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The treatment of effluents by biological means is of particular importance to any consideration of environmental biotechnology, since it represents the central point of the previously mentioned intervention triangle, having simultaneous relevance to manufacturing, waste management and pollution control.
A large number of industrial or commercial activities produce wastewaters or effluents which contain biodegradable contaminants and typically these are discharged to sewers. • The character of these effluents varies greatly, dependent on the nature of the specific industry involved, both in terms of the likely BOD loading of any organic components and the type of additional contaminants which may also be present.
Accordingly, the chemical industry may offer wastewaters with high COD and rich in various toxic compounds, while tannery water provides high BOD with a chromium component and the textile sector is another high BOD effluent producer, with the addition of surfactants, pesticides and dyes.
The direct human biological contribution to wastewater loading is relatively light. • Of course, the actual effluent arriving at a sewage works for treatment contains the nitrogen, phosphorus and other components.
Sewage Treatment • The aims of treatment can be summarised as the reduction of the total biodegradable material present, the removal of any co-existing toxic substances and the removal and/or destruction of pathogens.
The typical sewage treatment sequence normally begins with preliminary screening, with mechanical grids to exclude large material which has been carried along with the flow.
Primary treatment involves the removal of fine solids by means of settlement and sedimentation, the aim being to remove as much of the suspended organic solid content as possible from the water itself and up to a 50% reduction in solid loading is commonly achieved. • At various times, and in many parts of the world, discharge of primary effluent direct to the sea has been permissible, but increasing environmental legislation means that this has now become an increasingly rare option.
Throughout the whole procedure of sewage treatment, the effective reduction of nitrogen and phosphorus levels is a major concern, since these nutrients may, in high concentration, lead to eutrophication of the waterways. • Primary stages have a removal efficiency of between 5–15% in respect of these nutrients.
Secondary treatment phase, This contains the main biological aspect of the regime and involves the two essentially linked steps of initial bioprocessing and the subsequent removal of solids resulting from this enhanced biotic activity. • Aerobic bacteria are encouraged, thriving in the optimised conditions provided, leading to the BOD, nitrogen and ammonia levels within the effluent being significantly reduced. • Achieves nutrient reductions of between 30-50%.
In some cases, tertiary treatment is required as an advanced final polishing stage to remove trace organics or to disinfect effluent. • Tertiary treatment can add significantly to the cost of sewage management, not least because it may involve the use of further sedimentation lagoons or additional processes like filtration, microfiltration, reverse osmosis and the chemical precipitation of specific substances.
Process Issues • At the end of the process, the water itself may be suitable for release but, commonly, there can be difficulty in finding suitable outlets for the concentrated sewage sludge produced. • Spreading this to land has been one solution which has been successfully applied in some areas, as a useful fertiliser substitute on agricultural or amenity land. • Anaerobic digestion, which is described more fully in the context of waste management in Chapter 8, has also been used as a means of sludge treatment.
Sludge is readily biodegradable under this regime and generates sizeable quantities of methane gas, which can be burnt to provide onsite electricity. • Water resources are coming under increasing pressure. • This clearly makes the efficient recycling of water from municipal works of considerable importance to both business and domestic users. • The biological players and processes involved are little modified from what would be found in nature in any aquatic system which had become effectively overloaded with biodegradable material.
In this way, a microcosmic ecological succession is established. • Hence, heterotrophic bacteria metabolise the organic inclusions within the wastewater; carbon dioxide, ammonia and water being the main byproducts of this activity. • Inevitably, increased demand leads to an operational decrease in dissolved oxygen availability, which would lead to the establishment of functionally anaerobic conditions in the absence of external artificial aeration, hence the design of typical secondary treatments.
Land Spread • Treatment by land spread may be defined as the controlled application of sewage to the ground to bring about the required level of processing through the physico-chemical and biological mechanisms within the soil matrix. In most applications of this kind, green plants also play a significant role in the overall treatment process. • Inherent abilities of certain kinds of soil microbes to remediate a wide range of contaminants, either in an unmodified form, or with optimisation, enhancement or bioaugmentation.
The primary mechanisms for pollution abatement are physical filtration, chemical precipitation and microbiological metabolism. • The activity is typically concentrated in the upper few centimetres of soil, where the individual numbers of indigenous bacteria and other micro-organisms are huge and the microbial biodiversity is also enormous.
With so high a resident microbial biomass, unsurprisingly the availability of oxygen within the soil is a critical factor in the efficiency of treatment, affecting both the rate of degradation and the nature of the end-products thus derived. • Oxygen availability is a function of soil porosity and oxygen diffusion can consequently be a rate-limiting step under certain circumstances. • In general, soils which permit the fast influx of wastewater are also ideal for oxygen transfer.
In land that has vegetation cover, even if its presence is incidental to the treatment process, most of the activity takes place within the root zone. Some plants have the ability to pass oxygen derived during photosynthesis directly into this region of the substrate. This capacity to behave as a biological aeration pump. • In this respect, the plants themselves are not directly bioremediating the input effluent, but acting to bioenhance conditions for the microbes which do bring about the desired treatment.
Septic Tank • For homeowners in rural areas (or places with no connection to main sewage pipes), septic tanks are the principal means of waste water disposal. makes use of an intermediate form of land treatment. • Underground tank, collects and stores all the sewage arising from the household. At regular intervals, often around once a month dependent on the capacity, it requires emptying and tankering away, typically for spreading onto, or injection into, agricultural land.
By contrast, a septic tank is a less passive system, settling and partially digesting the input sewage, although even with a properly sized and well-managed regime the effluent produced still contains about 70% of the original nutrient input. • Since a system that is poorly designed, badly installed, poorly managed or improperly sited can cause a wide range of environmental problems, most especially the pollution of both surface and groundwaters, their use requires great care.
Limits to land application • Efficacy of the approach for human sewage and animal manures, its application to other effluents is less well indicated and the only truly ‘industrial’ wastewaters routinely applied to the land in any significant proportion. • A significant proportion of the water is used for washing purposes and thus the industry as a whole produces relatively large volumes of effluent, which though not generally dangerous to human health or the environment, is heavily loaded with organic matter.
The alternative options to land spreading involve either dedicated on-site treatment or export to an existing local sewage treatment works for coprocessing with domestic wastewater. • The choice between them is, of course, largely dictated by commercial concerns though the decision to install an on-site facility, tanker away to another plant or land spread, is not based on economic factors only.Regional agricultural practice also plays an important part.
Food and beverage industry, heavy potassium load. (for microbial metabolism and plant uptake) which obviously lends itself to rapid utilisation and in addition, the majority of effluents from this sector are comparatively low in heavy metals. • High levels of organic matter and nitrogen and, consequently, a low C/N ratio, which ensures that they are broken down very rapidly by soil bacteria under even moderately optimised conditions • Heavy sodium and chloride loadings originating from the types of cleaning agents commonly used.
The land application of such liquors requires care since too heavy a dose may lead to damage to the soil structure and an alteration of the osmotic balance. • Long-term accumulation of these salts within the soil produces a gradual reduction of fertility and ultimately may prove toxic to plants. • low carbon to nitrogen ratio tends to make these effluents extremely malodorous.
Nitrogenous Wastes • For those effluents, however, which are consigned to land treatment regimes, the fate of nitrogen is of considerable importance. In aerobic conditions, the biological nitrification processes within the soil produce nitrate from ammonia and organic nitrogen, principally by the chemotrophic bacteria, Nitrosomonas and Nitrobacter, which respectively derive first nitrites and then finally nitrates.
However, in anoxic conditions nitrate compounds can be reduced to nitrogen gas as a result of the activities of various species of facultative and anaerobic soil bacteria, in which the nitrate ion acts as an alternative electron acceptor to oxygen in respiration, as mentioned in Chapter 2. • Nitrogen losses via denitrification and plant uptake as control mechanisms for the nitrogenous component in wastewaters in land applications.
Approximately 20–30% of the applied nitrogen is lost in this way, a figure which may rise to as much as 50% under some circumstances,as factors such as high organic content, fine soil particles and water-logging all provide favourable conditions for denitrification within a soil.
Aeration • 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.
The systems used fall into one of two broad categories, on the basis of their operating criteria: •Diffused air systems. • Mechanical aeration.
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. • 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.
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.
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 Figure 6.3 Trickling filter
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. • 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.
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 should be durable and long lasting, resistant to compaction or crushing in use and resistant to frost damage such as clinker, blast-furnace slag, gravel, crushed rock and artificial plastic lattice material. • 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. • To maximise the treatment efficiency, it is clearly essential that the trickling filter is properly sized and matched to the required processing demands.
Activated Sludge Systems • 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. • 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. • Initial installation costs are higher and it requires greater maintenance and more energy than a trickling filter.
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. 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.
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. • Typically, collector arms rotate around the bottom of the tank to collect and remove the settled biomass solids. • 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.
