How difficult is it to become an environmental engineer if you're currently studying Environmental Science?

You are definitely on a right track. With your environmental science classes already taken, all you have do is to take same engineering department classes and some mathematics classes and you will be on a fast track to being an environmental engineer. There are other kinds of engineering that can still get you working as an environmental engineer. My own degree is in chemical engineering but I have been working as an environmental engineer since I graduated.
GOOD LUCK!!!!!

Without knowing how far along you are in your program, its difficult to answer this with precision. Environmental science programs tend to focus more on chemistry and biology affecting environmental media whereas environmental engineering programs tend to be more based in traditional civil engineering coursework like water resources engineering, air pollution control engineering, and solid and hazardous waste management and engineering. You could add these types of courses to your studies if you're not too far into your major; you could switch your major; or you could pursue an environmental engineering degree in graduate school. In my case, I was a physics major who then pursued a graduate degree in water resources and environmental engineering, and then worked the next 31 years in environmental engineering in the areas of water resources, hazardous waste managment, and air quality permitting and compliance. You're certainly on the right track and there is no "right" way to do it.

Hi, of course you can. Apply for environmental engineering positions, and start your career as an env engineer. You may be required to take some extra engineering courses specific to the company profile your work for. Environmental engineering is a broad area, so you have to decide in which industry you would like to start building your career. Good luck!

Ms. Achuko's response to your question is right on! I would add that your school counselor should be able to assist in signing up or finding another school if yours does not offer an environmental or general engineering degree. I also started as a chemical engineer, but ended up with 38 years as an environmental engineer and manager. The very large advantage of the engineering degree is that it opens up a much broader scope of activities and careers. Many employers will expect that the engineer will be able to not only understand the science of a problem, but to assist in its resolution. I wish you the best of luck in whatever path you choose.