Their most important and significant role is to solve environmental health issues. Adequately trained public health engineers can be employed to solve a lot of problems emerging in India these days.
About jobs, they can work with government agencies like Centers for Disease Control and Prevention (CDC), The National Institutes of Health (NIH), Occupational Safety and Health Administration (OSHA), Food and Drug Administration (FDA), The Environmental Protection Agency (EPA) etc.

Also in non-profit organizations like WHO