MIL-HDBK-1005/16
system and the number of hours per day scheduled for solids
wasting. Because of the effect of suspended solids on the
screen, the hydraulic capacity when screening primary solids is
estimated to be only about 10 percent of the hydraulic capacity
when screening raw wastewater. Typical screen opening size
varies between 0.1 and 0.25 inches (0.25 and 0.6 cm). Openings
below 0.1 inch (0.25 cm) will tend to bind with grease. The
presence of grease and scum make it essential that the screen
contain a positive cleaning mechanism. Fine screens typically
require a high-pressure (1,400 psi [9,653 kPa]) hot water or
steam wash at least once per day.
10.8
Solids Degritting. In small plants, separate grit-
removal facilities are frequently not provided at the head of the
plant, so grit settles in the primary clarifiers. In this case,
the primary solids are often degritted to protect downstream
processes such as pumps and digesters. Grit carryover to
secondary settling basins is generally not a problem that
warrants grit removal from secondary solids.
The use of hydrocyclones is the most practical method
of degritting solids. The solids stream is applied tangentially
to the cylindrical portion of the unit, which imparts a
centrifugal force. The heavier grit particles move to the
outside of the cylinder section and are discharged through a
conical section, while the organic solids stream is discharged
through a separate outlet. The efficiency of grit removal is a
function of the solids concentration of the influent solids.
Solids should be pumped to the hydrocyclone at no more than
1 percent solids concentration for effective grit removal.
10.9
Solids Blending. In small treatment plants of about
1 mgd [4 ML/d] or less, secondary waste activated sludge is
commonly wasted to the primary settling tanks, where it settles
and mixes with primary solids. If digestion is provided at the
plant, primary and secondary solids can also be fed separately to
the digester because the detention time in a digester is long
enough that a uniform feed mixture of primary and secondary
solids concentration is not required for process stability.
If a uniform blend of primary and secondary solids is
required upstream of a stabilization or dewatering process, a
separate solids blending tank should be provided. The detention
time of the tank will vary from a few hours to a few days,
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