3.2.1. Water Use Efficiency

MCDI is studied as CT pretreatment in order to maximize water use efficiency. The MCDI process settings that result in the largest water use efficiency are therefore of interest and are determined from the MCDI response surface model (RSM). More specifically *Umax* (Equation (3)) is determined from *WR* and *THout* model equations as function of process settings (*Qads*, *Iads* and *tads*) and feed water types. Feed water quality is reduced to four distinct feed water types (sweet/soft, sweet/hard, brackish/soft, brackish/hard) to allow to visualize *Umax*. Hardness limit (*THBDmax*; Equation (3)) is set to *TH* 225 ppm CaCO3 for soft water and *TH* 600 ppm CaCO3 for hard water. This accords to a maximal *COC* of 1.5 or a *Umax* of 0.33 for a CT without pre-treatment. Values are based on the operational conditions of existing Belgian power plants located at Brussels-Charleroi Canal and Ghent-Terneuzen Canal. Evaluation of the resulting volume plots (Figure 2) reveals that the highest *Umax* can be obtained with MCDI treated sweet/soft water. Optimal *Umax* for sweet/soft water type (*Umax* = 0.79) is the highest achievable value of *Umax* in the evaluated parameter space (*tads* = 900 s; *Iads* = 2.5 A; *Qads* = 90 mL min−1). Application of MCDI on sweet/soft water results in the lowest *THout* resulting in higher achievable water use efficiency when compared to high salinity or high hardness water types.

Lowest *Umax* values are found for hard/brackish water with a lower optimal *Umax* = 0.52 realized at low *Qads*, high *Iads* and intermediate *tads*. MCDI on intermediate water types results in intermediate optimal *Umax* (0.57 hard/sweet and 0.62 soft/brakish). An overall trend is observed for *Umax* as function of the studied operational parameters and different feed water types. Within a specific feed water type, flow rate is found to have the largest effect (inverse) on *Umax* (colour gradient varies strongest with *Qads*). Comparison amongs<sup>t</sup> different feed water types shows that the dependency of *Umax* on flowrate is strongest for sweet water. For a given flowrate, a ridge type optimum is found stretching from high *Iads* and low *tads* to low *Iads* and intermediate *tads*.

**Figure 2.** 4D volume plots of maximal utilization (*Umax*) for 4 feed water qualities ([Na+], *TH*). *Umax* is given as colormap slices in the volume plot of flowrate (*Q*), current (*I*) and duration (*t*) for MCDI adsorption phase.
