THE CANCER OF ACTIVATED SLUDGE PLANTS
Publication of IWWA Hoosier
Pipeline Magazine, Summer 1999
Copy Right© By Ann Bersbach
Wastewater operators often talk about Nocardia and the challenges they face while
trying to control the bacteria. Nocardia are short filaments encountered in activated
sludge plants. The bacteria form branches that are web like in appearance. They start
growing inside the floc particles and as they increase in number, foam and thick scum appear
in the aeration tanks. Floating scum will also form in the secondary clarifiers. Like Cancer
that should be removed radically, sludge containing Nocardia should be removed from the process.
This is accomplished by increasing the wasting rate until conditions once again become stable.
Since wasting sludge is a gradual procedure, recovery from Nocardia can be a lengthy process.
The best way to manage Nocardia is to avoid the conditions that trigger and enhance its growth.
Nocardia often begins to grow when the Food to Microorganisms ratio (F/M) suddenly changes.
Following are three examples of situations that caused the F/M ratio to suddenly change causing
the growth of Nocardia:
Nocardia formed after a city ordinance prohibited septic tankers from discharging into
a sewage plant. In this situation, the F/M ratio suddenly changed when the rich food
source from the septic waste was removed from the wastewater flowing to the aeration tanks.
The operator struggled with the problem for six months before conditions at the plant became
Widespread Nocardia growth was noticed in a plant every fall after the routine cleaning of
sewers that discharged to the plant. This Nocardia growth occurred because the plant received
much higher BOD (food) loads immediately following the cleaning activities than during normal
operation causing the F/M ratio to change suddenly.
Excess sludge was wasted from a plant causing the microorganisms count to drop suddenly.
This sudden drop in the microorganisms count caused the F/M ratio to suddenly change.
Nocardia formed at the plant shortly after the wasting was complete.
As shown in the above examples, the F/M ratio is an important control parameter in the activated
sludge process. The F/M ratio varies from plant to plant and from season to season. The best
way to find the F/M ratio for a plant is to calculate it during optimum operating conditions.
The food (F) is the pounds of BOD per day and the Microorganisms (M) are represented by pounds
of Mixed Liquor Volatile Suspended Solids, (MLVSS) in the aeration tanks. To prevent Nocardia growth,
conditions at the plant should be kept stable and consistent. Any changes to these operating parameters
should be introduced gradually to allow the process to adjust and compensate for the change.
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