Research On The Performance And Mechanism Of Nitrogen Removal In A Novel Biofilm Reactor Of FBBR Applied In Wastewater Treatment

Rapid development of modern industry, high-intensity agricultural production and sharply growth of population lead to a great amount of industrial wastewater,agricultural waste containing residual fertilizer as well as domestic sewage, which have severely aggravated nitrogen pollution to the surrounding water bodies. Nitrogen pollution not only has negative impact on urban water supply, but also can arise eutrophication of the surrounding waters.Traditional biological technologies of nitrogen removal are mostly based on activated sludge processes. These technologies usually need extended aeration time, adding nitrification liquid reflux equipment, and separated construction of nitrification and denitrification pools in order to maintain the performance. Therefore, traditional biological nitrogen removal technologies are characterized by long and complex processes, high operation cost and poor performance of nitrogen removal.To solve these problems, a self-designed Fluidized Bed Biofilm Reactor(FBBR)with novel Multiple Fluidized Dymanic(MFD) Biofilm Carrier was applied to wastewater treatment especially for nitrogen removal in this study, including optimization of operational factors in start-up and stable periods, removal perfomances of organics, ammonia and total nitrogen, mechanism of biological nitrogen removal.Firstly, several factors were investigated to find their influence on biofilm-forming time, attached biomass and system performance.It turned out that, the main influence factors of biofilm-forming time and attached biomass were carrier structure and filling ratio. Compared to PE02 carrier, FBBR with MFD carrier has a shorter biofilm-forming time of 7 days and a larger attached biomass(more than 70mgVSS·gCarrier-1),demonstrating that MFD carrier’s porous structure aided in carrier fluidization,and effective oxygen mass transfer.In addition, sludge inoculation methods also had effect on attached biomass. Fast sludge discharging released the competition between suspended and attached biomass and in consequently maintained stable growth of attached biomass. The optimized start-up parameters for FBBR with MFD carrier were:40% filling ratio, C/N ratio of 10:1,a concentration of 3 or 4mg·L-1 dissolved oxygen and fast sludge discharging method.Secondly, the influences of Surface area organic loading(SALR), HRT hydraulic retention time(HRT) and aeration volume on stable operation period of FBBR were analyzed and optimized by RSM(Response Surface Method). The main influence factor of organics removal efficiency was HRT; under the best HRT(8~9h), increasing aeration volume or low SALR could hardly increase COD removal efficiency. The main influence factors of total nitrogen removal efficiency and alkalinity consumption were HRT and SALR. When HRT was 8h and SALR was ranging from 4 to5gCOD·m-2·d-1, total nitrogen efficiency could be more than 60%; when HRT was ranging from 6 to 8h and organic loading was 5gCOD·m-2·d-1, alkalinity consumption was below 150mg·L-1; with the increase of SALR and HRT, the alkalinity consumption raised. In general, the optimized values for stable operation period’s parameters were SALR of 4.18gCOD·m-2·d-1, HRT of 8h, aeration volume of 1.66L·min-1(or the gas/water ratio was 35:1).This optimization predicted the removal efficiency of COD,ammonia and total nitrogen were 92.61%,84.63%,67.93% respectively and the predicted value of alkalinity consumption was 128.74mg·L-1.Results showed that prediction above were close to practical value and effluent indexes conformed to the first grade A standard, providing a reference for FBBR practical application in the future.Moreover, mechanisms of nitrification and nitrogen removal were discussed. It turned out that the top four species of bacteria in different operation stages of FBBR were basically the same, namely Proteobacteria, bacteroidetes, Verrucomicrobia and Firmicutes. Compared to the initial stage of start-up, nitrification bacteria strains had changed a lot in the end of start-up and stable operation stage with a significant reduction of autotrophic nitrifying and anammox bacteria and heterotrophic nitification-aerobic denitrification(HN-AD) bacteria became the main function bacteria for nitrogen removal. In view of the good performance of nitrogen removal in stable operation stage, it was speculated that the mechanism of nitrogen removal in FBBR was likely to be HN-AD. According to the microflora distribution under family level, the dominant HN-AD species in FBBR process were Comamonaceae, Rhizobiaceae,Pseudomonadaceae and Rhodobacteraceae.