1. INTRODUCTION
The Lower Rio Grande Valley Regional Wastewater Treatment Plant (technically designated as the South Water Reclamation Plant) represents a critical infrastructure node in one of the fastest-growing regions in the United States. Serving the rapidly urbanizing corridor of Hidalgo County, this facility is pivotal in managing the delicate hydrological balance of the Rio Grande basin. The plant treats an average of 8.5 million gallons per day (MGD) with a peak hydraulic design capacity exceeding 20 MGD.
Operated by the McAllen Public Utility, this facility is distinguished not only by its scale but by its aggressive integration of water reuse technologies. In a semi-arid region prone to drought, the plant serves as a model for “One Water” management, diverting significant effluent volumes for non-potable municipal reuse, thereby alleviating pressure on the Rio Grande river system. Following a recent multi-phase expansion, the facility now utilizes advanced biological nutrient removal and tertiary filtration to meet stringent Texas Commission on Environmental Quality (TCEQ) discharge standards.
2. FACILITY OVERVIEW
A. Service Area & Demographics
The facility serves the southern sector of McAllen and adjacent extraterritorial jurisdictions (ETJs) in the Lower Rio Grande Valley. The service area is characterized by a mixed demographic profile, including high-density residential zones, expanding commercial corridors along Expressway 83, and light industrial parks. The collection system spans over 600 miles of gravity sewer and force mains, supported by over 50 lift stations that convey flow to the regional headworks.
B. Operational Capacity
The plant is designed for a sustained average daily flow of 10.0 MGD, with hydraulic structures sized to accommodate a future expansion to 15.0 MGD. Historical flow trends indicate a steady annualized increase of 2-3%, driven by the region’s population boom.
- Design Flow: 10.0 MGD
- 2-Hour Peak Flow: 20.0 MGD
- Current Utilization: ~85%
C. Discharge & Compliance
Effluent is discharged under the Texas Pollutant Discharge Elimination System (TPDES). The receiving stream is typically an irrigation drainage canal or floodway that ultimately flows into the Laguna Madre, a hypersaline lagoon sensitive to nutrient loading. Consequently, the facility operates under strict limitations regarding Biological Oxygen Demand (BOD), Total Suspended Solids (TSS), and increasingly, Ammonia-Nitrogen.
3. TREATMENT PROCESS
The facility utilizes a conventional activated sludge process augmented by tertiary treatment for water reclamation. The treatment train is designed to handle high variations in hydraulic loading common to the region during tropical storm events.
A. Preliminary Treatment
Raw wastewater enters the headworks via large diameter force mains. Preliminary treatment consists of:
- Screening: Mechanically cleaned step-screens (6mm spacing) remove large debris and rags to protect downstream pumping equipment.
- Grit Removal: A vortex grit removal system extracts inorganic solids (sand, silt) which are classified and washed before landfill disposal.
- Odor Control: The headworks is enclosed and ventilated through a biotrickling filter system to mitigate H2S emissions, a critical requirement due to the plant’s proximity to residential developments.
B. Primary Treatment
The facility utilizes rectangular primary clarifiers to settle heavy organic solids. Primary sludge is pumped directly to the solids handling facility, while the supernatant flows to the aeration basins. This stage typically achieves 30-35% BOD removal and 50-60% TSS removal.
C. Secondary Treatment (Activated Sludge)
The core biological process occurs in parallel aeration basins. The plant utilizes a plug-flow activated sludge configuration with fine-bubble diffused aeration.
- Aeration: High-efficiency turbo blowers provide oxygen control via dissolved oxygen (DO) sensors, optimizing energy usage.
- Process Control: The Mean Cell Residence Time (MCRT) is managed to ensure nitrification, converting ammonia to nitrate.
- Secondary Clarification: Mixed liquor settles in large circular center-feed clarifiers. Return Activated Sludge (RAS) is recycled to the aeration basins, while Waste Activated Sludge (WAS) is sent to thickening.
D. Tertiary Treatment & Reuse
To meet Type I Reclaimed Water standards (Title 30 TAC Chapter 210), a portion of the secondary effluent undergoes tertiary filtration.
- Technology: Cloth media disk filters or rapid sand filters are utilized to reduce turbidity to < 3 NTU.
- Application: This water is diverted to the “Purple Pipe” system for irrigation of municipal parks, golf courses, and construction dust control.
E. Disinfection
The plant employs a dual disinfection strategy depending on the effluent destination:
- Chlorination/Dechlorination: For discharge into the receiving water body, gaseous chlorine is applied in contact basins, followed by sulfur dioxide for dechlorination to protect aquatic life.
- Ultraviolet (UV) Disinfection: For the reclaimed water stream, high-intensity UV systems provide pathogen inactivation without residual chemical byproducts.
F. Solids Handling
Biosolids management is a critical operational component:
- Thickening: Gravity belt thickeners concentrate WAS prior to digestion.
- Digestion: Anaerobic digesters stabilize the sludge, reducing volatile solids and pathogen content. Methane gas produced is flared or utilized for heating the digesters.
- Dewatering: Belt filter presses dewater the digested sludge to approximately 18-22% solids cake.
- Disposal: Class B biosolids are hauled to a sanitary landfill.
4. INFRASTRUCTURE & ENERGY
A. Physical Plant
The site encompasses approximately [XX] acres. Critical structures are elevated above the 100-year flood plain, a necessary resilience measure given the region’s susceptibility to hurricanes. The site includes a full SCADA command center and a NELAC-accredited laboratory for process control and compliance testing.
B. Energy Efficiency
Energy constitutes the second-highest operating cost. Recent retrofits have included the installation of Variable Frequency Drives (VFDs) on all major pumps and blowers. The replacement of coarse bubble diffusers with fine bubble membranes has resulted in an estimated 20% reduction in aeration energy consumption.
5. RECENT UPGRADES (2018-2024)
South Plant Expansion & Reuse Initiative – $45 Million
- Project Scope: Increased hydraulic capacity from 8.0 to 10.0 MGD; installation of new cloth media filters for tertiary treatment.
- Funding: Combination of Texas Water Development Board (TWDB) low-interest loans (Clean Water State Revolving Fund) and local revenue bonds.
- Key Drivers: Population growth in the southern sector and the strategic need to reduce potable water usage for irrigation.
- Outcome: The plant now supplies over 1.5 MGD of reclaimed water to the city’s non-potable reuse system, significantly extending the region’s potable water rights.
Upcoming: Lift Station & Force Main Rehabilitation
A $12 million capital improvement project is currently in the design phase to rehabilitate aging interceptors leading to the plant, specifically targeting corrosion caused by high-sulfate soils and wastewater typical of the region.
6. REGULATORY COMPLIANCE
The facility operates under TPDES Permit No. WQ0010539001. Current effluent parameters are strictly monitored:
- BOD (5-day): 10 mg/L (30-day average)
- TSS: 15 mg/L (30-day average)
- Ammonia-Nitrogen: 3 mg/L (Seasonal variation applies)
- E. coli: 126 CFU/100 ml
- Min. Dissolved Oxygen: 4.0 mg/L
The facility has maintained a “High Performer” classification with the TCEQ, with no significant non-compliance events in the last 36 months.
7. CHALLENGES & FUTURE PLANNING
Nutrient Loading on the Arroyo Colorado: The LRGV watershed faces challenges with dissolved oxygen impairment in the Arroyo Colorado. Future permit renewals are expected to impose stricter Total Phosphorus (TP) and Total Nitrogen (TN) limits, which may necessitate the retrofitting of biological nutrient removal (BNR) basins or chemical precipitation systems.
Climate Resilience: The region is prone to flash flooding. Future master planning includes hardening electrical substations and increasing emergency power generation capacity to maintain operations during grid failures associated with extreme weather events.
8. TECHNICAL SPECIFICATIONS SUMMARY
| Facility Data Sheet | |
|---|---|
| Facility Type | Advanced Secondary Activated Sludge |
| Design Capacity (Average) | 10.0 MGD |
| Peak Hydraulic Capacity | 20.0 MGD |
| Treatment Process | Screening, Grit Removal, Primary Clarification, Aeration, Secondary Clarification, Tertiary Filtration |
| Disinfection | Chlorine Gas / UV (for Reuse) |
| Biosolids Class | Class B (Anaerobically Digested) |
| Receiving Water | Drainage Canal / Laguna Madre |
| Reuse Capability | Type I (Unrestricted Urban Use) |
| Operating Authority | McAllen Public Utility |
| Staffing | ~25 Certified Operators and Maintenance Staff |
9. FAQ SECTION
Technical Questions
Q: What is the specific nutrient removal capability of the plant?
A: The plant is currently configured for nitrification (ammonia removal). While not currently designed for full Biological Nutrient Removal (BNR) for Phosphorus, chemical precipitation options are available if permit limits tighten.
Q: Does the plant accept septic hauler waste?
A: Yes, the facility has a designated septage receiving station that feeds directly into the headworks, subject to manifest tracking and sampling.
Q: What SCADA system is utilized?
A: The facility utilizes a Rockwell/Allen-Bradley platform with Wonderware interface, integrated via fiber optic ring to the central utility administration.
General Public Questions
Q: Does the plant smell?
A: The facility employs advanced biotrickling filters and chemical scrubbers at the headworks (where odors are strongest) to neutralize smells before they leave the property line.
Q: Is the treated water safe to drink?
A: No. While highly treated and clear, the effluent is “non-potable.” It is safe for irrigation and industrial use but is not purified to drinking water standards (Direct Potable Reuse).