The term filter in this unit operation is a misnomer, as the action of a trickling filter is not one of filtration, but rather it is a fixed film bio-reactor. Settled effluent to be treated is passed down through a packed bed counter-current to a flow of air. Micro-organisms ad hering to the packing matrix adsorb oxygen from the upflowing air and organic matter from the downflowing effluent; the latter is then metabolized and the effluent stream's BOD reduced.
A conventional trickling filter (Fig. 11.2) usually consists of a cylindrical concrete tank 2 to 3 m in depth and 8 to 16 m in diameter. Some filters are rectangular in shape, but a rotary system allows more uniform hydraulic loading (Bruce and Hawkes, 1983; Viessman and Hammer, 1993). The tank is packed with a bed of stone (usually granite) or special plastic packings, the bed being underlaid with drains. The packing material diameter should be 50 to 100 mm to give a specific surface of around 100 m2 m 3, and the material should be packed to give a voidage (% air space of total bed volume) of 45 to 55%, which should minimize the risk of the spaces between the packing material becoming blocked by the microbial film. Synthetic packing material, although more expensive, has a higher surface area and voidage, allowing higher treatment rates per unit volume of bed and reducing the likelihood of blockages. The trickling filter is always followed by a secondary sedimentation tank or humus tank to remove suspended matter (e.g. biofilm sloughed off the packing) from the treated effluent. In conventionally loaded or low-rate filters, the effluent from which the suspended solids have been removed, is fed on to the upper surface of the bed by spray nozzles or mechanical distributor arms (McKinney, 1962; Higgins, 1968). The effluent trickles gradually through the bed and a slime layer of biologically active material (bacteria, fungi, algae, protozoa and nematodes) forms on the surface of the support material. The large surface area created in the bed permits close contact between air flowing upwards through the bed, the descending effluent and the biologically active growth. The bacteria in the biological film remove the majority of the organic loading. Complex organic materials are broken down and utilized, nitrogenous matter and ammonia are oxidized to nitrates and sulphides and other compounds are similarly oxidized. The higher organisms (protozoa etc.) control the accumulation of the biological film (prevents the filter from blocking) and improve the settling characteristics of the solids (humus) discharged with the filter effluent. In low-rate filters the scouring action of the hydraulic load normally has a minor role in removal of any loose microbial film. The active slime takes time to develop and can be poisoned by the addition of toxic chemicals. The simple filter is inefficient when operated at abnormally high organic loading rates. Initially, there is a very rapid build up of
bacteria, fungi and algae at the top of the filter which cannot be controlled by the resident population of worms and larvae. The voids, therefore, block up, resulting in ponding (untreated effluent accumulating on the surface of the filter bed). Film growth can be limited by reducing the dosing frequency, which gives better liquid distribution deeper into the bed.
A trickling filter bed should remove 75 to 95% of the BOD and 90 to 95% of the suspended solids at organic loading rates of 0.06-0.12 kg BOD m 3 day"1 for conventional trickling filtration. When part of the treated effluent is being recirculated to dilute the feed and increase the hydraulic load placed on the unit the organic loading can be increased to 0.9-0.15 kg BOD m 3 day-1. The increase in hydraulic load thus applied causes greater hydraulic scouring of the bed (preventing blockage), but does not reduce treatment efficiency due to improved wetting of the packing surface and, thus, more efficient use of the biofilm (Forster, 1985). To achieve the Royal Commission (20:30) Standard together with a high degree of nitrification, filters being supplied with domestic sewage should receive organic loading rates of 0.07-0.1 kg BOD m 3 day"1 and hydraulic loading rates of 0.12-0.6 m3 m 3 clay 1 (Gray, 1989).
It is possible to modify the trickling filter to increase the capacity for organic loading by the use of two sets of filters and settling tanks in series; this is known as alternating double filtration (ADF). Effluent is applied to the first filter at a high hydraulic and organic loading rate, it passes from this filter through the first settling tank and then on to a second filter and settler. After a period of one to two weeks the sequence of the filters is reversed and the second filter receives the higher loading. In this way heavy film growth is promoted in the first filter to receive the effluent, but when the filter sequence is reversed it becomes nutrient limited, encouraging excess film removal. Loading rates of 0.32-0.47 kg BOD m 3 day"1 have been claimed (Forster, 1977), but recommended rates for design purposes are 0.15-0.26 kg BOD m 3 day"1 (Forster, 1985).
Alternatively, enclosed deep beds of 3.5 to 5.5-m depth may be used in which air is blown through the beds by fans. Loading rates up to 12 times that of the ordinary filter have been claimed (Abson and Tod-hunter, 1967).
Cook (1978) has stressed the need to consider the possible intermittency of a factory wastewater treatment process. It was shown that starving of a laboratory-scale trickling filter beyond 48 hours resulted in near failure of the filter. This indicated the need to supplement or artificially load the filter to maintain a viable biomass in the system.
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