Fundamental Study Of Completely Autotrophic Nitrogen Removal And Its Granulation In SBR Reactor

The accumulation of ammonia in water environment has resulted in serious deterioration in the quality of water environment. The problem that should be solved urgently is how to take proper measures to prevent the deterioration. The treatment of wastewater with high nitrogen concentrations and low C/N ratio in conventional method is rather cost-intensive, as both the oxygen demand during nitrification and the demand for organic substrates during denitrification depend on the respective nitrogen concentration in the wastewater, which goes against the sustainable development of wastewater treatment. Recently a new process named as deammonification is proposed, which describes the oxidation of nearly 50% of the NH4+ to NO2- and subsequent conversion of NH4+ and NO2- to molecular nitrogen (N2 gas). The main advantages of this process are lower oxygen demand (up to 25% energy savings during aeration) and the reduction of 40% organic substrate requirements for heterotrophic denitrification, so it becomes the highlight in the biological de-nitrogen study.Based on the latest research findings on biological nitrogen removal in both domestic and abroad, short-cut nitrification, anaerobic ammonium oxidation and the combined nitritation- anammox process are discussed in this paper. In addition, the cultivation of deammonification granular sludge is studied in SBR reactor.Two identical suspended growth systems (Continuous Stirred-Tank Reactor, CSTR) are conducted to study the restrained nitritation (R1) and the complete nitritation (R2). The difference between restrained nitritation and complete nitritation is: the former means that only partial ammonium in the wastewater is converted to nitrite (about 55%), otherwise, the latter means that all ammonium in the wastewater are converted to nitrite. The results show that the optimal operation of R1 are dissolved oxygen concentration controlled between 0.5 mg/Land 0.8 mg/L, pH value 7.5～8.3, temperature 30℃and influent ammonium concentration 350～400 mg/L; For R2, the optimal operation are dissolved oxygen concentration controlled between 0.8 mg/L and 1.0 mg/L, pH value 7.5～8.3, temperature 30℃and influent ammonium concentration 300～350 mg/L, the maximum nitrite accumulation rate reached above 90%.DO concentration is one of the most important parameters for partial nitritation. During the initial domestication period, both R1 and R2 reactors are operated in the condition of DO concentration of 1.5～2.0 mg/L. After 3 days'cultivation, nitrification activity increases in both reactors as shown by mass production of nitrate and low concentration of nitrite. During the phase of nitrifiers competition, restrained nitritation is obtained in R1 reactor when DO concentration is controlled between 0.5 mg/L and 0.8 mg/L. For complete nitritation, the DO concentration in R2 varies between 0.5 mg/L and 1.5 mg/L. These show that complete nitritation is easier controlled than restrained nitritation.Enrichment of anammox microorganism is carried out in a suspended growth SBR system. Methanogenic granular sludge is utilized to cultivate anammox granular biomass. Initially, the mineral medium (70 mg/L ammonium, 70 mg/L nitrite) is supplied to SBR with a HRT of 30 days. After 54 days'operation, the anammox activity presents in the reactor, which indicates that the Methanogenic granular sludge is a suitable inoculum and the HRT is an important factor for enriching Anammox bacteria. At t=90 days, anammox granules are cultivated successfully. NH4+-N and NO2--N are removed simultaneously and maximum volumetric removal rates reach 14.6 g NH₄⁺-N /(m³reactor·d) and 6.67 g NO₂^--N /(m3reactor·d) respectively. From t= 110 d, in order to further enrich anammox bacteria, the substrate load begin to increase by reducing the HRT. At t=156 d, the HRT is decreased to 5 days. A final concentration of enriched biomass of 4.5 g VSS dm^-3 was obtained, and the reactor is able to treat nitrogen loading rates up to 34 g N m^-3 d^-1. The ammonium and nitrite react in the stoichiometrical of 1:1.12. During the experiment period, the course of the start-up of anammox reactor can be monitored by the measuring of the ratio of nitrite/ammonium and the observation of the changes of sludge colour in the reactor.It is common knowledge that the chemolithoautotrophs mainly utilize inorganic carbon as carbon source. So the influent bicarbonate concentration is an important factor in the anammox enrichment. The anammox activity increases as the influent bicarbonate concentration increased from 1.0 g/L to 1.5 g/L and then is inhibited as the bicarbonate concentration approached 2.0 g/L. However, the activity restores by the reduction of bicarbonate concentration to 1.0 g/L, as shown by rapid conversion of ammonium and nitrite and nitrate production with normal stoichiometry. The optimization of the bicarbonate concentration in the reactor improves the anammox rate up to 66.4 mg N/ (L·d).Based on the prophase unsuccessful experiments on the cultivation of deammonification granular sludge from aerobic granular sludge, in this paper anaerobic granular sludge is used as seeding sludge, and the process of the cultivation of deammonification is also adjusted as follows: the first step is to enrich anammox bacteria under anaerobic condition in the SBR reactor; the second step is to control DO concentration (0.3～0.5 mg/L) to realize the conversion of anammox sludge to deammonification. It is observed that the higher influent CaCl2 concentration results in the increasing of inorganic matters in the sludge and the decreasing of relative biomass. However, when decreasing CaCl2 concentration in the influent, after about 30 days'cultivation, the sludge becomes brown gradually and the nitrogen removal rate is restored.