Miami Dade Water And Sewer South District Wastewater Treatment Plant

Facility Basic Information

• Plant Name: South District Wastewater Treatment Plant (SDWWTP)
• Location: 8950 SW 232nd St, Cutler Bay, FL 33190 (Miami-Dade County)
• Operating Authority: Miami-Dade Water and Sewer Department (WASD)
• Permitted Capacity (AADF): 112.5 MGD
• Peak Hydraulic Capacity: ~285 MGD
• Population Served: Approx. 800,000 residents
• Service Area: Southern Miami-Dade County (South of SW 88th St/Kendall Dr)
• Primary Discharge: Deep Injection Wells (Floridan Aquifer) & Atlantic Ocean (Phasing out)
• NPDES Permit Number: FL0057754
• Year Commissioned: 1983 (Original expansion from smaller package plants)

1. Introduction

The South District Wastewater Treatment Plant (SDWWTP) is a critical component of the Miami-Dade Water and Sewer Department’s (WASD) extensive infrastructure, serving as the primary treatment facility for the rapidly growing southern portion of the county. With a permitted Annual Average Daily Flow (AADF) of 112.5 million gallons per day (MGD), it is the second-largest wastewater facility in the Miami-Dade system, trailing only the Central District plant.

Located near Black Point Marina, the facility is currently at the center of one of Florida’s most significant environmental engineering undertakings: the compliance with the state’s Ocean Outfall Legislation (OOL). This mandate requires the elimination of routine ocean discharge by 2025. Consequently, the SDWWTP is undergoing massive capital improvements, shifting from ocean outfall disposal to High-Level Disinfection (HLD) with disposal via Deep Injection Wells (DIW). The plant is distinguished technically by its use of High-Purity Oxygen (HPO) activated sludge treatment, a process selected to handle high organic loading within a compact footprint.

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2. Facility Overview

A. Service Area & Coverage

The SDWWTP services a geographic area encompassing approximately 88 square miles south of Kendall Drive (SW 88th Street). This catchment area includes the municipalities of Cutler Bay, Palmetto Bay, Homestead, Florida City, and unincorporated areas of southern Miami-Dade. The collection system feeding the plant is extensive, relying on a network of regional pump stations to combat the flat topography of South Florida. The service area is characterized by a mix of dense residential developments and agricultural zones, creating a unique influent profile with significant diurnal flow variations.

B. Operational Capacity

The facility operates under a Florida Department of Environmental Protection (FDEP) permit allowing for 112.5 MGD on an annual average basis. Historical data indicates the plant treats an average of 95–100 MGD, placing it near 85% capacity utilization. This high utilization rate has triggered regulatory requirements for capacity expansion planning. During wet weather events—common in Miami’s tropical climate—peak hydraulic flows can surge toward 285 MGD, testing the hydraulic limits of the headworks and secondary clarifiers.

C. Discharge & Compliance

Historically, the plant utilized a 3.5-mile ocean outfall pipe discharging into the Atlantic Ocean. However, under the 2008 Ocean Outfall Legislation (Section 403.086(9), Florida Statutes), the facility is mandated to achieve zero routine ocean discharge by December 31, 2025. The facility is currently transitioning to 100% disposal via Class I Deep Injection Wells, which inject treated effluent approximately 3,000 feet below the surface into the “Boulder Zone” of the Lower Floridan Aquifer, well below drinking water sources.

3. Treatment Process

The SDWWTP utilizes a High-Purity Oxygen (HPO) activated sludge process, which differs significantly from conventional aeration. This selection allows for higher mixed liquor suspended solids (MLSS) concentrations and smaller aeration basin volumes.

A. Preliminary Treatment

Raw influent enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. Following screening, the flow enters aerated grit chambers where inorganic solids (sand, gravel) settle out. The facility recently upgraded its headworks to improve capture efficiency and protect downstream pumps. Odor control at the headworks is aggressive, utilizing chemical scrubbers (wet scrubbers) to neutralize hydrogen sulfide ($H_2S$), essential due to the proximity of the Black Point Marina recreational area.

B. Primary Treatment

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Flow proceeds to covered primary clarifiers. The covers are an integral part of the facility’s odor control strategy. Here, settleable solids are removed via gravity, reducing the BOD load on the secondary system by approximately 30-35% and TSS by 50-60%. Primary sludge is pumped to gravity thickeners before digestion.

C. Secondary Treatment (High-Purity Oxygen)

The biological treatment stage utilizes a High-Purity Oxygen (HPO) activated sludge system. Unlike conventional aeration which uses ambient air (21% oxygen), SDWWTP generates pure oxygen on-site via Cryogenic Oxygen Generation plants (and backup Pressure Swing Adsorption units).

• Reactor Configuration: The aeration basins are covered and staged. High-purity oxygen is fed into the headspace and mechanically mixed into the wastewater.
• Benefits: This process maintains high Dissolved Oxygen (DO) levels, supports a high biomass concentration, and improves sludge settling characteristics (low SVI).
• Clarification: The mixed liquor flows to secondary clarifiers where biological floc settles. Return Activated Sludge (RAS) is recycled to the reactor, while Waste Activated Sludge (WAS) is sent to solids handling.

D. Disinfection & Tertiary Treatment

To meet the requirements for Deep Injection Wells, the facility has implemented High-Level Disinfection (HLD). The effluent undergoes filtration followed by chlorination. The chlorination system is designed to meet specific contact times required to reduce fecal coliforms to non-detectable levels required for underground injection. While the facility does not currently perform advanced nutrient removal (BNR) for nitrogen/phosphorus to the levels of AWT, the deep well disposal method relies on geological confinement rather than nutrient reduction.

E. Solids Handling

Primary sludge and WAS are thickened and stabilized via anaerobic digestion.

• Digestion: The facility operates multiple anaerobic digesters (mesophilic range). The digestion process reduces volatile solids and produces methane-rich biogas.
• Dewatering: Digested biosolids are dewatered using high-speed decanter centrifuges to achieve a cake solid concentration suitable for transport.
• Disposal: The resultant Class B biosolids are hauled off-site for land application or composting, consistent with Florida’s biosolids management regulations.

4. Recent Upgrades & Major Projects

The SDWWTP is in the midst of a multi-year, multi-million dollar Capital Improvement Program (CIP), driven largely by the Ocean Outfall Legislation and the Consent Decree with the EPA/FDEP.

Ocean Outfall Legislation (OOL) Compliance Program (2015-2025)

• Project Scope: Construction of Deep Injection Wells (DIW) and associated pump stations to replace the ocean outfall.
• Budget: Part of a broader $600M+ allocation for the South District.
• Technical Highlights:
  • Drilling of multiple Class I injection wells to depths exceeding 3,000 feet.
  • Construction of a large-scale High-Level Disinfection (HLD) facility including filtration and chlorine contact basins.
  • Installation of large vertical turbine pumps capable of overcoming the hydrostatic head required for deep injection.

• Status: Several wells are operational; final phase nearing completion to meet the 2025 statutory deadline.

Electrical Distribution Building (EDB) & Substation Expansion

• Project Drivers: Aging electrical infrastructure and need for hurricane resilience.
• Scope: Construction of a new hurricane-hardened EDB, replacement of switchgear, and new transformers to support the increased power load from the new injection well pumps.
• Result: Enhanced reliability during grid instability events.

Oxygen Production Plant Rehabilitation

• Scope: Refurbishment of the cryogenic oxygen generation units (Cold Boxes).
• Importance: The HPO process is entirely dependent on reliable oxygen generation; these upgrades ensured process stability for the biological reactors.

5. Regulatory Compliance & Environmental Performance

Permit Requirements

The facility operates under FDEP Permit FL0057754.

• Effluent Limits (Ocean Outfall): Traditionally focused on BOD and TSS (30 mg/L monthly avg) and basic disinfection.
• Effluent Limits (Deep Well): Strict limits on Total Suspended Solids (TSS) and High-Level Disinfection criteria (fecal coliform non-detectable in 75% of samples).
• Nutrients: While not subject to the strict nutrient limits of surface water dischargers (like those discharging to the Everglades), the plant monitors Nitrogen and Phosphorus trends.

Consent Decree Status

In 2013, Miami-Dade County entered into a Consent Decree with the U.S. EPA and FDEP to address sanitary sewer overflows (SSOs) and aging infrastructure. The SDWWTP has been a focal point of this decree, requiring specific rehabilitation projects to be completed by fixed dates. The facility has successfully met the majority of its Consent Decree milestones regarding pump station rehabilitation and treatment process redundancy.

6. Infrastructure & Energy

Cogeneration (CHP)

The SDWWTP utilizes a Combined Heat and Power (CHP) system. Methane gas produced in the anaerobic digesters is captured and treated to fuel cogeneration engines. These engines generate electricity to offset grid consumption and provide heat for the digesters, closing the energy loop. This system is a critical component of WASD’s energy efficiency program.

Resilience Features

Given its coastal location, the plant is vulnerable to storm surge. Recent infrastructure projects have been designed with elevated base flood elevations. Critical electrical gear and emergency generators are housed in hardened structures designed to withstand Category 5 hurricane wind loads.

7. Technical Specifications Summary

8. FAQ

Technical/Professional Questions

1. Why does SDWWTP use High-Purity Oxygen (HPO) instead of conventional aeration?
HPO was selected to treat high organic loads in a smaller physical footprint. The partial pressure of oxygen in the covered basins allows for faster oxygen transfer rates, supporting higher mixed liquor concentrations compared to conventional atmospheric aeration.

2. What is the status of the Ocean Outfall elimination?
As of 2024, the facility is in the final stages of the transition. The Deep Injection Wells are largely constructed and operational. The facility is on track to meet the statutory requirement of eliminating routine ocean discharge by December 31, 2025.

3. How does the facility manage wet weather peak flows?
The plant utilizes robust hydraulic capacity in its headworks and can leverage the ocean outfall as an emergency backup during extreme hydraulic events (permitted under OOL for emergency/peak flow management only after 2025), alongside deep well injection.

4. Is the plant producing Reclaimed Water?
While the plant treats water to a high standard, the primary focus at SDWWTP is Deep Well Injection. However, Miami-Dade is implementing large-scale water reuse projects at the *South District* plant, primarily focused on aquifer recharge and potential cooling water for the nearby FPL Turkey Point power plant in future phases.

Public Interest Questions

5. Does the plant smell?
Odor control is a top priority due to the plant’s location near Black Point Park and Marina. The facility uses covered clarifiers and chemical scrubbers to treat air before release. While occasional odors may occur during maintenance, systems are in place to minimize impact.

6. Is the discharged water safe for the environment?
Yes. The water injected into the Boulder Zone (3,000 feet deep) is separated from the Biscayne Aquifer (drinking water source) by multiple confining layers of rock and clay. The water is treated and disinfected before injection.