Biomedical Engineering Reference
In-Depth Information
community, with bacteria feeding on the organic content in the effluent, which
are themselves consumed by various forms of attached, crawling and free
swimming protozoa, with rotifers also aiding proper floc formation by removing
dispersed biomass and the smaller particles which form. The action of aeration
also creates a circulation current within the liquid which helps to mix the
contents of the tank and homogenise the effluent while also keeping the whole
sludge in active suspension. Sludge tanks are often arranged in batteries, so that
the part-treated effluent travels though a number of aeration zones, becoming
progressively cleaned as it goes.
At the end of the central activated phase, the wastewater, which contains
a sizeable sludge component by this stage, leaves these tanks and enters the
clarifiers. These are often designed so that the effluent enters at their centre and
flows out over a series of weirs along the edge of the clarifier. As the wastewater
travels outward, the heavier biological mass sinks to the bottom of the clarifier.
Typically, collector arms rotate around the bottom of the tank to collect and
remove the settled biomass solids which, since it contains growing bacteria that
have developed in the aeration tanks, represents a potentially valuable reservoir
of process-acclimatised organisms.
Accordingly, some of this collected biomass, termed the Return Activated
Sludge (RAS), is returned to the beginning of the aeration phase to inoculate
the new input effluent. This brings significant benefits to the speed of processing
achieved since otherwise the wastewater would require a longer residence time
in which to develop the necessary bacteria and other microbes. It also helps to
maintain the high active biomass density which is a fundamental characteristic of
this system. The remaining excess sludge is removed for disposal and the clean
water flows over another final weir system for discharge, or for tertiary treatment
if required.
A similar treatment method sometimes encountered is called aerobic digestion
which uses identical vessels to the aeration tanks described, the difference being
operational. This involves a batch process approach with a retention period of 30
days or more and since they are not continuously fed, there is no flow through
of liquor within or between digesters. Under these conditions, the bacteria grow
rapidly to maturity, but having exhausted the available nutrients, then die off
leaving a residue of dead microbial biomass, rather than an activated sludge as
before. At the end of the cycle, the contents of the aerobic digesters are transferred
to gravity thickeners, which function in much the same way as the secondary
clarifiers previously described. The settled solids are returned to the aerobic
digester not as an innoculant but as a food-source for the next generation, while
the clear liquid travels over a separating weir and is returned to the general
treatment process.
In effect, then, the 'activated sludge' is a mixture of various micro-organisms,
including bacteria, protozoa, rotifers and higher invertebrates forms, and it is by
the combined actions of these organisms that the biodegradable material in the
incoming effluent is treated. Thus, it should be obvious that to achieve process
control, it is important to control the growth of these microbes, which therefore
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