Metropolitan St Louis Sewer District Lemay Wastewater Treatment Plant

1. INTRODUCTION

The Lemay Wastewater Treatment Plant (WWTP) is a cornerstone of the Metropolitan St. Louis Sewer District’s (MSD) infrastructure, serving as the second-largest treatment facility within the District’s expansive service area. Located near the confluence of the River Des Peres and the Mississippi River, the plant is engineered to manage the complex hydraulic challenges associated with a combined sewer system. With a peak secondary treatment capacity of 240 million gallons per day (MGD), the facility plays a critical role in protecting the water quality of the Mississippi River Basin.

Operated under the “MSD Project Clear” initiative, the Lemay plant has recently been the focus of substantial capital investment aimed at meeting stringent regulatory requirements and modernization goals. The facility is notable for its robust Trickling Filter/Solids Contact (TF/SC) process—selected for its ability to handle shock loads common in combined systems—and its state-of-the-art fluidized bed incineration complex, which represents a regional benchmark for sustainable biosolids management. As MSD executes its multi-billion dollar consent decree aimed at eliminating sewer overflows, the Lemay facility serves as the terminus for major deep tunnel projects, solidifying its status as a critical asset for the region’s environmental health.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Lemay WWTP service area encompasses approximately 82 square miles, covering the southern portions of St. Louis City and a significant swath of southern St. Louis County. The collection system is a hybrid of separate sanitary sewers and aging combined sewers (stormwater and wastewater in the same pipe). This distinct geography includes the heavily urbanized River Des Peres watershed. The influent is conveyed via massive interceptors, including the Lemay Interceptor and the Gravois Trunk Sewer, which consolidate flows from residential, commercial, and light industrial sources before reaching the headworks.

B. Operational Capacity

Hydraulic versatility is the defining characteristic of the Lemay facility. While the average dry weather flow hovers around 67 MGD, the plant is designed to ramp up aggressively during wet weather events.

• Average Daily Flow: 67 MGD
• Peak Secondary Capacity: 240 MGD
• Peak Hydraulic Capacity: Historically, flows exceeding secondary capacity were diverted or blended, but recent deep tunnel pump station upgrades are designed to capture and treat significantly higher volumes to reduce Combined Sewer Overflows (CSOs).

C. Discharge & Compliance

Treated effluent is discharged directly into the Mississippi River. The plant operates under a Missouri Department of Natural Resources (MoDNR) NPDES permit. The discharge point is critical due to the sheer volume of the Mississippi River, which allows for substantial dilution; however, recent regulatory shifts have imposed stricter disinfection requirements to protect recreational use downstream. The plant consistently achieves high removal rates for BOD and TSS, maintaining compliance with the Federal Clean Water Act and the specific mandates of MSD’s Consent Decree with the EPA.

3. TREATMENT PROCESS

The Lemay WWTP utilizes a Trickling Filter/Solids Contact (TF/SC) secondary treatment process. This hybrid configuration provides the energy efficiency of trickling filters with the effluent quality and clarification reliability of activated sludge, making it particularly effective for handling the variable loading rates inherent to St. Louis’s combined sewer system.

A. Preliminary Treatment

Raw wastewater enters the headworks where coarse debris is removed via mechanically cleaned bar screens. Following screening, flow enters aerated grit chambers where inorganic solids (sand, gravel, cinders) settle out. Given the combined nature of the collection system, grit loads can increase exponentially during storm events. The facility employs vortex grit removal technologies in newer upgrades to enhance separation efficiency before primary clarification.

B. Primary Treatment

Flow proceeds to primary clarifiers where gravity settling removes approximately 60-70% of Suspended Solids and 30-40% of BOD. Primary sludge is pumped to the solids handling complex, while the primary effluent flows to the secondary treatment stage. The primary clarifiers also facilitate scum removal (grease and oils).

C. Secondary Treatment (TF/SC Process)

The biological treatment stage is a two-step coupled process:

1. Trickling Filters: Primary effluent is distributed over plastic media trickling filters. A zoogleal film (biomass) on the media metabolizes dissolved organic matter. This stage is passive and energy-efficient, utilizing natural draft ventilation.
2. Solids Contact Basins: The effluent from the trickling filters enters aerated solids contact basins. Here, the process mimics activated sludge for a short retention time, flocculating the solids sloughed off the trickling filters and polishing the effluent to remove remaining soluble BOD.

Final clarification occurs in secondary clarifiers, where the biological floc settles. A portion of the sludge is returned (RAS) to the solids contact basins to maintain mixed liquor suspended solids (MLSS) concentrations, while waste activated sludge (WAS) is sent to processing.

D. Disinfection

Historically, many plants discharging to the Mississippi did not require year-round disinfection. However, under recent regulatory updates, Lemay has implemented a chemical disinfection system. Liquid chlorine (sodium hypochlorite) is used for pathogen inactivation, followed by dechlorination using sodium bisulfite prior to discharge to ensure no residual chlorine harms aquatic life in the river.

F. Solids Handling (Advanced Thermal Oxidation)

The Lemay plant serves as a regional solids processing hub. In a major departure from older multiple-hearth incineration, the facility now utilizes Fluidized Bed Incinerators (FBI).

• Thickening/Dewatering: Sludge is thickened (gravity belt thickeners) and dewatered using high-performance centrifuges to achieve optimal cake dryness (typically 25-28% solids) for autogenous combustion.
• Incineration: The FBI system injects air through a sand bed, creating a turbulent, suspended mass that burns biosolids at roughly 1,400°F. This technology significantly reduces supplemental fuel requirements and produces a stable, inert ash.
• Emissions Control: The system is equipped with advanced scrubbing systems, including venturi scrubbers and wet electrostatic precipitators (WESP), to meet strict MACT (Maximum Achievable Control Technology) air quality standards.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site occupies a significant industrial footprint along the riverfront. It includes distinct zones for liquid treatment, solids handling, and maintenance. The site geography is challenging, requiring flood protection levees to shield the plant from Mississippi River flood stages.

B. Energy & Heat Recovery

The transition to Fluidized Bed Incineration included the installation of waste heat recovery boilers. These systems capture thermal energy from the incineration exhaust to generate steam, which is reused for building heat and process heating, contributing to a circular energy loop and reducing the plant’s natural gas reliance.

C. Odor Control

Given its location near populated areas of South St. Louis, odor control is paramount. The headworks and sludge handling facilities utilize chemical scrubbers and carbon adsorption units. The newer incineration complex operates under negative pressure to prevent fugitive emissions.

5. RECENT UPGRADES & MAJOR PROJECTS

Current & Future: Tunnels and Pump Stations

The ongoing Jefferson Barracks Tunnel and associated deep pump stations are directing flows to Lemay. This massive deep-rock tunnel system acts as underground storage during storms, preventing overflows. The new Lemay Pump Station upgrades are designed to lift these captured flows from the tunnel depths into the headworks for full treatment, effectively increasing the wet-weather capture rate of the system.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

The Lemay WWTP operates under the regulatory oversight of the Missouri Department of Natural Resources (MoDNR) and the US EPA. The driving regulatory force is the consent decree entered into by MSD, aimed at eliminating illegal sanitary sewer overflows (SSOs) and reducing combined sewer overflows (CSOs).

• NPDES Compliance: The plant consistently meets limits for BOD5 (typically < 30 mg/L monthly avg) and TSS (< 30 mg/L monthly avg).
• Air Quality: The new incineration units operate under a Title V Air Permit, with Continuous Emissions Monitoring Systems (CEMS) tracking CO, NOx, SO2, and opacity in real-time.
• Biosolids: The resulting ash from incineration is tested for heavy metals and is typically disposed of in sanitary landfills, as it is inert and reduced in volume by over 90% compared to wet sludge.

7. CHALLENGES & FUTURE PLANNING

A. Wet Weather Management

The primary engineering challenge remains the “peaking factor.” The plant must transition from 60 MGD to 240 MGD rapidly during storms. Managing the biology of the TF/SC process during these wash-out events requires skilled operation and precise hydraulic control.

B. Aging Infrastructure

While major components like incineration and disinfection are new, the core civil structures (clarifiers, basins) dating back to the 1960s require aggressive asset management and concrete rehabilitation.

C. Nutrient Removal

While not currently under strict limits for Nitrogen and Phosphorus compared to the Chesapeake Bay or Long Island Sound, the Mississippi River basin is under increasing scrutiny regarding hypoxia in the Gulf of Mexico. Future master planning anticipates potential nutrient reduction limits, which would require significant process modifications (e.g., converting to BNR).

8. TECHNICAL SPECIFICATIONS SUMMARY

9. FAQ SECTION

Technical Questions

1. What distinguishes the TF/SC process used at Lemay?
The Trickling Filter/Solids Contact process is a hybrid system. The trickling filters handle the bulk of BOD removal with low energy usage, while the aerated solids contact basins improve flocculation and settleability. This provides the stability of attached growth systems with the effluent quality of suspended growth systems.

2. How does the plant handle Combined Sewer Overflows (CSOs)?
Lemay is the reception point for large deep tunnels (Project Clear). During storms, flow is stored in tunnels and pumped to the plant as capacity allows. The plant is designed to maximize throughput (up to 240 MGD) to treat as much wet weather flow as possible.

3. What is the advantage of Fluidized Bed Incineration over the old hearths?
FBI units provide more uniform combustion temperatures, result in lower NOx and CO emissions, require less supplemental fuel (autogenous burning at ~26-28% solids), and produce a higher quality ash.

Public Interest Questions

4. Does the Lemay Plant smell?
Wastewater treatment inherently generates odors, but MSD has invested heavily in odor control scrubbers at the headworks and utilizes enclosed systems for sludge processing to minimize impact on the surrounding South Broadway community.

5. Is the discharge safe for the Mississippi River?
Yes. The plant operates under a strict state permit. The effluent is treated to remove solids and organic matter and is disinfected to kill bacteria, protecting downstream water quality.