3.2.2. Cost Estimate

Estimated cost is determined from energy consumption and required electrode surface (Equation (4)). Both parameters depend largely on flowrate. Increasing *Qads* causes specific energy consumption (*SEC*) and *Ael* to lower causing *Cost* to decrease. With increasing flowrate also Umax decreases which makes it of interest to optimize *Umax* and *Cost* in terms of *Iads* and *tads*. MCDI *Cost* is made relative to CT evaporate flow to allow comparison of cooling capacity for the 4 previously defined feed water types. Iso-surfaces are plotted which represent a single optimal *Umax* value as a function of cost (€ m<sup>−</sup>3evap.), *Iads* and *tads* (Figure 3).

**Figure 3.** *Cost* (€ m<sup>−</sup>3evap.) at maximal utilization (*Umax*) versus current (*I*) and duration (*t*) of adsorption phase. For 4 distinct combinations of feed water composition ([Na+], *TH*), colormap indicates cost.

Comparison of different feed water types shows a notable effect of *THin* on cost. Cost increases strongly with hardness and to a lesser extent with increasing *[NaCl]in*. For each feed water type, an optimum setting exists for *tads* and *Iads* resulting in lowest cost for a given *Umax*. These optima (Table 4) are found at high *Iads* combined with low *tads* or low *Iads* combined with intermediate *tads* similar to

the previously described optima for *Umax*. For sweet/soft feed water, e.g., the optimum extends from low *Iads* (1 A) and optimal *tads* (~2000 s) up to high *Iads* (2.5 A) and an optimal *tads* (<1000 s). The existence of such optimum allows minimizing the cost by choosing optimal *Iads* and *tads* to achieve a given *Umax*. Since cost is determined by *Qads*, the objective could also be expressed as maximizing Qads to reach a specific *Umax*. *Qads* is a design parameter that largely depends on installation size for a given application. Alternative optima can be considered, e.g., maximizing *Umax* at fixed cost or minimizing footprint. Moreover, optimization of the product/waste cycle in MCDI can be of use to further maximize water recovery, since *WR* relates directly to *Umax* (Equation (2)).

**Table 4.** Optimal *Umax* for membrane capacitive deionization cooling towers (MCDI-CT) system (from screening tests) and corresponding operational conditions (*tads*, *Iads*, *Qads*), specific energy consumption (*SEC*), required electrode surface (*Ael*) and *Cost* (€/per m3evap) for 4 feed water qualities.


#### *3.3. MCDI on Real Feed Water*

MCDI tests are performed on real CT feed water samples (BC Canal, GT Canal, Eumes River and STP effluent). Treated STP effluent is included as a possible alternative source of cooling water [31]. The selected feed water types have a distinct composition (Table 5). BC Canal water has a relatively low [Na+] and a medium to high *TH* ([Ca<sup>2</sup>+] = 125.7 ppm, [Mg<sup>2</sup>+] = 15 ppm). GT Canal feed water *TH* is highly similar to BC Canal ([Ca<sup>2</sup>+] = 114 ppm, [Mg<sup>2</sup>+] = 43 ppm) while being higher in [Na+] (300 ppm Na<sup>+</sup>). Eumes river feed water is very low in *TH* ([Ca<sup>2</sup>+] < 0.05 ppm, [Mg<sup>2</sup>+] < 10 ppm) with a relatively high [Na+] and high pH compared to the other water types. STP effluent has a low sodium concentration (22.2 ppm) and relative low *TH* ([Ca<sup>2</sup>+] = 23.9 ppm, [Mg<sup>2</sup>+] = 3.3 ppm). In addition, total organic carbon (TOC) is not considered specifically in this study but could contribute to membrane fouling. Indicative *TOC* values for the cooling water samples (Table 5) are 70 mg C/L (BC Canal), 4.4 mg C/L (Eumes river) and <15 mg C/L (STP effluent).

**Table 5.** Chemical composition of cooling water samples.


MCDI tests with real CT feed water are used to reevaluate the RSM model. Since both Eumes river and STP effluent water compositions are far outside the factor ranges of the RSM (Table 1) they are not used for comparison with model predictions. RSM ranges for *THin* and NaCl are based on the average composition of BC and GT Canal water types. Despite of seasonal variation the current samples (Table 5) have a similar composition (*EC*, [Ca<sup>2</sup>+]) when compared to the ranges used for RSM ([Ca<sup>2</sup>+]: 56–180 ppm, *EC*: 1.2 mS/cm–4.6 mS/cm), the current BC Canal water sample however has a relative low [Na+]in. For GT Canal and BC Canal feed water types, *Umax*, *WR* and *THout* are calculated from test results and from RSM equations (predicted value and 95% confidence interval, Statistica prediction and profiling tool) and compared (Figure 4).

**Figure 4.** Comparison of *Umax*, *WR* and *THout* derived from RSM (±95% CI) and experimental values for GT Canal (**Top**) and GT Canal (**Bottom**) feed water.

Comparison results for GT Canal water shows that *Umax* and *WR* are well predicted by RSM. Prediction of parameter *THout* is less accurate and the inverse effect on *Umax* is notable (e.g., for 120-900-2.5). Average difference between RSM predicted and test results derived parameters is <5% for GT Canal water. Comparison of results for BC Canal shows a larger deviation; specifically, *THout* is overestimated by RSM. On average, RSM underestimates *Umax* by 20%, *WR* by 6% and overestimates *THout* by 89% for BC Canal water. Overestimation of *THout* is attributed to the relatively low [Na+]in in BC channel water (60.1 ppm) compared to the RSM factor range (Table 1) making the RSM model less predictive.

MCDI tests with real CT feed water are further used to evaluate the effect of feed water composition on MCDI-CT process parameters (*SEC*, selectivity, *Umax* and *Cost*). Desired properties of the MCDI process in relation to process efficiency are low cost, low energy consumption, high *Umax* and high selectivity (*S*) for bivalent ion removal (e.g., Ca2+, Mg<sup>2</sup>+). Process parameters are determined for the 4 selected water types and various MCDI process conditions and plotted for comparison (Figure 5).

**Figure 5.** MCDI-CT process parameters *SEC* (kWh/m3evap), *Umax* (-), S (-) and *Cost* (€/per m3evap) derived from MCDI tests on 4 types of real cooling water.

*Umax* is found to be highest for Eumes river water followed by STP effluent, BC Canal and GT Canal feed water types. This expected as *Umax* is inversely correlated with feed water EC and *TH* following their effect on *THout*. Variation of *Umax* among process conditions is small, except for BC Canal and GT Canal feed water where ((*tads*, *Iads*):(2000 s, 1 A)) *Umax* is lower in comparison to other test conditions. This is also mirrored in *Cost* (€ m<sup>−</sup>3evap.), which is inversely correlated with flowrate and *Umax* and to a much lesser extent energy consumption (kWh m<sup>−</sup>3evap) as can be seen for BC and GT Canal feed water. Energy consumption (kWh m<sup>−</sup>3evap) is high for BC Canal and GT Canal water types when compared to STP effluent and Eumes feed water. Selective removal of bivalent ions (Ca<sup>2</sup>+ and Mg<sup>2</sup>+) is a desired MCDI feature. Selectivity is found to some extend for BC Canal water (*S* = 1.35; Standard Deviation (*SD*) = 0.3; *N* = 4) and GT Canal water (*S* = 1.34; *SD* = 0.13; *N* = 4). No selectivity was found for STP effluent (*S* = 0.97; *SD* = 0.16; *N* = 4) while S could not be calculated for Eumes river water due to the lack of hardness ions. Preferential removal of bivalent ions is common in MCDI; it is observed in both synthetic feed mixtures [19,32] and real feed water [9]. This phenomenon is the result of diffusion kinetics and adsorption equilibria in MCDI and is attributed to the preferential storage of multivalent ions in ion exchange membranes [32,33]. Overall comparison shows that high flowrate (120 mL min−1) results in minimal *Cost* (€ m<sup>−</sup>3evap.) for all studied feed water types. Feed water with high *THin* (GT Canal, BC Canal) is preferably treated using short *tads* while applying high *Iads*. In addition, the gain in water use efficiency in relation to the base scenario where no pretreatment is in place needs to be considered. For BC Canal and GT Canal water utilization without treatment (*Umax*= 0.33) is relatively low making pretreatment of possible interest while for

STP e ffluent ( *Umax* = 0.76) and Eumes river ( *Umax* = 0.99), utilization is already high, and therefore, pretreatment is not useful. Application of MCDI on BC Canal and GT Canal water types results in a relatively high estimated cost per m<sup>3</sup> evaporate. This is partly due to the estimation procedure neglecting in part scale up e ffects. It generally indicates that the studied MCDI-CT scheme is currently only useful when CT feed water is costly or when legislative boundaries are present that limit water uptake. Legislative constraints on abstraction volumes have been reported to limit energy production in Southern Europe and the US [2]. This is specifically critical in countries where thermoelectric power generation is dominant, and regional water scarcity is a significant concern [1]. BC Canal and GT Canal water feed water cases are currently not severely impacted.

## *3.4. MCDI-CT Pilot Test*

The main purpose of the MCD-CT pilot test is to assess the e ffect of MCDI treated BC Canal feed water on cooling tower performance and acid consumption. BC Canal water was selected as single feed water source for pilot testing in view of relevance, availability and pilot duration (3 months). Equal ambient conditions for comparison are realized by simultaneously feeding one of Merades cooling tower circuits with untreated BC Canal water (reference) and the other one with MCDI treated BC canal water (Figure 6).

**Figure 6.** Schematic overview of the MCDI-CT pilot installation. MCDI container with inside view (**left**) and Merades cooling towers and peripheral equipment (**right**).

To allow comparison of both cooling tower circuits, a fixed MCDI product water quality (conductivity setpoint) is produced in each test run by allowing variable current (0–110 A) and fixed flow rate during adsorption phases, yielding a water recovery of 73%–88%. The process was controlled by the setpoint of the conductivity, i.e., either at 25% or 50% reduction of the incoming conductivity. This build in control strategy uses a variable current and adsorption time depending on the product water conductivity. The operating conditions of the pilot therefore di ffered from the optimal conditions determined from lab tests (Figure 5). In the pilot, specific flowrate was high (10 <sup>L</sup>/m2h), adsorption time was intermediate (1700 s) and time averaged current density was relatively low (1 <sup>A</sup>/m2) for 50% desalination compared to lab tests where following ranges for specific flowrate (5–10 L/hm2), adsorption time (900–2000 s) and current density (1.5 <sup>A</sup>/m<sup>2</sup> to 3.5 <sup>A</sup>/m2) were used. In practice, removal ratios di ffered quite significantly in the first 3 test runs due to software issues (Table 6). BC Channel feed water quality is seasonal and both conductivity (0.70–0.87 mS/cm) and [Ca<sup>2</sup>+] (88–108 ppm) are lower compared to previous samples (Table 5). The specific energy consumption was low (0.08–0.12 kWh m<sup>−</sup><sup>3</sup> produced) in part due to the relative high capacity of the MCDI system used (design flowrate = 0.2–1.8 m<sup>3</sup> <sup>h</sup>−1, applied flowrate = 0.42 m<sup>3</sup> <sup>h</sup>−1) but comparable to values found in literature [9,22]. The feed water was also found to have a significant fouling potential previously undetected during lab tests. A steady and consistent increase in hydraulic impedance (10<sup>8</sup> Pa s m<sup>−</sup>3) of the MCDI cell resulted in an increase in pressure drop over the cell by 2.8 bar in 24 h (at *Qads* = 0.42 m<sup>3</sup> <sup>h</sup>−1). Consequently, the MCDI system passes through an automatic cleaning cycle each 4 h. This type of fouling behavior (spacer fouling) is expectedly caused by small particles and colloidal matter (e.g., clay particles) present in the feed water, which adhere to the spacer fabric, indicating the applied pretreatment (5 μm bag filtration) is insu fficient for BC canal feed water. This indicates that fouling prevention remains an important aspect of MCDI operation despite the common notion that MCDI is less vulnerable to fouling compared to other membrane processes as also indicated recently by Choi and coworkers [34]. Overall the MCDI pilot was able to deliver the required volume for each of 6 test runs with Merades CT pilot (Table 6).

**Table 6.** MCDI removal ratios and operational parameters for Merades cooling tower at test conditions.


\* *TH* removal indicative (point samples).

The resulting *Umax* of the MCD-pilot at operational conditions (Table 6) is lower for MCDI treated water when compared to the reference untreated water. This is deceptive however since a di fferent operational pH is applied in all test runs (no acid dosing). CaCO3 scaling is highly pH dependent, and comparison of CT test conditions therefore requires taking into account both pH control (acid dosage) and water use e fficiency explicitly. An extrapolation from test data of CaCO3 scaling, acid dosage and operational pH is performed using ENGIE lab proprietary cooling water simulation software (Table 7).

**Table 7.** MCDI-CT pilot simulation of *COCmax*, *Umax*, acid dosage (g <sup>h</sup>−1) and feed water saved versus reference test at same pH for reference and test runs.


Comparing acid dosage and feed water savings for a given pH shows that MCDI technology allows a clear reduction of water consumption (74%–80%) when CT is operated at higher pH meanwhile strongly reducing acid dosage (63%–80%). MCDI pretreatment reduces acid consumption when operating at low pH but also increases feed water usage by 10%–30%. Under these conditions MCDI pretreatment is less useful. Comparison between operation at pH 8 without MCDI and pH 8.5 with MCDI shows 50% reduction in acid use for comparable *Umax*. It is concluded that the usefulness of MCDI for CT pretreatment depends strongly on operational conditions and feed water type. Specifically, CT feed water with a high hardness benefits from MCDI pretreatment. In addition, monitoring of total viable count (weekly) indicates that MCDI technology has no impact on biological growth in CT recirculation water, suggesting that nutrients (e.g., TOC) required for growth are not extensively removed by MCDI. LPR corrosion measurements show that MCDI treated BC Canal water is more corrosive (*x* = 2.0 mills/year, *N* = 25) in comparison to non-treated water (*x* = 1.4 mills year<sup>−</sup>1, *N* = 25). However, the ability to operate at higher elevated COC and pH counteracts this effect.
