From a study of the performance of the autothermal thermophilic aerobic stage (45 m3) of a full-scale dual digestion plant, operated with pure oxygen, it was found that: (1) VS and/or COD removal are poor parameters for estimating biological heat generation; (2) the biological heat generation is proportional to oxygen utilized at 13.0 MJ/kgO, a value which conforms closely to that obtained from bioenergetics and thermodynamics, (3) the maximum biological heat generation rate was limited by the maximum oxygen utilization rate of the sludge (OURbio), and (4) because the aerobic reactor is oxygen limited, the temperature could be completely and instantaneously controlled with oxygen feed rate up to OURbio; (5) CO2 generation in the vent gas was 0.68 mol CO2/mol O2 utilized instead of 1.0 usually assumed.

The use of process and two-dimensional clarifier modelling to investigate the implications of upgrading Melbourne Water's Eastern Treatment Plant to a nitrification/denitrification process is discussed. Results indicate that the existing clarification capacity is sufficient for the increased solids loading arising from operation at a nitrifying sludge age but that the existing diffuser system may need to be replaced in order to achieve the required oxygen transfer rate. The impact of step feeding into unaerated zones to reduce the aeration tank suspended solids concentration and to achieve denitrification is discussed and the installation of baffles to improve the performance of peripheral feed clarifiers is suggested.

The use of membrane bioreactors (MBR) continues to increase across the world. In dry countries such as Australia, they become the foundation technology for reclaimed water production facilities. However, there is a perception that an MBR uses significantly more energy than a conventional continuous wastewater treatment facility and this is incompatible with greenhouse gas concerns especially in Australia where a carbon tax is potentially going to be legislated. This paper reports on the performance of MBRs commissioned in the Australian City of Cairns, North Queensland, with particular reference to the nutrient removal performance and the measured power consumption in comparison to conventional processes with clarification and tertiary filtration. The MBRs were designed in accordance with design procedures outlined in 2008 which support the principle of avoiding high recycle rates between the membrane tank and the aeration zone of the main process bioreactor as this raises the MLSS concentration increasing the power required for process aeration. The design procedure integrates the performance requirements for BNR and the physical requirements for membranes to enable the design to be optimised with respect to reactor size and power requirements.

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