A two-step fed sequencing batch reactor combined with pre-nitritation for treating swine wastewater.

摘要

Scientists have identified agricultural fertilizers as a primary culprit behind the eutrophication phenomenon booming in lakes and gulfs, among which the wastewater flushed from confined swine production and applied to cropland as fertilizers is identified to be one of the major responsible agricultural sources. When the scale of swine production keeps rising, manure that cannot be land applied according to the plant and soil testing has to be treated before discharge.;Shortcut nitrification and denitrification (SND) is a novel nitrogen removal process that has drawn significant attention from researchers lately. In this study, the application of SND in swine wastewater treatment was investigated using two sequencing batch reactors (SBRs) connected in series in order to reduce the needs for energy and carbon uses. The first SBR produced a nitrite rich effluent that was subsequently fed to the second SBR where both nitrogen and phosphorus removal were taking place.;Shortcut nitrification (also termed nitritation) is the first step of shortcut nitrogen removal from swine wastewater. As such, stably obtaining an effluent with a significant amount of nitrite becomes the premise to realize SND. The possibility of accumulating nitrite from swine wastewater was firstly investigated by adopting a continuous feeding strategy in an activated sludge SBR. The results showed that free ammonia and free nitrous acid in the system could reduce the activities of nitrite oxidizing bacteria, generating an effluent with 13-23% of NH4-N, 15-21% of NO3-N, and 56-72% of NO2-N. Two cyclic modes with HRTs of 3 days (the ratio of aerobic feeding to total aerobic reaction time was 0.33) and 1.5 days (the ratio was 0.77) were employed. The cycle comparison between the two modes with different HRTs shows that there is no big difference with regard to the whole nitritation process, which is characterized by continuous conversion of loaded ammonium to nitrite and nitrate in the aerobic feeding period and no further conversion after the loading was terminated, resulting relatively stable levels for all the three nitrogen components in the entire cycle. Compared to 3-day HRT mode, 1.5-day HRT cyclic mode has doubled daily output in volume.;In order to better understand the process of nitrite accumulation, more bench experiments were performed including an effluent nitrogen composition stability test and a reducing load test. The nitrite production stability was tested using four different ammonium loading rates, 0.075, 0.062, 0.053, and 0.039 gNH4/gMLSS˙d in a 2-month running period. The TIN composition in the effluent was not affected when the ammonium load was between 0.053 and 0.075 g NH4/g MLSS˙d (64% NO2-N, 16% NO 3-N, and 20% NH4-N). Under 0.039 g NH4/g MLSS˙d, a little more NO2-N was transformed to NO3-N with an effluent of 60% NO2-N, 20% NO3-N, and 20% NH4-N. The reducing load test has revealed the relationship between a declining FNA concentration and the decreasing nitrite production. The NH4 + load was gradually decreased from 0.081 to 0.011 g/gMLSS˙d. When the NH4+ load was between 0.081 and 0.035 g/gMLSS˙d, the ratio of NO2-/(NO2-+ NO3-) was kept stable around 0.75. When the NH 4+ load dropped from 0.035 to 0.024 g/gMLSS˙d, the ratio dropped to 0.70, accompanied by an abrupt decline of FNA from 1.2 to 0.6. From that point forward, the nitrite dominance environment in the system was no longer existing. Combining the results from both reducing load and stability tests, it is concluded that an ammonium loading rate around 0.035 is the lower threshold for producing a nitrite dominance effluent from the activated sludge SBR.;In the denitrification step, three COD/NOx-N ratios (3.6, 4.8 and 6) and two solid retention times (SRTs), 16 and 23 days, were selected to test the influence of carbon availability and SRT on the total inorganic nitrogen (TIN) reduction and phosphorus removal efficiencies for the step-fed SBR. The best operating combination of parameters would consist of a COD/NO x-N ratio of 4.8 and an SRT of 23 days to achieve 97% TIN and 67% dissolved phosphorus (DP) removals.;Keywords. Swine wastewater, shortcut nitrification and denitrification, nitrite accumulation, SRT, COD/NOx-N ratio.