**1. Introduction**

With the application of nitrate-containing fertilizers, consumption of animal products, and industrial production activities, ever more ammonia and nitrate are being discharged into rivers and lakes, which may cause eutrophication and deterioration of aquatic environments. Traditionally, ammonia removal is achieved with biological processes such as nitrification, breakpoint chlorination, air stripping, reverse osmosis, zeolite adsorption, and so on. However, these processes either require high capital investment or are chemistry-intensive. Moreover, these processes focus on the removal instead of ammonia recovery. Ammonium nitrogen (N) is an important nutritional element in fertilizer besides phosphorus (P) and potassium (K). Recovering nutrients, instead of simply removing from wastewater, is drawing more attention to keep natural resources reliable and sustainable and minimize carbon footprint. Two processes focusing on nutrient removal and recovery have stood out in recent years and drawn attention from engineers, facility operators, and regulators.
