Environmental Engineering Reference
In-Depth Information
Aeration and mixing
Pretreated
influent
Effluent
Clarifier
Solids
recycle
Wasted sludge
Recycle sludge line
Figure 6.2
Activated sludge process
recirculation. In a complete mix reactor, the influent is dispersed immediately
upon introduction, so there is no concentration gradient in the tank. The reactor
contents are completely mixed, and it is assumed that there are no microorgan-
isms in the influent. Uniform concentration of the contaminants is maintained
throughout the reactor. In a plug-flow reactor, there is no longitudinal mixing,
and all particles entering the reactor stay in the reactor an equal amount of time.
Some particles may make more passes through the reactor because of recycle
but while they are in the tank, they all pass in the same amount of time.
Solid retention time (SRT) is the average time that the microorganisms spend
in the reactor. It is the ratio of mass of organisms in the reactor to the mass of
organisms removed from the reactor. In the design of activated sludge systems,
SRT is one of the most critical design parameter (Metcalf and Eddy 2002). An
increase in SRT will increase concentration of biomass in the reactor. If SRT is
too short, then the system is removing too many microorganisms and biomass
will wash out. SRT can be controlled by wasting more or less sludge from the
recycle line.
Hydraulic retention time (HRT) is the average amount of time that the wastew-
ater stays in the reactor. HRT is determined by dividing the reactor volume by
the wastewater flow rate. SRT is greater than HRT, and it is independent of HRT.
In successful operations, a minimum HRT must be met before SRT becomes a
controlling factor (Metcalf and Eddy 2002).
Food to microorganism ratio shows the organic load entering the reactor.
It is used as one of the operational design parameters. The ratio is calculated
by dividing the BOD load with the bacterial mass or biomass load expressed
as mixed liquor suspended solid or mixed liquor volatile suspended solids in
the reactor. It keeps the nutrients balance in the reactor and controls the over-
growth of filamentous microorganisms to avoid unsettling sludge in the secondary
clarifier.
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