| dc.description.abstract | Nitrogen is a common substance found in wastewater discharges. When it reaches a certain concentration, it can cause harm to ecosystems and human health. In Taiwan, regulations have been progressively tightened. From the year 2021 to 2027, there will be a 70% reduction in the discharge concentration of ammonia. Ammonia undergoes nitrification and converts into nitrate, which exists in water as nitrate ions. Most wastewater treatment systems employ heterotrophic denitrification to convert nitrate into nitrogen gas. However, some wastewater treatment plants experience poor denitrification efficiency due to insufficient carbon sources. In recent years, autotrophic denitrification has gained popularity as it uses inorganic substances as electron donors, eliminating the need for additional organic matter and reducing sludge production.
In this study, thiosulfate was used as an electron donor to compare the performance of sequencing batch reactors (SBR) and sequencing biofilm batch reactors (SBBR) in long-term experiments. After 108 days of domestication, the HDPE biofilm carriers did not achieve the desired biofilm formation due to their smooth surface and strong shear forces between the carriers. Therefore, SBBR could not effectively increase the reactor′s biomass and nitrogen loading rate.
In the batch test, the kinetics of the S/N ratio were investigated at different nitrogen loading rate. Both the long-term and batch test observed that when sufficient thiosulfate was present, the branched thiosulfate oxidation (BTO) reaction will occur. Resulting in the accumulation of elemental sulfur on the sludge surface after the reduction of thiosulfate.
Therefore, the S/N molar ratio needs to be maintained above 1.36 times (2.19) than theoretical value to provide enough electron donors for complete denitrification. If the S/N molar ratio decreases to 1.1 times than theoretical value (1.75), some nitrite will use accumulated elemental sulfur to reduce. Compared to thiosulfate, the reaction rate of elemental sulfur is only 0.17 times. In this study, single inflow experiments were conducted, so regardless of the S/N ratio, the BTO reaction occurred, resulting in an average sulfur conversion rate of only 70%. The accumulation of nitrite is mainly controlled by the S/N ratio and the characteristics of the different sulfur sources, which also alter microbial activity. Under sufficient S/N ratio, the change in nitrogen concentration follows zero-order kinetics, and as the electron donor concentration decreases, it gradually transitions to 0.5th t2nd-order kinetics.
In terms of microbial abundance, compared to the initial phase, Thiobacillus proportion increased by nearly twofold, while Sulfurimonas decreased by 10%. Autotrophic denitrifying bacteria accounted for more than 70%, and the presence of BTO bacteria Desulfobacterota was observed, accounting for 1.5% of the microbial community. | en_US |