Environmental Engineering Reference
In-Depth Information
the desired blanket thickness. Sampling and analysis of influent sludge,
supernatant, and thickened sludge are also required. Sludge addition
and withdrawal should be continuous if possible to achieve optimum
performance. Mechanical maintenance is also required. Expected per-
formance ranges for gravity and dissolved air flotation thickeners are:
• Primary sludge, 8-19% solids
• Waste activated sludge, 2-4% solids
• Trickling-ilter sludge, 7-9% solids
•
Combined sludges, 4-9% solids
Typical operational problems with sludge thickeners include odors,
rising sludge, thickened sludge below desired solids concentration,
dissolved air concentration that is too low, effluent low containing
excessive solids, and torque alarm conditions.
10.5.3 symptoms, Causes, and Corrective actions
for solids Concentrations Problems
10.5.3.1 Gravity Thickener
•
Odors and rising sludge are present.
Causal factors:
Sludge withdrawal rate is too low; overflow rate is
too low; thickener is septic.
Corrective actions:
Increase sludge withdrawal rate; increase
influent flow rate; add chlorine, permanganate, or peroxide to
influent.
symptom 1.
•
symptom 2.
The thickened sludge is below the desired solids
concentration.
Causal factors:
Overflow rate is too high; sludge withdrawal rate is
too high; short-circuiting is occurring.
Corrective actions:
Decrease influent sludge flow rate; decrease
pump rate for sludge withdrawal; identify cause and correct.
•
symptom 3.
The torque alarm is activated.
Causal factors:
There is a heavy sludge accumulation; collector
mechanism is jammed.
Corrective actions:
Agitate sludge blanket to decrease density;
increase sludge withdrawal rate; attempt to locate and remove
obstacle; dewater tank and remove obstacle.
10.5.3.2 Dissolved Air Flotation Thickener
•
symptom 1.
The float solids concentration is too low.
Causal factors:
Skimmer speed is too high; unit is overloaded; poly-
mer dose is insufficient; air-to-solids ratio is excessive; dissolved
air levels are low.
