I'm searching for reference material on Sewage, concerning treatment methods, environmental implication and monitoring methods
There's a link from the Intergovernmental Oceanographic Commission's webbsite to the OceanPortal. The address is http://ioc.unesco.org/oceanportal/
When you search the sites using the word sewage as searchword, you will find several interesting articles on your subject. There are also some articles to be found in the Ebsco database that might be of interest to you, but I think you will find the links from OceanPortal superb. You can use the Ebsco database for instance in the public libraries in Helsinki.