Trinity River Authority Central Regional Wastewater Treatment Plant Dallas

1. Introduction

The Trinity River Authority Central Regional Wastewater System (CRWS) stands as one of the largest and most critical wastewater infrastructure assets in the State of Texas. Located in western Dallas, this regional facility serves as the hydrological backbone for the Dallas-Fort Worth Metroplex, treating an average of 135 to 145 million gallons daily (MGD) for approximately 1.2 million residents. Commissioned in 1959 and operated by the Trinity River Authority (TRA), CRWS was one of the first regional wastewater systems in the nation, pioneering the concept of multiple municipalities utilizing a single, consolidated treatment facility to achieve economies of scale and superior environmental performance.

Currently rated for a design capacity of 162 MGD with a peak 2-hour flow capacity of 405 MGD, the plant is a complex engineering marvel encompassing over 500 acres. Following substantial capital investments exceeding $1 billion over the last two decades, including the massive Phase III expansion, CRWS utilizes advanced secondary and tertiary treatment processes to meet stringent Texas Commission on Environmental Quality (TCEQ) standards. The facility plays a dual role: protecting public health and providing a critical base flow for the Trinity River, which serves as a water source for downstream populations, including the City of Houston.

2. Facility Overview

A. Service Area & Coverage

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The CRWS operates as a wholesale service provider, treating wastewater for a massive 486-square-mile service area. Rather than collecting from individual households, the TRA maintains a vast network of large-diameter interceptors that receive flow from municipal collection systems. The “Contracting Parties” include major cities such as Arlington, Bedford, Carrollton, Dallas, Duncanville, Euless, Farmers Branch, Grand Prairie, Irving, and the DFW International Airport. The demographics are mixed, handling heavy industrial loads from the airport and industrial parks, alongside dense residential zones.

B. Operational Capacity

The facility is designed to handle significant hydraulic variations:

• Permitted Annual Average Flow: 162 MGD
• Peak 2-Hour Flow: 405 MGD
• Hydraulic Peak: The plant is engineered to manage extreme wet-weather events, a critical requirement given the expansive collection system that is subject to inflow and infiltration (I/I).

Historical trends indicate a steady rise in loading consistent with DFW’s explosive population growth. The plant generally operates at 80-85% of its average daily design capacity, triggering ongoing master planning for future expansions.

C. Discharge & Compliance

Treated effluent is discharged directly into the Trinity River (Segment 0805). In dry summer months, the effluent from CRWS and neighboring plants can constitute the majority of the river’s flow. Consequently, the TPDES (Texas Pollutant Discharge Elimination System) permit imposes strict limitations on biochemical oxygen demand (CBOD), total suspended solids (TSS), and nutrients to prevent eutrophication and maintain dissolved oxygen levels for aquatic life.

3. Treatment Process

The CRWS employs a sophisticated treatment train designed to handle high-strength municipal wastewater and deliver tertiary-quality effluent. The plant layout is generally divided into multiple parallel trains to manage hydraulic throughput.

A. Preliminary Treatment

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Raw wastewater enters the facility through massive interceptors (up to 90+ inches in diameter). The headworks facility utilizes:

• Coarse and Fine Screening: Mechanically cleaned bar screens remove large debris, rags, and non-flushables to protect downstream pumping equipment.
• Grit Removal: Aerated grit chambers are utilized to settle out sand, gravel, and coffee grounds. The velocity is controlled to allow organics to pass through while capturing inorganic abrasives.
• Odor Control: The headworks are enclosed and vented to chemical scrubbers and biotrickling filters to mitigate H₂S emissions, crucial given the plant’s proximity to Loop 12 and commercial zones.

B. Primary Treatment

The flow transitions to primary clarification. CRWS utilizes a battery of large circular primary clarifiers. These tanks reduce the velocity of the wastewater, allowing settleable solids to drop to the bottom as primary sludge and floatables (grease/oils) to be skimmed from the surface. Typical removal efficiency at this stage targets 30-35% of BOD and 50-60% of TSS.

C. Secondary Treatment (Activated Sludge)

The biological heart of the plant is the Activated Sludge process. CRWS operates multiple aeration basins equipped with fine-bubble diffusion systems to maximize oxygen transfer efficiency.

• Process Type: The facility utilizes a variation of the conventional activated sludge process, optimized for nitrification.
• Aeration: Large centrifugal blowers supply air to the basins, maintaining the dissolved oxygen levels necessary for aerobic bacteria to metabolize organic matter and convert ammonia to nitrate.
• Secondary Clarification: Following aeration, the mixed liquor flows into circular secondary clarifiers. Here, the biological floc settles out. A portion is returned to the aeration basins (RAS) to maintain the biological population, while the excess is removed as waste activated sludge (WAS).

D. Tertiary Treatment (Filtration)

To meet the high-quality standards required for the Trinity River, CRWS employs tertiary filtration. The facility uses deep-bed mono-media (anthracite) and dual-media gravity filters. This stage physically removes remaining suspended solids that escaped secondary clarification, significantly reducing turbidity and particulate-bound pollutants.

E. Disinfection

Disinfection is achieved through chlorination. Gaseous chlorine is applied to the filtered effluent in contact basins to neutralize pathogenic organisms. Prior to discharge into the river, the water undergoes dechlorination using sulfur dioxide (SO₂) to ensure zero residual chlorine, protecting aquatic life in the receiving stream.

F. Solids Handling (Biosolids)

CRWS is renowned for its biosolids management program:

• Thickening: Primary and waste activated sludges are thickened (often using gravity belt thickeners or dissolved air flotation) to reduce water content.
• Stabilization: The facility has historically utilized Anaerobic Digestion, producing biogas which can be used for heating or power generation.
• Dewatering: Digested sludge is dewatered using high-performance belt filter presses or centrifuges.
• Beneficial Reuse: TRA is a leader in beneficial reuse. A significant portion of the Class B biosolids is land-applied as a soil conditioner and fertilizer on TRA-owned farms and authorized application sites, supporting agricultural production and soil reclamation projects.

4. Infrastructure & Facilities

A. Physical Plant

The site spans approximately 560 acres along the West Fork of the Trinity River. The campus includes extensive maintenance shops, a central administration building, and a NELAC-accredited laboratory that performs thousands of analyses annually for process control and regulatory reporting.

B. Energy Systems

As one of the largest energy consumers in the region, CRWS has implemented aggressive energy management strategies. The plant utilizes Variable Frequency Drives (VFDs) on major pumps and blowers to match energy use with hydraulic demand. The facility has also explored cogeneration technologies, utilizing methane gas produced in the anaerobic digesters to offset purchased electricity and provide process heating.

C. Odor Control

Odor control is a top priority. TRA utilizes a multi-stage approach including covering primary emission sources (headworks, primary clarifier weirs, solids handling) and treating the foul air through chemical scrubbers (wet scrubbers using hypochlorite/caustic) and biological towers. A centralized SCADA system monitors hydrogen sulfide levels in real-time at the fence line.

5. Recent Upgrades & Major Projects

The CRWS requires continuous modernization to handle the Metroplex’s growth. Recent major Capital Improvement Projects (CIP) include:

6. Regulatory Compliance & Environmental Performance

The CRWS operates under TPDES Permit No. WQ0010313001. The facility maintains an exemplary compliance record with the EPA and TCEQ.

A. Permit Limits (Typical Parameters)

• CBOD (5-day): Strict limits, often averaging < 7-10 mg/L depending on seasonal permit variations.
• TSS: Generally limited to 15 mg/L daily average, though the plant consistently achieves single digits.
• Ammonia-Nitrogen (NH3-N): Critical due to toxicity; seasonal limits are strictly enforced (often < 2-3 mg/L).
• E. coli: Disinfection must maintain geometric means below 126 CFU/100mL.

B. Environmental Stewardship

The TRA has received numerous National Association of Clean Water Agencies (NACWA) Peak Performance Awards for the CRWS, recognizing years of 100% permit compliance. The high-quality effluent provides essential base flow for the Trinity River, supporting a rebounding ecosystem that now hosts diverse fish populations and recreational activities downstream.

7. Operational Excellence

The facility is staffed by over 100 personnel, including highly licensed wastewater operators (Class A and B), industrial mechanics, electricians, and instrument technicians. The TRA emphasizes professional development, with a strong focus on safety and certification.

Technology: The plant utilizes a robust SCADA system integration, allowing for centralized control of the massive campus. Real-time data analytics are used for chemical dosing optimization (chlorine and sulfur dioxide) to prevent over-dosing and save costs.

8. Challenges & Future Planning

A. Inflow and Infiltration (I/I)

The primary challenge for CRWS is the management of peak flows caused by I/I in the aging collection systems of customer cities. During heavy rain events, flow can spike from 140 MGD to over 400 MGD in hours. TRA works closely with contracting cities to identify and remediate I/I sources.

B. Nutrient Standards

Future regulatory trends indicate potential tightening of Total Phosphorus (TP) and Total Nitrogen (TN) limits. CRWS is actively planning for potential biological nutrient removal (BNR) upgrades or chemical precipitation enhancements to meet future nutrient criteria without requiring massive new infrastructure footprints.

9. Technical Specifications Summary

10. FAQ

1. What is the treatment capacity of the TRA Central Regional Wastewater System?

The plant has a rated annual average daily flow of 162 MGD and a peak 2-hour wet weather flow capacity of 405 MGD.

2. Which cities send wastewater to the CRWS?

The system serves 21 contracting parties, including major municipalities like Dallas, Irving, Arlington, Grand Prairie, and the DFW International Airport.

3. Does the facility use UV disinfection?

No, the CRWS currently utilizes chlorination for disinfection followed by dechlorination with sulfur dioxide before discharge.

4. How are biosolids handled at the facility?

Biosolids are processed through anaerobic digestion and dewatering. The resulting Class B biosolids are primarily used for beneficial reuse through land application on agricultural land.

5. Is the plant under a consent decree?

While the TRA works cooperatively with regulators, the specific contracting cities that feed the plant often face their own regulatory actions regarding sanitary sewer overflows (SSOs) within their collection systems. The CRWS plant itself maintains a high level of compliance.

6. Can the public tour the facility?

Yes, TRA is well-known for its public education efforts and offers scheduled tours for schools, universities, and professional groups to educate the public on the water cycle.

7. What happens to the water after treatment?

The high-quality effluent is discharged into the Trinity River, where it flows downstream towards Lake Livingston and eventually Trinity Bay, serving as a water source for wildlife and downstream communities.

8. How does CRWS mitigate odors?

The facility uses a combination of covered process units, chemical scrubbers, and biological filters to treat odorous air extracted from the headworks and solids handling facilities.