Swapan Basu, Ajay Kumar Debnath, in Power Plant Instrumentation and Control Handbook (Second Edition), 2019
2.6.4 Water Treatment Plant (WTP)
WTP including an effluent treatment plant: There are three different sections in a WTP: a pretreatment (PT) plant, a posttreatment or demineralized water (DM) plant, and a waste treatment or effluent treatment (ET) plant. The ratio of numbers of analog to digital measurement is near unity, and because at present there is hardly much difference between the DCS and PLC, so the selection of control could go either way. However on account of many sequential controls pertinent to each of the exchanger units in a DM plant and effluent plants (and cost also), the PLC is preferred as the control system. It has been found that in some cases, a number of PLCs are utilized for a total WTP plant. Also, some smaller units use PLCs with SLC (Section 4.3). A WTP encompasses everything from a raw water pump house to a DM storage tank, and the location of the pretreatment plants is far from the DM plant control room, where normally the main PLC and operator station are placed for control and operation. Sometimes it is better for pretreatment plants to have a separate PLC with remote I/Os for a raw water plant. Again, a chlorination plant is almost an independent plant and may be controlled by a micro PLC. Depending on plant size and configuration, the ET plant may be an elaborate system involving many I/Os, so a separate PLC with communication to the main system may be economical. Finally, all these PLCs may be integrated or linked to have relevant data available at the DM control room. Alternatively, these may be achieved by deploying remote I/Os and/or via a fieldbus to reduce cable cost. In a nutshell, there is a need to have an integrated LAN with a soft link or fieldbus. In terms of automation, there are differences in control systems deployed for PT, DM, and ET. In pretreatment plants, there will be a number of pumps, and their automatic selections and associated logics are major issues. Also, there will be a few measurements such as flow measurement by a Parshall flume. For a large power house, there may be a large number of pumps and valves, so these automatic selections are not very simple and a dedicated PLC may be a better option. Similarly, in a DM plant, there will be sequential operation of solenoid-operated valves for backwash and regeneration for the exchanger units. Regeneration can be done in various modes such as DP across an exchanger, time-based, or a conductivity-based system, so the PLC needs to take care of such selections. As in a DM plant, there are controls for chemical handling (for example, brine tank automation). The sequence control may be fully automatic, semiautomatic, or manual. A PLC in a DM plant needs to handle many analog parameters (for example, pH, Cond. analyzer) for monitoring and for modulating control (for example, degasser control) also. Therefore, the trending of parameters is important. In case of small DM plants, the use of a local dedicated sequencer is not uncommon. Selection of the IP rating for a PLC and local enclosure is quite important as the environment is very corrosive. In these plants, local operation of a few drives is common, for example, the clariflocculator agitator, so local panels with operator interface and a link to the LAN may have to be considered.