Environmental Engineering Reference
In-Depth Information
The decanting stage most commonly involves the slow lowering of a scoop or “trough” into the
basin. This has a piped connection to a lagoon where the final effluent is stored for disposal to a
wetland, tree growing lot, ocean outfall, or to be further treated for use on parks, golf courses, etc.
The operating principles of a batch activated sludge process, or SBR, are characterized in six
discrete periods:
1. Filling
2. Reacting
3. Settling
4. Decanting
5. Idling
6. Sludge wasting
32.4.3.1 Fill
During this stage the SBR tank is filled with the influent wastewater. To maintain suitable F/M (food
to microorganism) ratios, the wastewater should be admitted into the tank in a rapid, controlled
manner. This method functions similarly to a selector that encourages the growth of certain
microorganisms with better settling characteristics.
32.4.3.2 react
This stage involves the utilization of biochemical oxygen demand (BOD) and ammonia nitrogen,
where applicable, by microorganisms. The length of the aeration period and the sludge mass
determines the degree of treatment. The length of the aeration period depends on the strength of the
wastewater and the degree of nitrification (conversion of the ammonia to a less toxic form of nitrate
or nitrite) provided for in the treatment.
32.4.3.3 settle
During this stage, aeration is stopped and the sludge settles, leaving clear, treated effluent above the
sludge blanket. Duration of settling varies from 5 to 60 minutes depending on the number of cycles
per day and sludge settling characteristics.
32.4.3.4 decant
At this stage of the process, effluent is removed from the tank through the decanter without
disturbing the settled sludge.
32.4.3.5 Idle
The SBR tank waits idle until it is time to commence a new cycle with the filling stage.
32.4.3.6 sludge Wasting
Excess activated sludge is wasted periodically during the SBR operation. As with any activated sludge
treatment process, sludge wasting is the main control of the effluent quality and microorganism
population size. This is how the operator exerts control over the effluent quality by adjusting the
mixed liquor suspended solids (MLSS) concentration and the mean cell residence time (MCRT).
32.5 munIcIPal sludGe treatment
Sludge (solids) stabilization processes are very important for reliable performance of any wastewater
treatment plant. For successful operation of solids treatment facilities, the sources, characteristics,
and quantities of the treated solids have to be carefully evaluated, for the design of these facilities to
offer optimum performance for minimum investment and operational costs.
