WASTEWATER COLLECTION SYSTEM ALTERNATIVES
CHAPTER 6
WASTEWATER COLLECTION SYSTEM ALTERNATIVES
In a comparison to IWSs involving on-lot treatment and disposal, a centralized
treatment facility that serves a whole community or a decentralized cluster treatment
system that serves discrete neighborhoods necessitate the installation of street
sewers to collect and convey wastewater to an offsite location for treatment and
disposal. This chapter describes alternative collection systems applicable to small
communities.
A prime factor in evaluating the cost-effectiveness of a wastewater collection system
is the terrain and subterranean characteristics of the area. In rolling terrain,
conventional gravity sewers are more costly to install because they must be buried
deep in the ground. Further, excavation costs are higher in areas with shallow
groundwater due to dewatering activities, and in areas where hard pan rock exists.
The use of collector sewers at the large 5-acre and adjoining 1-acre agricultural
parcels of Kapoho Farm Lots is not deemed practical and cost-effective. Low
population density and high ground elevation make it practical to dispose septic tank
effluent in a drain field situated in a soil mantle zone well above the groundwater
table. The evaluation of alternative collection sewers will therefore focus on the
coastal subdivision lots comprising Kapoho Beach Lots and Vacationland Estates.
6.1 CONVENTIONAL GRAVITY SEWER SYSTEM
Gravity sewers serving communities similar in size and density to Kapoho Beach
Lots and Vacationland Estates typically consist of jointed pipe laid in a trench in a fall
line that induces a motive force to attain a self-scour flow velocity of two feet per
second or more.
Typically, 4-inch diameter laterals transport sewage from
household plumbing fixtures to a sewer lateral and collector main in the road. At
junctures where collector mains intersect, manholes are installed for maintenance
pur