"As someone looking to break into this field as a water treatment operator, my obvious weakness is lacking direct experience with the machines and systems I would be operating each and every day. I want you to know that I'm very competent and comfortable learning new software and maintenance programs and have proven my ability to do this over the last five years in my current job. If I'm fortunate enough to be offered this position, I will come astutely focused on learning the equipment I will be using and ask as many questions as possible to ensure I'm up and running in short order."

It is important to answer this question honestly but also keep the answer positive. Talk less about the weakness you feel you have and more about what you will do to improve on the weakness if hired as their next water treatment operator.