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.
The filter is designed to permit good drainage and ventilation and in addition sedimentation and settling tanks are generally associated with the system. Efflu-ent, 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 intermit-tently, 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 micro-organisms 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 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 signifi-cance 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, 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 increas-ingly inhibited.
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.
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