Mangroves should not be used as sewage disposal sites. It is killing them. Mangroves grow in highly saline, coastal waters, representing a very unique ecosystem.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/effluents-killing-mangroves-study
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/deccan-herald-bangalore
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/soil-salinisation
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/mangroves
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/new-zealand