1. Introduction

Human activities appear to be the major contributor to water pollution, for instance, agricultural, industrial, and municipal activities. Nitrogen surplus released into the environment has been proven to cause negative impacts on water qualities, human health, and ecosystems [1]. Nonetheless, a wide range of technologies is available to reduce the release of ammonia nitrogen into the environment, such as ammonia stripping [2], breakpoint chlorination [3], ion exchange [4], electrodialysis [5], and biological nitrification [6]. -e ammonia stripping method has several plus points as it is a relatively simple process and cost-effective to remove ammonia in wastewater [7]. Besides, the valuable ammonia stripped from wastewater can be recovered from the stripping process. Due to the stability of this process, the ammonia stripping process has been deemed as an appropriate method in remediating wastewater that contains high concentration of ammonia and toxic compounds [8]. Consequently, ammonia stripping has emerged as a strong interest research area among researchers and industrial community. As such, numerous labscale and pilot-scale studies have been performed, especially for the wide range of industrial wastewater that demands costeffective remediation.

This paper looks into several emerging issues pertaining to remediating ammonia nitrogen by using the ammonia stripper technique, along with some significant operating parameters. In addition, this paper reviews the recent progress in ammonia stripping method with advanced gas-liquid contactors, as conducted by some researchers. Next, a comparison was made between the advanced liquid contactors and the conventional packed tower. Finally, this paper explores the future prospects of this ammonia stripping process.

2. Ammonia Stripping Process

The ammonia stripping process is based on the principle of mass transfer. It is a process, by which wastewater is contacted with air to strip the ammonia gas present in the wastewater. The presence of ammonia in wastewater can be found in two forms, namely, ammonium ions and ammonia gas. The relative concentrations of ammonia gas and ammonium ions are subjected to the pH and the temperature of wastewater [9]. The formation of ammonia gas is favored by increasing the pH, which shifts the chemical equilibrium to the right, thus inducing the formation of ammonia gas. Since high pH is required for eective ammonia stripping, lime is used to increase the pH values of wastewater prior to ammonia stripping [9]. In fact, various types of configurations for ammonia stripping process have been applied to remediate the varied types of wastewater containing ammonia nitrogen. For instance, O’Farell et al. conducted a study on nitrogen removal by stripping on a secondary e uent of a municipal wastewater treatment plant [10]. Figure 1 illustrates a schematic diagram of lime precipitation process and ammonia stripping process. Lime is incorporated to hike the pH of the influent prior to stripping, and this is followed by a recarbonation process for neutralization. Aside from raising the wastewater pH, calcium oxide (lime) generates calcium carbonate in the wastewater and serves as a coagulant for hard and particulate matters. Additionally, O’Farell et al. discovered that the ammonia stripping method could remove as much as 90% of ammonia from the secondary e uent [10].

Meanwhile, Raboni et al. investigated the efficiency of the ammonia stripping technique for remediation of groundwater polluted with leachate [11] (Figure 2). In the study, polyelectrolyte, sodium hydroxide, and iron (iii) chloride were added for the coagulation-flocculation and sedimentation processes at pH higher than 11 [11]. e system also comprised of a heater to heat the wastewater at 38° C and ammonia recovery via absorption with sulphuric acid. Lastly, the e uent was neutralized after adding sulphuric acid. As a result, they found that the ammonia stripping system for groundwater polluted with leachate displayed removal efficiency of 95.4% with initial ammonia concentration at 199.0 mg/L. Next, Saracco and Genon investigated the performance of air-stripping system to treat ammonia nitrogen from industrial e uent (Figure 3) [12]. ey suggested this route as feasible only if the industrial e uent was characterized by relatively high temperature and ammonia concentration. The stripping process was followed by absorption and crystallization processes. Saracco and Genon concluded that the ammonia stripping and the recovery system, along with its internal air recycle, had been technically feasible and easy to control [12].