Environmental Engineering Reference
In-Depth Information
used to increase the solids content of the sludge by removing a portion
of the liquid fraction. By increasing the solids content, more economical
treatment of the sludge can be effected.
10.5.1 sludge Thickening Processes
Sludge thickening processes include gravity thickeners, flotation
thickeners, and solids concentrators.
10.5.1.1 Gravity Thickening
gravity thickening
is most effective on primary sludge. In operation,
solids are withdrawn from primary treatment (and sometimes second-
ary treatment) and pumped to the thickener. The solids buildup in the
thickener forms a solids blanket on the bottom. The weight of the blan-
ket compresses the solids on the bottom and squeezes the water out.
By adjusting the blanket thickness, the percent solids in the underflow
(solids withdrawn from the bottom of the thickener) can be increased
or decreased. The supernatant (clear water) that rises to the surface is
returned to the wastewater flow for treatment. Daily operations of the
thickening process include pumping, observation, sampling and test-
ing, process control calculations, maintenance, and housekeeping.
Note:
The equipment employed in thickening depends on the specific
thickening processes used.
Equipment used for gravity thickening consists of a thickening tank
that is similar in design to the settling tank used in primary treatment.
Generally, the tank is circular and provides equipment for continuous
solids collection. The collector mechanism uses heavier construction
than that in a settling tank because the solids being moved are more
concentrated. The gravity thickener pumping facilities (i.e., pump and
flow measurement) are used for withdrawal of thickened solids.
The solids concentrations achieved by gravity thickeners are typi-
cally 8 to 10% solids from primary underflow, 2 to 4% solids from waste
activated sludge, 7 to 9% solids from trickling-filter residuals, and 4 to
9% from combined primary and secondary residuals. The performance
of gravity thickening processes depends on various factors, including:
•
Type of sludge
•
Condition of influent sludge
•
• Blanket depth
Solids loading
Temperature
•
• Hydraulic loading
• Solids retention time
Hydraulic detention time
•
