Point Loma Wastewater Treatment Plant San Diego

1. INTRODUCTION

The Point Loma Wastewater Treatment Plant (PLWTP) serves as the terminus and treatment hub for the Metropolitan Sewerage System (Metro System), processing wastewater for the City of San Diego and 12 surrounding agencies. Perched on a 40-acre bluff overlooking the Pacific Ocean, this facility is unique among major American treatment plants due to its regulatory status and treatment methodology. Operating under a modified NPDES permit (301(h) waiver), PLWTP utilizes Advanced Primary Treatment—specifically Chemically Enhanced Primary Treatment (CEPT)—rather than conventional secondary biological treatment.

Treating approximately 130 to 145 million gallons daily (MGD), the facility is a marvel of energy efficiency, often generating 100% of its energy needs through onsite biogas utilization and hydroelectric power. As the City of San Diego transitions toward the “Pure Water” program—a massive potable reuse initiative—the role of Point Loma is evolving from a primary disposal facility to a component of a sophisticated, closed-loop regional water strategy.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The PLWTP services the Metro Joint Powers Authority (JPA) area, covering 450 square miles. This includes the City of San Diego and participating agencies such as Chula Vista, Coronado, Del Mar, El Cajon, Imperial Beach, La Mesa, National City, Poway, the Lemon Grove Sanitation District, the Otay Water District, the Padre Dam Municipal Water District, and the County of San Diego (Winter Gardens, Lakeside, Alpine, Spring Valley, East Otay Mesa). The system manages flow from a mix of residential (high density), commercial, and significant military infrastructure.

B. Operational Capacity

While the plant has a rated design capacity of 240 MGD and a peak wet weather hydraulic capacity of 432 MGD, actual flows have trended downward over the last decade despite population growth. This reduction is attributed to aggressive water conservation mandates and the diversion of flows to upstream reclamation facilities (North City and South Bay Water Reclamation Plants). The facility currently operates at approximately 55-60% of its hydraulic design capacity.

C. Discharge & Compliance

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology

Treated effluent is discharged into the Pacific Ocean through the Point Loma Ocean Outfall (PLOO). This critical infrastructure extends 4.5 miles (7.2 km) offshore to a depth of roughly 320 feet. The outfall terminates in a Y-shaped diffuser structure designed to maximize dilution. The plant operates under a waiver from the secondary treatment requirements of the Clean Water Act, contingent upon meeting stringent removal efficiencies for Total Suspended Solids (TSS) and Biochemical Oxygen Demand (BOD) via chemical enhancement.

3. TREATMENT PROCESS

Note: PLWTP employs Advanced Primary Treatment (CEPT). It does not utilize conventional biological secondary treatment (e.g., activated sludge).

A. PRELIMINARY TREATMENT (Headworks)

Wastewater enters the facility through the South Metro Interceptor. The headworks facility, critical for protecting downstream pumps and tanks, includes:

• Screening: Five mechanical bar screens remove large debris, rags, and non-dispersibles.
• Grit Removal: Following screening, flow enters aerated grit chambers where heavy inorganic materials (sand, gravel, coffee grounds) settle out.
• Odor Control: The headworks is fully enclosed and ventilated to chemical scrubbers, a necessity given the plant’s location near Cabrillo National Monument and high-value residential zones.

B. ADVANCED PRIMARY TREATMENT (CEPT)

This is the core process mechanism at PLWTP, allowing it to achieve removal rates significantly higher than standard primary treatment.

• Chemical Addition: Before entering sedimentation tanks, the influent is dosed with Ferric Chloride and Anionic Polymers.
  • Ferric Chloride: Acts as a coagulant, neutralizing charges on particles to form micro-flocs.
  • Polymer: Acts as a flocculant, bridging micro-flocs into larger, heavier aggregates that settle rapidly.

• Sedimentation Basins: The chemically conditioned water flows into 12 rectangular sedimentation basins. The enhanced settling velocity provided by the chemicals allows the plant to consistently achieve >80% TSS removal and >58% BOD removal, satisfying the specific conditions of their modified NPDES permit.
• Scum Removal: Surface skimmers remove fats, oils, and grease (FOG), which are collected and pumped to the digesters.

C. SECONDARY TREATMENT

Not Applicable. The PLWTP operates under a section 301(h) waiver of the Clean Water Act. The “Pure Water San Diego” program aims to reduce flows to Point Loma significantly by diverting wastewater to new advanced purification facilities upstream, eventually reducing the reliance on the ocean discharge.

D. DISINFECTION

While the ocean outfall provides significant distance for natural attenuation, the effluent is chlorinated using Sodium Hypochlorite (Bleach) prior to entering the outfall structure. This is primarily done to prevent biofouling in the outfall pipe and to reduce bacterial counts in the immediate discharge zone.

E. SOLIDS HANDLING

Solids removed from the sedimentation basins (raw sludge) are thickened and pumped to the onsite digestion complex.

• Anaerobic Digestion: The facility operates eight anaerobic digesters. The process reduces volatile solids and produces methane-rich biogas.
• Digested Biosolids Transport: Unlike many plants that dewater onsite, PLWTP pumps digested biosolids 17 miles through a dedicated pipeline to the Metro Biosolids Center (MBC) near the Miramar Landfill for dewatering (centrifuges) and final disposal/beneficial reuse.

4. INFRASTRUCTURE & ENERGY

A. Energy Self-Sufficiency

Point Loma is an industry leader in energy recovery, frequently achieving “net-zero” energy consumption from the grid.

• Biogas Utilization: Methane produced in the digesters is captured, treated, and utilized in the onsite Gas Utilization Facility (GUF). Two 2.25 MW Caterpillar internal combustion engines and one 4.5 MW Solar Turbine utilize this gas to power the plant.
• Hydroelectric Power: The treated effluent drops approximately 90 feet down a cliff shaft to the ocean outfall. A 1.35 MW Hydroelectric Plant captures this kinetic energy, generating additional electricity for onsite use or export to the grid.

B. Odor Control

Due to the sensitive location, PLWTP employs one of the most extensive odor control systems in the nation. The strategy involves containment, ventilation, and treatment via wet chemical scrubbers (utilizing bleach and caustic soda) and dual-bed carbon adsorption towers.

5. RECENT UPGRADES & MAJOR PROJECTS

Grit Processing Improvement Project (GPIP) – $38 Million (2018-2022)

• Project Scope: Replacement of the aging grit removal system with modern aerated grit tanks and grit washers.
• Drivers: The previous system (commissioned in the 1960s) struggled with efficiency, leading to grit deposition in the digesters, which reduced digestion capacity and increased maintenance costs.
• Technical Highlights: Installation of new grit pumps, cyclone separators, and classifiers to produce a cleaner, drier grit product for disposal.
• Results: Significantly reduced grit accumulation in downstream digesters and reduced wear on sludge pumps.

Pure Water Program Phase 1 (Impact on PLWTP) – Ongoing

While physically located at North City, this $1.5+ billion program directly alters PLWTP’s future. By diverting 30 MGD initially (and up to 83 MGD eventually) for potable reuse, the solids loading and hydraulic loading at Point Loma will shift, requiring operational adjustments to the CEPT process to maintain permit compliance with lower flows but potentially higher concentrations.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

The 301(h) Waiver

Point Loma operates under a waiver from secondary treatment standards, authorized under Section 301(h) of the Clean Water Act. This waiver is granted based on the facility’s ability to demonstrate that its discharge does not harm the balanced indigenous population of marine life in the receiving water. This is achieved through:

• Deep Ocean Discharge: The 4.5-mile long outfall disperses effluent below the thermocline.
• Chemical Enhancement: High efficiency removal of suspended solids (>80%) via Ferric Chloride and Polymer.
• Extensive Ocean Monitoring: The City conducts one of the most comprehensive ocean monitoring programs in the world, sampling 80+ stations for water quality, sediment chemistry, and benthic ecosystem health.

7. CHALLENGES & FUTURE PLANNING

A. Coastal Erosion & Site Stability

Located on a sedimentary bluff directly abutting the Pacific Ocean, the facility faces long-term risks from cliff erosion and sea-level rise. Retaining walls and bluff stabilization measures are continuously monitored.

B. The Transition to Potable Reuse

The “Pure Water” program is the long-term solution to the 301(h) waiver capability. As flows are diverted for recycling, the remaining wastewater arriving at Point Loma becomes more saline and concentrated. Engineering staff must adapt the CEPT chemical dosing strategies to handle this changing influent chemistry.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. RELATED FACILITIES

• Metro Biosolids Center (MBC): Located near the Miramar Landfill, this facility receives digested sludge from Point Loma and raw sludge from North City for dewatering and disposal.
• North City Water Reclamation Plant (NCWRP): A 30 MGD tertiary plant that treats wastewater to recycled water standards, reducing load on Point Loma.
• South Bay Water Reclamation Plant (SBWRP): Located near the international border, relieving flow from the southern portion of the Metro System.

10. FAQ SECTION

Technical Questions

Q: Why doesn’t Point Loma use secondary treatment (activated sludge)?
A: The plant operates under a federal 301(h) waiver, which allows for modified requirements based on the depth and distance of the ocean outfall and the lack of adverse environmental impact. The plant substitutes biological treatment with advanced chemical precipitation (CEPT).

Q: What chemicals are used for the CEPT process?
A: The plant primarily uses Ferric Chloride as a coagulant and an anionic polymer as a flocculant aid to enhance sedimentation.

Q: How does the hydroelectric plant work?
A: The plant is situated on a bluff roughly 90 feet above sea level. As treated effluent drops down the shaft to the ocean outfall level, it passes through a turbine, capturing the kinetic energy of the falling water to generate up to 1.35 MW of electricity.

Public Interest Questions

Q: Can the public tour the facility?
A: Yes, the City of San Diego offers tours of the PLWTP to school groups, organizations, and the public by appointment. It is a popular destination due to its scenic location.

Q: Does the plant smell?
A: While wastewater plants inherently have odors, PLWTP invests millions in advanced odor control scrubbers and carbon filters. Under normal operating conditions, odors are contained within the site boundary to protect visitors at the nearby Cabrillo National Monument.