Subject: How can I count the number of microorganisms in a drop of river water?

For my science fair project, I need to observe and count the number of 
microorganisms in a sample of river water after certain purification methods 
have been applied to the water. How can I count or estimate the number of 
organisms, if any, that are left in the sample. When I look at the sample 
through a microscope, the live organisms are moving around and hard to count 
accurately. 

Re: How can I count the number of microorganisms in a drop of river water?