Formation, Controlling And Characteristics Of The Granular Sludge Partial Nitrification System

Partial nitritation means to controll partial nitrification process, so that approximately 50% of the influent ammonia is oxidized to nitrite; the effluent can be directly used as water of anaerobic ammonia oxidation. Partial nitritation is the pre-process of the combined anaerobic ammonia oxidation short-cut biological nitrogen removal process. The combined process have the advantages of saving oxygen and carbon source, and reducing the amount of excess sludge, etc. They are especially suitable for high ammonia and low C/N wastewater treatment. However, in the reactor,biomass are not easy to improve due to the slow growth of the autotrophic ammonia-oxidizing bacteria. It will influence the denitrification in the system. Granular sludge have the advantages of good settling properties and resistance to shock loading,etc. It can maintain a very high biomass in the reactor that enhancing the denitrification capability of the system.The cultivation of nitritation granular sludge by adding Ca2+was investigated in a sequencing batch reactors(SBR) under aerobic conditions(DO>1.9mg·L-1) with seeding conventional activated sludge. On this basis, changing conditions induced the realization of partial nitrification. While the controlling and nitrogen transformation characteristics in the system were studied. The results obtained were as follows:(1)In a sequencing batch reactors(SBR)with normal activated sludge and under aerobic conditions(DO>1.9 mg·L-1), after 30 days, the nitrate concentration in effluent was reduced to 39 mg·L-1, the nitrite concentration was increased to 190 mg·L-1and the nitrite accumulation reached 83% by gradually increasing the influent ammoniaconcentration from 100 mg·L-1to 300 mg·L-1. It indicates that the successful implementation of the nitritation. Then, the conversion rate of ammonia reached 98%~99%, and the nitrite accumulation has remained at 80% to 85%, the nitritation system was stable.(2)The cultivation of nitritation granular sludge was strengthened by shortening the settling time and adding Ca2+. Granules with diameters larger than 200 μm accounted for around 79% after 90 days’ continuous operation. It indicates that the formation of granular sludge. During the formation of granular sludge, the Zeta potential of the sludge surface increased from-21.4 mV to-13.6 mV and the amount of extracellular proteins increased from 26.82 mg·g-1to 51.99 mg·g-1. On one hand, the electrostatic repulsion among the flocs were reduced with the increment of the Zeta potential, on the other hand, the increased extracellular proteins can be combined to form biological polymers under the bridging effect of Ca2+. The collaboration of both accelerated the formation of nitritation granular sludge.(3)Based on the realization of nitritation, maintaining constant influent ammonia concentration and controlling the molar ratio of inorganic carbon and nitrogen was 1, after 30 days, the ammonia conversion rate dropped to about 60%, the nitrite accumulation rate was around 75%, the effluent ammonia/nitrite was 1:1.3. It indicates that the partial nitrification was achieved successfully. In the next 100 days, the reactor was stable.(4)The effects of different TIC/N on ammonia oxidation process was investigated. The results showed that when the TIC/N was 0.5, the conversion rate of ammonia was 34%, nitrite accumulation rate was 75% and the effluent NO2--N/NH4+-N was 0.4. When the TIC/N was 1.0, the ammonia conversion rate was 58%, nitrite accumulation reached 82% and the effluent NO2--N/NH4+-N was 1.3. When the TIC/N was 1.5 and 2.0, ammonia conversion was 100%, nitrite accumulation was 88%. We can see, under aerobic conditions, TIC/N is an important controlling parameter of the partial nitrification.(5)The release characteristics of N2 O in the autotrophic granular sludge partial nitrification system was studied. Single-cycle emissions of N2 O was 2.97 mg, N2O-N conversion rate(N2O-N/NH4+-N) was 0.22%. Compared with the previous study ofN2 O emissions in flocculent sludge partial nitrification system, we can draw that emissions of N2 O in the granular sludge system decreased. This may because that granular sludge have larger particle size, and partial N2 O produced inside the particles have sufficient time to react. Then the N2 O entering the liquid and releasing into the atmosphere decreased.