Aerobes and Effluents 131input 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 inoculant 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 invertebrate 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
makes some understanding of the microbiology of activated sludge essential.
Bacteria account for around 95% of the microbial mass in activated sludge and
most of the dispersed growth suspended in the effluent is bacterial, though ideally
there should not be much of this present in a properly operating activated sludge
process. Generally speaking this tends only to feature in young sludges, typically
less than 3 or 4 days old, and only before proper flocculation has begun. Ciliates
are responsible for much of the removal of dispersed growth and adsorption onto
the surface of the floc particles themselves also plays a part in its reduction.
Significant amounts of dispersed growth characterises the start-up phase, when
high nutrient levels are present and the bacterial population is actively growing.
However, the presence of excessive dispersed growth in an older sludge can
often indicate that the process of proper floc formation has been interrupted in
some way. When floc particles first develop they tend to be small and spherical,
largely since young sludges do not contain significant numbers of filamentous
organisms and those which are present are not sufficiently elongated to aid in the
formation process. Thus, the floc-forming bacteria can only flocculate with each
other in order to withstand shearing action, hence the typical globular shape. As
the sludge ages, the filamentous microbes begin to elongate, their numbers rise
and bacterial flocculation occurs along their length, providing greater resistance