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Effect of saltwater intrusion on activated sludge flocculation

Saltwater addition negatively impacted the settling properties of the activated sludge in a sequencing batch reactor, which in turn caused the effluent quality and the dewatering properties to deteriorate. Experiments showed that increasing sodium ion concentrations above 1 meq/L interfered with bio‐flocculation by ion exchange of calcium ions displaced by sodium in mixed liquor flocs, causing increases in the sludge volume index, total suspended solids, and chemical oxygen demand concentrations of the final effluent and in the times of the time to filter test of the mixed liquor. Experiments were also conducted to reverse mixed liquor de‐flocculation by adding calcium ions in wastewater with different sodium chloride concentrations and a positive relationship was found between calcium ion added and improved effluent quality. Calcium ion concentrations above 1.6 meq/L proved to be beneficial for adhesion of microbial aggregates leading to the formation of stable flocs and improved sludge dewatering by reversing the ion exchange phenomenon observed with sodium concentrations above 1 meq/L. Monovalent‐to‐divalent ratios did not provide predictive information on de‐flocculation. Hence, individual ion concentrations in addition to monovalent‐to‐divalent cation ratios are important in the onset of de‐flocculation effects while a system‐specific concentration threshold was measured under which no adverse effects were observed.