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Engineering aspects and mechanisms of redox-stratified membrane bioreactors for completely autotrophic nitrogen removal from wastewater

Final Report Summary - RSMBR (Engineering aspects and mechanisms of redox-stratified membrane bioreactors for completely autotrophic nitrogen removal from wastewater)

A lab-scale redox-stratified membrane aerated biofilm reactor (RSMBR) was constructed with a working volume of 0.59 L. A gas-permeable silicone membrane (12 mm diameter, 300 mm length) was used as the support for biofilm growth. The influent flow rate was kept at 1.77 L d-1, which gave a hydraulic retention time (HRT) of 8.4 h. It took three months to investigate the hydrodynamic characteristics and the oxygen transfer rates with distilled water. Hereafter, the RSMBR was inoculated with nitrifying activated sludge (Day 0), and its nitrogen conversion performance was investigated. The influent ammonium-nitrogen (NH4+-N) concentration was controlled at 71.1±6.5 mg NH4+-N L-1 (influent NH4+-N loading rate: 11.1±1.0 g NH4+-N m-2 d-1) from Day 1 to Day 52. During this period of time, nitrate (NO3-) was the main nitrogen species in the effluent and its concentration increased from 20.5 mg NO3--N L-1 (Day 8) to 57.3 mg NO3--N L-1 (Day 53). Meanwhile, the effluent nitrite-nitrogen (NO2--N) concentration was below 5.3 mg N L-1, indicating that complete nitrification occurred when the NH4+-N loading rate was less than 11.1 g NH4+-N m-2 d-1. In order to stimulate NO2- accumulation and shortcut nitrogen removal in the RSMBR system, DO concentration in the bulk liquid was controlled below 0.05 mg L-1, and the influent NH4+-N concentration was increased from 71.1 mg NH4+-N L-1 to 262.0 mg NH4+-N L-1 from Day 53 to Day 60 and maintained at 237.1±20.3 mg NH4+-N L-1 from Day 61 to Day 140. The effluent NH4+-N concentration rose remarkably from about 11.0 mg NH4+-N L-1 (Day 60) to about 182.0 mg NH4+-N L-1 (Day 80), then decreased gradually to 67.8±27.2 mg NH4+-N L-1 (from Day 90 to Day 140), which indicates a gradual increase of the NH4+-N removal rate in the RSMBR. The NO2--N concentration rose from 3.2 mg NO2--N L-1 (Day 60) to 22.3±5.3 mg NO2--N L-1 (from Day 100 to Day 140). The NO3--N concentration was maintained at about 46.6±14.3 mg NO3--N L-1 (from Day 80 to Day 140). The nitrogen loss in the RSMBR system increased from 18.0±9.6 mg N d-1 (Day 20-Day 53) to 128.9±61.7 mg N d-1 (Day 80-Day 140), which suggests that denitrification was enhanced by increasing the influent NH4+-N loading rate. It is speculated that simultaneous nitrification/nitritation and denitritation occurred in the RSMBR system.
The spatial distribution of ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) in the membrane aerated biofilms was investigated by using fluorescence in situ hybridization (FISH) and confocal laser scanning microscopy (CLSM) techniques. The stratification of nitrifying and denitrifying populations was observable in the biofilms, when the biofilm was thick enough to prevent oxygen diffusion through the biofilm into the bulk liquid. The anoxic zone appeared near the biofilm-bulk liquid interface as expected. AOB and NOB were located at the base of the biofilm, i.e. close to the surface of the membrane or the supply source of oxygen; other heterotrophic bacteria, including denitrifying bacteria, were located at the top of the biofilm close to the bulk liquid and with lower DO and higher NH4+-N concentrations.
Functional genes for ammonium and nitrite oxidizing (amoA gene and nxrA gene) were quantified with the real-time quantitative PCR technique. On the whole, the abundance of amoA gene was two orders magnitude higher than the abundance of nxrA gene in the extracted DNA samples. The abundance of amoA gene increased from 0.70±0.15×106 copies/ng DNA (Day 20) to 2.80±0.42×106 copies/ng DNA (Day 80), then decreased a little bit to 2.10±0.29×106 copies/ng DNA (Day 120). Correspondingly, the abundance of nxrA gene increased from 1.15±0.49 ×104 copies/ng DNA (Day 20) to 3.21±0.58×104 copies/ng DNA (Day 40), then decreased to 2.34±0.65×104 copies/ng DNA (Day 120). These results reflect that AOB were dominant populations and NOB were almost outcompeted by AOB, but some NOB populations still survived in the biofilms.