Where can i find a solution manual for Principles of Environmental Engineering and Science?

Solution manuals can typically only be gotten by sending a letter to the publisher on school letterhead requesting one. You might be much better off talking to the professor and seeing if he can recommend any other texts that have more examples in them that you can read up on. Old homework and exam questions and solutions can also be helpful as examples to work through, and professors often make them available for studying anyway, so you might be able to get some.