| An increasing population and industrialization will increase both our water demand and wastewater generation, placing even more pressure on water resources and wastewater treatment. Conventional wastewater treatment plants have not been designed for nitrogen removal, and many plants do not meet the current discharge limits. However, with the more stringent regulations on controlling nutrient discharge into receiving waters, nitrogen removal has been implemented in more and more countries to respond to the harmful effects of nutrients, e.g. typical effluent standards requires that nitrogen concentration in effluent must be less than 3mg total-nitrogen/1. In some regions, more stringent nitrogen discharge limit was set. Therefore, the advanced technology for nitrogen removal is urgently required and many processes for nitrogen removal have been developed. Biological nitrogen removal process is one of the most economically feasible and widely practiced methods for nitrogen removal and has received maximum attention. However, conventional biological nitrification and denitrification processes have some limitations, such as sludge bulking, large space of treatment plant, washout of nitrifying granules, higher production of waste sludge, etc. Therefore, new methods for nitrogen removal are still highly desired.Aerobic granule technology has been studied for over 10 years for wastewater treatment. Compared to conventional activated sludge, aerobic granule has regular and compact physical structure, diverse microbial species, good settling property, high biomass retention, and great ability to withstand shock load or shock of toxic compound. Therefore, aerobic granule is becoming a very promising technology for wastewater treatment. So far, a few operating modes of granular sludge systems for nitrogen removal have been reported on the basis of lab-scale experiments, such as alternating oxic-anoxic mode (OA), continuous or on/off aeration with controlled DO for simultaneous nitrification and denitrification (SND), anaerobic-controlled oxic mode (AO), and alternating anaerobic-oxic or anaerobic-oxic-anoxic mode (AO or AOA) using denitrifying polyphosphate accumulating organisms (DNPAOs). However, there are still some technological constraints that hinder further application of the technology, such as poor nitrogen removal at low carbon concentration condition, poor capability to treat wastewater with high nitrogen concentration (e.g. over 100 mgN/l) etc.. In this study, nitrogen removal in aerobic granular sludge system was investigated in order to tackle some of the unsolved problems in current study.Generally, there are two kinds of nitrogen-containing wastewater, i.e. low strength wastewater such as municipal wastewater with nitrogen concentration lower than 100 mg/l and high strength wastewater with nitrogen concentration higher than 500 mg/l, e.g. wastewater from composting facility, chemical industry, tannery and leachate etc.. Our study is conducted to aim to these two kinds of nitrogen-containing wastewater: one focuses on the synthesis wastewater stimulating municipal wastewater from Jurong west wastewater treatment plant in Singapore; the other focuses on ammonia removal from high nitrogen wastewater. The results are as follows:A novel operating strategy was developed for efficient nitrogen removal and stable performance by granule sludge, i.e. alternating anoxic/oxic mode combined with step-feeding, which was developed based on the characteristics of granules with average size of 1.5mm and 0.7mm, respectively. The application of this operating mode to the low-strength wastewater treatment and the effects of the novel operating mode on long-term stability of the system were also tested. The results shows that nitrogen removal efficiency reached 93.0-95.9% under alternating anoxic/oxic phases combined with step-feeding, while 67.9-71.5% nitrogen removal efficiency under the alternating anoxic/oxic mode and 75.0-80.4% under the alternating anoxic/oxic mode with DO controlled at 2 mg/l were obtained. Obviously, the newly developed operational strategy had much higher nitrogen removal efficiency compared with other strategies reported in literature. In addition, it was found that granule density is another important factor apart from granule size to influence the rate of nitrification and denitrification. the ratio of aerobic zone to anoxic zone in granule greatly depends on both granule size and granule density, which impacts the efficiency of nitrification and denitrification in granules. Relying on anoxic zone in granule for denitrification is unreliable because it is impossible to control granule size and granule density during the operation. However, It was found that alternating anoxic/oxic mode combined with step-feeding was the optimal mode for nitrogen removal, which is independent of density and size of granules. Thus, anoxic/oxic mode combined with step-feeding was applied to the synthesis wastewater with low C/N ratios (5:1 and 3:1) to test the feasibility of the method. Furthermore, the system with C/N ratio of 5:1 in the feed was run for over 2 months in order to find out the influence of the strategy on long term operation. The nitrogen removal efficiency at low C/N ratio was high (over 90%) and the characteristics of the granules were stable during the 70 days of operation. This reveals that the alternating anoxic/oxic condition combined with step-feeding mode has high potential in treating low carbon source wastewater with good stability.Biological nitrogen removal by activated sludge in wastewater treatment plants is widely applied, but the process is quite vulnerable due to the slow growth rate, uneasy retention in the reactor and high sensitivity of nitrifying bacteria to toxic compounds or fluctuation of environmental conditions. However, for granular sludge, nitrifying bacteria could be easily retained in reactor due the high density of granular sludge and long SRT of granular sludge. the high concentration of nitrifying bacteria could accelerate nitrification rate. However, most nitrifying granules cultivated presently were not fed with high concentration ammonia, e.g. over 500 mgN/l, which is common in high strength nitrogen-containing wastewater. In this study, the nitrifying granules which are able to treat ammonia as high as 1000 mg N/l were successfully cultivated in two identical reactors (N1 and N2). The result shows that the evolution of nitrifying granules was a gradual process with acclimation or lag phase, granulation phase and maturation phase. It was observed that the main characteristics of the nitrifying granules in N1 and N2 such as treatment capacity (99%), settling ability (around 10-15 ml/g of SⅥ), biomass concentration and dominant microbial community were almost same under the same operational conditions. However, the nitrifying granules in N1 broke into small flocs and then recovered, while the nitrifying granules in N2 had a more sustainable development in terms of size and morphology. This demonstrates that nitrifying granules had a more flexible developing mode than mixed culture granules, which make the system more stable and promising in practice.In order to find out the tolerance of nitrifying granules to higher loading pressure, two different nitrogen loading increasing strategies, i.e. shortening cycle time and increasing influent ammonia concentration, were applied to furter increase the nitrogen loading rate on two nitrifying granule system. It was found that the nitrifying granules in the both reactors exhibited outstanding performances with over 99% ammonia nitrogen removal efficiency. In the meantime, the nitrifying granules maintained compact and dense structure throughout the whole operation process, with good settling ability (10-15 ml/g of SⅥ), high biomass retention (around 9 g/l of MLVSS), and increased size of 468μm in N1 and 490μm in N2 respectively at the end of the operation. At the highest loading pressure (3.4 kgNH4+-Nm-3·d-1 in N1 and 3.8 kgNH4+-Nm-3·d-1in N2), complete nitrification was successfully converted to partial nitrification in the two reactors by adjusting pH from 7.5-8.5 to 7.0-8.0 through lowering bicarbonate concentration. The size of the nitrifying granules steadily increased to 863μm in N1 and 757μm in N2, and SVI of the granules was around 10 ml/g, and kept stable in both reactors. However, ammonia removal efficiency and the ratio of MLVSS/MLSS in Nl fluctuated due to the abrupt increase of the specific loading rate in N1 during the temporarily short-time decrease of MLSS, while the nitrifying granules in N2 maintained stable performance. With the adjustment of settling time in Nl, the performance of the nitrifying granules in N1 recovered in one month. The results clearly demonstrate that nitrifying granules are able to withstand nitrogen loading pressure as high as 3.8 kgNH4+-Nm-3·d-1 with good performance and stability. Thus, it is feasible to take nitrifying granular SBR as an alternative for partial nitrification to combine with other processes for total nitrogen removal.
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