*2.5. MCDI-CT Pilot*

The MCDI-CT pilot installation consists of a 30 m<sup>3</sup> feed tank, MCDI container and a mobile cooling tower unit (Merades). The MCDI 10-foot container holds a prefiltration device (5 μm bag filter, Filtermat) coupled to a Voltea CAP-DI IS2 MCDI unit with a capacity of 0.2–1.8 m<sup>3</sup> h−1. The pilot is fully automated (Siemens PLC) and remotely controllable, Selenium webdriver with Python is used for data collection. 0.1 M NaOH and 0.1 M citric acid solutions are used for cleaning in place (CIP). Merades consists of two parallel and independent cooling tower loops simulating semi-open cooling circuits. Each circuit consists of a cooling tower basin, circulation pump and a 12 m length condenser pipe. Water is pumped from the cooling water basin through the condenser where water temperature is slowly increased, condenser outlet temperature is controlled, and is subsequently sent to the cooling tower where it is cooled before flowing down back into the cooling tower basin. The cooling tower is equipped with a nozzle to spray the water through a fill. The temperature in the cooling tower basin is regulated by varying the forced air flow (ventilator) in the cooling tower. The *COC* of the cooling circuit is regulated by a blowdown pump. Merades is equipped with automated injection system and continuous inline monitoring. Make-up and circulation water are monitored for pH and *EC* (continuously), hardness (TAC and THCa) and chlorides (hourly), LPR corrosion measurement (3 times/week), free and total ATP and total viable count (weekly).

The pilot installation was operated for 6 test runs during a 12-week test period (May–July 2018, Linkebeek Belgium). Feed water is abstracted from Brussels-Charleroi Canal and transported to the pilot location (Engie Laborelec, Linkebeek, Belgium) on ~weekly basis. In each test, Merades loop 1 is fed with untreated Canal water (reference) while loop 2 is supplied with MCDI treated water. *COC* of both Merades loops is gradually increased until scaling occurs to determine the maximal achievable *COC*. Condenser inlet pH is maintained constant at pH 8.0 (H2SO4) in each test. Scaling onset is determined from circulating water [Ca<sup>2</sup><sup>+</sup>]/[Cl−] ratio. When minimal blowdown flowrate is reached (4 × 10−<sup>3</sup> m<sup>3</sup> <sup>h</sup>−1, *COC* 4 to 5) without scaling occurring, pH is gradually increased to induce scaling. Fixed operational CT parameters include condenser outlet *T* (37 ◦C), Δ *T* in condenser (10 ◦C), CT water *T* (27 ◦C), water spread in condenser pipes 1.46 m s<sup>−</sup>1, circulation flow rate 1.9 m<sup>3</sup> <sup>h</sup>−1, packing spraying flow rate 8 m<sup>3</sup> m<sup>−</sup><sup>2</sup> <sup>h</sup>−1, hydraulic halftime 0.25 h. Stainless-steel condenser pipe and film fill (height 1.5 m) are used. Following each test, the cooling water circuits are cleaned (concentrated HNO3, pH < 2.0 for 2 h minimum). MCDI CIP is performed following each test and intermittently when required. After cleaning the system and modules are thoroughly rinsed with demineralized water.

## **3. Results and Discussion**
