The largest wastewater treatment plants can be defined in several ways.
The largest in term of capacity, both during dry and wet-weathers, is the Jean-R.-Marcotte Wastewater Treatment Plant in Montreal. With full secondary treatment of effluents it would be the Deer Island Waste Water Treatment Plant of Boston. In terms of area occupied it is by far Melbourne's Western Treatment Plant.
Wastewater treatment capacities are expressed in cubic meters per day. 1000 cubic meters is 1 ML (mega liter). The area occupied by the plants are in square kilometers.