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While flow-electrode capacitive deionization (FCDI) is an emerging desalination technology, reduction in water hardness using this technology has so far received minimal attention. In this study, treatment of influents containing both monovalent and divalent cations using FCDI was carried out with flow-electrodes operated in short-circuited closed-cycle (SCC) configuration. Divalent Ca cations were selectively removed compared to monovalent Na with the selectivity becoming dominant when the FCDI unit was operated at lower current densities and hydraulic retention times. Results showed that SCC FCDI operation was much more energy-efficient for brackish water softening compared to operation in isolated closed-cycle (ICC) mode, particularly with implementation of energy recovery. This finding was largely ascribed to (i) charge neutralization of the flow-electrodes in SCC configuration and (ii) regeneration of the active materials to maintain pseudo "infinite" capacity during electrosorption. In addition, mixing of the flow-electrodes in SCC operation significantly inhibited pH excursion in the flow-electrode with resultant alleviation of calcium precipitation on the carbon surface.

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Lab- and pilot-scale simultaneous nitrification, denitrification and phosphorus removal-sequencing batch reactors were operated under cyclic anaerobic and micro-aerobic conditions. The use of oxygen, nitrite, and nitrate as electron acceptors by Candidatus Accumulibacter phosphatis during the micro-aerobic stage was investigated. A complete clade-level characterization of Accumulibacter in both reactors was performed using newly designed qPCR primers targeting the polyphosphate kinase gene (ppk1). In the lab-scale reactor, limited-oxygen conditions led to an alternated dominance of Clade IID and IC over the other clades. Results from batch tests when Clade IC was dominant (i.e., >92% of Accumulibacter) showed that this clade was capable of using oxygen, nitrite and nitrate as electron acceptors for P uptake. A more heterogeneous distribution of clades was found in the pilot-scale system (Clades IIA, IIB, IIC, IID, IA, and IC), and in this reactor, oxygen, nitrite and nitrate were also used as electron acceptors coupled to phosphorus uptake. However, nitrite was not an efficient electron acceptor in either reactor, and nitrate allowed only partial P removal. The results from the Clade IC dominated reactor indicated that either organisms in this clade can simultaneously use multiple electron acceptors under micro-aerobic conditions, or that the use of multiple electron acceptors by Clade IC is due to significant microdiversity within the Accumulibacter clades defined using the ppk1 gene.