Electronic Thesis and Dissertation Repository

Thesis Format

Alternative Format

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Nakhla, George

Abstract

Anaerobic digestion (AD) is a widely implemented technology for biosolids treatment, enabling both waste stabilization and renewable energy generation. However, conventional AD faces limitations such as long hydraulic retention times (HRT), ammonia inhibition, and inefficient nutrient recovery. This thesis explores the potential of IntensiCarb™ (IC), a novel vacuum-enhanced AD system, to overcome these challenges by integrating thickening, digestion, and ammonia recovery.

The digestion of a 50/50 mix of primary and thickened waste-activated sludge at OLRs ranging from 3.47 to 11.3 kgCOD/m³·d (2–6 times conventional AD) demonstrated that IC technology achieved stable methane yields of 0.20 mL-CH₄/gCODfed and 50% COD destruction. Stability was attributed to ammonia toxicity mitigation and a microbial shift toward Methanosarcina, an ammonia-tolerant methanogen. IC recovered 43%-46% of influent TKN as ammonia, raising the inhibition threshold from 1.9 gN/L (conventional AD) to 5.6 gN/L. Despite high OLR, IF6 maintained an ammonia concentration of 1,500 mgN/L, well below inhibition levels. High OLRs led to propionate accumulation, limiting hydrogenotrophic methanogenesis. Batch tests showed propionate degradation rates were 1.92 to 2.82 times higher in conventional AD due to Smithella and Syntrophobacter, while IC favored alternative propionate degraders.

IC application was further extended to activated waste sludge digestion, and it was found that the capability of IC for 4x OLR was higher than that of conventional AD. The ammonia was maintained at a level below 1.5 gN/L. A holistic IC-AD model in SUMO accurately predicted COD, TN, ammonia, methane, and solids within ~10% error. The proposed model was the first one to integrate the current AD model with ex-situ vacuum.

Comparing IC-AD to THP-AD at OLR 8–8.7 kgCOD/m³·d, IC maintained stable methane production (0.22 L-CH₄/gCODfed, 55% COD destruction), while THP-AD failed due to ammonia and propionate accumulation. IC recovered 49%-56% ammonia, preventing toxicity and fostering a methanogenic community 7.2 times higher than THP.

IC-AD represents a breakthrough in AD intensification, sustaining stable operation at six times the OLR of conventional systems while ensuring high methane yields, enhanced inhibitor tolerance, and ammonia recovery. This positions IC-AD as a transformative solution for wastewater treatment and industrial-scale biogas production.

Summary for Lay Audience

Anaerobic digestion (AD) is a widely used process for breaking down organic waste, such as sewage sludge, to produce biogas—a renewable energy source. However, traditional AD systems face several challenges, including long processing times, ammonia buildup, and inefficient nutrient recovery, which can limit performance. To address these issues, this study explores a new technology called IntensiCarb™ (IC), which improves AD efficiency using a vacuum system.

IC-AD combines digestion, thickening, and ammonia recovery in a single unit, making the process faster and more effective. The study tested IC technology at much higher organic loading rates (OLR) than conventional AD—up to six times higher—to see if it could still maintain stable operation. The results showed that IC achieved similar methane production and waste breakdown compared to conventional AD, even under extreme conditions. A key reason for this success was its ability to recover ammonia, preventing it from reaching toxic levels that would otherwise disrupt the process. Another challenge in high-rate digestion is propionate accumulation, which can slow down methane production. Tests showed that in conventional AD, certain bacteria (Smithella and Syntrophobacter) break down propionate more effectively than in IC-AD. However, IC compensates with ammonia-tolerant microbes like Methanosarcina, which thrive under high ammonia levels.

When compared to another widely used AD intensification technology, thermal hydrolysis (THP), IC proved to be a better option. THP struggled with high ammonia and propionate buildup, leading to system failure, while IC maintained stable methane production and efficiently recovered ammonia.

In summary, IC-AD offers a faster, more resilient, and more efficient solution for converting waste into energy. It could help wastewater treatment plants and industries produce more biogas while reducing environmental impact, making it a promising innovation for sustainable waste management.

Download

Available for download on Thursday, December 31, 2026

Share

COinS