- City of San Diego -

Point Loma Wastewater Treatment Plant (PLWTP)

Headworks and Grit System Improvements
Design Completion: 2011
Construction: 2012 – 2016
Cost: $23 MM

CaseStudy-PointLomaBanner

 

SITUATION
Point Loma Wastewater Treatment Plant in the City of San Diego’s primary wastewater treatment facility and is located on idyllic cliffside bluffs, next to the City’s famed Mission Bay. 

The treatment plant was originally constructed in 1963 to treat approximately 61 mgd of sewage, whereas now, the plant is currently treating up to 240 mgd.

LEE + RO was selected as the prime consultant for the design of a $23 million headworks improvement project. The objective of this project was twofold:

  • to address the ongoing problem of excessive grit accumulation in Point Loma’s primary influent tunnels, which threatens downstream processes and requires intensive effort to keep clean and
  • to increase the hydraulic capacity of the plant by removing hydraulic bottlenecks in the plant’s influent channels.
Challenge
First, determining the nature and location of the hydraulic bottlenecks is very difficult due to the scale and complexity of this large treatment facility.

To meet this technical challenge, LEE + RO developed a scale model of PLWTP’s influent channels to determine the flow distribution patterns, calculated hydraulic profile, most-efficient grit tank section for optimum air/water rolls, and efficient aerated grit removal tank dimensional configuration.

This creative approach allowed our engineers to pinpoint hydraulically deficient portions of the existing structure and come up with tailored solutions to these problem areas. This novel approach ensured an efficient, effective design, without requiring an extensive and costly physical investigation of the existing facility, saving the City time and money.

Grit removal is a critical process at PLWTP and the design and construction of a new large grit facility would need to be performed in parallel to a working grit removal process.

Solution
In this case, our solution was to design and construct an interim grit removal facility, directly upstream of the permanent facility to handle grit removal during construction. The interim facility was designed, constructed, and placed into service to provide the flexibility and performance that Point Loma requires.


The permanent grit removal facility is housed in a 140’ L X 40’ W X 49’, two-story concrete structure and includes: a new grit pump gallery; new grit processing/dewatering equipment, grit storage, and truck loading facility; new agitation blowers; new foul air collection and odor control system; new dual-grit slurry piping from all six grit tanks; improved hydraulics performance to better split flows to six bar screens/grit tanks. The new south grit tanks have an improved tank geometry based on the pilot model testing results. The project required extensive instrumentation design effort to comply with the existing Westinghouse DCS system.

Construction coordination was demanding, and one incident in particular, demonstrates how the team came together under trying circumstances. During a Friday afternoon, one of the Contractor’s specialty sub-contractors was drilling tiebacks in close proximity to the main sewage influent channel to support the project’s deep excavation. Unfortunately, the driller missed their mark and accidentally drilled into the treatment’s plants main sewage influent channel. Fortunately, the driller left the drill bit inserted into the channel. And just as in the fabled story of the Dutch boy keeping his fingers in the dike to keep the town from flooding, the drill rig was left in this precarious position temporarily blocking the sewage from entering the excavation.

LEE + RO was called to take emergency charge of the situation. Under LEE + RO’s leadership, an emergency plan was formed and implemented within hours. Since the sewage flows could not be fully stopped, the situation called for divers. We were only able to provide a three-hour window in the middle of the night to get the current in the sewage channel low enough to allow for safe diving conditions.

During these short-duration windows, the divers made a series of dives into the sewage channel in adverse conditions over the next week while working under conditions with no visibility. Based on only the diver’s assessment by feeling with their hands, we were able to provide a temporary fix to stop the immediate threat of sewage spill, and then later on provided a more permanent fix. The whole team rose to the challenge, and prevented environmental damage to the City’s coastline, and the incident result in ZERO spill of sewage.

Results

This project was awarded APWA Project of the Year. This demanding project was only possible through successful partnering between LEE + RO, City of San Diego Staff, the Contractor, sub-contractors, and third party DCS programming engineers.   The new system is currently working as designed and is efficiently removing grit at Point Loma 24 hours per day, 7 days a week.

More Solutions

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