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UBC Theses and Dissertations

Anoxic-aerobic digestion of waste activated sludge : a lab scale comparison to aerobic digestion with and without lime addition Jenkins, Christopher Jay


A lab-scale study of anoxic-aerobic digestion of waste activated sludge was performed, using 6 litre digesters, and operated in a semi - continuous (fed-once-a-day) manner with solids retention times (SRTs) of 20, 15 and 10 days and mixed-liquor temperatures of 20 °C and 10 °C. Raw sludge was obtained from a pilot-scale biological phosphorus removal facility operating at U.B.C. Fresh sludge was obtained daily and digested by three different digestion modes: anoxic-aerobic, aerobic with lime addition and aerobic. Two aerobic control digesters were run in parallel with the anoxic-aerobic digesters. One of the aerobic digesters received a daily dose of lime slurry. All three digesters were operated under identical conditions (except for the cycling of air supply to the anoxic-aerobic digesters) so that direct comparison could be made between the three digestion modes. Comparisons were made on the basis of five main parameters related to: (1) digestion kinetics, (2) digested sludge characteristics, (3) supernatant quality, (4) ORP monitoring, and (5) an overall rating system. Percent volatile suspended solids (VSS) reduction was used as one performance variable. Despite using only 42 percent of the air required by the two controls, anoxic-aerobic digestion showed comparable percent VSS reductions. All three digestion modes showed increased solids reduction with increasing SRT and temperature. There was a linear relationship between percent TVSS and the product of SRT and temperature. All three digestion modes had a propensity to retain their percent nitrogen and phosphorus within their solids. However, with respect to retaining phosphorus, the aerobic controls were the least effective. Anoxic-aerobic digestion maintained neutral mixed-liquor pH (MLpH) throughout. Lime controls were maintained at MLpH close to neutral. Aerobic digestion, in general, resulted in MLpH levels below 5.0, however, there were periods when the MLpH of the aerobic digesters varied widely between 4.2 and 6.8. Supernatant quality was superior for the anoxic-aerobic digesters. Due to the incorporation of non-aerated periods, there was almost 100 percent denitrification of nitrates produced during the aerated time. This nitrification-denitrification resulted in very low soluble nitrogen levels in the effluent, as well as considerable removal of nitrogen gas. Neither of the controls showed this ability. The lime and aerobic controls produced high levels of effluent nitrates, as well as occasional measurements of ammonia and nitrite. Phosphorus levels were lowest for the lime control and anoxic-aerobic digesters. Presumably, due to reduced pH levels, the soluble phosphorus levels from the aerobic digesters were 2 to 3 times those in the lime or anoxic-aerobic digesters. Alkalinity was conserved in the anoxic-aerobic digesters as well as the lime control. However, the purely aerobic digesters consumed alkalinity until very little buffering capacity remained. Oxidation-reduction potential (ORP) was used as a means of monitoring the anoxic-aerobic digesters on a real time basis. ORP was particularly useful during the non-aerated periods, due to the fact that, at those times, dissolved oxygen was undetectable. Characteristic real time ORP profiles were revealed. Slope changes correlated well with events of theoretical and engineering interest; the' disappearance of ammonia and nitrates, as well as the (dis)appearance of detectable dissolved oxygen, could be predicted by these slope changes. As a result of the findings, ORP may prove to be an ideal parameter for the control of the anoxic-aerobic digestion process. Finally, an overall rating system was developed. The results of this study suggest that, for the digestion of waste activated sludge, anoxic-aerobic digestion out-performed both lime-control and conventional digestion modes.

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