Intracellular free Ca2+ are canonically well known as significant "second messenger" in cells and regarded as critical regulators of bacterial metabolism. We investigated the influence of Ca2+ dosage on the restoration of anammox consortium, in which nearly 80% cells were dead or badly damaged. Chemical analysis and flow cytometry (FCM) demonstrated that Ca2+ dosage was of primary importance and the restoration process was apparently faster with increasing Ca2+ as its concentration was ranged 0.02-0.5 mM in feeding. Using FCM and Fura red fluorescence labeling for analysis of intracellular free Ca2+, we found a strong correlation between external Ca2+ concentration in feeding and the levels of steady-state intracellular free Ca2+, the abundance of which was considered as the intrinsic causes for favoring anammox consortium restoration. This study provides new insight into the ions effects on rapid restoration of damaged anammox consortium, targeting efficient nitrogen removal from wastewater with anammox process. (C) 2013 Elsevier Ltd. All rights reserved.