## 2 System Identification of the Pilot-Scale Seawater RO Desalination Plant  
The pilot-scale RO desalination plant considered in this paper is installed in the Control and Automation Laboratory of the Pontifical Catholic University of Peru, located in Lima. This installation is used in the development of research related to the design and implementation of new controllers, as well as in the preparation of personnel trained in the operation of RO processes, control, optimization, monitoring, fault detection, and management of brackish and seawater desalination commercial plants. Figure 1 shows a view of this plant, which in order to prevent water losses, the water flows inside the plant in a closed loop system; therefore, it does not have energy recovery devices and the post-treatment stage is not necessary. The schematic representation of Figure 2 shows the different components of this pilot-scale setup, which are: the pre-treatment system, the high pressure feed pump, the RO membranes rack, the brine and permeate tanks, and the storage and mixing system of the permeate and brine flowrates, that is integrated by the mixing valve, the feedback pump and the storage tank. The pre-treatment system is integrated by the feed pump, the UV filter, the multimedia filter, the active carbon filter, the fine filter, and the two additives tanks. The high-pressure feed pump is of the stainless steel model with variable pressure of up to 50 bar. This pump is connected to a speed variator that controls the inlet pressure to the RO membrane's rack. The pilot-scale seawater RO desalination plant is equipped with an instrumental platform composed of a pH sensor (pHT), two pressure sensors (PT), a temperature sensor (TT), two flow sensors (FT), two conductivity sensors (CT), three ultrasonic level sensors (LT), two additive dosing pumps, four pressure gauges (PG), and four proportional control valves (PV). The flow sensor (FT1) and the conductivity sensor (CT2) located at the RO membranes rack outlet allow to control the flow and conductivity of the permeate, and therefore, the conversion rate. The permeate conductivity is controlled by the adjustment of the position of the proportional control valve (PV3) located at the exit of the brine flowrate. This pilot-scale plant was designed for normal operation with the brine flowrate control valve opening (PV3) 50% with a nominal pressure of 18 bars. In this plant, we use drinking water mixed with a certain minimum amount of salt, that is, we use brackish water and therefore the pressure of 18 bar is enough to desalt the incoming (raw) water. To avoid membranes damages, the proportional control valve PV3 should not be completely closed. This plant also has an automatic  
Figure 1: Pilot-scale reverse osmosis (RO) desalination plant of the Pontifical Catholic University of Peru. flushing and start-up system that protects the membranes from scaling and fouling. The recovery rate of this plant is 46%. The supervision and control system of this plant includes two control stations: the local control one, and the one of remote supervision. The local control station includes a programmable automation controller (PAC) ControlLogix 5570 (Rockwell Automation, Milwaukee, WI, United States), which combines the robustness of a programmable logic controller (PLC) with the advanced calculation capacity of an industrial computer, and a PanelView 1500 (Rockwell Automation, Milwaukee, WI, United States), for local operation of the implemented control systems. The remote supervision station is based on a personal computer (PC). Both stations have a SCADA (data acquisition and supervisory system) application programmed in FactoryTalk View (Rockwell Automation, Milwaukee, WI, United States). The communication between these stations is done through the TCP/IP protocol. The controllers of this plant are implemented in the PAC using the structured text language that allows the implementation of advanced controllers. Different control strategies and setpoint changes can be applied through the SCADA application.