Only half of Delhi is connected to sewer lines, which means that the other half’s waste flows, untreated, into the Yamuna.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/treating-untreated-waste-flows-yamuna
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/yamuna
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/water-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/sewage-pollution
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi-jal-board-djb
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/high-court-sewage-treatment-plants-stp
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/centre-science-and-environment-cse
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/central-pollution-control-board-cpcb