My strategy is to browse the reference manual quickly at first, so that i know where is everything and what they are generally talking about (you do not need to understand everything).
Then I use the environmental engineering questions and solutions book, look at the question and look at the answers, find where the topic is covered in the reference manual. Mark it up. Do it for every question and then you will be more clear.
Third step, go over the exam highlight you have in the step two, and try to answer the question again as much as you can. Last step, do it one or two more times, you will be very clear which topic the exam is covering.
Refresh class will be helpful after you get a general idea. I spent around 200 hours to review, so it is definitely time consuming. Strategy is important but there is no shortcut.
Please check more useful recommendations here:
Also check my PE environmental engineering summary under here, it covers the main point from reference manual, problems and questions and my personal summary, It helps a lot during the exam preparation,
Good luck! Please leave a reply if you would like to know more about PE and I will try to write some blogs for those topitcs