The more than 800 study questions and answers in this study companion represent all aspects of liquid treatment processes and help operators prepare for the first three levels of certification examinations. Practicing these questions will allow operators to practically measure and improve their knowledge of the basics of liquid treatment as well as critical aspects of biological treatment, nutrient removal, and disinfection. These questions are also included as "Test Your Knowledge" questions in the Wastewater Treatment Fundamentals I--Liquid Treatment training manual and online course developed in collaboration with the Association of Boards of Certification (ABC). This separate study guide is intended for those operators who do not have the opportunity to use the training manual or online course for study. The peer-reviewed resources in the Wastewater Treatment Fundamentals series represent the expertise of hundreds of water quality professionals. They align with updated Need-to-Know Criteria from the Association of Boards of Certification and are based on WEF's extensive existing resource collection, including Operation of Water Resource Recovery Facilities, MOP 11.