| Budget Amount *help |
¥2,800,000 (Direct Cost: ¥2,800,000)
Fiscal Year 2006: ¥1,200,000 (Direct Cost: ¥1,200,000)
Fiscal Year 2005: ¥1,600,000 (Direct Cost: ¥1,600,000)
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| Research Abstract |
Recently, a biological phosphorus removal (BPR) process has been getting more attention, because it is a key factor to solve eutrophication of closed water bodies and to recover phosphorus from wastewater. However, it is well known that BPR process often becomes unstable. Therefore, it is an urgent subject to clarify the mechanism of instability. This study focuses on nitrite that is known as an inhibitor both to growth and respiration of ordinary heterotrophic bacteria. Since nitrite is often present in full-scale wastewater treatment plants as a by-product of nitrification and/or an intermediate of denitrification, nitrite could be one of the factors deteriorating biological phosphorus removal.
Three sequencing batch reactors were fed with acetate to cultivate phosphate-accumulating organisms (PAOs) with different anoxic phosphate uptake activities. Two of them were operated under anaerobic-oxic conditions, and one of them was added with small amount of nitrite at the start of aerobic
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condition. The rest was operated under anaerobic-anoxic condition with nitrate as a final electron acceptor. The enriched PAOs sludge was used for nitrite inhibition batch tests to investigate the effect of nitrite exposure on aerobic phosphate uptake, respiration, growth, and acetate uptake.
The experimental results indicated several important findings. One of them is that the responses of respiration and phosphate uptake to nitrite exposure were well identical, and neither phosphate release nor acetate uptake showed any inhibitory effect, though inhibition of growth was not clear. From these results, it is concluded that respiration must be the main mechanism of inhibition of aerobic phosphate uptake. More interesting observation is that nitrite disappeared without nitrate production. Since this is not a chemical reaction and nitrate never reduced under the same condition, it is confirmed with other experimental evidences that some PAOs can conduct aerobic nitrite denitrification. Moreover, the size of inhibition was smaller in case of the enriched PAOs with higher anoxic activity than that of the PAOs with lower anoxic activity. This demonstrated that some PAOs could reduce the inhibitory effect of nitrite by aerobic nitrite denitrification. In fact, the results indicated the meaningful increase of P/O ratio in the presence of nitrite. From all these results, finally, the model describing nitrite inhibitory effect was newly developed with the size of nitrite addition and specific concentration of nitrite. Less
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