Environmental Engineering Reference
In-Depth Information
6
ChaPTer
PriMary TreaTMenT
(sediMenTaTion)
6.1 inTroduCTion
The purpose of primary treatment (primary sedimentation or pri-
mary clarification) is to remove settleable organic and floatable solids.
Each primary clarification unit can be expected to remove 90 to 95% set-
tleable solids, 40 to 60% total suspended solids, and 25 to 35% BOD
5
.
Note:
Performance expectations for settling devices used in other areas
of plant operation are normally expressed as overall unit performance
rather than settling unit performance.
Sedimentation may be used throughout the plant to remove set-
tleable and floatable solids. It is used in primary treatment, secondary
treatment, and advanced wastewater treatment processes. In this chap-
ter, we focus on primary treatment, or primary clarification, which uses
large basins in which primary settling is achieved under relatively qui-
escent conditions. Within these basins, mechanical scrapers collect the
primary settled solids into a hopper, from which they are pumped to a
sludge-processing area. Oil, grease, and other floating materials (scum)
are skimmed from the surface. The effluent is discharged over weirs into
a collection trough.
6.2 ProCess desCriPTion
In primary sedimentation, wastewater enters a settling tank or
basin. Velocity is reduced to approximately 1 foot per minute (fpm).
Solids that are heavier than water settle to the bottom, while solids that
are lighter than water float to the top. Settled solids are removed as
sludge, and floating solids are removed as scum. Wastewater leaves the
