## 1 Introduction  
With industrial development and population growth, there has been a rapid increase in the amount of wastewater containing ammonia from various sources, including sewage sludge, cattle excrement, food waste, and biomass [1]. Ammonia containing wastewater is also generated in the energizing process of waste resources [2, 3]. Ammonia in wastewater should be removed because it causes eutrophication and ecotoxicity when it leaks into ecosystems [4, 5].  
There are various methods for removing ammoniacal nitrogen from solution, including physical, electronic, and biological methods. Ammonia stripping is a particularly efficient method for ammonia recovery from high concentration wastewater [6, 7]. In ammonia stripping system, ammoniacal nitrogen is removed as a gas by supplying gas such as air or steam [6, 7, 8, 9]. The reactor is usually a fixed-bed column or packed bed tower [6, 7, 8, 9]. In tower-type reactors, ammonia mass transfer is achieved by gas-liquid contact within the packing material [10]. Therefore, the liquid-gas ratio should be carefully considered to optimize the operating conditions of packed towers.  
In this regard, substantial research and development have focused on the operating conditions and other factors affecting stripping towers for different types of wastewater [11, 12, 13, 14, 15, 16]. For example, [PERSON] et al. [12] performed ammonia stripping from landfill leachate using a packed tower at room temperature and pH 11 and achieved a removal efficiency of 98% after 24 h of stripping at a liquid-gas ratio (L/G) of 6.7-20 L/m3. [PERSON] et al. [13] used a continuous flow packed tower to strip ammoniacal nitrogen from anaerobic digestate at a liquid-gas ratio of 2.5 L/m3, achieving a removal efficiency of 55% at 50 \({}^{\circ}\)C and pH 10. [PERSON] et al. [14] evaluated the ammonia removal efficiency and rate for the urine with temperature, pH, concentration, and liquid-gas ratio. [PERSON] et al. [15] and [PERSON] et al. [16] also analyzed ammonia removal rate by temperature, pH and air supply condition.  
Conventional ammonia stripping processes are typically performed at a temperature between room temperature and 50 \({}^{\circ}\)C and in a pH range of 10-12 [11; 12; 13; 14; 15; 16]. These process require a large amount of reagents to adjust the pH [13; 17]. In the ammonia stripping process, a basic reagent is used to increase the pH. After ammonia removal, the pH must be adjusted to neutral for wastewater treatment and discharge. One method for reducing the use of these reagents is high-temperature stripping; at a high temperature, the same removal efficiency can be achieved with a smaller quantity of reagent [17]. Although there is a problem that a heat is required for high-temperature stripping, the heat energy consumption can be minimized by using waste heat. For example, the hydrothermal thermal carbonization (HTC) reaction is carried out at 180-260 \({}^{\circ}\)C [18; 19], and it is possible to raise the temperature of the HTC wastewater up to 70-80 \({}^{\circ}\)C by using HTC heat. In addition, the high-temperature process can be operated more economically by reducing the air supply [8].  
The purpose of this study is to design a high-temperature stripping tower and derive the optimal operating factors that will reduce the use of reagents and air supply while maintaining the ammonia fraction at an operating temperature of 70 \({}^{\circ}\)C. To this end, the ammonia removal efficiency and mass transfer coefficient are assessed at different temperatures, pH, ammonium hydroxide concentration, and airflow volume. Specifically, by deriving the optimal liquid-gas ratio and air supply volume for efficient liquid-to-gas contact in the packing layer, this study provides useful data on the key factors in the design of high-temperature ammonia stripping devices.