Year

2014

Degree Name

Master of Philosophy

Department

School of Civil, Mining and Environmental Engineering - Faculty of Engineering and Information Sciences

Abstract

Anaerobic digestion (AD) is the most widely used treatment process for sewage sludge stabilisation over concerns of public health. In addition, the production of methane (CH4), a renewable fuel, has also shaped the prospective of AD within the context of energy security and global warming. This dissertation thus aimed to evaluate two main aspects of the AD process of sewage sludge. First, the potential of enhancing biogas production was assessed when co-digesting sewage sludge with glycerol. Glycerol is a by-product of biodiesel production and then rich in organic carbon. Second, the capacity of removing trace organic compounds (TrOCs) was examined.

Results reported here suggest the stability of anaerobic conversion determined the ultimate CH4 yield, greatly affecting the assessment of CH4 potential. Alkalinity buffer and inoculum over substrate (I/S) ratio were demonstrated to be important factors in maintaining steady state of the AD process. The addition of NaHCO3 resulted in an increase of CH4 production of sewage sludge that could be ascribed to well-buffered conditions for methanogens. A significant enhancement in CH4 yield from sewage sludge was achieved with an increase of I/S ratio from 1/9 to 1/1. Adequate quantity of inoculum for degrading substrate was responsible for such improvement. Moreover, no disruption of CH4 production was found since only 0.25% and 0.5% glycerol of digested sludge (by volume) was applied. The obtained ultimate CH4 yields of three types of glycerol were comparable, indicating that any glycerol was possibly used as a potential co-substrate of sewage sludge. Additionally, the identification of appropriate values of alkalinity and I/S ratio for an optimal CH4 production was dependent on key characteristics (i.e. pH and alkalinity) of sewage sludge and glycerol.

Experimental results from anaerobic co-digestion of raw primary sludge and glycerol show that the addition of 0.5% and 1.0% of glycerol to raw primary sludge (by volume) could improve the anaerobic sludge conversion in terms of the daily and cumulative CH4 production. Carbon-rich content and high solubility of glycerol led to an increase only within the first seven days. Despite the buffer supplementation, the instability of the AD process was clearly observed in this batch test through characteristics of the feeds and digestates (acidic pH and low alkalinity for example), suggesting a shock of organic loading due to insufficient inoculum and extra organic matter from glycerol. The feasibility of glycerol as a co-substrate of raw primary sludge was satisfactory to give rise to CH4 generation. On the other hand, excessive addition of glycerol as a co-substrate may destabilise the AD process, and thus be counter-productive.

The removal of TrOCs by AD was evaluated. The evolution of pH and alkalinity as well as CH4 yields obtained from experiment data and kinetic analysis indicated a steady state of methanogenesis in TrOC-spiked bottles after 1.6 days. In a two-phase matrix (i.e. sludge and water), TrOCs could partition (adsorb) to the solid phase as well as the water phase. Their distribution in these phases was examined using a twophase fate model, which considers mass transfer (i.e. sorption and desorption). In this study, the removal of the investigated TrOCs is defined as anaerobic biodegradation. Their various removal efficiencies were consistently related to their molecular properties in terms of functional groups rather than other physicochemical properties. Compounds with the inclusion of halogen, methoxy and amine/amide groups exhibited an effective removal while low or no removal efficiencies were observed in compounds possessing alkyl functional groups. Regarding compounds possessing high biodegradability (kb> 0.001 L/gVS.d), water-solid partition coefficient (kp) determined from the proposed two-phase fate model could describe their distribution in the sludge phase better than their hydrophobicity (in terms of log D). For biologically persistent compounds (kb< 0.001L/gVS.d), sorption properties, which can be predicted by log D values, were responsible for the fate of organic contaminants in sludge matrix.

An economic design at laboratory scale of biomethane potential testing could rapidly assess the AD of sewage sludge. Results also demonstrated increased methane yields when using glycerol as a co-substrate of sewage sludge and trace organic removal efficiency under anaerobic conditions. These findings are of great importance for scaling up AD facilities in real wastewater treatment plants. Keywords: anaerobic digestion, anaerobic co-digestion, biomethane potential, sewage sludge, glycerol, trace organic compounds, sorption.

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