Comparative evaluation fluidized bed airlift bioreactor with conventional systems for nitrification of seawater

Abstract

This work investigated the performance of various filters in the nitrification process. The modeled systems included the submerged filter, the packed bed airlift filter, the fluidized bed airlift filter. The commonly used trickling filter was also employed as a base experimental case for the purpose of comparison. Firstly, the investigation on the hydrodynamics and gas-liquid mass transfer of the various filters was carried out. It was found that the gas holdup in all the systems increased with an increase in the gas throughput to the system. The submerged filter seemed to be the best among all the systems under investigation as it provided the highest gas holdup and gas-liquid mass transfer rate. The fluidized bed airlift filter, on the other hand, was the poorest among the three filters. This was because the operating conditions employed in the nitrification process were not adequate for the perfect fluidized bed condition. Hence, there existed a "still bed condition" where a large fraction of packing was not fluidized and this caused high pressure drop in the system. This led to a larger gas bubbles which reduced the gas-liquid mass transfer area. Also the gas holdup in this filter was the lowest because large bubbles were formed which left the system more rapidly than smaller bubbles found in other systems. However, certain conditions could lead to a perfect fluidized bed condition and the performance of the system at this condition was found to be comparative to the packed bed airlift system. In terms of nitrification rate, the packed bed airlift filter was found to be the most effective system. The removal rate from the submerged filter was observed to be relatively high at all operating conditions. The performance of fluidized bed airlift filter, however, depended on several factors. At low Ad/Ar, the system was not effective in removing ammonia. This was because the system could not reach full fluidizing condition in the range of gas superficial velocity employed in this work. Increasing Ad/Ar helped create a better fluidizing condition in this system and augmented the removal rate in the fluidized bed airlift filter. However, the specific ammonia removal rate obtained from the fluidized bed at high Ad/Ar was still lower than that obtained from the packed bed airlift filter at low Ad/Ar.