City Of Austin Walnut Creek Wastewater Treatment Plant
1. INTRODUCTION
The Walnut Creek Wastewater Treatment Plant (WWTP) serves as a cornerstone of the City of Austin’s hydrological infrastructure, managing the wastewater needs of the rapidly expanding northern and eastern sectors of the city. With a permitted design capacity of 75 million gallons per day (MGD), it is one of Austin Water’s two major liquid-stream treatment facilities. Commissioned in 1977, the plant acts as a critical environmental safeguard for the Colorado River, discharging high-quality effluent downstream of Longhorn Dam.
Operated by Austin Water, the facility is distinct in its operational integration with the Hornsby Bend Biosolids Management Plant. Unlike many municipal facilities that process sludge on-site to final disposal, Walnut Creek focuses on liquid stream treatment, pumping generated residuals to Hornsby Bend for stabilization and composting. Following a series of capital improvements aimed at modernization and resiliency, the plant continues to maintain rigorous compliance with Texas Commission on Environmental Quality (TCEQ) standards while preparing for a service area population that continues to grow exponentially.
2. FACILITY OVERVIEW
A. Service Area & Coverage
The Walnut Creek WWTP service area encompasses approximately the northern half of the City of Austin and portions of Travis County. The collection system feeding the plant serves a diverse mix of residential neighborhoods, major commercial districts (including The Domain), and significant industrial corridors. As Austin acts as a major technology hub, the influent profile includes contributions from semiconductor manufacturing and other high-tech industries, necessitating robust industrial pretreatment monitoring.
B. Operational Capacity
The facility is currently permitted for an annual average flow of 75 MGD. Historical flow data indicates the plant typically operates between 60% and 75% of its rated capacity during dry weather conditions, treating approximately 45-55 MGD. However, the facility is designed to handle significant hydraulic surges associated with Central Texas flash flood events, with a peak 2-hour flow capacity of 165 MGD. Future master planning by Austin Water anticipates a potential re-rating or expansion to 100 MGD to accommodate projected population density increases in the Walnut Creek basin.
C. Discharge & Compliance
Treated effluent is discharged into the Colorado River, a vital water resource for downstream agricultural and municipal users. The discharge point is located roughly five miles downstream of Longhorn Dam. The facility operates under TPDES Permit No. WQ0010543011, administered by the TCEQ. The permit enforces strict limitations on Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), Ammonia-Nitrogen, and E. coli, reflecting the ecological sensitivity of the Colorado River ecosystem.
3. TREATMENT PROCESS
Walnut Creek WWTP utilizes an advanced secondary treatment process with tertiary filtration. A unique operational characteristic is the absence of anaerobic digestion on-site; all solids are thickened and pumped off-site for processing.
A. PRELIMINARY TREATMENT
Influent wastewater enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics that could damage downstream pumps. Following screening, the flow enters aerated grit chambers. These chambers reduce the velocity of the water, allowing inorganic solids like sand, gravel, and coffee grounds to settle 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 managed via chemical scrubbers to mitigate nuisance odors for nearby communities.
B. PRIMARY TREATMENT
Flow proceeds to primary clarifiers, which are large circular settling tanks. Here, the hydraulic velocity is further 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 sludge is collected by scraper mechanisms and pumped to the sludge holding tanks. This stage typically removes 50-60% of TSS and 30-40% of BOD.
C. SECONDARY TREATMENT (Activated Sludge)
The core biological treatment occurs in the aeration basins using a conventional activated sludge process. Settled wastewater is mixed with Return Activated Sludge (RAS) to form mixed liquor.
- Aeration: The basins are equipped with fine-bubble diffusers that introduce compressed air into the mixture. This oxygen supports the aerobic bacteria that consume the dissolved organic matter and convert ammonia to nitrate (nitrification).
- Secondary Clarification: The mixed liquor flows into large circular secondary clarifiers. The biological floc (biomass) settles to the bottom, separating from the clear treated water. A portion of this settled biomass is returned to the aeration basins (RAS), while the excess growth is removed as Waste Activated Sludge (WAS).
D. TERTIARY TREATMENT (Filtration)
To meet stringent effluent requirements, Walnut Creek employs tertiary deep-bed granular media filters. These gravity filters polish the secondary effluent, removing remaining suspended solids and particulate-bound pollutants. This step is critical for ensuring the clarity of the water discharged into the Colorado River and ensures compliance with low TSS permit limits.
E. DISINFECTION
The filtered effluent undergoes disinfection using chlorination (sodium hypochlorite) to eliminate pathogenic organisms. Following the chlorine contact basins, the water is treated with sodium bisulfite for dechlorination to remove residual chlorine, which is toxic to aquatic life, before the final discharge into the river.
F. SOLIDS HANDLING
Walnut Creek operates as a liquid-stream-focused facility. Primary sludge and Waste Activated Sludge (WAS) are thickened (using gravity thickeners or dissolved air flotation) and blended. Instead of on-site digestion, the thickened sludge is pumped via a dedicated force main to the Hornsby Bend Biosolids Management Plant. At Hornsby Bend, the sludge undergoes anaerobic digestion, is composted (Dillo Dirt program), or land applied. This centralization of solids handling allows Walnut Creek to minimize its footprint and odor potential.
4. INFRASTRUCTURE & FACILITIES
A. Physical Plant
The site spans extensive acreage east of downtown Austin. The layout is designed for hydraulic efficiency, moving flow via gravity where possible. Key structures include the influent pump station, headworks building, multiple batteries of primary and secondary clarifiers, aeration basins, filtration complex, and chemical storage facilities for disinfection.
B. Energy Systems
As a major consumer of electricity, primarily for aeration blowers and pumping systems, Walnut Creek participates in Austin Energy’s load management programs. The facility has undergone retrofits to replace constant-speed drives with Variable Frequency Drives (VFDs) on large motors, allowing energy consumption to scale directly with flow and biological demand.
C. Odor Control
Given the facility’s proximity to residential developments and the Colorado River Park, odor control is a priority. The plant utilizes dual-stage chemical scrubbers at the headworks and primary clarifier weirs. These systems typically employ sodium hypochlorite and caustic soda to oxidize hydrogen sulfide and other odorous compounds from the air stream before release.
5. RECENT UPGRADES & MAJOR PROJECTS
Austin Water maintains a robust Capital Improvement Program (CIP) to ensure asset renewal and capacity assurance.
Primary Clarifier & Flow Equalization Upgrades
Timeline: 2018–2022
Approximate Cost: $20–30 Million
Project Scope: Rehabilitation of existing primary clarifiers, including mechanism replacement and concrete repair. Installation of new flow equalization capabilities to better manage diurnal peaks and wet weather flows, ensuring consistent biological treatment performance.
Technical Highlights: Improved hydraulic distribution and modern scum removal mechanisms reduced maintenance intervals and improved primary capture rates.
Electrical Substation & Power Distribution Improvements
Timeline: 2019–2023
Approximate Cost: $50+ Million
Project Scope: Complete replacement of the main electrical substation and distribution gear. This project was critical to address aging electrical infrastructure and increase the reliability of power supply to critical process equipment.
Drivers: Increasing resiliency against grid instability and replacing equipment nearing end-of-life.
Upcoming: 100 MGD Expansion (Planning Phase)
Timeline: Projected 2025–2030
Status: Master Planning / Preliminary Design
Overview: Austin Water’s “Water Forward” integrated water resource plan identifies the need to expand Walnut Creek from 75 MGD to 100 MGD. This expansion will likely involve new aeration basins, additional secondary clarifiers, and upgraded filtration capacity to handle the explosive population growth in the northern service territory.
6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE
A. Permit Requirements
Operating under TPDES Permit WQ0010543011, the facility faces stringent discharge limits tailored to the Colorado River. Key parameters typically include:
- CBOD5: 5 mg/L (30-day average)
- TSS: 5 mg/L (30-day average)
- Ammonia-Nitrogen (NH3-N): 2 mg/L (30-day average)
- E. coli: 126 CFU/100 ml (Geometric Mean)
- Dissolved Oxygen (DO): Minimum 6.0 mg/L
B. Compliance History
Walnut Creek WWTP consistently demonstrates high compliance rates. The facility has received multiple National Association of Clean Water Agencies (NACWA) Peak Performance Awards (Silver and Gold) for permit compliance. Occasional challenges have historically been related to wet-weather hydraulic overloads, which recent equalization projects have aimed to mitigate.
7. OPERATIONAL EXCELLENCE
A. Staffing & Certification
The facility is staffed 24/7 by a team of licensed wastewater operators, maintenance technicians, and instrumentation specialists. Texas regulations require shift supervisors to hold Class A or B Wastewater Treatment Operator licenses issued by the TCEQ. Austin Water emphasizes continuous education, with an on-site training program to ensure staff remains current on biological process control and safety protocols.
B. Technology & Automation
The plant utilizes a comprehensive SCADA (Supervisory Control and Data Acquisition) system for real-time monitoring. Operators track Dissolved Oxygen (DO), Oxidation-Reduction Potential (ORP), and Mixed Liquor Suspended Solids (MLSS) trends to optimize aeration and sludge wasting rates. This automation allows for precise chemical dosing for disinfection and dechlorination, minimizing chemical costs and environmental impact.
8. CHALLENGES & FUTURE PLANNING
A. Current Challenges
Urban Growth: The most pressing challenge is the rapid densification of the sewershed. Managing increased loading while maintaining effluent quality requires constant process optimization.
Aging Infrastructure: With major components dating back to the 1977 commissioning, the ongoing rehabilitation of concrete structures and mechanical equipment is a significant budgetary focus.
B. Future Planning
Austin Water is exploring Indirect Potable Reuse (IPR) and Direct Potable Reuse (DPR) concepts. While currently a discharging facility, future iterations of Walnut Creek may involve advanced membrane treatment to contribute to Austin’s water supply portfolio, reducing reliance on the Colorado River and Highland Lakes.
10. TECHNICAL SPECIFICATIONS SUMMARY
| Parameter | Specification |
|---|---|
| Facility Type | Advanced Secondary with Tertiary Filtration |
| Design Capacity | 75.0 MGD (Annual Average) |
| Peak 2-Hour Flow | 165.0 MGD |
| Treatment Process | Activated Sludge, Tertiary Media Filtration |
| Disinfection | Chlorination / Dechlorination |
| Biosolids Processing | Off-site (Pumped to Hornsby Bend BMP) |
| Permitted Effluent BOD | 5 mg/L |
| Permitted Effluent TSS | 5 mg/L |
| Permitted Effluent NH3-N | 2 mg/L |
| Receiving Water | Colorado River |
| Operating Authority | Austin Water |
| TPDES Permit | WQ0010543011 |
11. RELATED FACILITIES
Hornsby Bend Biosolids Management Plant: All sludge generated at Walnut Creek is pumped to this facility for anaerobic digestion and composting. This symbiotic relationship is central to Austin’s wastewater master plan.
South Austin Regional (SAR) WWTP: Austin’s other major liquid-stream facility serving the southern portion of the city.
12. FAQ SECTION
Technical Questions
1. What is the hydraulic retention time (HRT) at Walnut Creek?
While variable based on flow, typical aeration HRT in conventional activated sludge modes at this facility ranges between 4 to 6 hours.
2. Does Walnut Creek have on-site anaerobic digesters?
No. Walnut Creek thickens sludge and pumps it via force main to the Hornsby Bend Biosolids Management Plant for stabilization.
3. What type of filtration is used?
The facility uses deep-bed granular media gravity filters (typically sand/anthracite) for tertiary polishing.
General Questions
4. Does the plant smell?
While wastewater treatment inherently generates odors, Walnut Creek employs chemical scrubbers at the headworks and primary treatment areas to treat air before release. Occasional odors may occur during maintenance or extreme weather.
5. Is the water released safe?
Yes. The effluent meets or exceeds all state and federal standards for discharge into the Colorado River. It is disinfected to kill bacteria and is often cleaner than the river water itself regarding suspended solids.