[Purification Efficiency and Microbial Characteristics of Four Biofilters Operated Under Different Conditions]

To explore the effects of aeration, microbial acclimation period, and pool shape or flow pattern change on the purification efficiency of biofilters at the microbiological level, four biofilters with different working conditions, that is, MAVF, NAVF, NVF, and BHF, were studied. The first three are vertical-flow biofilters and the last one is a baffled-flow biofilter. The four filters filled with the same ceramsite were made of organic glass. The MAVF filter was connected with the BHF filter in series and was operated for one year prior to the trial. The NAVF and NVF filters are newly activated filters. The four filters that were used to treat domestic sewage were synchronously operated in batch mode in this study. The MAVF and NAVF filters were intermittently aerated in contrast to the other two. During the period of microbial acclimation of the newly activated filters, the purification efficiency of the four filters was continuously monitored and the microbial community structure characteristics were analyzed at the end of microbial acclimation. The results show that the purification efficiency of the three vertical-flow filters is significantly higher than that of the horizontal-flow one and aeration significantly enhances the purification efficiency. However, aeration has a weaker effect on the efficiency than the microbial maturity of the filter. An apparent accumulation of nitrates or nitrites in the four filters was not observed, indicating that the denitrification was rather thorough. The 16S rDNA high-throughput sequencing analysis shows that the diversity index of the four filters is BHF > MAVF > NAVF > NVF, indicating that the more mature the filter is, the higher is the diversity index. Most of the packing microorganisms are facultative heterotrophic bacteria and the most abundant are heterotrophic denitrifying bacteria. Heterotrophic nitrification occurs in the NVF and BHF filters and aeration promotes the enrichment of aerobic ammonium-oxidizing bacteria. Aerobic phosphorus-accumulating organisms were not detected in the four filters. Therefore, phosphorus was mainly removed via denitrifying phosphorus accumulation. Under the test conditions, the removal rate of total nitrogen was not high, mainly because nitrifying bacteria were not enriched in the filter or their abundance was insufficient. The latter resulted in the limited ammonium-oxidizing ability of the filter, thus affecting the removal of total nitrogen. The above-mentioned results show that the adjustment of different working conditions will affect the redox status and associated enrichment of functional bacteria inside the biofilter, which will ultimately affect the purification efficiency.