Hers and my class took a field trip to a water purification facility.
She and I took a field trip with our class to a water purification facility.
genitive is a term that includes a larger range of relationships than "possessive"—ownership (my dog), temporary possession (his cookie), a natural trait (his tail), a family relationship (his mom), origin (his city), performer of an action (his arrival) and more.
genitive determiner – Its her house. (my, his, their, our, your)
genitive pronoun – Its hers. (mine, his, theirs, ours, yours)
water purification – a process that makes water safe to drink
University of California, Berkeley Professor Julian Fulton presents to students at the Freeport Water Intake purification facility. By Samantha (Wiki Ed), Wikimedia. Licensed under CC BY-SA 4.0. Cropped and resized. Accessed on 15 Jan 2017.