Protozoans are single-celled organisms. They can serve as indicator organisms to identify the health of a wastewater system. They can also help identify toxic events and recovery from upsets.
Metazoans are multi-cellular organisms and a step higher on the evolutionary chair from protozoans. They are present in more mature sludge ages and can help indicate treatments and health for the overall wastewater system.
One of the most interesting parts about looking under a microscope is not knowing what is going to be under the coverslip. Here are some other life forms and interesting things we have been able to experience during microscopic exams: