**1. Introduction**

Nitrification and denitrification is a major process route for nitrogen cycles in ecological systems. It is essential for nitrogen removal from water and wastewater. In the past, there were tremendous efforts and a significant amount of research on this topic in regard to microbial species, effects of operating parameters such as temperature, pH, dissolved oxygen (DO), types of carbon sources, and hydraulic and mass loadings. In this book, a comprehensive review of conventional technologies was carried out and innovative technologies such as *anaerobic ammonium oxidation* (*Anammox*) were focused upon. A unique species *Alcaligenes faecalis* No. 4 was reviewed and experimented for heterotrophic nitrification and aerobic denitrification. Aerobic biofiltration for ammonia removal and an anoxic membrane bioreactor (MBR) for nitrate removal were selected as application examples.

However, the effect of salinity on nitrification and denitrification was not discussed. Some industrial wastewater contains high concentrations of salinity and nitrate, that is, flue-gas desulfurization (FGD) wastewater contains nitrate (about 50 mg/L as N), high total dissolved solids (TDSs) (1–5% of chloride), and other contaminants. Elevated TDS levels present in industrial wastewater may have profound effect on denitrification and there had been little research on this subject matter. To remove nitrate from such wastewater, it is important to understand the effects of salinity on the process kinetics, selection of carbon sources, periodical salinity fluctuations, and microbial communities for the process selection and engineering design.
