The Collaborative International Dictionary of English v.0.48:

Sewerage \Sew"er*age\, n.
   1. The construction of a sewer or sewers.
      [1913 Webster]

   2. The system of sewers in a city, town, etc.; the general
      drainage of a city or town by means of sewers.
      [1913 Webster]

   3. The material collected in, and discharged by, sewers. [In
      this sense sewage is preferable and common.]
      [1913 Webster]

WordNet (r) 3.0 (2006):

sewerage
    n 1: waste matter carried away in sewers or drains [syn:
         sewage, sewerage]
    2: a waste pipe that carries away sewage or surface water [syn:
       sewer, sewerage, cloaca]