Environmental Engineering Reference
In-Depth Information
sedimentation tank over an effluent weir and moves on to the next step
in treatment. Detention time, temperature, tank design, and condition
of the equipment control the efficiency of the process.
Note:
The velocity is based on minutes instead of seconds, as was the case
in the grit channels. A grit channel velocity of 1 fps would be 60 fpm.
6.2.1 overview of Primary Treatment
•
Primary treatment reduces the organic loading on downstream
treatment processes by removing a large amount of settleable, sus-
pended, and floatable materials.
Primary treatment reduces the
•
velocity of the wastewater through
a clarifier to approximately 1 to 2 fpm so settling and flotation can
take place. Slowing the flow enhances removal of suspended solids
in wastewater.
•
Primary
settling tanks remove floated grease and scum, remove
the settled sludge solids, and collect them for pumped transfer to
disposal or further treatment.
The clarifiers may be rectangular or circular. In
•
rectangular
clari-
fiers, wastewater flows from one end to the other, and the settled
sludge is moved to a hopper at one end, either by flights set on par-
allel chains or by a single bottom scraper set on a traveling bridge.
Floating material (mostly grease and oil) is collected by a surface
skimmer. In
circular
tanks, the wastewater usually enters at the
middle and flows outward. Settled sludge is pushed to a hopper
in the middle of the tank bottom, and a surface skimmer removes
floating material.
•
Factors affecting primary
clarifier performance include:
Rate of flow through the clarifier•
•
•
Wastewater characteristics (strength, temperature, amount and
type of industrial waste, and the density, size, and shapes of
particles)
Performance of pretreatment processes
•
•
Nature and amount of any wastes recycled to the primary
clariier
Key factors in primary
•
clarifier operation include:
Volume (gal)24hr/day
F
×
low(gpd)
RetentionTime(hr)
=
q
(gpd)
Surface Area (ft )
Surface LoadingRate(gpd/ft
2
=
2
Solids into Clarifier(lb/da
y)
2
=
Solids LoadingRate
(lb/day/ft )
2
Surface Area (ft )
(gpd)
Weir Length (linear ft
q
Weir Overflow Rate (gp
d/linear ft)
=
)
