San Antonio Water System Dos Rios Wastewater Treatment Plant

FACILITY BASIC INFORMATION

Official Name: Steven M. Clouse Water Recycling Center (Formerly Dos Rios Water Recycling Center)

Location: 3495 Valley Road, San Antonio, Bexar County, Texas 78221

Operating Authority: San Antonio Water System (SAWS)

Design Capacity: 125 MGD (Average Daily Flow)

Current Average Flow: ~75–85 MGD

Population Served: Approx. 1.2 million (Greater San Antonio Metro)

Service Area: Central, South, and West San Antonio (Bexar County)

Receiving Water Body: San Antonio River (Segment 1901) and Medina River confluence

NPDES/TPDES Permit Number: WQ0010137033

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Year Commissioned: 1987 (Major expansion/renaming in subsequent decades)

1. INTRODUCTION

The Steven M. Clouse Water Recycling Center (formerly known as the Dos Rios Water Recycling Center) stands as the largest and most technically advanced wastewater treatment facility operated by the San Antonio Water System (SAWS). Treating an average of 80 million gallons daily (MGD) with a peak hydraulic capacity exceeding 200 MGD, this facility serves as the backbone of sanitation infrastructure for over 1 million residents in the greater San Antonio metropolitan area. Renamed in honor of former SAWS COO Steven Clouse in 2019, the plant is a nationally recognized model for large-scale water reuse and sustainable biosolids management.

Strategically located at the confluence of the Medina and San Antonio Rivers, the facility plays a dual role: protecting public health through rigorous sanitation and maintaining the base flow of the San Antonio River—vital for the city’s famed Riverwalk and downstream ecosystems. The plant is particularly renowned in the engineering community for its pioneering biogas-to-energy program, created in partnership with Ameresco, which became the first project in the nation to process biogas from a municipal wastewater treatment plant for sale to a commercial natural gas grid.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Steven M. Clouse WRC serves the majority of the San Antonio metropolitan area, specifically capturing flows from the central, western, and southern portions of the collection system. The service area is characterized by a dense urban core surrounded by rapidly expanding suburban developments. The collection system feeding the plant includes massive interceptors, most notably the W-6 Upper Segment sewer main, a critical infrastructure artery designed to transport flows from the city’s high-growth western sectors.

B. Operational Capacity

While the facility has a permitted annual average flow design of 125 MGD, current dry weather flows average between 75 and 85 MGD, providing the utility with a capacity utilization buffer of approximately 35%. This excess capacity is critical for managing peak wet weather events, common in South Texas, where inflow and infiltration (I&I) can cause instantaneous flow spikes. The plant is designed with a peak 2-hour flow capacity significantly higher than its average rating to prevent sanitary sewer overflows (SSOs).

C. Discharge & Compliance

Effluent management is unique at this facility. A significant portion of the treated effluent contributes to the San Antonio River’s base flow. During dry months, the plant’s discharge can account for nearly the entire flow of the river, making treatment reliability critical for aquatic life and downstream users. Additionally, the facility feeds the SAWS Recycled Water System—the largest direct recycled water delivery system in the nation—supplying “purple pipe” water to golf courses, parks, commercial customers, and the Toyota manufacturing plant.

3. TREATMENT PROCESS

The Steven M. Clouse WRC utilizes a complex treatment train designed to meet strict Texas Commission on Environmental Quality (TCEQ) standards for Type I reclaimed water and nutrient limitations for river discharge.

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A. Preliminary Treatment

Raw 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. These chambers reduce the velocity of the water, allowing heavier inorganic materials (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The removed grit is washed and dewatered before being hauled to a landfill. Odor control at the headworks is managed through chemical scrubbers to mitigate nuisance odors for nearby communities.

B. Primary Treatment

Flow proceeds to large circular primary clarifiers. Here, gravity separation occurs; settleable solids sink 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 Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD). The primary sludge is pumped directly to the anaerobic digesters.

C. Secondary Treatment (Activated Sludge with BNR)

The biological treatment stage is the core of the facility. The plant employs an activated sludge process configured for Biological Nutrient Removal (BNR). This is critical for complying with TPDES permit limits regarding ammonia-nitrogen and phosphorus.

• Aeration Basins: The facility utilizes fine-bubble diffusion to maximize oxygen transfer efficiency. The basins are operated to create alternating aerobic and anoxic zones, facilitating nitrification (conversion of ammonia to nitrate) and denitrification (conversion of nitrate to nitrogen gas).
• Secondary Clarification: Mixed liquor flows to secondary clarifiers where biological floc settles. A portion of this activated sludge is returned to the aeration basins (RAS), while the excess biomass (Waste Activated Sludge or WAS) is thickened and sent to digestion.

D. Tertiary Treatment & Filtration

To meet Type I reuse standards (safe for contact recreation and irrigation in public areas), the secondary effluent undergoes tertiary filtration. The facility utilizes cloth media disk filters and/or gravity sand filters depending on the specific flow train. This step polishes the water, reducing turbidity to typically less than 2 NTU, ensuring high-clarity effluent suitable for the recycled water distribution system.

E. Disinfection

Disinfection is achieved through chlorination using sodium hypochlorite or chlorine gas, providing the necessary contact time in serpentine chlorine contact basins to inactivate pathogens. For effluent discharged to the San Antonio River, the water must be dechlorinated using sulfur dioxide or sodium bisulfite to prevent toxicity to aquatic life. Residual chlorine levels are strictly monitored to ensure they remain below permit limits (typically < 0.1 mg/L) at the outfall.

F. Solids Handling & Biogas Generation

The solids handling train is a standout feature of the plant:

• Thickening: WAS is thickened using gravity belt thickeners or dissolved air flotation (DAF) prior to digestion.
• Anaerobic Digestion: Primary sludge and thickened WAS are blended and fed into anaerobic digesters. These heated tanks break down volatile solids, reducing volume and stabilizing the sludge.
• Biogas Recovery: The digestion process produces roughly 1.5 million cubic feet of biogas daily. Through a partnership with Ameresco, this gas is treated to remove moisture, siloxanes, and hydrogen sulfide, then injected into the commercial natural gas pipeline.
• Dewatering & Disposal: Digested sludge is dewatered using high-solids belt filter presses. The resulting biosolids are Class B (and occasionally Class A equivalent via composting) and are beneficially reused through land application on agricultural fields.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site spans several hundred acres in South San Antonio. It houses administrative offices, a sophisticated water quality laboratory, and extensive maintenance shops. The layout is designed with redundancy in mind; multiple treatment trains allow operators to take individual basins or clarifiers offline for maintenance without disrupting overall plant capacity.

B. Energy Systems & Sustainability

The Steven M. Clouse WRC is a net energy producer regarding natural gas equivalents. The Ameresco biogas facility, operational since 2010, processes over 900,000 standard cubic feet of biogas per day. The facility generates enough renewable natural gas to fuel approximately 1,200 homes annually or power a fleet of vehicle operations. This reduces carbon dioxide emissions by roughly 20,000 tons per year.

5. RECENT UPGRADES & MAJOR PROJECTS

SAWS continuously invests in the Clouse WRC to maintain compliance and extend asset life.

Treatment Plant Reliability Improvements (2018–2023)

• Scope: Comprehensive rehabilitation of primary and secondary clarifier mechanisms, replacement of aging electrical switchgear, and upgrades to the aeration blower systems.
• Project Budget: Part of a broader multi-year Capital Improvement Plan (CIP) exceeding $50 million for treatment facilities.
• Technical Highlights: Installation of high-efficiency turbo blowers to reduce electrical consumption in the aeration basins by up to 30%.

W-6 Upper Segment Interceptor (Associated Infrastructure)

• Scope: While off-site, this massive tunneling project was critical for the plant. It involved constructing 5.5 miles of 104-inch diameter sanitary sewer pipe to convey flows from rapidly growing areas to the Clouse WRC.
• Drivers: Accommodating population growth and preventing SSOs in the collection system.

Nutrient Removal Optimization

• Scope: Chemical feed systems (Alum/Ferric) upgrades to enhance phosphorus removal capabilities in anticipation of stricter TPDES limits.
• Status: Ongoing optimization of the BNR process to minimize chemical costs while ensuring permit compliance.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under TPDES Permit No. WQ0010137033. Key effluent limitations typically include:

• CBOD (5-day): 5-10 mg/L (varies by season/flow)
• TSS: 15 mg/L
• Ammonia-Nitrogen (NH3-N): 2-3 mg/L
• E. coli: 126 CFU/100 ml (Geometric Mean)
• Phosphorus: 0.5 – 1.0 mg/L (Strict monitoring required due to river discharge)

B. Environmental Stewardship

The plant has received the National Association of Clean Water Agencies (NACWA) Platinum Peak Performance Award multiple times, recognizing facilities that achieve 100% compliance with NPDES permits for five consecutive years or more. The high-quality effluent is instrumental in sustaining the habitat for the Texas wild rice and other endangered species downstream in the San Antonio River system.

7. OPERATIONAL EXCELLENCE

The Steven M. Clouse WRC is staffed 24/7 by a team of licensed wastewater operators (Class A and B TCEQ licenses), maintenance mechanics, instrument technicians, and chemists. SAWS employs a robust SCADA (Supervisory Control and Data Acquisition) system that provides real-time monitoring of thousands of data points, allowing for automated adjustment of dissolved oxygen levels, return sludge rates, and chemical dosing.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

The primary challenge facing the facility is the rapid urbanization of Bexar County. As impervious cover increases, peak flow management during storm events becomes more critical. Additionally, the aggressive nature of wastewater causes corrosion in concrete structures, necessitating continuous “maintenance of plant” (MOP) projects to rehabilitate digesters and headworks concrete.

B. Future Planning

Future CIPs for the facility are focused on:

1. Advanced Nutrient Removal: Preparing for potential future reductions in total nitrogen and phosphorus limits.
2. Biosolids Diversification: Exploring additional markets for compost and soil amendments.
3. Smart Water Technologies: Implementing AI-driven process controls to optimize energy usage in the aeration basins further.

9. COMMUNITY & REGIONAL IMPACT

The facility is an economic engine for the region. By providing a guaranteed source of recycled water, the plant allows industries like Toyota and data centers to operate in San Antonio without depleting the Edwards Aquifer (the city’s primary drinking water source). The plant also hosts frequent tours for university engineering students and industry professionals, serving as a living laboratory for wastewater education.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

The Steven M. Clouse WRC works in conjunction with other major SAWS facilities to manage regional flows:

• Leon Creek Water Recycling Center: A smaller facility (~46 MGD) that also discharges to the regional system.
• Medina River Water Recycling Center: Serves the far south/west portions of the county.
• H2Oaks Center: A unique facility focused on brackish groundwater desalination and aquifer storage and recovery, complementing the water portfolio.

12. FAQ SECTION

Technical/Professional Questions

1. Why was the plant renamed from Dos Rios to Steven M. Clouse WRC?
In 2019, the SAWS Board of Trustees voted to rename the facility to honor Steven M. Clouse, a long-serving Chief Operating Officer who was instrumental in modernizing San Antonio’s wastewater infrastructure and leading the response to the EPA consent decree.

2. Does the facility achieve total nitrogen removal?
The facility utilizes nitrification/denitrification zones in the aeration basins to reduce ammonia and nitrogen levels. While not a “Limit of Technology” (LOT) nitrogen removal plant (like those in the Chesapeake Bay), it achieves significant reduction to protect the San Antonio River.

3. How is the biogas utilized?
Unlike many plants that use biogas for cogeneration (Combined Heat and Power) onsite, this facility processes the gas to natural gas pipeline quality standards and sells it to the commercial grid via a third-party partner, Ameresco.

4. What is the sludge retention time (SRT)?
While variable based on season, the facility typically targets an SRT sufficient to maintain nitrifying bacteria populations, usually in the range of 10 to 15 days.

Public Interest Questions

5. Does the plant smell?
While wastewater treatment naturally generates odors, the facility employs advanced odor control scrubbers at the headworks and covers various process tanks to capture and treat foul air. Buffer zones also separate the plant from residential areas.

6. Is the water released into the river safe?
Yes. The effluent meets strict state and federal standards for water quality. In fact, the water discharged is often cleaner and clearer than the river water it flows into.

7. Can I tour the facility?
SAWS offers tours of its facilities, including the Steven M. Clouse WRC, primarily for educational groups, civic organizations, and industry professionals. Requests can be made through the SAWS website community relations department.