Study On Single-Stage Nitrogen Removal Process With Airlift Biofilm Reactor

The single-stage nitrogen removal biofilm process combined short-cut nitrification and anammox has been a hot area of research for its great advantages. We inoculate the aerobic nitrifying biofilm in the riser of airlift reactor and meantime put the anaerobic anammox biofilm in the downer of airlift reactor. In order to undestand the effect of operation conditions and process parameters on nitrogen removal performance of the reactor, three aspects of research are conducted as follows.(1) CFD simulation of airlift reactor is performed and the impact of superifical gas velocity on gas volume fraction and liquid circulation velocity is investigated. When the superifical gas velocity rises from 0.01 m/s to 0.12 m/s, the gas volume fraction of riser rises from 0.02 to 0.14 and the liquid circulation velocity rises from 0.15 m/s to 0.34 m/s. The analysis of reactor flow field indicates that the eddy can be found in the bottom and top zone. The gas volume fraction and volumetric oxygen transfer coefficient of the reactor are measured.(2) The research of short-cut nitrification is performed. Usually the control of short nitrification is hard, so the experiment of short nitrification with activated sludge is done. The result of batch experiment indicates the dissolved oxygen concentration is a key operation parameter for the control of short nitrification. When the dissolved oxygen concentration in the bulk liquid is about 7 mg/L, the phenomena of nitrite accumulation can't be observed. When the dissolved oxygen concentration is about 3 mg/L, the nitrite begins to accumulate. In order to determine the suitable operation condition of nitrite accumulation, the model based on the concept of minimum substrate concentration is built. The results indicate the substrate concentration and direct inhibition of pH is not effective control methods, but the inhibition of free ammonia and nitrous acid is effective. The control of short-cut nitrification in biofilm system is different from suspended system. The Monte Carlo simulations based on biofilm model have been done and the impact of DO, thickness of mass transfer boundary layer, ammonia surface loading, temperature and influent ammonia concentration on the washout of nitrite oxidizer is investigated. The results indicate the ratio of ammonia and DO concentration in biofilm is crucial for the short-cut nitrification.(3) The mixing time, oxygen mass transfer rate and reaction characteristics of reactor are solved by Aquasim and Matlab software. The mixing time increases with the rise of liquid velocity and axial diffusivity coefficient. The aomount of oxygen that transfers from gas to liquid can be neglected.The liquid circulation velocity, dissolved oxygen volume mass transfer coefficient, thickness of boundary layer, ammonia surface loading and biofilm area ratio have great effect on the nitrogen removal performance of airlift biofilm reactor.