Fly ash integration for enhanced partial nitrification stabilization and aerobic granular sludge stability under low-temperature conditions

Abstract

Anaerobic ammonium oxidation requires the influent NO₂⁻–N/NH₄⁺–N ratio to be 1.32 : 1. At low temperatures, poor sludge settling performance, expansion, and loss are profound. The effluent water quality cannot meet the influent requirements of the anaerobic ammonium oxidation stage. In this study that was operated for more than 300 days, the SBR was used to inoculate flocculent sludge and cultivate it to form aerobic granular sludge (AGS), which was then domesticated into partially nitrosated granular sludge (PNGS), and gradually reduced to low-temperature for intensive cultivation. During the cooling process, the ratio of NO₂⁻–N/NH₄⁺–N in the effluent was maintained by controlling the aeration time and the operating cycle of the SBR to achieve the best partial nitrosation efficiency. PNGS cultured increased the volumetric load of NH₄⁺–N removal from 0.24 kg (m⁻³ d⁻¹) to 0.35 kg (m⁻³ d⁻¹) at 15 °C compared with 30 °C. Fly ash was used as the crystal nucleus and carrier to prevent the disintegration of AGS at low temperatures (15 °C). The effect of fly ash dosages 50, 100, 150, 200, 250, 300 mg L⁻¹ on partial nitrification efficiency was determined. It accelerates the formation of new AGS and improves partial nitrification performance. Compared with no fly ash addition at 15 °C, when fly ash dosage of 250 mg L⁻¹ was added, the NO₂⁻–N accumulation rate increased from 75% to 85%, and NH₄⁺–N volume removal load increased from 0.35 kg (m⁻³ d⁻¹) to 0.45 kg (m⁻³ d⁻¹). Effluent NO₂⁻–N/NH₄⁺–N increased from 0.55 : 1 to 1.20 : 1. Effluent NO₂⁻–N/NH₄⁺–N meets anammox influent requirements. This study can be used to build sustainable wastewater treatment in low-temperature regions worldwide.