Linux

Really? If it's a big enough treatment works to warrant a SCADA, then I doubt an automation engineer with the experience to set it all up would be asking this question, but here goes. You've a couple of obstacles:

• every contract I've ever seen for industrial automation has either specified which control plane they want directly, or they'll have a list of approved suppliers which you must use. Someone after you will have to maintain this. Those maintainers will only accept the things that they have been trained on. Those things are Windows PCs running Windows software. They will reject anything else. The people running network security on those machines will have a very short list of the acceptable operating systems for running SCADA systems. That list will be a couple of versions of Windows Server. They will also reject anything else.

• that's not nearly enough information to make a recommendation. Which PLCs? Allen Bradley, Siemens, Mitsubishi, ...? I can't think of a job I've ever been on where the local HMI hasn't matched the PLCs. The SCADA software almost invariably matches the PLCs used in the main motor control centre, with perhaps a couple of oddball PLCs for proprietary panels and such like. Could maybe ask the supplier if they've a Linux alternative? Siemens will laugh at you and Mitsi won't understand the question, but AB just might.

Sorry - I'm a Linux evangelist, but I don't think it's a good fit for here. SCADA performance generally isn't bad due to Windows Server - it's fine, does what it's intended to - but because eg. STEP 7 is an appallingly slow and bloated piece of software which would bring a mainframe to its knees. Which is bizarre - the over-the-wire protocol connecting the machines is generally a short binary blob described in the PLC configuration - these bits are the drive statuses, these bits are an int or a float for an instrument readout - and it shouldn't be at all slow updating it all, but slow it is.