reinforce that knowledge through technical presentations, facility tours, workshops, and peer exchange. The section’s annual awards program also reflects the profession’s emphasis on practical achievement and public service.
Building Skills for Long-Term Reliability
A commissioning project can lose its benefits if training ends when construction is complete. The agency created standard operating procedures for startup, low-temperature operation, wet-weather response, sensor failure, low alkalinity, and suspected nitrifier loss. Each procedure identified normal ranges, alarm thresholds, immediate actions, and escalation points.
Cross-training was equally important. Operators learned how recycle changes affected anoxic conditions, while electricians and instrumentation technicians learned how probe fouling or signal drift could influence automated aeration. Supervisors scheduled refresher sessions after the first seasonal cycle, when staff had real operating examples to discuss.
The agency also established an annual process review. This included verification of oxygen transfer performance, calibration of nutrient sensors, review of chemical feed readiness, and comparison of actual results with design assumptions. Leadership continuity helps sustain these practices; the organization’s section leadership history offers a broader example of how professional institutions preserve knowledge across changing personnel and priorities.
Practices That Strengthen Similar Projects
A successful nutrient-removal upgrade depends on disciplined preparation as much as on equipment selection. The following practices helped the facility control risk and achieve stable performance:
- Build the design from verified operating data, including seasonal ammonia, nitrate, flow, alkalinity, and solids information.
- Preserve hydraulic flexibility so parallel treatment trains can remain balanced during peak and maintenance conditions.
- Commission instrumentation and control loops before changing biological operating conditions.
- Protect solids retention time during startup, upset recovery, and wet-weather events.
- Train operators with real plant data and document response steps for sensor, blower, recycle, and clarifier problems.
The project also showed why early communication matters. Operators should participate in process selection, control narrative development, equipment testing, and acceptance reviews. Their practical knowledge often identifies access limitations, maintenance conflicts, or alarm strategies that are invisible in a design office.
Water and wastewater professionals can apply these lessons through peer learning and continuing education. Technical gatherings, automation workshops, MOC certification courses, and facility tours create useful opportunities to compare nutrient-removal strategies with colleagues facing similar regulatory, staffing, and infrastructure conditions.
For organizations planning a nitrogen-control project in the Los Angeles Basin, LABS of CWEA provides a direct connection to professionals working across engineering, operations, consulting, and public agencies. Explore upcoming educational opportunities, participate in section activities, and bring these implementation lessons into the next process review or capital planning discussion.