Nitrogen removal and metabolic pathways of nitrifying coal ash propelled by temperature change under high nitrogen loading

Abstract

Coal ash, owing to its cost-effectiveness and greater adhesion with nitrifiers, has been used as a carrier for immobilizing nitrifiers when treating wastewater with a low C/N ratio and high nitrogen loading. Various environmental conditions significantly affect the nitrogen metabolic pathways of nitrifying coal ash. In this study, the nitrogen metabolic pathways of nitrifying coal ash propelled at different temperatures were investigated to evaluate the impact of seasonal variation on nitrifying coal ash. Results showed average NH₄⁺-N effluent concentrations increased with temperature, and reached 3.37, 5.16, and 76.06 mg N/L with 1158 mg N/L influent under 15 ℃, 25 ℃, and 35 ℃ systems. The total nitrogen removal rate at a cooling temperature of 15 ℃ was higher than that at 25 ℃ and 35 ℃. Further analysis showed <i>Nitrosomonas</i> and <i>Nitrobacter</i> were enhanced, whereas a significant evolution of autotrophic denitrifiers, <i>Acidovorax</i>, was enriched at 15 ℃. An in-depth investigation of N-metabolism pathways suggested higher temperatures of 25 ℃ and 35 ℃ reduced the abundance of key genes involved in ammonia oxidation (amoABC, <i>hao</i>) and nitrite oxidation (<i>nar</i>GH). Network analysis between N-metabolic functional genes and genera presented stronger relevance and richer sources between nitrifiers/denitrifiers and nitrification/denitrification-associated genes under cooler temperatures.