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The effect of operational and environmental parameters on the acid-phase anaerobic digestion of primary sludge Elefsiniotis, Panagiotis


This research explored the effect of certain operational and environmental parameters on the acid-phase anaerobic digestion of primary municipal sludge. The operational parameters included the hydraulic retention time (HRT) and the solids retention time (SRT), while pH, reactor configuration and influent characteristics were the environmental factors of interest. Moreover, an attempt was made to identify the most significant metabolic pathways involved in the conversion of the major components of primary sludge (carbohydrates, proteins and lipids) to short-chain volatile fatty acids (VFAs) and other soluble end-products. The experiments were conducted using two continuous-flow three-liter reactors having different configurations: a completely mixed reactor (CMR) with a clarifier and sludge recycle, and an upflow anaerobic sludge blanket (UASB) reactor. Both systems were run at an ambient liquid temperature between 18 and 22 °C. The research program evolved into the following four stages: In Stage 1 the role of HRT was investigated, while Stage 2 focused on the effect of SRT. The issues of replication and the source of influent sludge were the targets in Stage 3. Finally, in Stage 4 the effect of pH was explored. During the last stage, dilute solutions (0.02N) of HC1 or NaOH were continuously added through an automated pump system to keep the pH at selected values. Favorable conditions for acidogenic digestion were established and maintained resulting, generally, in high VFA and low gas generation rates. The net VFA concentration and the specific production rate increased, in both reactors, with an increase in HRT up to 12 hours, but decreased slightly at longer HRTs. The same pattern was followed not only by the COD concentration but also by the specific solubilization rates of COD and TOC. Variation in SRT had a profound effect on VFA production rate only at the lower (5 day) SRT. At longer SRTs a plateau in acid production appeared to be reached. A decrease in pH from 5.1 to 4.5 did not have an effect on the rate of VFA generation, but an increase to pH 6.1 resulted in significantly lower rates (25 to 30%) of acid production. Acetic acid and propionic acid were the most prevalent VFAs produced and accounted for 45 and 31% (on average) of the total respectively. Butyric acid followed with an average value of 9%. The percent VFA distribution appeared to be independent of HRT, but it was a function of both SRT and pH. Besides VFAs, small amounts of formic acid, ethanol and lactic acid were regularly detected in both systems. Results showed that the steady-state operation of the acid-phase digestion can be replicated and that the seasonal changes in the study (summer-winter) did not affect the process. The use of a different source of influent sludge had an effect on lipid and carbohydrate utilization patterns, which was also reflected in the corresponding VFA production rates. In general, protein degradation percentages were moderate and significantly lower than those obtained for the other two groups of organic compounds. The utilization of all three substrates increased with an increase in HRT, but (with the exception of proteins) was essentially independent of SRT. The reactor configuration played a role in substrate degradation as well. Although both systems showed a fairly similar behavior in protein utilization, the degradation of carbohydrates and lipids was distinctly and consistently different. Lipids were broken down more efficiently in the CMR system, while higher rates of carbohydrate dissimilation were observed in the UASB reactor.

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