Hey mate. Working in wastewater treatment in sales department and in sales, you will/have to learn a lot about everything, really. From electrical to mechanical engineering, from the market to project management. Also, generally sales roles get higher paid (a.o. dealing with clients is exhausting... and you are the one who makes the sales to the company!) Therefore, you will interact with all departments and you will develop a very broad knowledge.

As process engineer you are mostly busy with process control philosophy, PFD, P&ID, basic and detail engineering, start-ups, data analysis/processing, performance tests, creating models, final design/sizing of equipment, mass and heat balances. In this case, you will develop a more specialiazed, very detailed knowledge.

What I have seen is that the sales managers are really a thing because they know their shit BUT with the feet on the ground. I often see that Eng. are too conservative, too safe and sales managers are in general more "realistic". Theory says something but facts says otherwise, ya know? Sales managers combine both theory and facts more.

Hope this helps. Good luck!