"Breaking The Wall": Mechanistic Insights into Low-Intensity Ultrasound Recovery of Calcified CANDAN Granular Sludge

In the hydroxyapatite (HAP)-based CANDAN-HAP process, the coupling of HAP with granular sludge enhances granule settleability and mechanical strength while enabling partial phosphorus removal through HAP formation. However, prolonged exposure to high-calcium environments may lead to excessive HAP accumulation and subsequent calcification, causing granule deactivation. This study evaluated the effectiveness of low-intensity ultrasound (LIU) in restoring the activity of deactivated granular sludge through cavitation effects. In this study, calcified CANDAN granular sludge was treated with LIU, achieving complete activity recovery after 133 days of long-term operation. The results demonstrate that after LIU treatment, the nitrogen removal capacity of the granules was significantly restored, with the total nitrogen removal efficiency (TNRE) increasing from 74.79% to 90.21%. The granule surface changed from smooth and flat to rough and uneven after ultrasound treatment, while porosity increased from 74.87% to 80.32%, restoring mass transfer channels. Further studies revealed that proteins (PN) in extracellular polymeric substances (EPS) bind with inorganic precipitates to form a "wall" that blocks mass transfer channels in the granules. LIU can break this "wall," causing minor short-term damage to microorganisms but promoting faster long-term recovery of microbial activity. Due to the protective effect of the outer denitrifying bacteria (DB) layer on the inner anammox bacteria (AnAOB), AnAOB were not negatively affected by LIU. Ultimately, the granules achieved full recovery. Therefore, this study provides new insights into the recovery of calcified granular sludge and the effects of LIU on granular sludge, while establishing a theoretical foundation for the practical operation and risk control of the CANDAN-HAP process.