Mobile Area Water And Sewer System C C Williams Wastewater Treatment Plant

TARGET AUDIENCE

• Municipal consulting engineers evaluating High-Purity Oxygen (HPO) systems
• Wastewater treatment plant operators and utility managers
• Regulatory compliance officers (ADEM/EPA)

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• Engineering firms pursuing coastal resiliency and nutrient removal projects
• Environmental researchers studying Mobile Bay estuary protection

1. INTRODUCTION

The C.C. Williams Wastewater Treatment Plant serves as the cornerstone of wastewater infrastructure for the Mobile Area Water and Sewer System (MAWSS). Located near the Port of Mobile, this facility is critical to the environmental health of Alabama’s Gulf Coast. With a design capacity of 28 million gallons per day (MGD), it is the largest treatment facility in the MAWSS network, handling the majority of the municipal and industrial flow from the metro area.

Distinguished by its use of High-Purity Oxygen (HPO) activated sludge technology, the plant is designed to treat high-strength waste within a compact footprint—a necessity given the urban density and industrial contributions in its catch basin. Following substantial capital investments in tertiary treatment, specifically Deep Bed Denitrification Filters, C.C. Williams has positioned itself as a leader in nutrient reduction, directly supporting the conservation efforts of the Mobile Bay National Estuary Program. The facility operates under strict oversight from the Alabama Department of Environmental Management (ADEM), maintaining high compliance standards amidst the unique hydrological challenges of one of the wettest cities in the United States.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The C.C. Williams WWTP serves the core metropolitan area of Mobile, Alabama, including the downtown business district and significant industrial corridors along the Mobile River. The collection system feeding the plant is complex, comprised of gravity sewers and major force mains that convey wastewater from both residential neighborhoods and commercial zones. The service area is characterized by older infrastructure, necessitating rigorous Inflow and Infiltration (I/I) management programs to mitigate wet-weather peaking.

B. Operational Capacity

The facility is permitted for a monthly average flow of 28.0 MGD. However, due to Mobile’s subtropical climate and heavy rainfall (averaging over 65 inches annually), the plant must manage significant hydraulic variations.

• Average Daily Flow: Typically ranges between 16–20 MGD.
• Peak Hydraulic Capacity: The plant can process surges up to approximately 60 MGD during storm events.
• Utilization: Under dry weather conditions, the plant operates at approximately 60-70% of its design capacity, allowing for operational redundancy and maintenance rotation.

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C. Discharge & Compliance

Treated effluent is discharged into the Mobile River, a short distance upstream from Mobile Bay. Given the ecological sensitivity of the bay—a major estuary for seafood and recreation—the plant operates under a stringent NPDES permit (AL0023086). The discharge is monitored for Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), Ammonia-Nitrogen, Total Nitrogen, and pathogens (E. coli/Enterococci).

3. TREATMENT PROCESS

The C.C. Williams WWTP utilizes a sophisticated treatment train centered around High-Purity Oxygen activated sludge, followed by tertiary filtration for nutrient control.

A. Preliminary Treatment (Headworks)

Raw wastewater enters the facility through large interceptors. The headworks facility, which has undergone recent rehabilitation, focuses on the removal of inorganic debris to protect downstream pumps and process equipment.

• Screening: Heavy-duty mechanical bar screens remove rags, plastics, and large debris.
• Grit Removal: Vortex grit chambers utilize centrifugal force to separate sand, gravel, and heavy grit from the organic wastewater. Extracted grit is washed and dewatered prior to landfill disposal.
• Odor Control: The headworks are enclosed and ventilated to chemical scrubbers (wet scrubbers) to neutralize hydrogen sulfide (H₂S) and organic odors, a critical requirement given the plant’s proximity to populated areas.

B. Primary Treatment

Flow proceeds to rectangular primary clarifiers. Here, flow velocity is reduced to allow settleable solids to drop to the bottom as sludge, while oils and grease (scum) float to the surface.

• Mechanism: Chain-and-flight mechanisms scrape bottom sludge to hoppers and skim surface scum.
• Efficiency: Typically removes 30-40% of BOD and 50-60% of TSS, significantly reducing the organic load on the secondary biological system.

C. Secondary Treatment: High-Purity Oxygen (HPO)

The heart of the C.C. Williams plant is the High-Purity Oxygen (HPO) activated sludge process (often associated with UNOX technology). Unlike conventional aeration which pumps air (21% oxygen), this system utilizes covered aeration basins and injects >90% pure oxygen.

• Oxygen Generation: A Pressure Swing Adsorption (PSA) or VPSA on-site oxygen generation system produces the high-purity oxygen required.
• Process Advantages: The HPO system allows for a higher Mixed Liquor Suspended Solids (MLSS) concentration, faster reaction rates, and greater ability to handle shock loads compared to conventional air systems. This is particularly effective for the industrial components of Mobile’s wastewater.
• Secondary Clarification: Following the aeration basins, the mixed liquor flows to circular secondary clarifiers where the biological floc settles. Return Activated Sludge (RAS) is recycled to the reactor, while Waste Activated Sludge (WAS) is sent to solids handling.

D. Tertiary Treatment: Denitrification Filters

To address nutrient loading in Mobile Bay, MAWSS installed Deep Bed Denitrification Filters.

• Technology: These down-flow sand filters serve a dual purpose: they physically filter out remaining suspended solids and biologically convert nitrate to nitrogen gas (denitrification) under anoxic conditions.
• Carbon Source: A supplemental carbon source (typically methanol or similar) is added to fuel the denitrifying bacteria within the filter media.
• Performance: This stage ensures compliance with strict Total Nitrogen limits and polishes the effluent to high clarity.

E. Disinfection

The plant utilizes Ultraviolet (UV) Disinfection. This modernization replaced traditional chlorine gas, eliminating the safety hazards associated with chemical storage and the need for dechlorination chemicals. The UV system alters the DNA of pathogens, rendering them unable to reproduce.

F. Solids Handling

Sludge from primary and secondary treatment is processed to reduce volume and pathogen content.

• Thickening: Gravity thickeners are used for primary sludge, while dissolved air flotation (DAF) or rotary drum thickeners are typically used for waste activated sludge.
• Dewatering: The facility utilizes mechanical dewatering, typically belt filter presses or centrifuges, to produce a cake with 18-25% solids.
• Disposal: Dewatered biosolids are transported to a sanitary landfill or a dedicated composting facility, depending on current MAWSS management plans.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant & Power

The site covers extensive acreage adjacent to Three Mile Creek and the Mobile River. The facility includes an administration building with a SCADA control room and a certified water quality laboratory. To ensure resilience during hurricane season, the plant is equipped with massive standby diesel generators capable of running the entire treatment process during grid outages, a critical redundancy feature for the Gulf Coast.

B. Odor Control Systems

MAWSS places a high priority on being a “good neighbor.” The C.C. Williams plant utilizes multi-stage chemical scrubbers at the headworks and primary treatment areas. These systems continually monitor air quality and adjust chemical dosing (typically sodium hypochlorite and sodium hydroxide) to oxidize odorous compounds before venting.

5. RECENT UPGRADES & MAJOR PROJECTS

MAWSS has executed a strategic Capital Improvement Plan (CIP) focused on regulatory compliance and asset renewal.

Denitrification & Headworks Upgrade (Approx. $22 Million)

• Project Scope: Construction of the Deep Bed Denitrification Filter complex and rehabilitation of the headworks facility.
• Drivers: Compliance with ADEM nutrient criteria for Mobile Bay and aging infrastructure replacement.
• Technical Highlights: The installation of the Tetra Denite® (or equivalent) filter system allowed the plant to achieve low total nitrogen levels without expanding the biological reactor footprint significantly.
• Outcome: Significant reduction in nitrogen loading to the estuary and improved removal of fine suspended solids.

UV Disinfection Retrofit

• Project Scope: Replacement of the chlorine gas disinfection system with a high-intensity UV system.
• Drivers: Enhanced safety (elimination of hazardous gas) and environmental stewardship (elimination of disinfection byproducts).
• Outcome: Consistent compliance with E. coli limits without the risk of residual chlorine toxicity in the receiving water.

Upcoming/Ongoing: Collection System Rehabilitation

While not inside the plant fence line, MAWSS is investing heavily in the C.C. Williams basin collection system to reduce Inflow and Infiltration. By lining pipes and rehabilitating manholes, the hydraulic peaking factor at the plant is stabilized, improving treatment consistency.

6. REGULATORY COMPLIANCE

The C.C. Williams WWTP operates under NPDES Permit AL0023086. The facility has seen a marked improvement in compliance consistency following the installation of the denitrification filters and UV system.

• Nutrients: The plant adheres to seasonal limits for nitrogen to prevent hypoxia in Mobile Bay.
• Wet Weather Management: As with many Gulf Coast utilities, the primary regulatory challenge involves Sanitary Sewer Overflows (SSOs) in the collection system during extreme tropical rain events. MAWSS operates under a Consent Decree structure aimed at minimizing these occurrences through systematic infrastructure renewal.
• Awards: MAWSS facilities frequently receive Peak Performance Awards from the National Association of Clean Water Agencies (NACWA) for permit compliance.

7. CHALLENGES & FUTURE PLANNING

A. Climate Resilience

Located in a hurricane zone, the C.C. Williams plant faces threats from storm surge and extreme precipitation. Future planning integrates climate adaptation, including hardening of electrical switchgear, elevating critical pumps, and ensuring fuel autonomy for generators.

B. Aging Infrastructure

While the process technology has been upgraded, the physical concrete structures and underground piping of the original plant require ongoing asset management. MAWSS utilizes a comprehensive GIS-based asset management program to prioritize rehabilitation projects.

C. Emerging Contaminants

MAWSS engineers are monitoring EPA regulatory trends regarding PFAS (Per- and polyfluoroalkyl substances). While not currently regulated in wastewater effluent at the federal level, the facility is participating in baseline monitoring to understand the fate and transport of these compounds within the HPO process.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. FAQ SECTION

Technical Questions

Q: Why does C.C. Williams use High-Purity Oxygen (HPO) instead of conventional aeration?
A: HPO systems are ideal for facilities with limited space that treat high-strength wastewater. They allow for higher biomass concentrations and faster treatment rates, which is beneficial given the mix of municipal and industrial waste in Mobile.

Q: Does the plant remove phosphorus?
A: While the primary nutrient focus for the Mobile Bay estuary is Nitrogen, the biological process and solids removal achieve incidental phosphorus reduction. Specific limits are dictated by the NPDES permit.

Q: How does the plant handle hurricane shutdowns?
A: The facility is equipped with full-capacity emergency diesel generators. The plant is designed to continue treating wastewater even if the municipal power grid fails, preventing untreated discharge during storms.

Public Interest Questions

Q: What causes odors near the plant?
A: Wastewater naturally generates hydrogen sulfide. MAWSS uses chemical scrubbers to treat the air from the headworks and primary tanks. Occasional odors may occur during maintenance or extreme weather, but they are actively managed.

Q: Is the treated water safe to drink?
A: The effluent is treated to a level safe for discharge into the river and supports aquatic life, but it is not potable (drinking) water. MAWSS provides drinking water from separate reservoirs and treatment plants.