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Activated Sludge Systems
This approach was first developed in Manchester, just prior to the outbreak of the First World War, to deal with the stronger effluents which were being produced in increasingly large amount by the newly emerging chemicals industry and were proving too toxic for the currently available methods of biological processing. Treatment is again achieved by the action of aerobic microbes, but in this method, they form a functional community held in suspension within the effluent itself and are provided with an enhanced supply of oxygen by an integral aeration system. This is a highly biomass-intensive approach and consequently requires less space than filter to achieve the same treatment. The main features are shown in Figure 6.4.
The activated sludge process has a higher efficiency than the previously described filter system and is better able to adapt to deal with variability in the wastewater input, both in terms of quantity and concentration. However, very great changes in effluent character will challenge it, since the resident microbial community is generally less heterogeneous than commonly found in filters. Additionally, as a more complex system, initial installation costs are higher and it requires greater maintenance and more energy than a trickling filter of comparable throughput.
In use, the sludge tanks form the central part of a three-part system, comprising a settlement tank, the actively aerated sludge vessels themselves and a final
clarifier for secondary sedimentation. The first element of the set-up allows heavy particles to settle at the bottom for removal, while internal baffles or a specifically designed dip pipe off-take excludes floating materials, oil, grease and surfactants.
After this physical pretreatment phase, the wastewater flows into, and then slowly through, the activated sludge tanks, where air is introduced, providing the enhanced dissolved oxygen levels necessary to support the elevated micro-bial biomass present. These micro-organisms represent a complex and integrated 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 suspen-sion. 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 they contain growing bacteria that have developed in the aeration tanks, represent 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 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 to shearing, which in turn favours the floc-forming bacteria. As these thrive and produce quantities of sticky, extracellular slime, larger floc particles are formed, the increasingly irregular shape of which is very apparent on microscopic exam-ination of the activated sludge. Mucus secretions from rotifers, which become more numerous as the sludge ages, also contribute to this overall process. Inter-ruption of this formative succession may occur as a result of high toxicity within the input effluent, the lack of adequate ciliated protozoan activity, excessive inter-tank shearing forces or the presence of significant amounts of surfactant.
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