Document V3rD9bMLoekZYJ6vwk53VqeQZ

Region 6 Compliance Assurance and Enforcement Division INSPECTION REPORT Inspection Date(s): Media: Regulatory Program(s) Company Name: Facility Names: Facility Physical Locations: (city, state, zip code) Mailing address: (city, state, zip code) County/Parish: Facility Contact: 1/28/2026 Water Clean Water Act City of Mercedes City of Mercedes 1501 Mile 8 N Mercedes, Texas 78570 400 South Ohio Mercedes, Texas 78570 Hidalgo County Oscar D. Montoya Sr. Mayor mayor.montoya@cityofmercedes.com 956 565-3114 FRS Number: Identification/Permit Number: Media Number: NAICS: SIC: TX0021547/TXR05V314 4952 Personnel participating in inspection: Michael Williams EPA/ 6EN-WMH Alejandro Solis City of Mercedes Inspector Chief Operator 281-983-2150 956-793-5176 EPA Lead Inspector Signature/Date Supervisor Signature/Date MICHAEL WILLIAMS 2026.03.09 09:27:29 -05'00' MICHAEL WILLIAMS Michael D. Williams Date ROBERTO BERNIER Date: 2026.03.09 13:08:52 -05'00' Digitally signed by ROBERTO BERNIER Roberto Bernier Date City of Mercedes TX0021547/TXR05V314 Inspection Date 1/28/2026 Section I - Introduction PURPOSE OF THE INSPECTION EPA Region 6 inspector, Michael Williams, arrived at the City of Mercedes Wastewater Treatment Plant (WWTP) at 9:55 am on January 28, 2026, for an announced National Pollutant Discharge Elimination System (NPDES) inspection. During the opening conference, I met Alejandro Solis, the Chief Wastewater Operator. Mr. Solis was informed that this was an EPA lead compliance evaluation inspection to determine compliance with the facility's NPDES permit TX0021547 and Stormwater permit TXR05V314, facility operations and maintenance requirements under the Clean Water Act (CWA). The inspection was conducted under the authority of the NPDES and Stormwater General permit programs, in accordance with the Federal CWA. The information presented in this report is based on the materials and information supplied by the City of Mercedes representatives, observations made during the inspection, and records or reports maintained by the permittee, permittee contract laboratory, the State of Texas and EPA. FACILITY DESCRIPTIONS (TX0021547) The City of Mercedes Wastewater Treatment Plant (WWTP) is designed to treat up to 5 million gallons per day (MGD) of municipal wastewater using a conventional activated sludge biological treatment process. Raw influent entering the facility first passes through the headworks, where preliminary treatment removes large debris. Screening is provided by both a mechanical bar screen and a manual bar screen, which capture rags, plastics, and other coarse materials. Screened wastewater then flows into a grit chamber, where inorganic materials such as sand, gravel, and small stones are removed to prevent abrasion and accumulation within pumps and process basins. Following preliminary treatment, the wastewater is directed to one of three racetrack-style aeration basins. In these basins, air is introduced via surface aerating discs to support microbial activity that biologically degrades dissolved and suspended organic matter. The aeration process maintains adequate dissolved oxygen levels to allow microorganisms to metabolize pollutants and form biological floc. The mixed liquor from the aeration basins then flows to three secondary clarifiers operating in parallel with each racetrack. Within these clarifiers, the biological solids settle by gravity, separating treated effluent from the activated sludge biomass. Clarified effluent exits the top of the clarifiers while the settled solids are collected at the bottom. Waste activated sludge from the clarifiers is conveyed to the facility's belt filter press for mechanical dewatering. As a backup sludge management option, solids may also be directed to 16 drying beds where natural evaporation and drainage further reduce moisture content. The clarified effluent undergoes ultraviolet (UV) disinfection to inactivate pathogenic organisms prior to discharge. After disinfection, the treated wastewater is released to an unnamed drainage ditch that ultimately flows into the Arroyo Colorado, the receiving stream. 2 City of Mercedes TX0021547/TXR05V314 Inspection Date 1/28/2026 Dewatered biosolids generated at the facility are transported off-site for beneficial land application by Denali Water Solutions. Section II- Observations This inspection revealed the following observations for TX0021547: Racetrack Aeration Basin No. 1 Out of Service At the time of the inspection, Racetrack Aeration Basin No. 1 was not in operation. Facility staff reported that the associated secondary clarifier receiving flow from this basin had been taken offline due to a structural crack in the center well. Because the clarifier could not reliably perform solids separation under these conditions, influent flow to Racetrack No. 1 was suspended until repairs to the clarifier structure could be completed and the unit returned to service. Partial Mechanical Failure in Racetrack Aeration Basin No. 2 Racetrack Aeration Basin No. 2 was in service; however, one of the two-disc rotors used for aeration and mixing was not operational at the time of inspection. With only one rotor functioning, the basin may experience reduced oxygen transfer and mixing efficiency, which could potentially affect biological treatment performance if sustained for an extended period. East Plant Racetrack Basin Fully Operational The third racetrack basin, referred to by plant personnel as the East Plant, was observed to be fully operational during the inspection. All associated aeration and process equipment appeared to be functioning as designed, and the basin was actively treating wastewater flow at the time of observation. Lack of Solids Storage or Stabilization Capacity The facility currently does not have a dedicated solids holding tank or sludge digester to provide operational flexibility in managing waste activated sludge. Without intermediate storage or stabilization capacity, the plant has limited ability to regulate or equalize sludge waste from the secondary clarifiers, which can make solids management more challenging during periods of fluctuating flows, equipment downtime, or maintenance activities. Section III- Areas of Concern This inspection revealed the following areas of concern for TX0021547: The City of Mercedes failed to ensure that the wastewater treatment facility and all components of its collection, treatment, and disposal systems were properly operated and maintained at all times, as required under the conditions of the facility's TPDES permit. Specifically, during the inspection it was observed that Racetrack Aeration Basin No. 1 was not in operation because the secondary clarifier receiving flow from this basin had been taken offline due to a crack in the center well structure. Additionally, Rotor No. 4 associated with Racetrack Aeration Basin No. 2 was not operational, reducing the available aeration and mixing capacity for that treatment unit. 3 City of Mercedes TX0021547/TXR05V314 Inspection Date 1/28/2026 It is also noted that the City has previously been cited by the Texas Commission on Environmental Quality (TCEQ) for issues associated with Racetrack No. 1 (also referred to as an oxidation ditch) during a prior inspection conducted in 2024. The recurrence of operational deficiencies associated with this treatment unit indicates a continuing concern regarding the proper maintenance and reliability of critical treatment infrastructure at the facility. Section IV - Stormwater TXR05V314 Observations & Areas of Concern The City of Mercedes obtained coverage under the Texas Pollutant Discharge Elimination System (TPDES) Multi-Sector General Permit for Industrial Stormwater, which became effective on November 24, 2006. At the time of the inspection, the facility maintained a Stormwater Pollution Prevention Plan (SWP3) on-site. The plan was available for review and appeared to be complete and properly maintained. Key elements of the SWP3 were documented and implemented, including the facility's organizational structure and responsible personnel, employee training program, routine monthly facility inspections, quarterly visual monitoring of stormwater discharges, rainfall monitoring, and benchmark monitoring data. Records reviewed during the inspection indicated that these compliance activities were current and maintained in accordance with permit requirements. Additionally, the Best Management Practices (BMPs) outlined in the SWP3 appeared to be implemented effectively and were functioning as intended to minimize potential pollutant discharges in stormwater runoff from the facility. Section V - LIST OF APPENDICES Appendix A - Photos 4 City of Mercedes TX0021547/TXR05V314 Inspection Date 1/28/2026 Appendix A Photograph Log UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 1 County/Parish: Hidalgo State: Texas Bar Screen UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 2 County/Parish: Hidalgo State: Texas Racetrack #1 Out of Service UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 3 County/Parish: Hidalgo State: Texas Clarifier #1 Out of Service with Cracked Well UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 4 County/Parish: Hidalgo State: Texas Racetrack #2 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 5 County/Parish: Hidalgo State: Texas Clarifier #2 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 6 County/Parish: Hidalgo State: Texas Clarifier #3 UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 7 County/Parish: Hidalgo State: Texas UV Disinfection UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 8 County/Parish: Hidalgo State: Texas Drying Beds UNITED STATES ENVIRONMENTAL PROTECTION AGENCY Photograph Log Location: City of Mercedes City: Mercedes Photo No. 9 County/Parish: Hidalgo State: Texas Outfall 001