Anaerobic digestion intensification and ammonia recovery using side stream vacuum evaporation
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.