Background

The City of San Diego (City) owns and operates a water recycling facility known as the South Bay Water Reclamation Plant (SBWRP) located in the Tijuana River Valley within San Diego’s South Bay. The plant began operations in May 2002 and treats up to 15 million gallons of wastewater per day to Title 22 California Code and Regulations standards (Title 22). The SBWRP provides safe and reliable reclaimed water to the South Bay communities thereby reducing San Diego’s reliance on imported water.

One of SBWRP’s final treatment stages is the disinfection of the reclaimed water before distribution. The plant uses an ultraviolet light (UV) system where submerged UV lamps kill or render pathogens unable to multiply. The previous UV disinfection system had been in operation since 2004 and was reaching the end of its useful life. In addition, the City preferred a more energy-efficient system requiring less maintenance.

The City partnered with LEE + RO on the design of a new UV system that not only is more efficient resulting in reduced maintenance time and cost, but also ensures that the reclaimed water is safe and ready for distribution in the future.

Situation

The existing Trojan UV4000 disinfection system had been identified by the City to be beyond its useful life and the units no longer met current energy efficiency standards. The objective of this project included evaluation of alternatives to replace the existing UV system with the latest technology available and prepare design documents based on the recommended UV system.

The new UV system required specific manufacturer-supplied electrical equipment used for controlling and powering the UV lamps and, ultimately, the UV disinfection process. Additionally, the new UV system required several ancillary power distribution and protection devices not provided by the manufacturer. The new system was designed and integrated with the existing wastewater treatment facility.

"Establishing worker safety during COVID-19 was a top priority."

CHALLENGES

A driving factor for the project schedule was the allowable outage period for the UV disinfection system. With the UV disinfection system out of service during construction, the City is not able to provide reclaimed water to the end users and the City must discharge the effluent to the ocean after secondary treatment. Because the City provides recycled water to several customers, the outage period had to occur during the peak rainy period as demand for recycled water used for irrigation is the lowest during that time.

Another critical item driving the outage period was the time required by the State of California to review the test results and issue the permit to allow the City to once again start supplying recycled water. As most of the project construction took place during the COVID-19 pandemic, that had to be taken into account as well.

"Innovative design, planning, scheduling, and team coordination ensured for minimum system downtime."

Solutions

1. Provide reclaimed water with a minimal shutdown period while battling COVID-19

San Diego County imports more than 80% of its water. Shutting down the system for an extended period would add additional costs. As most of the construction took place during COVID-19, protocols that were put in place to ensure a safe work environment adversely impacted the delivery of materials as well as the management of the project teams. Additional accommodations had to be made for the subcontractors that were out of state and for the border crossing of the UV system, which was coming from a Canadian supplier.

LEE + RO’s innovative design of the new system allowed the project to have an aggressive construction schedule that not only minimized downtime but took into account the seasonal demand for recycled water.

"A detailed evaluation of alternative UV systems ensured that the City would be able to provide reclaimed water at a substantially lower energy cost."

2. Provide recommendation for the most energy-efficient UV system

Energy efficiency was a critical component for this project. LEE + RO conducted a detailed evaluation of alternatives for UV system replacement. The objective was to find the latest, most energy-efficient technology available that fit within the City’s budget. The new UV system:

• Improves efficiency and consumes less electricity while providing a higher level of disinfection.
• Requires less frequent lamp replacement and is easier to maintain, leading to lower operations and maintenance costs.
• Helps the City comply with Title 22 permit requirements in a more efficient manner by providing better dosage monitoring, a more reliable and maintenance-friendly design, faster response to flow changes, and reduction in algae growth.

"Structural modifications were required to accommodate the new UV system."

3. Structural and Mechanical Modifications to Existing Concrete Channel

The existing concrete UV channel had to be structurally modified to accommodate the new UV system, which is smaller than the old UV system in overall dimensions. The system hydraulic grade lines were evaluated for optimum performance and the existing channel sluice gates and motorized weir gate were replaced.