Mass transfer coefficient of ammonia in the air stripping process for municipal wastewater: An experimental study

This study evaluated the effects of different operating conditions and the air-to-water ratio (G/L) on the kinetics and the mass transfer coefficient of ammonia (KL) in the air stripping method for removing ammonium ions (NH4+) from wastewater with low concentrations in municipal wastewater treatment plants (WWTPs). The impact of operating conditions including the temperature, initial ammonium ion concentration, pH, and air-to-water ratio (G/L) of <2000:1 (60:1, 70:1, and 80:1) on KL in the air stripping method was investigated using artificial wastewater at laboratory scale. The NH4+ concentrations in the wastewater samples were determined with the Nesslerization method (the standard method for the examination of water and wastewater). According to the results, the minimum (0.0528 h-1) and maximum (0.64825 h-1) of KL were obtained within 1 to 4 h in the operating status that included an initial ammonium ion concentration of 33.63-52.81 mg/l, a temperature of 34-45.7 °C, a pH of 9.48-12.2, and an air-to-water ratio of 60:1-80:1. A comparison of the results of three regression models showed that the air-to-water ratio was the most effective factor on KL. Furthermore, in Model 3 (multivariate linear regression model/comparing four parameters), the effects of the air-to-water ratio, pH, and temperature increased, leading to the acceleration and conversion of ammonium ions (NH4+) to a gaseous form (NH3). Also, the initial NH4+ concentration and pH in Model 4 (multivariate linear regression model by subgroup) at a low (60:1) and high (80:1) G/L ratio were the most influential factors on KL, respectively. The results of this study revealed that the air-to-water ratio (60:1, 70:1, and 80:1) could be used successfully for the elimination of ammonium ions from municipal WWTPs, leading to lower energy costs for the required aeration in the air stripping method.