Membrane bioreactors (MBR)s exhibit a higher treatment capacity compared to conventional activated sludge (CAS) systems, since MBRs can handle mixed liquor suspended solids (MLSS) concentration of approximately 10 to 12 g/L. However, as the MLSS concentration increases, there is a higher need for dissolved oxygen to satisfy the respiration demand of the system. Diffused aeration systems are relatively inefficient in supplying dissolved oxygen (DO), with maximum oxygen transfer efficiencies (OTE)s of approximately 6 to 9% per meter of submergence in clean water. The OTEs are even much lower in mixed liquor. The higher the MLSS concentration, the lower the OTEs; even negligible OTEs have been reported at MLSS concentration of approximately 30 to 40 g/L. The Speece cone, a concentrated oxygen supply technology, is a pressurized vessel where a stream of water (or mixed liquor) is super-saturated using pure oxygen; the oxygen-saturated water (or sludge) stream is then directed back to the basin to be oxygenated where the supersaturated stream is mixed/diluted with the process water. The Speece cone technology has been evaluated in the context of hypolimnetic aeration/oxygenation; however, it has not been evaluated in the context of wastewater treatment. The effects of the Speece cone operational parameters on the oxygen transfer performance both in clean water and mixed liquor were evaluated in a pilot-scale MBR provided with a Speece cone aeration system. The evaluated operational parameters included: (i) the operational pressure inside the cone; (ii) the inlet velocity at the top of the cone; (iii) the recirculation flow rate to the cone; and (iv) the high-purity oxygen flow to the cone. When working with clean water, the Speece cone system was able to reach a maximum oxygen transfer rate (OTR) of 3.8±0.2 kgO2/d, a maximum OTE of 96%, and an oxygen mass transfer rate coefficient (KLa) of 2.1 h-1. When working with mixed liquor at CAS relevant MLSS concentrations, the Speece cone was able to reach a maximum OTR of 2.2±0.3 kgO2/d, a maximum OTE of 78%, and a KLa of 1.5 1/h. A maximum alpha factor (impact of the MLSS on the OTEs due to the presence of sludge) of 0.96 was reported at a MLSS concentration of approximately 5 g/L at the evaluated operational conditions in the Speece cone. According to these results, the Speece cone demonstrated to be a highly efficient technology for supplying DO at the evaluated experimental conditions.

Evaluation of the Speece cone oxygen transfer performance in clean water and mixed liquor in a pilot-scale membrane bioreactor

255th National Meeting (2018)

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