Miami Dade Water And Sewer Central District Wastewater Treatment Plant

1. INTRODUCTION

The Central District Wastewater Treatment Plant (CDWWTP) is the oldest and one of the most critical infrastructure assets within the Miami-Dade Water and Sewer Department (WASD) system. Located on Virginia Key, a barrier island separating Biscayne Bay from the Atlantic Ocean, the facility operates with a permitted capacity of 143 million gallons per day (MGD). It serves the economic core of the region, including Downtown Miami, the financial district, and high-density coastal communities.

Originally commissioned in 1956 as a primary treatment facility, the plant has undergone extensive expansion to become a secondary treatment facility utilizing High Purity Oxygen (HPO) activated sludge technology. Currently, the CDWWTP is the focal point of a massive capital improvement program driven by two primary factors: a federal Consent Decree to address aging infrastructure and Florida’s Ocean Outfall Legislation (OOL), which mandates the elimination of normal wastewater discharge to the ocean. With over $1 billion in active and planned upgrades, the facility is transitioning toward deep injection well disposal and increased resiliency against sea-level rise.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The CDWWTP serves a high-density urban environment covering approximately 35 square miles. The service area encompasses the City of Miami, the Village of Key Biscayne, the City of Coral Gables, and portions of Miami Beach. The collection system feeding the plant is complex, involving large-diameter force mains crossing Biscayne Bay (including a 72-inch force main under Government Cut) and major pump stations (such as Pump Station 1 and 2) that are critical to preventing sanitary sewer overflows (SSOs) in the sensitive marine environment.

B. Operational Capacity

The facility is permitted for an Annual Average Daily Flow (AADF) of 143 MGD. The plant handles significant diurnal peaks due to the commercial and tourist nature of the service area.

• Design Capacity: 143 MGD
• Peak Hourly Flow: >300 MGD
• Historical Trends: Flows have stabilized due to water conservation efforts, despite population growth. However, wet weather events significantly impact hydraulic loading due to Inflow and Infiltration (I&I) in the older collection systems of Miami.

C. Discharge & Compliance

Historically, treated effluent has been discharged via an ocean outfall extending 3.6 miles into the Atlantic Ocean at a depth of approximately 90 feet. The facility operates under NPDES Permit FL0024805. While compliance has generally been maintained for secondary treatment standards (CBOD5 and TSS), the facility is currently executing a state-mandated transition. By 2025, Florida law requires the facility to cease the use of the ocean outfall for normal disposal, necessitating the construction of deep injection wells to discharge treated effluent into the Boulder Zone of the Lower Floridan Aquifer.

3. TREATMENT PROCESS

The CDWWTP employs a secondary treatment process distinguished by its use of High Purity Oxygen (HPO) rather than conventional air aeration. This technology was selected to handle high organic loading within a limited physical footprint on the island.

A. PRELIMINARY TREATMENT

Raw sewage enters the headworks where it undergoes physical screening.

• Screening: Mechanically cleaned bar screens remove large debris, plastics, and rags to protect downstream pumps.

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology
• Grit Removal: Aerated grit chambers facilitate the settling of sand, gravel, and heavy particulates.
• Odor Control: The headworks are enclosed and ventilated to chemical scrubbers (wet scrubbers/packed towers) to mitigate H₂S emissions, critical due to the plant’s proximity to recreational areas on Virginia Key.

B. PRIMARY TREATMENT

Flow moves to rectangular primary settling tanks.

• Clarifiers: The facility utilizes multiple batteries of rectangular clarifiers equipped with chain-and-flight sludge collectors.
• Efficiency: Designed to remove approximately 30-40% of BOD and 50-60% of TSS.
• Scum Removal: Surface skimmers remove grease and oils, which are collected for separate disposal or digestion.

C. SECONDARY TREATMENT (High Purity Oxygen)

The core biological treatment occurs in covered aeration basins using the UNOX or similar HPO process.

• Oxygen Generation: A cryogenic oxygen plant (Cryo Plant) on-site separates nitrogen from air to produce >90% pure oxygen.
• Aeration Basins: The pure oxygen is introduced into covered, multi-stage aeration tanks. The high partial pressure of oxygen increases the transfer rate, allowing for a higher Mixed Liquor Suspended Solids (MLSS) concentration and shorter hydraulic retention times compared to conventional air systems.
• Secondary Clarifiers: Mixed liquor flows to circular secondary clarifiers where biological floc settles.
• RAS/WAS: Return Activated Sludge is pumped back to the aeration basins; Waste Activated Sludge is thickened and sent to digestion.

D. DISINFECTION

Effluent is disinfected using sodium hypochlorite (chlorination) in chlorine contact chambers. For the ocean outfall discharge, dechlorination is not typically required due to the dilution factors allowed in the open ocean, but monitoring for chlorine residual is strictly enforced. As the plant transitions to Deep Injection Wells (DIW), disinfection requirements will align with High-Level Disinfection (HLD) or industrial wastewater injection standards depending on the specific well classification.

E. SOLIDS HANDLING

• Thickening: Gravity thickeners reduce the water content of primary sludge; dissolved air flotation (DAF) or centrifugal thickening is used for WAS.
• Digestion: Anaerobic digesters operate in the mesophilic range. These floating-cover tanks stabilize the sludge and produce methane-rich biogas.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges, producing a cake suitable for land application or disposal.
• Biosolids Class: The facility typically produces Class B biosolids, which are land-applied for agriculture or marketed as fertilizer.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site occupies a significant portion of Virginia Key. Due to its location on a barrier island, the plant is exposed to corrosive marine environments and potential storm surge. Recent infrastructure hardening includes elevating electrical switchgear and constructing flood walls around critical assets. The site includes extensive maintenance shops, a certified environmental laboratory, and administration buildings.

B. Energy Systems & Cogeneration

CDWWTP is a major energy consumer, primarily due to the cryogenic oxygen production and pumping requirements.

• Cogeneration (CHP): The facility operates a Combined Heat and Power system. Biogas produced in the anaerobic digesters is scrubbed (siloxane and H₂S removal) and used to fuel engine generators.
• Heat Recovery: Waste heat from the engines is recovered to heat the anaerobic digesters, maintaining the mesophilic process temperature.
• Grid Integration: The system operates in parallel with the FPL grid, providing peak shaving capabilities and emergency backup power.

5. RECENT UPGRADES & MAJOR PROJECTS

WASD is currently executing the largest Capital Improvement Program (CIP) in Miami-Dade history, largely driven by the 2013 Consent Decree and the 2008 Ocean Outfall Legislation.

Consent Decree Projects (2013–Present)

Scope: A federally mandated program to eliminate sanitary sewer overflows and upgrade aging infrastructure.

• Plant 2 Digester Cluster Upgrades ($XX Million): Comprehensive rehabilitation of anaerobic digester clusters, including new mixing systems, cover replacements, and gas piping improvements to maximize biogas capture and system safety.
• Oxygen Production Facility Rehab: Refurbishment of the cryogenic oxygen plant cold boxes and compressors to ensure reliable supply for the secondary treatment process.
• Headworks Improvements: Installation of robust screening and grit removal systems to handle variable flows and protect downstream process equipment.

Ocean Outfall Legislation (OOL) Compliance Projects

Deep Injection Well System ($100+ Million)

• Description: Construction of deep injection wells to replace the ocean outfall. These wells discharge treated effluent approximately 3,000 feet below the surface into the “Boulder Zone,” a cavernous hydrogeologic unit effectively isolated from the drinking water aquifer.
• Status: Several wells have been drilled and are in testing or operational phases. The deadline for full diversion of baseline flow is 2025.
• Technical Highlight: The project requires massive high-service pumping stations to overcome the hydrostatic pressure of the deep aquifer.

Resiliency and Hardening

Ongoing projects involve hardening the plant against Category 5 hurricanes and projected sea-level rise. This includes elevating motor control centers (MCCs), installing submersible pumps capable of operating during flood events, and hardening building envelopes.

6. REGULATORY COMPLIANCE

NPDES Permit (FL0024805): The permit regulates the discharge of treated domestic wastewater.

• Effluent Limits: Strict limits on CBOD5 (25 mg/L annual avg), TSS (30 mg/L), and fecal coliform.
• Monitoring: Continuous monitoring of flow and pH; daily/weekly composites for nutrients and metals.
• Ocean Outfall Requirement: The permit incorporates the schedule for compliance with the Leah Schad Memorial Ocean Outfall Program, requiring the cease of discharge by December 31, 2025, except for peak flow management.

The facility operates under a Consent Decree (Case No. 1:12-cv-24400-FAM) with the U.S. EPA and FDEP. Regular status reports verify the completion of capital improvement milestones to reduce untreated discharges and SSOs.

7. CHALLENGES & FUTURE PLANNING

A. Climate Change and Sea Level Rise

Located on a barrier island, CDWWTP is “Ground Zero” for climate risks. King Tides already threaten access roads (Rickenbacker Causeway). Future planning involves not just protecting the plant, but ensuring the conveyance system (pump stations) can overcome higher hydraulic heads caused by rising tides.

B. Aging Infrastructure

Much of the plant’s core structural concrete dates to the 1950s and 1960s. Concrete restoration, rebar corrosion mitigation, and pipe replacement are constant operational realities.

C. Emerging Contaminants

As the plant transitions to injection wells and potential future reuse applications, monitoring for PFAS (Per- and polyfluoroalkyl substances) and pharmaceuticals is becoming a higher priority for regulators and utility planners.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. RELATED FACILITIES

The CDWWTP is part of a tri-regional system operated by Miami-Dade WASD:

• North District Wastewater Treatment Plant (NDWWTP): Located in North Miami, also utilizing High Purity Oxygen technology with a capacity of 112.5 MGD.
• South District Wastewater Treatment Plant (SDWWTP): Located in the Black Point area, this facility uses conventional aeration and features a massive High-Level Disinfection (HLD) deep injection well system, treating 112.5 MGD.

10. FAQ SECTION

Technical Questions

Q: Why does CDWWTP use High Purity Oxygen (HPO) instead of conventional aeration?
A: HPO systems allow for a higher biomass concentration and faster reaction rates, significantly reducing the footprint of the aeration basins. This is critical for the Virginia Key site where land availability is limited.

Q: What is the status of the Ocean Outfall shut down?
A: Under Florida state law, the routine use of the outfall must cease by December 31, 2025. The facility is constructing deep injection wells to divert this flow. The outfall will likely be retained as a backup for peak wet-weather flow management.

Q: How are biosolids managed?
A: Sludge is thickened, anaerobically digested, and dewatered via centrifuges. The resulting Class B biosolids are typically hauled off-site for land application in agricultural areas.

Public Interest Questions

Q: Does the plant smell?
A: While wastewater treatment naturally generates odors, the CDWWTP employs extensive odor control systems, including chemical scrubbers, to treat air from the headworks and sludge processing areas. The department actively monitors odors due to the proximity of Virginia Key Beach Park and Key Biscayne.

Q: Is the plant safe during hurricanes?
A: The facility is equipped with backup power generation (both diesel and biogas cogeneration). Recent upgrades have focused on raising critical electrical gear above FEMA flood limits to ensure the plant can continue pumping and treating water during storm surges.