What automation system is best for Waste water treatment plant ?

hi, currently i'm working as an automation engineer at a wastewater plant. I think you need to be a little more specific, how big is the plant, how many I/O, do you need to store data for a number of year, are there always operators at the plant or only during daytime, do you wish to intergrade any pumping stations to the system etc. Normally I would say some kind of PLC with an overlaying SCADA system. but if it's a big plant that often changes i think you should consider a DCS system for example ABB800xA. 

I have commissioned one of the waste water treatment plant. It consists of multiple PLC's in different locations interconnected with each other through a fibre optic link. SCADA is also controlling and monitoring the equipment through the same FO link. Inter PLC interlocking also commence through the same FO link. 

Hi,

I worked in WWTP that uses PLC.  You need to read sensors, DO, pH, temperature.  ANd you need to control (on/off, speed) of pumps, valves, compressors/blowers.  I think PLC is the most suitable.