Electronic Thesis and Dissertation Repository

Thesis Format

Integrated Article

Degree

Master of Engineering Science

Program

Civil and Environmental Engineering

Supervisor

Nakhla, George

Abstract

The effect of food waste (FW) co-digestion with wastewater biosolids (WWB) on microbial communities was investigated through running thirteen lab-scale digesters for 100 days at different operational conditions i.e. organic loading rates (2 and 4 kgCOD/m3·day), feed types (WWB and FW), and FW content (10%, 90%, 100%). Methanosarcina in FW digesters in contrast to Methanosaeta in WWB, Compared with mono-digestion of WWB, FW co-digestion enhanced biogas production by 13% and COD degradation rates by up to 101%. Among fermentative bacteria/acetogens, Syntrophomonas was the dominant genus in FW digesters in contrast to the dominance of Clostridium in WWB digesters. COD degradation rates and methane yields were well correlated with Bacteroidetes population. Methane production rate was well correlated with Clostridium for FW digesters, with syntrophs for WWB digesters, and with aceticlastic methanogens for both digesters. Synergism was associated with hydrolytic bacteria, Clostridium, Syntrophomonas, syntrophs, Methanosarcina, and Methanobacterium. The impact of biochar in anaerobic mono-digestion and co-digestion of FW with hydrothermally pre-treated and untreated thickened waste activated sludge (TWAS) was investigated by running ten lab-scale digesters for 153 days at different operational conditions i.e. organic loading rates (OLR of 2, 3, 4 and 8 kgCOD/m3·day), FW to pre-treated TWAS and untreated TWAS of 30% and 70% by volume respectively. Biochar was added to five reactors while the other five reactors served as controls. The presence of biochar enhanced methane production by 15%, between phase 1 (OLR 2kgCOD/m3/d) to 89% in phase 4, (OLR 8kgCOD/m3/d) for 100%FW+biochar as compared to the control. For untreated TWAS, methane production increased by 14% in phase 1 to 88% in phase 4, 4% in phase 1 to 92% in phase 4 for 100% pre-treated TWAS+biochar, 10% in phase 1 to 45% in phase 4 for 30%FW+70% untreated TWAS+biochar and finally, 6% to 65% for 30%FW+70% pre-treated TWAS+biochar. This value is also consistent with percentage COD destruction efficiencies.

Since there was no evidence of ammonia and volatile fatty acids VFA inhibition in biochar added reactors throughout the study, adsorption tests were done on acetic, butyric and propionic acids as well as ammonia chloride using 0.8g biochar for VFA analysis and 1.6g biochar for ammonia adsorption in other to evaluate the adsorptive capacity of biochar. From the result, biochar showed good adsorption characteristics for both ammonia and VFA. The adsorption of acetic was less pronounced compared to butyric acid and propionic acid. The concentration of butyric acid was reduced from 1200mg/L to 240mg/L, propionic from 1200 mg/L to 405 mg/L, and the concentration of ammonia was also reduced from 3000mg/L to 830mg/L, thus rationalizing the improved system performance at high loading rates. Throughout the study, the pH of the biochar added reactors was within the range of 7.2-7.8 without alkalinity supplementation.

Summary for Lay Audience

The effect of digesting food waste (FW) with wastewater biosolids (WWB) on microbial communities was investigated by running 13 oxygen-free bioreactors in the laboratory for 100 days at different inflow rates (2 and 4 kgCOD/m3·day). FW and WWB were digested as a single substrate (mono-digestion) and as a combined substrates (co-digestion). Food waste was added to WWB by volume at ratios 10%, and 90%, compared with mono-digestion of WWB, FW co-digestion improved methane production by 13% and organic matter degradation by up to 101%. The impact of biochar in anaerobic mono-digestion and co-digestion of FW with hydrothermally pre-treated, and untreated thickened waste activated sludge (TWAS) was investigated. Biochar enhanced digestion of pre-treated sludge digesters more by increasing the methane production by 92% at an organic loading rate of 8 kgCOD/m3·d in phase 4. Biochar reduced ammonia and volatile fatty acids (VFA) inhibition by reducing the concentration of butyric acid from 1200 mg/L to 240 mg/L, propionic acid from 1200 mg/L to 405 mg/L and ammonia concentration from 3000 mg/L to 830 mg/L. As the OLR increased, the impact of adding FW as co-substrate increased while biochar indicated an insignificant FW co-substrate effect.

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