In general, you will need to have some prerequisite education and will need to pass an exam.
Water and wastewater treatment plant and system operators typically need at least a high school diploma or equivalent and a license to work.
Water and wastewater treatment plant and system operators must be licensed by the state in which they work. Requirements and standards vary widely depending on the state.
Most states have multiple levels of licenses for water and wastewater treatment plant and system operators. Each increase in license level allows the operator to perform more complicated processes without supervision.
At the largest plants, operators who have the highest license level work as shift supervisors and may be in charge of large teams of operators.
(Source: bls.gov)
You can find state specific information in our article: Where can I find information and courses specific to my state?