Evaluation Of Up-flow Two-stage Biological Aerated Filters

Recently, biological aerated filter (BAF) has been paid close attentions due to its advantages of less land requirement, small footprint, and high removal efficiency. It is used widely for secondary and advanced treatment as well as reuse of sewage in many wastewater treatment plants.In order to evaluate performances of up-flow two-stage BAFs (UBAFs) at bench-scale where ceramisite was packed as medium, a series of experiments were carried out with a synthetic sewage water mainly containing organic and ammonium as substrates, which included the start-up performance of UBAFs reactor, the effects of operation conditions on removal efficiencies of COD, ammonia, the nitrogen loss and simultaneous nitrification and denitrification performance in the UBAFs reactor, and the effects of backwash experiments in the UBAFs.The results showed that the start-up of UBAFs reactor was successful by the meathod of sludge acclimation, static and continuous culture, which lasted 15 days (excluding sludge acclimation). The COD and ammonia removal efficiencies stably reached 80% and 75%, respectively.Performances of UBAFs for sewage treatment were pretty good. Effluent concentrations of both COD and ammonium met the requirememt in the Urban Miscellaneous Water Quality Standard of Urban Wastewater Reclamation and Reuse (GB/T 18920-2002) when the inffluent concentrations of COD and ammonia were 200~363 mg/L and 16.8~31.3 mg/L, respectively.The profiles for the concentration and removal rates of COD and ammonia with the media layer heights was consistent at the filtration velocities of 12.5, 24.6 and 49 m/d. 70% COD was removed within the media layer height of 0~1.0 m due to a large amount of reproduction of short generation time heterotrophic microorganism at the bottom of media that had a plenty of nutrients in the UBAFs. While ammonia removal mainly happened within the media layer height of 1.0~2.0 m, which showed well affects of nitrification and de-nitrification because of some types of matured and stable nitrification bacterias in the second stage of UBAFs.Operation results indicated that UBAFs reactor was able to endure a stong COD shock loadings. 90% COD removal was maintained with flexible variation of influent COD loadings at 1.21~5.07 kg COD/ (m~3·d). Meanwhile, UBAFs were able to resist a certain ammonia shock loadings. Ammonia removal rate was below 80% and nitrification performace was also decreased greatly when ammonia loadings exceed 0.617 kg ammonia / (m~3·d).Analysis of the nitrogen balance and scanning electron microscope (SEM) were used to investigate nitrogen removal in UBAFs. In the first stage of the UBAFs, anaerobic heterotrophic de-nitrification occurred where anoxic condition existed. In the second stage, aerobic heterotrophic de-nitrification was caused by heterotrophic nitrifying bacteria.The effect of using combined air-water backwashing system in the UBAFs under the most unfavorable condition was remarkable. After 120 min of bacwashing, both of COD and ammonia concentrations reached the target level.These results show that UBAFs are suitable for the advanced treatment and reclamation of domestic wastewater in China.