We have developed a biological nitrogen and phosphorus removal system ( BNPS ) with a net-sheet carrier for use in sewage treatment. It is a promising method with practical advantages over other bio-film systems. It requires half the space of other activated sludge systems and it is easier to maintain. This paper presents a study that determined the oxygen transfer and flow characteristics required to design an effective and efficient aeration tank for this system. Results of a pilot-scale experiment yielded the following: (1) A slow gas flow rate (U{sub}g) less than about 2.2×10{sup}(-2) m/disc/s was a prerequisite to increase the oxygen transfer efficiency ( η) by at least 20%. A higher η was obtained when the ratio of aeration area to the area of the bottom of the tank ( diffuser area A{sub}d) ranged from 11.7 to 15.4%. (2) To avoid the formation of a dead zone which is created by an incomplete mixture of wastewater and bacteria, either a large diffuser area or a high gas velocity was required. Quantitatively, these combinations worked effectively: a) diffuser area of 11.7% and U{sub}g = 3.7 ×10{sup}(-2) m/disc/s, orb) diffuser area of 15.4% and U{sub}g> 1.7 × 10{sup}(-2) m/disc/s. The continuous flow pattern in this aeration tank was simulated and the results were helpful in understanding the global flow pattern.
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