Fast granulation of halophilic activated sludge treating low-strength organic saline wastewater via addition of divalent cations

Granulation of halophilic activated sludge is an important solution to solve the problem of solid-liquid separation in biological treatment of saline wastewater. This study demonstrated that by adding divalent cations into the saline influent with low organic load, halophilic granular sludge with an average diameter of 910 ± 10 μm can be cultivated. The close correlation between divalent cations and particle size indicated that Ca2+ played a major role in the granulation process. Ca2+ was accumulated in halophilic granular sludge, which provided an inorganic carrier for microbial aggregation and leaded to the dominance of halophilic bacteria of the family Flavobacteriaceae. The halophilic bacteria secreted a large amount of extracellular polymeric substances (EPS), which contained 70.0 ± 0.02% protein. By enhancing the EPS network of protein and Ca2+, halophilic granular sludge was formed. The addition of Mg2+ enhanced the network of Mg2+ and loosely bound EPS, which could be destroyed due to Na+ substitution. This study provides an effective granulation method for halophilic activated sludge. [Display omitted] •Estuarine sediment inocula formed halophilic granule by addition of Ca2+ and Mg2+.•Enriched halophiles of Flavobacteriaceae secreted a large amount of PN-EPS.•Mg2+ prevents Na+ substitution reaction and binds EPS that may be lost.•Ca2+ provided an inorganic carrier for microbial adhesion.•Cations addition is a solution of granulation in low organic saline wastewater.