Impact of Increased Chloride Concentration on Nitrifying-Activated Sludge Cultures

The impact of increased chloride concentration on continuous flow nitrifying-activated sludge cultures is reported in this paper. To examine further the effect of chloride concentration on nitrifying-activated sludge, a nitrifying batch culture with increased chloride concentration was also investigated. The variables tested included the ammonia removal efficiency, the specific nitrification rate (SNR), and the dominant species of the nitrifiers. A continuous culture of nitrifying-activated sludge with a gradual increase in the chloride concentration in the influent was found to perform better than one with a fixed influent chloride concentration. However, a down-and-up variation in the ammonia removal efficiency was observed in this culture after the influent chloride concentration reached $18\text{,}200\mathrm{mg} \mathrm{Cl} L^{-1}\text{.}$ This trend was confirmed by the response of the SNR of the batch culture when the initial chloride level was increased from 10,000 to $18\text{,}200\mathrm{mg} \mathrm{Cl} L^{-1}\text{.}$ When the chloride concentration was below $10\text{,}000\mathrm{mg} \mathrm{Cl} L^{-1}\text{,}$ the dominant species of ammonia-oxidizers and nitrite-oxidizers in both the batch and continuous cultures were the same. Nitrosomonas europaea-lineage, Nitrosomonas eutropha, Nitrosomonas halophila, and Nitrosococcus mobilis-lineage were the dominant ammonia-oxidizers while Nitrobacter was the dominant nitrite-oxidizer. When chloride concentration was increased toward $18\text{,}200\mathrm{mg} \mathrm{Cl} L^{-1}\text{,}$ only Nitrosococcus mobilis-lineage survived as the dominant ammonia-oxidizer, while the nitrite-oxidizers disappeared. Thus, $10\text{,}000-18\text{,}200\mathrm{mg} \mathrm{Cl} L^{-1}$ chloride concentration was the critical level for the nitrifier population.