Hydrodynamic cavitation for efficient removal of ammoniacal nitrogen: Optimization of process parameters and geometric configurations

Abstract

The present study investigates the effectiveness of hybrid hydrodynamic cavitation (HC) process with the aid of aeration for effective removal of ammoniacal nitrogen. Custom-designed hydrodynamic cavitation reactor with orifice plates ranging from 2 to 2.5 mm in diameter and varied number of circular holes was used in the present investigation. The experiments were conducted at different upstream pressure (2 to 7 kg/cm²) in hydrodynamic cavitation with initial ammoniacal nitrogen concentration of 2000 mg/L. Potassium persulphate (KPS) was also evaluated under selected conditions for comparative assessment of oxidative pathways. The experimental findings demonstrate the effectiveness of hydrodynamic cavitation which showed a remarkable removal efficiency (70%) of ammoniacal nitrogen in 90 min of operation at optimal pressures of 3 kg/cm², with initial pH 10.5 and air flow rate maintained at 5 litres per miniute (LPM). During cavitation, the solution temperature increased from ambient (~34°C) to approximately 68°C, representing high-temperature conditions, while the alkaline pH (≥10) ensured dominance of free ammonia for stripping. The volumetric mass-transfer coefficient (K_La) increased from 0.0057 h⁻¹ at 2 kg/cm² to 0.0126 h⁻¹ at 3 kg/cm² and reached 0.0216 h⁻¹ at 7 kg/cm², confirming enhanced gas liquid mass transfer with increasing cavitation intensity. This study further examines the removal mechanisms of ammoniacal nitrogen using hydrodynamic cavitation, particularly under high-temperature and high-pH conditions.