Work Performed and Main Achievements
(-) Development of Process Conditions for Efficient Pre-Treatment
o Optimized process conditions for air-assisted US pre-treatment were identified:
Ultrasound power density: 0.15 W/mL
Specific energy input: 9.9 kJ/g TS
Sonication duration: 12 minutes
Air injection rate: 0.078 L/min
Aeration duration: 3 minutes at the start of US application
Three continuous anaerobic digesters were operated under mesophilic conditions across three distinct phases. The sludge retention time (SRT) was maintained at 16 days, while the organic loading rate (OLR) was progressively increased from 1.33 gVS/L·d to 2.00 gVS/L·d, and ultimately to 2.50 gVS/L·d.
• R1 (Control): Fed with untreated sewage sludge.
• R2: Fed with ultrasound (US)-pretreated sewage sludge.
• R3: Fed with air-US pretreated sewage sludge
Optimization of Air- assisted ultrasound (air-US) Pre-Treatment
- Achievements:
o 1.7-fold increase in total organic carbon (TOC) solubilization when air injection.
o Enhanced solubility of heavy metals such as Pb (78%) and Ni (>58%) from the solid structure of sewage sludge.
o Resulted in an 18.9% increase in CH4 yield compared to US pretreatment alone with the specific energy input (SE) of 14.85 kJ/g TS.
o Achieved over a 41% improvement in CH4 yield compared to un-pretreated sewage sludge.
o The findings were submitted to a peer-reviewed journal.
o Research findings were presented at Campus PhD Seminars and PETLab seminars, engaging with the academic community and facilitating interdisciplinary discussions.
Optimization of Micro-Aeration-Based Anaerobic Fermentation (Air-AF):
o Operational parameters for Air-AF were established:
Four continuous anaerobic digesters were operated under mesophilic conditions. The sludge retention time (SRT) was maintained at 16 days, and the OLR was kept constant at 3.35 g VS/L·d.
• R1 (Control): Fed with untreated sewage sludge.
• R2: Fed with ultrasound (US)-pretreated sewage sludge.
• R3: Fed with air-US pretreated sewage sludge.
• R4: Operated under air-AF, where a small amount of air was introduced to regulate the oxidation-reduction potential (ORP)
ORP maintained between -500 mV and -420 mV to suppress methanogenesis.
- Achievements:
o Lead to maximize short chain carboxylic acids (VFA) (5724 ± 722 mg COD/L).
o Resulted in additional 12% removal of Zn and Cu for already pre-treated sludge
o The digestate obtained from the integrated strategy (air-US + air-AF) enhanced plant growth compared to both the control and conventional anaerobic digestion-derived digestate. Germination rates increased by 17%, while total plant height was 30% higher than that observed with conventional digestate.
(-) Influence of Micro-pollutants on the Performance of Conventional Anaerobic Digestion and Air-AF
Four reactors were operated: a control reactor (R1) and three test reactors (R2, R3, and R4), each subjected to specific inhibitory compounds.
• R1 (Control): Operated under standard conditions without any inhibitory compounds.
• R2: Supplemented daily with 50 mg/L of CHD in addition to the substrate.
• R3: Received a daily addition of 50 mg/L of HTAB along with the feedstock.
• R4: Supplemented daily with 25 mg/L of CHD and 25 mg/L of HTAB, with additional micro-aeration to maintain the oxidation-reduction potential (ORP) between -500 mV and -420 mV
- Achievements:
o Prolonged exposure of conventional AD to micro-pollutants, led to severe methanogenesis inhibition, reducing CH4 production to near-zero levels due to microbial toxicity.
o Air-AF emerged as a viable alternative, as micro-pollutant exposure led to significant VFA accumulation (>12,000 mg COD/L), and thus, facilitating pollutant transformation.
