Improving Anaerobic Digester Performance: What Happens When Mixing Fails

When it comes to anaerobic digesters, sound mixing isn’t optional — it’s the heartbeat of the process. It drives biogas production, keeps solids in suspension, and maintains a stable biological environment.

But as one major municipal utility learned, even a well-planned upgrade can deliver disappointing results if flow behavior isn’t fully understood.

When a Mixing Upgrade Backfires

After installing new linear-motion mixers and gas-holder covers, the utility expected higher efficiency and reduced maintenance. Instead, gas production fell by nearly half, digesters began to foam, and one stainless-steel cover started to deform under pressure.

It quickly became clear that the issue wasn’t biological — it was mechanical and hydraulic in nature. The system wasn’t mixing the sludge effectively.

The Root Cause: Flow and Rheology

The investigation showed that the new mixers produced turbulence only near the impeller, leaving large portions of the sludge volume stagnant. Computational Fluid Dynamics (CFD) modeling confirmed the problem: strong velocity in the core, almost no motion near the perimeter.

Illustrative representation of Computational Fluid Dynamics (CFD). This diagram is conceptual and not derived from project data.

The sludge’s rheology — its resistance to shear and flow — was another factor. Anaerobic sludge behaves as a non-Newtonian fluid, meaning its viscosity changes depending on how fast it’s stirred. When flow slows down, the sludge thickens, creating dead zones and trapping gas.

The gas-holder covers compounded the issue. Built with 4-foot skirts (half the standard 8-foot depth) and shorter guide columns, they flexed under gas pressure, creating uneven stress across the system.

“Testing and flow modeling confirmed that mixing was concentrated near the impeller, leaving large zones of sludge essentially stagnant. Once those regions lose motion, solids settle, gas gets trapped, and overall digester efficiency drops sharply.”

— Greg Benz, P.E.
Bioreactor Mixing and Process Engineering, LEC Partners

LEC Partners’ Investigation

The utility brought in LEC Partners to perform an independent, data-driven review. The multidisciplinary team — John Forcier, P.E., Greg Benz, P.E., and Terry Mazanec, Ph.D. — conducted field inspections, laboratory testing, and advanced modeling to pinpoint the source of the performance loss.

The Investigation Included:

• Sludge Rheology Testing – Measuring viscosity, yield stress, and solids content to build accurate CFD inputs.

• CFD Flow Modeling – Visualizing internal flow to identify stagnant zones and optimize impeller placement.

• Pilot Testing – Evaluating center-mounted axial-flow impellers to achieve full-tank mixing.

• Structural Review – Assessing gas-holder cover skirt height and guide length for safety and pressure balance.

Project Completion Date: October 2025

Read the related case study here → Anaerobic Digester Mixing and Performance Investigation

Key Insights

Here’s what this project revealed for digester operators and designers:

• Mixing performance must reflect the real properties of sludge, not those of clean water.

• CFD modeling can identify flow problems before costly retrofits are required.

• Cover geometry directly affects digester pressure stability and mechanical reliability.

• Phased rehabilitation allows incremental improvement with lower capital risk.

Results and Outlook

While full implementation is still pending, modeling and pilot testing indicated strong recovery potential across both process and energy metrics:

• Throughput restored to approximately 100,000 gallons per day per digester

• Biogas yield increased 25–30%, improving renewable gas production potential

• Reduced sludge hauling and disposal costs through improved solids destruction

• Improved cover stability with redesigned skirts and guide geometry

• Co-generation of electricity: modeled performance suggests restoring biogas supply sufficient to meet up to 33% of the wastewater treatment plant’s total power demand

These outcomes demonstrate how data-driven analysis of mixing, rheology, and structural design can unlock significant process efficiency — and advance energy self-sufficiency for wastewater utilities.

Frequently Asked Questions

Why did performance drop after the upgrade?
The linear-motion mixers didn’t provide full-tank mixing. Stratified sludge and trapped gas reduced digestion efficiency and increased cover stress.

How can CFD help prevent this?
CFD provides a 3D visualization of internal flow, revealing dead zones, circulation loops, and gas hold-up before equipment is installed.

Which mixing system works best in high-solids sludge?
Center-mounted axial-flow impellers are generally more effective in high-viscosity sludge, providing consistent motion and fewer stagnant zones.

Does cover design really affect process safety?
Yes — short skirt heights or weak guide columns can lead to unbalanced gas pressures and long-term mechanical deformation.

About LEC Partners

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

Our team bridges science, engineering, and investment to help clients develop, fund, and optimize sustainable technologies in anaerobic digestion, biogas, biofuels, carbon capture, and renewable chemicals.

From digester evaluations to process design and techno-economic analysis, LEC delivers independent insight and hands-on expertise that improve performance, reduce risk, and move clean technologies from concept to commercial success.

Meet the Project Team

Ready to Improve Your Digester Performance?

If you’re facing mixing inefficiencies, foaming, or low gas yield, LEC Partners can help identify the root cause and engineer a practical solution.

Contact us to discuss your anaerobic digester performance and learn how CFD, rheology testing, and process redesign can optimize your results.