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炮打总统府金虫 (小有名气)
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[求助]
求高手翻译，环境工程水处理方面！
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Processes for biological nutrient removal (BNR) typically adopt alternating anaerobic, anoxic, and aerobic zones in order to remove nitrogen and phosphorous, as well as organic compounds, from the wastewater. In these BNR processes,nitrification reaction relies on slow-growing, autotrophic bacteria that require a long solids retention time (SRT) and a relatively high oxygen concentration, while the denitrification reaction requires an organic electron donor that is not always sufficient in the influent wastewaters . Noting the benefits of shortcut biological nutrient removal (SBNR) process, many researchers have tried to obtain consistent nitrite accumulation in nitrification, which is a key prerequisite for successful SBNR . From a biological viewpoint, two distinct approaches are possible. First, the SHARON (single reactor system for high activity ammonia removal over nitrite) process employs a short (1–2days) SRT and a high reaction temperature (35 8C) in order to selectively wash out nitrite oxidizers . SHARON process is the most suitable for wastewaters already at high temperature and with little organic carbon. The second approach uses a high pH and, consequently, a high unionized ammonium concentra-tion or a low DO to slow nitrite oxidation preferentially and washout nitrite-oxidizing bacteria. The second approach is the most suitable for insufficiently buffered wastewaters at lower temperatures (<20 8C). A key operating factor for any SBNR process using either approach to eliminate nitrite oxidation is to avoid retarding ammonium oxidation toomuch. The focus of this work was on the second approach. A number of researchers have shown that a high concentration of unionized ammonia, or free ammonia (FA),inhibits nitrite oxidizers. Anthonisen et al. reported that inhibition of nitrite oxidation by FA began at 0.1–1.0 mg FA/L, while that of ammonium oxidation at 10–150 mg/L. Thus, a selective inhibition of nitrite oxidation should be achieved within a FA concentration of 1.0–10 mg/L. One challenge of the second approach is that the threshold concentration of FA seems to increase gradually with time .Villaverde et al. concluded that the nitrifica-tion–denitrification process via nitrite is not likely to be stable or feasible for long periods of time due to the acclimation of nitrite oxidizers to FA. Nitrite oxidizers may also be more sensitive to low DO (0.5 mg/L) than ammonium oxidizers, depending on SRT . Another potential pathway in shortcut denitrification is to have rapid denitrifica-tion of nitrite as soon as it is produced by the ammonium oxidizers. Simultaneous denitrification with nitrification is now well established n aerobic reactors in which the DO level is poised at a suitably low level, typically 0.5–1 mg/L. Rapid scavenging of nitrite for denitrificationaugments the benefits of FA or low DO for inhibiting nitrite oxidizers. A hybrid shortcut biological nutrient removal (HSBNR) process integrates biofilm within a suspended-growth system to maximize the hold-up and SRT of ammonium oxidizers, while operating with relatively high FA and low DO concentrations to inhibit nitrite oxidation and enhance rapid denitrification of nitrite . The HSBNR offers the potential of three selective pressures against nitrite oxidation: high FA, low DO, and rapid direct denitrification. The HSBNR also should have the advantage of a short hydraulic retention time (HRT) and small system volume, due to the accumulation of biofilm biomass along with suspended biomass. The hybrid system was tested in a batch mode to demonstrate its promise for stabilizing SBNR . This study evaluates the hybrid reactor in the continuous-flow mode, which tests the long-term performance of shortcut nitrogen removal and investigates the relative importance of factors affecting nitrite accumulation in the HSBNR reactor: concentrations of FA, DO, and direct denitrification. |
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