Enhanced aerobic floc-like granulation and nitrogen removal in a sequencing batch reactor by selection of settling velocity

Abstract

In order to enhance the solidÐliquid separation properties and nitrogen removal efficiency of SBR, the aerobic floc-like granules were cultivated under temporal alternating aerobic and anoxic conditions without the presence of carrier material in a SBR having 15 H/D (height/diameter) ratio. Two different effluent port positions were applied to the SBR for different selection of minimum settling velocities (over 0.6 and 0.7 m/h) of granules retained in the SBR during aerobic floc-like granule formation. The effect of different minimum settling velocities as an operational parameter on the size and solidÐliquid separation properties of floc-like granules and also the COD and nitrogen removal of SBR were evaluated. The reactor was operated 6 hours per cycle (aerobic 4.75 hours, anoxic 1.25 hours) under chemical oxygen demand (COD) loading rate of 2.5 kg/m3·d (1.3 kg acetate-COD and 1.2 kg glucose-COD). When increasing the minimum settling velocity by 0.1 m/h, the following results were observed at steady state. The nitrification efficiency was not changed at about 97% but the denitrification efficiency was improved from 78 to 97%. The COD removal efficiency was improved from 82 to 97% and the concentration of biomass in the reactor was retained at lower level at about 3,000 mg MLSS/L. The average sludge volume index (SVI) value of granules was decreased about 85 to 50 mL/g and the granule sizes were increased 0.1-0.5 mm to 1.0-2.0 mm. The required time to form granules and reach steady state was significantly shortened. Based on the results, the selection of the minimum settling velocity had a significant effect on both the physical properties of granules and the SBR performance, so it is suggested to use the minimum settling velocity as an operational parameter.