The largest tributary to the Ganga is the Ghaghara, which meets it before Patna, in Bihar, bearing much of the Himalayan glacier melt from Northern Nepal.
The Gandak, which comes from near Kathmandu, is another big Himalayan tributary. Other
important rivers that merge with the Ganga are the Son, which originates in the hills of
Madhya Pradesh, the Gomati which flows past Lucknow.
Upstream from Varanasi, one of the major pilgrimage sites along the river, the water is
comparatively pure, having a low Biochemical oxygen demand and faecal Coliform count.
Studies conducted in 1983 on water samples taken from the right bank of the Ganga at
Patna confirm that ischemia coli (E.Coli.), faecal streptococci and vibro cholera organisms
die two to three times faster in the Ganga than in water taken from the rivers Son and
Gandak and from dug wells and tube wells in the same area.
Patna city covering an area of 100 sq.km is subdivided into 5 district zones (
Digha, Beur, Saidpur, Pahari, Karmali chak ). The old Pahari Zone of Patna as per the
existing sewerage scheme is now divided in two zones namely Zone-IV A-S & V. This zone
have a STP within its boundary at Pahari STP ( 25 MLD plant) site.