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A wastewater treatment plant is situated in temperate climate zone with a minimum water temperature of 16oC. At the facility, aerobic digesters are employed to stabilise sludge prior to dewatering by centrifuges. Thickened waste activated sludge of 2,000 m3/day and 1.5% dry solids are continuously fed to the digesters. Post digestion, the sludge is dewatered to 20% dry solids by the centrifuges.
Two widely used design criteria for aerobic digesters are that:
• 400 degrees days (oC x days) to achieve Grade B stabilisation • 40% VSS destruction based on a digester feed VSS percentage of 0.75 g per g TSS.
(a) Calculate hydraulic retention time (HRT) required for the aerobic digesters. [3 marks]
(b) Estimate the combined volume of the aerobic digesters. [3 marks]
(c) Re-estimate the volume of the aerobic digesters if the wasted activated sludge is thickened to 2.5% dry solids content. [5 marks]
(d) The centrifuges are known to recover 95% of the incoming solids. Calculate the quantity of sludge (in wet tonne) produced. [4 marks]
(e) Supernatant from the centrifuges is returned to the upstream biological process, estimate the flow and TSS concentration in the supernatant stream. [5 marks]