| Summary: | 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. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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