Miami Dade Water And Sewer North District Wastewater Treatment Plant

FACILITY BASIC INFORMATION

Plant Name: North District Wastewater Treatment Plant (NDWWTP)

Location: 2575 NE 151st Street, North Miami, Miami-Dade County, Florida

Operating Authority: Miami-Dade Water and Sewer Department (WASD)

Design Capacity: 120.0 MGD (Annual Average Daily Flow)

Current Average Flow: ~85-95 MGD

Population Served: Approx. 2.4 million (System-wide), NDWWTP serves Northern region

Service Area: North Miami-Dade County, including Miami Gardens, North Miami, North Miami Beach, Opa-Locka, and Hialeah

Receiving Water Body: Atlantic Ocean (Outfall) & Floridan Aquifer (Deep Injection Wells)

NPDES Permit Number: FL0032182

Read more4D-Printed Smart Materials For Water Treatment

Year Commissioned: 1979 (Major expansion/modernization)

TARGET AUDIENCE

• Municipal consulting engineers evaluating High Purity Oxygen (HPO) systems
• Wastewater treatment plant operators and managers
• Regulatory compliance officers tracking Ocean Outfall Legislation (OOL)
• Engineering firms pursuing Capital Improvement Program (CIP) contracts
• University researchers studying deep well injection technologies

1. INTRODUCTION

The North District Wastewater Treatment Plant (NDWWTP) is a critical component of the Miami-Dade Water and Sewer Department (WASD) infrastructure, which operates one of the largest water and wastewater utilities in the United States. Situated on a 245-acre site in North Miami, the facility is permitted for an Annual Average Daily Flow (AADF) of 120 million gallons per day (MGD), serving the densely populated municipalities of the northern county region. The plant is distinguished by its use of a pure oxygen activated sludge process, a technology selected to maximize treatment efficiency within a constrained footprint while handling high organic loading.

Currently, the NDWWTP is at the center of a historic infrastructure pivot. Driven by Florida’s Ocean Outfall Legislation (OOL) of 2008, the facility is transitioning from discharging treated effluent into the Atlantic Ocean to utilizing Deep Injection Wells (DIW) into the Boulder Zone of the Floridan Aquifer. Simultaneously operating under a federal Consent Decree to eliminate sanitary sewer overflows (SSOs), the plant is undergoing hundreds of millions of dollars in capital improvements, making it a focal point for engineering innovation in coastal resilience, nutrient management, and advanced effluent disposal.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The NDWWTP serves a highly urbanized catchment area in northern Miami-Dade County. The collection system feeding the plant includes hundreds of miles of gravity mains and force mains, supported by a network of regional pump stations. Key municipalities served include the City of North Miami, North Miami Beach, Miami Gardens, Opa-Locka, and portions of Hialeah. The service area is characterized by a mix of high-density residential zones and significant commercial/industrial corridors, contributing to a variable influent profile that requires robust biological treatment capabilities.

B. Operational Capacity

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With a permitted design capacity of 120 MGD, the plant typically treats an average daily flow between 85 and 95 MGD, providing a capacity utilization of approximately 75%. This buffer is critical for managing wet-weather flows common in South Florida’s tropical climate. During peak storm events, the plant’s hydraulic peak capacity can exceed 200 MGD. The facility has seen steady flow patterns over the last decade, with water conservation efforts balancing population growth.

C. Discharge & Compliance

Historically, the NDWWTP has discharged secondary treated effluent via an ocean outfall extending approximately 2 miles offshore into the Atlantic Ocean. However, compliance with Florida Statutes (Section 403.086(9)) mandates the elimination of routine ocean discharge by 2025. Consequently, the facility is shifting to Class I municipal injection wells. The plant operates under NPDES Permit FL0032182 and an Underground Injection Control (UIC) permit for its deep wells. The facility consistently meets secondary treatment standards for Carbonaceous Biochemical Oxygen Demand (CBOD5) and Total Suspended Solids (TSS).

3. TREATMENT PROCESS

A. PRELIMINARY TREATMENT

Influent wastewater 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 designed to settle out inorganic solids such as sand, gravel, and coffee grounds while keeping organic matter in suspension. The removed grit and screenings are washed, compacted, and hauled to a landfill. Odor control at the headworks is a priority, utilizing chemical scrubbers (typically wet scrubbers using sodium hypochlorite and sodium hydroxide) to neutralize hydrogen sulfide (H₂S).

B. PRIMARY TREATMENT

The screened and degritted wastewater flows into rectangular primary sedimentation tanks. Here, flow velocity is reduced to allow settleable organic solids to drop to the bottom as primary sludge, while fats, oils, and grease (FOG) float to the surface for skimming. The primary treatment stage typically removes 50-60% of TSS and 30-40% of BOD, significantly reducing the load on the subsequent biological stage.

C. SECONDARY TREATMENT (High Purity Oxygen)

The core of the NDWWTP is its High Purity Oxygen (HPO) activated sludge system. Unlike conventional aeration basins that use ambient air (21% oxygen), this facility utilizes covered aeration tanks injected with >90% pure oxygen.
Key Technical Features:

• Cryogenic Oxygen Plant: An on-site cryogenic air separation plant generates pure oxygen, which is piped directly into the headspace of the covered aeration basins.
• Mechanical Aerators: Surface aerators mix the oxygen-rich headspace with the mixed liquor, facilitating rapid oxygen transfer.
• Performance: The HPO process allows for higher Mixed Liquor Suspended Solids (MLSS) concentrations and shorter hydraulic retention times (HRT) compared to conventional air systems, making it highly efficient for the plant’s limited footprint.

Following aeration, the mixed liquor flows to secondary clarifiers where the biomass settles. Return Activated Sludge (RAS) is pumped back to the aeration tanks, while Waste Activated Sludge (WAS) is sent to solids handling.

D. EFFLUENT MANAGEMENT

The plant does not currently employ tertiary filtration for its primary discharge stream but is implementing High-Level Disinfection (HLD) protocols required for deep well injection.

E. DISINFECTION

Disinfection is achieved through chlorination. The plant utilizes chlorine gas or high-strength sodium hypochlorite in contact basins to ensure pathogen inactivation. For the effluent stream directed to the ocean outfall, the water retains a chlorine residual to prevent regrowth in the pipe. For the deep injection well stream, specific monitoring ensures compatibility with aquifer geochemistry.

F. SOLIDS HANDLING

Primary sludge and thickened WAS are blended and pumped to anaerobic digesters. The digestion process stabilizes the solids, reducing volume and volatile content while producing biogas (methane).
Dewatering: Digested sludge is dewatered using high-solids centrifuges, producing a cake that is typically 20-25% solids.
Disposal: The biosolids are hauled off-site for land application or landfill disposal, depending on the class of biosolids achieved and current regulatory allowances.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The 245-acre site is dominated by the enclosed HPO reactor trains, large rectangular clarifier banks, and the towering anaerobic digesters. The site includes a dedicated Administration Building, a fully equipped process control laboratory, and extensive maintenance shops.

B. Energy Systems & Cogeneration

The NDWWTP is a significant energy consumer, primarily due to the cryogenic oxygen generation and influent pumping. To offset this, the facility operates a Cogeneration (Combined Heat and Power) plant. Methane gas produced during anaerobic digestion is scrubbed and used to fuel internal combustion engines that generate electricity. Waste heat from these engines is recovered to heat the digesters, maintaining the mesophilic temperature range required for optimal biological activity. This system offsets a portion of the plant’s grid energy demand and reduces the carbon footprint.

C. Odor Control

Given the plant’s location near residential zones and the Biscayne Bay campus of Florida International University, odor control is paramount. The facility utilizes a multi-stage approach, covering primary emission sources (headworks, thickeners) and treating foul air through packed-bed wet chemical scrubbers.

5. RECENT UPGRADES & MAJOR PROJECTS

Miami-Dade WASD is currently executing a multi-billion dollar Capital Improvement Program (CIP) driven by the Ocean Outfall Legislation (OOL) and a federal Consent Decree.

6. REGULATORY COMPLIANCE

A. Permit Requirements

The facility operates under FDEP permit FL0032182. Key effluent parameters for the ocean outfall typically include:

• CBOD5: 25 mg/L (Annual Avg)
• TSS: 30 mg/L (Annual Avg)
• pH: 6.0 – 8.5 s.u.
• Enterococci: Monitoring required for ocean discharge.

For the Deep Injection Wells, the focus shifts to High-Level Disinfection (HLD) monitoring to ensure no migration of fluid into underground sources of drinking water (USDW).

B. Consent Decree

Since 2013/2014, Miami-Dade WASD has operated under a Consent Decree with the U.S. EPA and FDEP to address sanitary sewer overflows. While much of this work involves collection system pipes and pump stations, the NDWWTP has seen specific upgrades to ensure it can process peak flows without bypassing treatment stages, ensuring full compliance during wet weather events.

7. OPERATIONAL EXCELLENCE

A. Staffing

The NDWWTP is staffed 24/7 by a team of Florida-licensed wastewater operators (Class A, B, and C). The team includes maintenance mechanics, electricians, instrumentation technicians, and laboratory analysts. WASD maintains a robust internal training program to facilitate licensure advancement.

B. Technology

The plant utilizes a centralized SCADA system for real-time monitoring of all process variables (DO levels, tank levels, pump status, chlorine residuals). Recent integrations include advanced metering infrastructure (AMI) data to better predict diurnal flow patterns.

8. CHALLENGES & FUTURE PLANNING

A. Climate Resilience

Located in a coastal zone, the NDWWTP faces risks from sea-level rise and storm surge. Future planning involves “hardening” the facility—elevating critical assets, improving stormwater management on-site, and ensuring backup power reliability during extended grid outages.

B. Transition to 100% Injection/Reuse

The immediate challenge is the complete operational switch from ocean discharge to deep wells by 2025. This changes the hydraulic profile of the plant (pumping down a hole vs. gravity/pumping out to sea) and requires rigorous testing of the well capacity and redundancy.

C. Saltwater Intrusion

The collection system suffers from Infiltration and Inflow (I/I), specifically saltwater intrusion from rising groundwater tables. High chloride levels can inhibit biological treatment processes and accelerate corrosion of mechanical equipment. The county is actively lining pipes to reduce this impact.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. RELATED FACILITIES

The NDWWTP is one of three regional treatment facilities operated by Miami-Dade WASD. The other two are:

• Central District Wastewater Treatment Plant (CDWWTP): Located on Virginia Key, treating flows from Miami and Key Biscayne.
• South District Wastewater Treatment Plant (SDWWTP): Located in the Black Point area, serving the southern portion of the county and featuring a significant water reclamation component.

11. FAQ SECTION

Technical Questions

1. Why does NDWWTP use High Purity Oxygen (HPO)?
HPO was selected to treat high organic loads in a smaller physical footprint compared to conventional air activated sludge. The high partial pressure of oxygen accelerates the biological breakdown of waste.

2. What is the status of the Ocean Outfall at NDWWTP?
Per Florida law, the routine use of the ocean outfall must cease by 2025. The plant is transitioning to Deep Injection Wells for effluent disposal.

3. Does the plant have nutrient removal capabilities?
The plant achieves secondary treatment standards. While it removes some nutrients through biomass synthesis, it is not currently designed as an Advanced Nutrient Removal (BNR) facility, relying instead on the geological confinement of deep injection wells to protect surface waters.

4. How is the plant powered?
The plant relies on grid power from FPL but supplements this with on-site Cogeneration engines fueled by methane gas produced in the anaerobic digesters.

Public Interest Questions

5. Can the public tour the facility?
Tours are generally restricted due to security and safety regulations but may be arranged for educational institutions or engineering groups by contacting the Miami-Dade WASD Public Affairs office.

6. Does the plant smell?
Wastewater treatment inherently generates odors, but NDWWTP utilizes extensive chemical scrubbers and covered process tanks to capture and treat foul air, minimizing impact on the surrounding community.