Biological Filters

Biological filters can also be used for treating meat industry wastes. In this process the aerobic microorganisms grow as a slime or film that is supported on the surface of the filter medium. The wastewater is applied to the surface and trickles down while air percolates upwards through the medium and supplies the oxygen required for purification (Fig. 6). The treated water along with

Clogged Sewer Clean Out
Figure 6 Typical biological filtration treatment system.

any microbial film that breaks away from the support medium collects in an under-drain and passes to a secondary sedimentation tank where the biological solids are separated. Trickling filters require primary treatment for removal of settleable solids and oil and grease to reduce the organic load and prevent the system blocking. Rock or blast furnace slag have traditionally been used as filter media for low-rate and intermediate-rate trickling filters, while high-rate filters tend to use specially fabricated plastic media, either as a loose fill or as a corrugated prefabricated module. The advantage of trickling filters is their low energy requirement, but the disadvantage is the low loading compared to activated sludge, making the plant larger with a consequent higher capital cost. Hydraulic loading rates range from 0.02-0.06 gal/ft2 day (0.001-0.002 m3/ m2 day) for low-rate filters to 0.8-3.2 gal/ft2 day (0.03-0.13 m3/m2 day) for high-rate filters. Organic loading ra2es range from 5-25 lb B0D/103 ft2day to 100-500 lb B0D/103 ft2day (0.020.12 kg/m2 day to 0.49-2.44 kg/m2 day). The overall BOD removal efficiency can be as great as 95%, but this is dependent on the loading applied and the mode of operation. A

typical performance envelope for biological filters operating with a plastic support medium is given in Fig. 7.

Because of the relatively high strength of slaughterhouse wastewater, biological filters are more suited to operation with effluent recirculation, which effectively increases surface hydraulic loading without increasing the organic loading. This gives greater control over microbial film thickness. In the United States, high-rate single-stage percolating filters with high recirculation ratios have been used. An overall BOD removal of 92-98% was reported using a high-rate filter with a BOD loading of 2.6-3.8 lb BOD/103 gal media day (0.31-0.45 kg BOD/ m3 media day) and a recirculation ratio of about 5:1 for treating preliminary treated slaughterhouse wastes [71]. Dart [18] reported that a high-quality effluent with 11 mg/L of BOD and 25 mg/L SS could be obtained using alternating double filtration (ADF) at a loading rate of 2.8 lb/103 gal day (0.34 kg BOD/m3 day) for treating screened and settled abattoir waste; the influent was diluted 1:1 with recirculated effluent. Higher loadings with a BOD of between 17 and 33 lb/103 gal (2-4 kg BOD/m3) and a surface hydraulic loading of 884 gal/ft2 day (1.5 m3/m2 hour) and recirculation ratios of 3-4 are given as a typical French design guideline aimed at providing a roughing treatment in reactors 13.1 ft (4m) high [14]. Such a design is likely to give a BOD removal of less than 75% (Fig. 7) and not to provide any nitrification.

V. BOD REMOVAL

Figure 7 Performance envelope for high rate biological filtration.

V. BOD REMOVAL

Figure 7 Performance envelope for high rate biological filtration.

Figure 8 Schematic for a completely mixed continuous flow activated sludge plant.

Dart [31] summarized the performance of some high-rate filtration plants treating meat industry wastes (Table 11).

Biological filters have not been widely adopted for the treatment of slaughterhouse wastewaters despite the lower operating costs compared with activated sludge systems. Obtaining an effluent with a low BOD and ammonia in a single-reactor system can provide conditions suitable for the proliferation of secondary grazing macro-invertebrate species such as fly larvae, and this may be unacceptable in the vicinity of a slaughterhouse. There is also the need for very good fat removal from the influent wastewater flow, as this will otherwise tend to coat the surface of the biofilm support medium. The use of traditional biological filtration for abattoir wastewater treatment is discussed by Philips [72], and further reviewed by Parker et al. [73].

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