Fagbohungbe, Michael and Semple, Kirk and Dodd, Ian and Li, Hong (2015) Optimisation of small scale anaerobic digestion technology. PhD thesis, Lancaster University.
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Abstract
The recent advances in anaerobic digestion (AD) technology and changes in government policies have contributed to the gradual increase in the establishment of on-site smallscale anaerobic digesters in developed regions, particularly in Europe. However, these advances have not completely eradicated some of the challenges with operating AD system. The project is aimed at investigating the potential of optimizing small-scale AD through high solid digestion (HSAD) and reduction of substrate induced inhibition (SII). The study of different inocula, changes to environmental conditions, adsorption of inhibitors and reactor modification was explored. To investigate these possibilities, an onsite mono-substrate such as citrus fruit waste (CFW) with an average dry matter of 16% was used as the substrate, biochar material (rice husk, coconut shell and wood biochar) were used as adsorbent while an operating temperature from 35 - 55 ⁰C were also investigated. Limonene is an inhibitory compound and a constituent of CFW, this was used as the inhibitor, a compartmentalized anaerobic reactor (CAR) was designed to improve HSAD while selected inocula from digested sewage sludge, compost and landfill leachate and their mixture were used as an inoculant. In the first study, the acclimation rate of different inocula to increasing concentration of limonene compound was investigated and the mixed inocula recorded the highest recovery rate and methane yield with a value of 544 ± 21 ml CH4. The mixed inocula benefited from the synergistic effect of using a broader microbial community to mitigate limonene inhibition. This was followed up with the biochar study on AD of CFW and the result showed that microbial lag phase reduced by 50% which was attributed to sorption of limonene compound and biofilm formation on the biochar material. The study on AD of CFW at a different operating temperature of 35-55 ⁰C showed that the higher temperature of 45 and 55 ⁰C outperformed the other incubation with no detectable microbial lag phase. Finally, the optimization option for HSAD was investigated using a CAR and compared against the conventional continuous stirred tank reactor and a 34%, 43.3%, 48.5% and 79.9% higher cumulative methane production for organic loading rates of 1.42, 2.85, 4.00 and 5.00 gVSL-1 day -1 , respectively was achieved. This performance was attributed to the lower compartment of the CAR which facilitated leachate treatment and distribution. The result showed that limonene a constituent of CFW and an example of SII can be counteracted by (i) inoculating with a mixture of inocula (ii) addition of biochar (iii) operation at high temperature of 45 and 55 ⁰C and (iv) the single stage compartmentalized reactor improved HSAD and reduced limonene suppression.