Greater Lawrence Sanitary District North Wastewater Treatment Plant

1. INTRODUCTION

The Greater Lawrence Sanitary District (GLSD) Wastewater Treatment Facility acts as the environmental backbone for the Merrimack Valley, serving a diverse metropolitan area of over 214,000 residents and a dense industrial base. Located on the banks of the Merrimack River in North Andover, Massachusetts, this 52-MGD secondary treatment plant is a critical component of regional water quality management in the northeast United States.

Commissioned in the late 1970s following the passage of the Clean Water Act, GLSD has evolved from a conventional treatment plant into a model of energy resilience and sustainability. The facility is particularly renowned in the engineering community for its pioneering “Organics-to-Energy” program, which utilizes anaerobic co-digestion of food waste to generate electricity, positioning the plant near net-zero energy consumption. Operating under strict NPDES parameters to protect the Merrimack River—a drinking water source for downstream communities—GLSD balances complex hydraulic loading dynamics with advanced process control and resource recovery.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The District serves a 66-square-mile area comprising five Massachusetts communities (Lawrence, Methuen, Andover, North Andover, and a portion of Dracut) and one New Hampshire community (Salem). The service area includes a mix of dense urban residential zones, commercial corridors, and significant industrial dischargers, necessitating a robust industrial pretreatment program. The collection system includes large interceptors that convey flow to the Riverside Pump Station, the primary lift station delivering wastewater to the plant.

B. Operational Capacity

The facility is designed for an average daily flow of 52 million gallons per day (MGD). Current average flows typically range between 28 and 32 MGD, providing adequate capacity for future regional growth. However, the plant is engineered to handle substantial wet weather events, with a peak hydraulic capacity of 135 MGD. Like many historic New England systems, the collection network experiences significant Inflow and Infiltration (I/I), causing rapid flow peaking during storm events that challenges hydraulic retention times and process stability.

C. Discharge & Compliance

Treated effluent is discharged via a submerged outfall into the Merrimack River (Class B water body). The facility operates under NPDES Permit MA0100447, administered by the EPA Region 1 and MassDEP. The permit dictates stringent limits on Total Suspended Solids (TSS), Biochemical Oxygen Demand (BOD), pH, and Total Residual Chlorine (TRC). The plant consistently achieves high removal efficiencies, typically exceeding 85-90% removal for BOD and TSS, playing a vital role in the ecological restoration of the Merrimack River watershed.

3. TREATMENT PROCESS

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The GLSD facility utilizes a conventional activated sludge process augmented by advanced solids handling and energy recovery systems. The treatment train is designed to handle high-strength industrial loads while managing diurnal flow variations.

A. Preliminary Treatment

Raw wastewater enters the facility via the Riverside Pump Station force mains. The headworks facility includes mechanical bar screens to remove large debris, rags, and plastics, protecting downstream pumps. Following screening, flow passes through aerated grit chambers where inorganic solids (sand, gravel, coffee grounds) settle out. The grit is removed, washed, and disposed of off-site. The facility also employs odor control scrubbers at the headworks to mitigate nuisance odors for neighboring residential areas.

B. Primary Treatment

Wastewater flows into rectangular primary sedimentation tanks. Here, the velocity of the water is 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 clarifiers remove approximately 60% of suspended solids and 30-35% of BOD. Primary sludge is pumped directly to the gravity thickeners.

C. Secondary Treatment

The biological treatment stage utilizes a conventional activated sludge process. The settled sewage enters large aeration basins equipped with fine-bubble diffused aeration systems. Microorganisms consume the remaining dissolved organic matter and ammonia. The facility uses centrifugal blowers to maintain dissolved oxygen levels necessary for biological activity.

Following aeration, the mixed liquor flows to secondary clarifiers. The biological floc settles to the bottom, separating from the clear treated water. A portion of this settled biomass is returned to the aeration tanks as Return Activated Sludge (RAS) to maintain the biological population, while the excess is removed as Waste Activated Sludge (WAS).

D. Disinfection

The clarified effluent undergoes chemical disinfection using sodium hypochlorite (chlorine bleach) to eliminate pathogenic bacteria and viruses. To protect aquatic life in the Merrimack River, the chlorinated water is then dechlorinated using sodium bisulfite prior to final discharge. The plant utilizes baffled contact tanks to ensure adequate contact time for effective pathogen inactivation.

E. Solids Handling & Biosolids

GLSD employs a sophisticated solids handling train:

• Thickening: Primary sludge is thickened in gravity thickeners, while WAS is thickened using Dissolved Air Flotation (DAF) or gravity belt thickeners.
• Anaerobic Digestion: Thickened sludge is pumped to anaerobic digesters (mesophilic operation). Here, volatile solids are reduced, and biogas (methane) is produced. The facility practices co-digestion, accepting high-strength organic food waste to boost gas production.

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• Dewatering: Digested sludge is dewatered using high-solids centrifuges to achieve a cake solid concentration suitable for drying.
• Heat Drying/Pelletization: The dewatered cake is processed at the on-site heat drying facility (operated by a third-party partner, typically NEFCO). The sludge is dried into Class A biosolids fertilizer pellets, which are sold for land application, closing the nutrient loop and eliminating landfill disposal.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site encompasses a significant footprint along the river. Major structures include the Headworks Building, Primary and Secondary Clarifier batteries, Aeration Basins, the Solids Processing Building, the Gravity Thickener complex, and the distinctive egg-shaped or cylindrical Anaerobic Digesters. The facility also houses a full-scale laboratory for process control and compliance testing.

B. Energy Systems & Organics-to-Energy

GLSD is a leader in the “Net Zero” movement for wastewater utilities. The facility captures biogas generated during anaerobic digestion to power Combined Heat and Power (CHP) cogeneration engines. The waste heat from the engines is recovered to heat the digesters and facility buildings.

C. Odor Control

Given the proximity to residential neighborhoods and the processing of imported food waste, odor control is paramount. The facility utilizes a combination of chemical wet scrubbers and activated carbon adsorption units at critical emission points, including the headworks, sludge holding tanks, and the sludge dryer facility exhaust.

5. RECENT UPGRADES & MAJOR PROJECTS

Organics-to-Energy & CHP Improvements (2016-2020)

• Project Scope: Installation of food waste receiving tanks, pumping equipment, and upgrade of CHP cogeneration engines.
• Budget: ~$25 million
• Funding: Combination of SRF loans, Clean Energy Center grants, and District capital funds.
• Technical Highlights: The project allowed the plant to accept up to 9,000 gallons per day (or more) of organic waste. New cogeneration engines increased electrical generation capacity to over 3 MW.
• Results: The facility reduced its energy purchase costs by millions annually and diverted tons of food waste from landfills.

Riverside Pump Station Improvements (Ongoing/Recent)

• Project Scope: Rehabilitation of the main lift station serving the plant, including pump replacement, electrical gear upgrades, and screening improvements at the station level.
• Driver: Aging infrastructure reliability and flood resiliency.
• Benefit: Ensures reliable conveyance of wastewater from the interceptor system to the treatment plant headworks.

Secondary Clarifier Rehabilitation

• Scope: Replacement of internal mechanisms, weirs, and scum baffles in the secondary clarifiers.
• Technical Outcome: Improved settling characteristics and lower effluent TSS concentrations, ensuring consistent permit compliance during peak flow events.

6. REGULATORY COMPLIANCE

A. Permit Requirements

Under NPDES Permit MA0100447, GLSD must adhere to strict seasonal limits. Key parameters include:

• CBOD5: Monthly average limits typically around 25 mg/L.
• TSS: Monthly average limits typically around 30 mg/L.
• TRC (Chlorine): Strict limits due to the freshwater receiving stream; often <0.1 mg/L average monthly.
• Nutrients: Monitoring requirements for Nitrogen and Phosphorus to assess impact on the Merrimack River estuary.

B. Environmental Stewardship

GLSD has maintained a strong record of compliance. The facility proactively manages Combined Sewer Overflows (CSOs) within its member communities through long-term control plans (LTCP), although the plant itself focuses on maximizing treatment of wet weather flows to minimize bypass events. The biosolids program ensures 100% beneficial reuse of sludge, diverting thousands of dry tons from landfills annually.

7. OPERATIONAL EXCELLENCE

The facility is staffed by approximately 45-50 professionals, including licensed Massachusetts wastewater operators (Grades 5-7), maintenance technicians, laboratory analysts, and administrative staff. The laboratory is state-certified, conducting daily analyses for process control and regulatory reporting.

GLSD utilizes a robust SCADA system for real-time monitoring of dissolved oxygen, tank levels, flow rates, and methane production. The integration of the food waste co-digestion process requires careful biological monitoring to prevent digester upset (acidification), showcasing the high technical competency of the operations team.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

• Inflow & Infiltration (I/I): Older collection systems in Lawrence and Methuen contribute significant clear water to the system during storms, hydraulically stressing the plant.
• Aging Infrastructure: Much of the concrete and mechanical infrastructure dates to the late 1970s, requiring a continuous cycle of capital reinvestment.
• Emerging Contaminants: Like all modern WWTPs, GLSD faces future regulatory uncertainty regarding PFAS (forever chemicals) in both effluent and biosolids.

B. Future Planning

The District’s Capital Improvement Plan (CIP) focuses on resiliency. Future projects include hardening the facility against climate change (flood protection measures along the Merrimack), further optimizing the aeration system for energy efficiency, and potential upgrades to nutrient removal technologies should permit limits tighten regarding Nitrogen discharge.

9. COMMUNITY & REGIONAL IMPACT

GLSD enables the economic viability of the Merrimack Valley. By providing reliable sanitation for industries in Lawrence and Andover, the District supports thousands of manufacturing and commercial jobs. The facility’s commitment to the “Clean Water” mission has been instrumental in the revitalization of the Merrimack River, transforming it from a heavily polluted industrial waterway into a recreational asset used for boating and fishing.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

Riverside Pump Station: The primary lift station for the District, located upstream of the main plant. It is one of the largest sewage pump stations in New England, responsible for lifting the majority of the service area’s flow into the headworks.

Biosolids Drying Facility: Located on the GLSD campus but often operated via public-private partnership (currently associated with NEFCO), this facility processes the dewatered cake into marketable fertilizer pellets.

12. FAQ SECTION

Technical Questions

Q: What is the specific energy output of the CHP system?
A: The Combined Heat and Power system, fueled by biogas from co-digestion, has a capacity of approximately 3.2 MW, capable of meeting a significant portion (and at times 100%) of the plant’s electrical demand.

Q: Does GLSD perform nutrient removal?
A: The facility is primarily designed for carbonaceous BOD and TSS removal. While it achieves incidental nutrient reduction, it is not currently configured as a dedicated Biological Nutrient Removal (BNR) facility, though it monitors nitrogen and phosphorus levels closely.

Q: How is the food waste introduced to the digesters?
A: Source-separated organics (SSO) are delivered by truck to a dedicated receiving station, processed into a slurry, and pumped directly into the anaerobic digesters to blend with thickened sludge.

Public Interest Questions

Q: Can the public tour the facility?
A: GLSD occasionally offers tours for educational groups, schools, and professional organizations. Interested parties should contact the District administration directly to inquire about availability.

Q: Does the plant smell?
A: While wastewater treatment naturally produces odors, GLSD employs advanced chemical scrubbers and carbon filters at the headworks and solids handling buildings to capture and neutralize odors before they leave the site boundary.

Q: What happens to the “waste” after treatment?
A: The solid waste is digested, dewatered, and heat-dried into fertilizer pellets. These pellets are sold and used as a soil amendment, meaning very little waste from the plant ends up in a landfill.

Disclaimer: This article is for informational purposes for engineering and industry professionals. Data presented is based on publicly available records, NPDES permits, and engineering reports as of 2023-2024. For official operational data or regulatory inquiries, please contact the Greater Lawrence Sanitary District directly.