Anaerobic Digester Mixing and Performance Investigation for Large Wastewater Utility

Case Study
Client
Large Municipal Wastewater Utility
Project Number
25080
Industry
Bioenergy
Service
Investigative Assessment
Location
North America
Team Members

John Forcier, P.E. 

Greg Benz, P.E. 

Terry Mazanec, Ph.D. 

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Project Snapshot

Independent investigation and modeling to restore anaerobic digester performance for a major municipal wastewater utility.

  • Biogas Yield: +25–30% improvement projected
  • Throughput: Restored to ~100,000 gallons/day per digester
  • Structural Reliability: Stabilized gas-holder covers with new skirt/guide design
  • Energy Recovery: Co-generation potential up to 33% of plant power

Project Completion Date: October 2025

Overview

A large municipal wastewater utility experienced significant performance losses and safety concerns following the upgrade of its anaerobic digester equipment.

After installing linear-motion (LM) mixers and gas-holder covers, the digesters began operating at roughly 60% of design capacity, producing substantially less biogas. Operators observed surface foaming, sludge stratification, and visible deformation of stainless-steel covers — all signs of incomplete mixing and structural stress.

The utility engaged LEC Partners to conduct an independent investigation and develop a data-driven plan to restore process performance, safety, and energy recovery efficiency.

Challenge

The new mixing system created turbulence only near the impeller, leaving large portions of the digester volume stagnant. Vendor-supplied Computational Fluid Dynamics (CFD) modeling confirmed limited mixing reach and poor flow circulation.

Compounding the issue, the new gas-holder covers were designed with 4-foot skirts and short guide columns — half the industry standard. Under normal gas pressure, the covers flexed and tilted, introducing mechanical strain and structural safety concerns.

The result: reduced throughput, lower biogas output, and increased maintenance risk.

LEC Partners’ Approach

LEC Partners assembled a multidisciplinary team to lead a comprehensive field and analytical investigation, including:

  • Sludge Rheology Testing: Laboratory characterization of viscosity and yield stress to model actual flow behavior.

  • CFD Flow Modeling: Simulation of circulation patterns to identify stagnant zones and optimize impeller placement.

  • Pilot Trials: Evaluation of center-mounted axial-flow impellers to achieve full-tank mixing in high-solids sludge.

  • Structural Assessment: Review of cover geometry, with recommendations for skirt height and guide length to restore stability and pressure balance.

This evidence-based method provided the client a phased rehabilitation roadmap without requiring a full system replacement.

Projected Results

Implementation of LEC’s recommendations is expected to produce measurable gains across process, energy, and safety metrics:

Metric Anticipated Result
Throughput Restored ≈ 100,000 gallons/day per digester
Biogas Yield Improvement +25–30%
Sludge Volume Reduction Significant decrease in disposal tonnage
Structural Safety Improved cover stability and reliability
Co-Generation of Electricity Restore supply meeting up to 33% of WWTP power demand

Key Takeaways

  • Mixing performance must reflect true sludge rheology, not water-like assumptions.

  • CFD modeling can identify problem zones before costly rebuilds.

  • Cover geometry directly influences mixing stability and mechanical safety.

  • A phased, data-driven rehabilitation delivers measurable improvements at lower capital cost.

LEC Partners Perspective

This investigation highlights LEC Partners’ ability to combine real-world operating insight with advanced modeling to solve complex anaerobic digester challenges. By integrating field experience, CFD analysis, and structural engineering, the team provided practical, data-backed solutions that enhance performance, safety, and energy sustainability.

About LEC Partners

LEC Partners is a Cambridge, Massachusetts–based consulting and engineering firm advancing the bioeconomy through practical, data-driven solutions.

The firm supports clients across the anaerobic digestion, biofuels, renewable chemicals, and waste-to-energy sectors—delivering independent insight, techno-economic analysis, and process optimization to improve performance and reduce risk.

Project Team

John Forcier, P.E. — Senior Consultant, Anaerobic Systems and Wastewater Treatment

Over 50 years of experience in anaerobic digestion, biogas systems, and renewable energy integration. John has led more than 75 digester projects across North America.

Greg Benz, P.E. — Process Engineer, CFD and Mixing Analysis

Veteran engineer with 40+ years of experience in bioreactor design, agitation, and mixing systems for fermentation, wastewater, and biofuels.

Terry Mazanec, Ph.D. — Technical Advisor, Process and Mechanical Systems

Technology leader with 40 years of experience in catalysis, thermochemical conversion, and biogas-to-chemicals development.