Abstract
For enhancement of removals of nitrogen and phosphorus in wastewater treatment using activated sludge, two different types of bubble columns with a draft tube are proposed. One is a bubble column with a relatively small and short draft tube, and the other is a column with a long draft tube whose upper end is located a little above the liquid surface at aeration. In both columns, the annulus between column and draft tube is anaerobic. Using these two columns in addition to the standard bubble column without a draft tube, wastewater treatment experiments are performed. It is found that the proposed columns have higher removals of nitrogen and phosphorus than those of standard one. The liquid circulation flow rate in the column and the gas-liquid mass transfer volumetric coefficient are measured. By using these data and the oxygen consumption rate of activated sludge, the distribution of dissolved oxygen concentration in column is calculated. Based on the calculated results of dissolved oxygen distribution, the contribution of anaerobic region volume to the removals is examined. Independently of column type, the removals of nitrogen and phosphorus have maximum values when the volume fraction of anaerobic region is 40–60%.
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