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UBC Theses and Dissertations

The effect of primary and secondary sludge mix ratios on VFA production in thermophilic aerobic digestion using pilot scale ATAD units Fothergill, Samantha


Research was undertaken to determine if a mixture primary and secondary sludge could provide increased volatile fatty acid (VFA) production, as compared with primary sludge alone, in a thermophilic aerobic digestion process. In addition, pre-solubilization of the secondary sludge, with NaOH, was investigated for its effect on VFA production. Previous research has demonstrated that VFA production can be enhanced during the thermophilic digestion of primary sludge with reduction in both aeration and retention time. Secondary sludge was predicted to further enhance production by providing the required substrate for process micro-organisms 'pre-packages' in the correct ratios. Pre-solubilization of the secondary sludge was intended to make this substrate readily available to process micro-organisms, eliminating a potentially rate-limiting step. Pre-solubilization of feed sludge has been shown to enhance anaerobic digestion. Experiments were carried out at UBC's Wastewater Treatment Pilot Plant. Primary and secondary (Bio-P) sludges were generated on-site, by a modified UCT process, and metered to feeding tanks daily for use in the autothermal thermophilic aerobic digestion (ATAD) reactors. Configured in parallel, the two, 125 L reactors were each operated as first stage reactors, in semi-continuous mode, with an average retention time of 3 days. Based on TS, primary/secondary mix sludge ratios of 100/0, 65/35, 35/65, and 0/100 were tested in parallel with primary sludge in the control reactor. The 35/65 and 0/100 streams were additionally pre-solubilized, with 15 meq/L of NaOH, and tested in parallel with non-solubilized mix ratios of 35/65 and 0/100, respectively, in the control reactor. Through all experimental runs: feed consistency was maintained around 1% TS; reactor temperatures stayed between 42°C to 50°C, ie. within the thermophilic range; and a "micro-aerobic" environment was sustained with a constant supply of air into the reactor contents (< 1 mg/L DO, and consistent ORP values between -200 mV and -450 mV). The incorporation of secondary sludge, in mixed sludge feed, resulted in increased production and accumulation of VFA. The greatest production and accumulation of VFA was produced with the digestion of 100% secondary sludge. Although chemical pre-solubilization of sludge resulted in increases in VFA concentrations in the feed tanks, no conclusions could be made with respect to its efFect on VFA production in ATAD. The addition of NaOH did produce large fluctuations in reactor pH. Based on this impact on digester stability, and the positive results obtained without chemical pre-solubilization of feed sludge, further investigations were not undertaken with NaOH. Analysis of nutrient species confirmed that, both the mixing of primary and secondary sludge, and further, the thermophilic aerobic digestion of mixed and secondary sludges, results in the release of stored phosphorus and increases in ammonia nitrogen. Post-treatment, of some type, would be required before recycle to nutrient removal processes.

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