Effluent sewer is a wastewater collection system that pumps only the liquid portion of sewage. At each home, a buried tank collects and passively separates solids from the liquid effluent. High head pumps then pump the effluent through small diameter pipes (typically 2" to 4") to downstream treatment. Because the system is pressurized, pipes can be laid just under the surface, along the ground's contour

Because an effluent stream has a lower waste strength than raw sewage, the downstream treatment plant capacity can often be downsized.

Clusters of just three homes, neighborhood developments, and whole communities with thousands of connections have been serviced by effluent sewer collection.

http://en.wikipedia.org/wiki/Effluent_sewer