The Mechanism Of NO And N₂O Accumulation During The Nitrite-based SOAD

Sulfide-oxidizing autotrophic denitrification（SOAD）was widely concerned by scholars at home and abroad for its advantages of green,high efficiency,no secondary pollution and low sludge yield.However,there are still many controversies about the mechanism of N₂O accumulation in the SOAD process,and there is a serious lack of monitoring data on the NO accumulation and emission.In this study,nitrite-basedSOAD was set up in a sequence batch biofilm reactor and several batch tests were carried out at the different original concertrations of S^2-and NO₂^-.In the batch tests,NO,N₂O,disolved oxygen（DO）and pH were monitored on line,and S^2-,NO₂^-,SO₄^2-were analysised.The mechanism and characteristics of nitrite-based SOAD as well as NO and N₂O accumulation would be discussed,and several conclusions was shown as following:SOAD proceeded in two stages.S^2-was the main electronic donor in the first stage,while S⁰ was the main electronic donor in the second stage.pH varitions can be a good indication of the two stages of SOAD.When the mass ratio of S/N was 4 in the influent,the electronic donor S^2-was so excess that it could inhibit the second stage of S⁰ oxidation.Thus S⁰ was the main oxidation products of sulfide at the S/N ratio of 4.The TN removal rate was the fastest at the S/N ratio of 2.NO accumulation was result from the high nitrite concentration.Though,S^2-significantly simulated the NO accumulation in SOAD,the low mass ratio was proved to promote the NO accumulation.The highest NO accumulation was observed at the influent nitrite concentration of 30 mg NO₂^--N/L and mass S/N ratio of 1.Along with the NO accumulation,remarktable DO reading was observed,thus NO dismutation may occur.The accumulation of N₂O is mainly caused by the double inhibitions of S^2-/H₂S and free nitrite acid（FNA）in SOAD.As the electronic donor,S^2-promoted the whole denitrification process,but also inhibited the activity of N₂O reductase（Nos）.When the concentration of S^2-was higher than 5 mg S^2--S/L,there may be a constant inhibition on Nos activity.When the S^2-concentration was lower than 5 mg S^2--S/L,there may be a non-competitive inhibition which was associated with H₂S concentration.The inhibitory effect of FNA would be weakened by S^2-reduction at high S^2-concentration,and would be enhanced by H₂S/S^2-inhibition at low S^2-concentration.The functional bacterial genera were Thiobacillus,Ottowia and Thiovirga in the nitrite-based SOAD biofilm reactor that was seeded with the sluge from a local landfill leachate treatment plant located in Xi’an City,Shaanxi province.Thiobacillus became absolutely dominant genus in the reactor seeded with ordinary mixed sludge from municipal wastewater treatment plant.Although the functions genera are different,the same characteristics of SOAD were achieved in the two reactors.