Environmental Biotechnology - Theory and Application - G. M. Evans & J. C. Furlong

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as factors such as high organic content, fine soil particles and water-logging all provide favourable conditions for denitrification within a soil.

Though amelioration processes involving land spreading or injection clearly have beneficial uses for some kinds of wastewaters, in general effluents, particularly those of industrial origin, require more intensive and engineered solutions. In this respect, whether the liquors are treated on-site by the producers themselves, or are tankered to external works is of little significance, since the techniques involved will be much the same irrespective of where they are applied. The contribution of environmental biotechnologies to the safe management of effluents principally centres on microbial action, either in anaerobic digestion where the carbon element is fully reduced, or in aerobic processes which lead to its oxidation. As has been mentioned earlier, the former is covered elsewhere in this book; the rest of this chapter will largely address the latter.

Aeration

Introducing air into liquid wastes is a well-established technique to reduce pollutant potential and is often employed as an on-site method to achieve discharge consent levels, or reduce treatment costs, in a variety of industrial settings. It works by stimulating resident biomass with an adequate supply of oxygen, while keeping suspended solids in suspension and helping to mix the effluent to optimise treatment conditions, which also assists in removing the carbon dioxide produced by microbial activity. In addition, aeration can have a flocculant effect, the extent of which depends on the nature of the effluent. The systems used fall into one of two broad categories, on the basis of their operating criteria:

•Diffused air systems.

•Mechanical aeration.

This classification is a useful way to consider the methods in common use, though it takes account of neither the rate of oxygen transfer, nor the total dissolved oxygen content, which is occasionally used as an alternative way to define aeration approaches.

Diffused air systems

The liquid is contained within a vessel of suitable volume, with air being introduced at the bottom, oxygen diffusing out from the bubbles as they rise, thus aerating the effluent.

These systems can be categorised on the basis of their bubble size, with the crudest being coarse open-ended pipes and the most sophisticated being specialised fine diffusers. Ultra-fine bubble (UFB) systems maximise the oxygen transfer effect, producing a dense curtain of very small bubbles, which consequently have a large surface area to volume ratio to maximise the diffusion.

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Table 6.3 Horticultural waste process liquor analysis before and after 85-day aeration treatment and the associated percentage reductions achieved

Determinant	Baseline	Post-treatment	% reduction
pH	5.8	8.8	–
Conductivity @20 ◦ C	6 950	6 320	9.1
BOD total + ATU	15 800	198	98.7
COD	27 200	1 990	92.7
Solids particulate 105 ◦ C	6 200	28	99.5
Total dissolved solids	13 700	293	97.9
Ammoniacal nitrogen	515	316	38.6
Total oxidised nitrogen	1.7	0.3	82.4
Kjeldahl nitrogen	926	435	53.0
Nitrite	0.79	0.04	94.9
Nitrate	0.9	0.3	66.7
Sulphate	194	63.4	67.3

All in mg/l except pH (in pH units) and conductivity (in mS/cm). Results courtesy of Rob Heap, unpublished project report.

The UFB system is the most expensive, both to install in the first place and subsequently to run, as it requires comparatively high maintenance and needs a filtered air supply to avoid air-borne particulates blocking the narrow diffuser pores. Illustrative UFB aeration results, based on operational data, obtained from the amelioration of post-anaerobic digestion liquor from a horticultural waste processing plant, are shown in Table 6.3.

Though the comparatively simple approaches which produce large to medium sized bubbles are the least efficient, they are commonly encountered in use since they offer a relatively inexpensive solution.

Mechanical aeration systems

In this method, a partly submerged mechanically driven paddle mounted on floats or attached to a gantry vigorously agitates the liquid, drawing air in from the surface and the effluent is aerated as the bubbles swirl in the vortex created.

Other variants on this theme are brush aerators, which are commonly used to provide both aeration and mixing in the sewage industry and submerged turbine spargers, which introduce air beneath an impeller, which again mixes as it aerates. This latter approach, shown in Figure 6.2, can be considered as a hybrid between mechanical and diffused systems and though, obviously, represents a higher capital cost, it provides great operational efficiency. A major factor in this is that the impeller establishes internal currents within the tank. As a result the bubbles injected at the bottom, instead of travelling straight up, follow a typically spiral path, which increases their mean transit time through the body of the liquid and hence, since their residence period is lengthened, the overall efficacy of oxygen diffusion increases.

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Figure 6.2 Turbine sparger aeration system

Table 6.4 Illustrative oxygen transfer rates for aeration systems at 20 ◦ C

System	Transfer rate
	(kg O2/kWh)
Diffused air
Coarse bubble	0.6–1.2
Medium bubble	1.0–1.6
Fine bubble	1.2–2.0
Brush aerator	1.2–2.4
Turbine sparger Aerator	1.2–2.4

The design of the system and the processing vessel is crucial to avoid problems of oxygen transfer, liquid stratification and foaming, all of which can be major problems in operation. The time taken to effect treatment depends on the regime used and the nature of the effluent. In this context, Table 6.4 shows typical oxygen transfer rates for aeration systems at 20 ◦C.

The value of aeration in the treatment process is not restricted to promoting the biological degradation of organic matter, since the addition of oxygen also plays an important role in removing a number of substances by promoting direct chemical oxidation. This latter route can often help eliminate organic compounds which are resistant to straightforward biological treatments.

Trickling Filters

The trickling or biological filter system involves a bed, which is formed by a layer of filter medium held within a containing tank or vessel, often cast from concrete, and equipped with a rotating dosing device, as shown in a stylised form in Figure 6.3.

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Figure 6.3 Trickling filter

The filter is designed to permit good drainage and ventilation and in addition sedimentation and settling tanks are generally associated with the system. Effluent, which has been mechanically cleaned to remove the large particles which might otherwise clog the interparticulate spaces in the filter bed, flows, or is pumped, into the rotating spreader, from which it is uniformly distributed across the filter bed. This dosing process can take place either continuously or intermittently, depending on the operational requirements of the treatment works. The wastewater percolates down through the filter, picking up oxygen as it travels over the surface of the filter medium. The aeration can take place naturally by diffusion, or may sometimes be enhanced by the use of active ventilation fans.

The combination of the available nutrients in the effluent and its enhanced oxygenation stimulates microbial growth, and a gelatinous biofilm of microorganisms forms on the filter medium. This biological mass feeds on the organic material in the wastewater converting it to carbon dioxide, water and microbial biomass. Though the resident organisms are in a state of constant growth, ageing and occasional oxygen starvation of those nearest the substrate leads to death of some of the attached growth, which loosens and eventually sloughs, passing out of the filter bed as a biological sludge in the water flow and thence on to the next phase of treatment.

The filter medium itself is of great importance to the success of these systems and in general the requirements of a good material are that it should be durable and long lasting, resistant to compaction or crushing in use and resistant to frost damage. A number of substances have been used for this purpose including clinker, blast-furnace slag, gravel and crushed rock. A wholly artificial plastic lattice material has also been developed which has proved successful in some

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applications, but a clinker and slag mix is generally said to give some of the best results. The ideal filter bed must provide adequate depth to guarantee effluent retention time, since this is critical in allowing it to become sufficiently aerated and to ensure adequate contact between the microbes and the wastewater for the desired level of pollutant removal. It should also have a large surface area for biomass attachment, with generous void spaces between the particles to allow the required biomass growth to take place without any risk of this causing clogging. Finally, it should have the type of surface which encourages splashing on dosing, to entrap air and facilitate oxygenation of the bed.

The trickling filters in use at sewage works are squat, typically around 8 – 10 metres across and between 1 – 2 metres deep; though these are the most familiar form, other filters of comparatively small footprint but 5 to 20 metres in height are used to treat certain kinds of trade effluents, particularly those of a stronger nature and with a more heavy organic load than domestic wastewater. They are of particular relevance in an industrial setting since they can achieve a very high throughput and residence time, while occupying a relatively small base area of land.

To maximise the treatment efficiency, it is clearly essential that the trickling filter is properly sized and matched to the required processing demands. The most important factors in arriving at this are the quality of the effluent itself, its input temperature, the composition of the filter medium, detail of the surface-dosing arrangements and the aeration. The wastewater quality has an obvious significance in this respect, since it is this, combined with the eventual clean-up level required, which effectively defines the performance parameters of the system. Although in an ideal world, the filter would be designed around input character, in cases where industrial effluents are co-treated with domestic wastewater in sewage works, it is the feed rate which is adjusted to provide a dilute liquor of given average strength, since the filters themselves are already in existence. Hence, in practice, the load is often adjusted to the facility, rather than the other way about.

The input temperature has a profound influence on the thermal relations within the filter bed, not least because of the high specific heat capacity of water at 4200 J/kg/ ◦C. This can be of particular relevance in industrial reed bed systems, which are discussed in the following chapter, since a warm liquor can help to overcome the problems of cold weather in temperate climes. By contrast the external air temperature appears to have less importance in this respect. The situation within the reaction space is somewhat complicated by virtue of the nonlinear nature of the effect of temperature on contaminant removal. Although the speed of chemical reactions is well known to double for every 10 ◦C rise, at 20 ◦C, in-filter biodegradation only represents an increase of 38% over the rate at 10 ◦C. Below 10 ◦C, the risk of clogging rises significantly, since the activity of certain key members of the microbial community becomes increasingly inhibited.

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The general properties of the filter media were discussed earlier. In respect of sizing the system, the porosity and intergranular spaces govern the interrelation between relative ease of oxygen ingress, wastewater percolation and nutrient to biofilm contact. Clearly, the rougher, pitted or irregular materials tend to offer the greatest surface area per unit volume for microbial attachment and hence, all other things being equal, it follows that the use of such media allows the overall filter dimensions to be smaller. In practice, however, this is seldom a major deciding factor.

In the main, filter systems use rotational dosing systems to ensure a uniform dispersal of the effluent, though nozzles, sprays and mechanised carts are not unknown. The feed must be matched to the medium if the surface aeration effect is to be optimised, but it must also take account of the fluidity, concentration and quality of the wastewater itself and the character of the resident biofilm.

Since the biological breakdown of effluents within the filter is brought about by aerobic organisms, the effectiveness of aeration is of considerable importance. Often adequate oxygenation is brought about naturally by a combination of the surface effects as the wastewater is delivered to the filter, diffusion from atmosphere through the filter medium and an in-filter photosynthetic contribution from algae. Physical air flow due to natural thermal currents may also enhance the oxygenation as may the use of external fans or pumps which are a feature on some industrial units.

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

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Figure 6.4 Schematic activated sludge system

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 microbial 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 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 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

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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

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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 examination of the activated sludge. Mucus secretions from rotifers, which become more numerous as the sludge ages, also contribute to this overall process. Interruption 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.

Process disruption

Toxicity is a particular worry in the operational plant and can often be assessed by microbiological examination of the sludge. A number of key indicators may be observed which would indicate the presence of toxic components within the system, though inevitably this can often only become apparent after the event. Typically, flagellates will increase in a characteristic ‘bloom’ while higher life forms, particularly ciliates and the rotifers, die off. The particular sensitivity of these microbe species to toxic inputs has been suggested as a potential method of biomonitoring for toxic stress, but the principle has not yet been developed to a point of practical usefulness.

The floc itself begins to break up as dispersed bacterial growth, characteristic of an immature sludge, returns, often accompanied by foaming within the bioreactor, the progressively reducing growth of microbial biomass leading to a lowered oxygen usage and hence to poor BOD removal. If the toxic event is not so severe as to poison the entire system, as new effluent input washes through the tanks, increasingly diluting the concentration of the contaminating substances and the process recovers, excessive filament formation may occur leading to a condition known as ‘filamentous bulking’. As a result, it is sometimes said that toxic inputs favour filamentous bacteria but, with the exception of hydrogen sulphide contamination, this is not strictly true. It is, however, fair to say that the disruption caused by a toxic influx permits their burgeoning growth, particularly since they are generally the fastest group to recover.

By contrast, ‘slime bulking’ can often occur in industrial activated sludge settings, where the effluent may commonly be deficient in a particular nutrient, most typically either nitrogen or phosphorus. This results in altered floc formation, reduced settling properties and, in some cases, the production of the slimy, greyish foam at the surface of the aeration vessel, which gives this event its name. This greasy, extracellular polymer interferes with the normal settling processes, altering the sludge buoyancy by entrapping air and encouraging foaming. The situation can generally be managed simply by adding appropriate quantities of the missing nutrient, though where relatively easily biodegradable soluble BOD is readily available, it may be necessary to deliberately create higher levels of nitrogen and phosphorus within the system than a straightforward analysis might otherwise indicate.

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Foaming can be a significant and unsightly nuisance in operational facilities and, as has been discussed, may occur as a result of either nutrient deficiency or the growth of specific foam-generating filamentous organisms. Microscopic examination of the fresh foam is often the best way to determine which, and thus what remedial action is necessary.

Typical protozoans present in the sludge include amoebae, ciliates and flagellates and, together with rotifers, they play secondary roles in the activated sludge treatment of wastewaters. The presence or absence of particular types can be used as valuable biological indicators of effluent quality or plant performance. In this way, the incidence of large numbers of amoeba often suggests that a shock loading has taken place, making large quantities of food available within the system, or that the dissolved oxygen levels in the tanks have fallen, since they are better able to tolerate conditions of low aeration. A large flagellate population, particularly in mature sludges, suggests the persistence of appreciable quantities of available organic nutrients, since their numbers are usually limited by competition with bacteria for the same dissolved foodstuff. Since ciliates, like rotifers, feed on bacteria, their presence indicates a healthy sludge, as they typically blossom after the floc has been formed and when most of the effluent’s soluble nutrients have been removed. As protozoa are more sensitive to pH than floc-forming bacteria, with a typical optimum range of 7.0 – 7.4 and tolerating 6.0 – 8.0, they can also provide a broad measure of this parameter in the system.

The population of rotifers seldom approaches large numbers in activated sludge processes, though they nevertheless perform an important function. Their principal role is the removal of dispersed bacteria, thus contributing to both the proper development of floc and the reduction of wastewater turbidity. Taking the longest time of all members of the microbial community to become established in the sludge, their presence indicates increasing stabilisation of the organic components of the effluent.

Organic loadings

Calculating the organic loadings for a given activated sludge system is an important aspect of process control. Measuring the BOD of the incoming wastewater gives a value for the amount of biodegradable matter available for microbial use, which can be used together with an estimate of the resident biomass to derive a relationship termed the food to micro-organism (F/M) ratio. This, which is also sometimes known as the organic loading rate, is given as follows:

F/M = mass of BOD applied to the biological phase each day total microbial biomass in the biological phase

The F/M ratio is a useful indication of anticipated micro-organism growth and condition, a high F/M value yielding rapid biomass increase, while a low one suggests little available nutrients and consequently slow growth results. Clearly,