LABS of CWEA

Retrofitting an old plant with advanced oxidation processes

Aging wastewater treatment plants are being asked to address contaminants and discharge limits that were not part of their original design. Facilities built around primary clarification, activated sludge, and conventional disinfection may now need to reduce trace organic compounds, improve effluent quality, or prepare for potable reuse without replacing every major process unit.

This case study follows an anonymized Southern California water reclamation facility that added an advanced oxidation process (AOP) to an operating plant. The project focused on UV-based oxidation with hydrogen peroxide, using existing tanks, electrical infrastructure, and hydraulic channels wherever practical. Its experience offers a useful framework for engineers, operators, consultants, and agency managers evaluating similar upgrades.

The central lesson was that successful AOP implementation depended less on installing sophisticated equipment than on controlling the conditions around it. Pretreatment, ultraviolet transmittance, peroxide dosing, hydraulics, monitoring, and operator training all influenced the final result.

Establishing the retrofit target

The facility treated approximately 45 million gallons per day during average dry-weather conditions. Its original treatment train included screening, grit removal, primary clarification, biological nutrient removal, secondary clarification, filtration, and chlorination. The plant met its existing discharge requirements, but its owner was preparing for stricter limits on trace contaminants and considering future indirect potable reuse.

The project team first defined the treatment objective instead of selecting equipment prematurely. The immediate goal was to reduce a group of indicator compounds, including pharmaceuticals and personal-care product residues, after filtration. The process also had to maintain reliable pathogen control and avoid creating an excessive peroxide residual or unwanted oxidation byproducts.

Sampling showed that the secondary and filtration processes already removed most suspended solids. However, ultraviolet transmittance varied significantly after storms and during periods of unusual industrial loading. Since UV light must pass through the water to activate the oxidant, these fluctuations became a major design consideration.

Working with existing infrastructure

The retrofit had to fit within a crowded site with limited space for new concrete basins. The engineering team selected a former chlorine contact channel for the UV reactor and converted an adjacent chemical area into a peroxide storage and feed room. This approach reduced civil construction, shortened the schedule, and allowed much of the existing process to remain in service.

A hydraulic profile was developed before equipment procurement. The analysis examined peak flow, bypass conditions, upstream filter performance, reactor head loss, and the available elevation difference between filtration and discharge. The plant could not tolerate a large pumping requirement, so the UV reactor was specified with low head loss and multiple parallel channels.

The electrical review identified another constraint. The existing motor control center had little spare capacity, and the original power distribution system was not designed for high-output UV lamps. The project added a dedicated electrical section, variable-frequency drives for selected pumps, emergency power connections, and interlocks tied to flow, UV intensity, and peroxide dosing.

The team also reviewed lessons from UV disinfection practice, particularly the relationship between UV performance, effluent quality, and residual management. That experience helped the facility separate disinfection requirements from the more demanding oxidation objectives.

Selecting and piloting the AOP train

Several treatment configurations were considered, including ozone followed by biological filtration, UV with hydrogen peroxide, and UV combined with a catalyst. Ozone offered strong oxidation potential but would have required additional gas-generation equipment, off-gas control, and new contact basins. UV and peroxide could use the existing channel and had a smaller construction footprint.

A six-month pilot compared several UV fluence levels and peroxide-to-flow ratios. Operators tested the system under normal conditions, high turbidity events, variable alkalinity, and different levels of dissolved organic carbon. The pilot showed that increasing peroxide did not automatically improve treatment. At excessive doses, peroxide scavenged hydroxyl radicals and raised chemical consumption without producing proportional contaminant reduction.

The selected design used staged peroxide injection. A smaller initial dose was mixed upstream of the UV reactor, with a second trim point available when influent conditions required it. Online instruments measured flow, UV intensity, transmittance, and peroxide residual. Laboratory testing verified the destruction of target compounds and checked for potentially problematic transformation products.

Design consideration Existing condition Retrofit response Operational benefit
Hydraulic capacity Limited elevation and narrow channels Parallel low-head-loss UV reactors Reduced pumping and easier maintenance
UV transmittance Seasonal and storm-related variation Online monitoring with automatic dose adjustment More stable oxidation performance
Chemical storage Small former chlorine area Secondary containment and new peroxide feed system Safer chemical handling
Electrical service Minimal spare capacity Dedicated power distribution and controls Better reliability and isolation
Process control Mostly manual final-stage adjustments PLC logic with alarms and interlocks Faster response to changing water quality
Sampling Limited post-treatment points New influent, intermediate, and final sample locations Better process verification

The design also accounted for trace-contaminant planning. The facility reviewed PFAS detection and treatment separately because AOPs are not a universal solution for per- and polyfluoroalkyl substances. That distinction prevented the owner from treating oxidation as a substitute for source control, adsorption, ion exchange, or other PFAS-specific technologies.

Building the project without losing treatment capacity

Construction was divided into stages so that at least one treatment path remained available. Temporary piping and bypass arrangements were installed before the old chlorine channel was isolated. The contractor then completed concrete repairs, pipe supports, chemical containment improvements, electrical work, and instrument installation in separate work zones.

Commissioning began with dry testing. Every valve, pump, UV module, level instrument, flow meter, alarm, and emergency shutdown circuit was checked before water was introduced. Wet commissioning followed with clean water, then filtered secondary effluent, and finally normal plant flow. This sequence helped identify wiring errors and control-loop problems before chemicals were added.

One early issue involved inaccurate flow signals at low nighttime rates. Because peroxide dosing was flow-paced, the error could have caused overdosing during extended low-flow periods. The controls team corrected the meter configuration, added a low-flow dosing limit, and created an operator alarm for conditions outside the normal calibration range.

The plant also established a bypass protocol. If UV intensity fell below the validated operating range or peroxide feed became unavailable, the system could isolate the AOP stage while maintaining baseline disinfection. This arrangement protected treatment continuity during maintenance and reduced pressure on operators during abnormal events.

Measuring results and managing the process

After stabilization, the retrofit achieved the project’s target reduction for the selected indicator compounds under typical operating conditions. UV transmittance remained the most important day-to-day performance variable. During storm-related deterioration, the control system reduced throughput or adjusted operating parameters according to the approved operating envelope rather than allowing the reactor to operate outside validated conditions.

Chemical use was lower than the initial conservative design estimate because staged dosing responded to actual water quality. The facility also reduced unnecessary lamp operation during low-flow periods by using automated bank control. These changes lowered energy and peroxide consumption while preserving treatment performance.

Operators received hands-on training covering peroxide unloading, leak response, UV module cleaning, lamp replacement, instrument verification, and data review. The standard operating procedures included clear action levels for low UV intensity, high peroxide residual, abnormal transmittance, and loss of communication with the programmable logic controller.

Performance reporting combined continuous data with laboratory confirmation. Monthly reviews compared flow, UV dose, transmittance, peroxide consumption, alarms, maintenance hours, and contaminant results. This allowed the team to distinguish equipment problems from changes in upstream treatment and gave management a clearer basis for future capital planning.

Practical recommendations for water agencies

Retrofitting an existing plant requires a broader evaluation than comparing vendor equipment. The following practices helped this facility control risk and preserve flexibility:

AOP should also be placed within the plant’s wider treatment strategy. Improved upstream solids removal can make oxidation more consistent, while source control may reduce the contaminant load more economically than downstream treatment. For agencies planning reuse, the process should be evaluated alongside membranes, biological treatment, activated carbon, monitoring, and regulatory requirements.

The project’s strongest outcome was operational confidence. The facility gained a new treatment barrier without abandoning the assets that still performed well. Its staff could explain why the process was needed, how it was controlled, and what conditions required intervention—three factors that are essential when advanced treatment becomes part of routine plant operations.

Water and wastewater professionals evaluating similar upgrades can use this case as a starting point for pilot planning, retrofit design, and operator engagement. LABS of CWEA provides a practical forum for exchanging those lessons through technical presentations, facility tours, workshops, and professional development programs. Explore upcoming opportunities to connect with peers and turn advanced treatment concepts into dependable plant performance.