Towers

Because trickling filters do not have both a high specific area and a high voidage, they are less suitable for the treatment of large volumes of strong industrial effluents (Table 11.2). Large areas of land would be required for the expensive and extensive traditional filter beds. Towers, 6-9 m in height, packed with lightweight (40-80 kg m"3) plastic multi-faced modules or small random packing units have provided to be a space saving and relatively inexpensive solution to the problem. These packings have a relatively open structure for oxygen transfer (specific surface of 100-300 m2 m"3) and high voidage (90-98%), but are expensive compared with the conventional filter packings. They are capable of coping with high BOD loadings. At a loading of 3.2 kg BOD m"3 day"1 a 50% BOD removal may be achieved, and at 1.5 kg m"3 day"1 70% removal is possible (Ripley, 1979). The biological film is similar to that formed on the conventional packing and scouring is due to the hydraulic load applied rather than the predation of higher organisms.

BIOLOGICALLY AERATED FILTERS (BAFS)

Biologically aerated filters are a relatively recent development based on the trickling filter. They consist of a packed bed which provides sites for microbial growth through which air is passed but, unlike trickling filters, the reactor volume is flooded with the effluent to be treated which is passed upwards or downwards through the reactor (i.e. co- or counter-currently to the air supply) depending on the design. The packing matrix may be natural (e.g. pumice) or synthetic (e.g. polyethylene), and may be either a fixed structure or randomly packed.

The combination of aeration and filtration allows high rates of BOD and ammonia removal together with solids capture, so that sedimentation tanks may not be required. However, regular backwashing is essential to remove filtered solids and excess biomass. Organic loading rates for 90% BOD removal are significantly greater than those obtained for trickling filters, being in the range 0.7-2.8 kg BOD m"3 day-1 (Stephenson et al., 1993). They are versatile treatment systems, and of those currently in operation, design capacities vary between 600 and 70,000 m3 day-1. In addition to their use as a secondary treatment process they can also be utilized for tertiary treatment or modified to allow denitrification in a manner similar to that of activated sludge systems.

ROTATING BIOLOGICAL CONTACTORS (ROTATING DISC CONTACTORS)

In this treatment method (Fig. 11.3) a unit composed of closely spaced discs (2 to 3 m diameter with 1 to 2 cm spacing between discs), on a central drive shaft are rotated slowly (0.5 to 15 rpm) through the effluent so that 40 to 50% of the disc surfaces are submerged (Borchardt, 1970; Pretorious, 1973). The discs, usually made from synthetic material (e.g. polystyrene, PVC), are arranged in stages or groups separated by baffles to minimize short circuiting or surging (Forster, 1985) and to enhance specific treatment requirements such as nitrification. The discs may be flat or corrugated to increase surface area. A microbial film forms on the discs; this is aerated during the exposed part of the cycle and absorbs nutrients during the submerged part. Shear forces produced as the discs rotate through the liquid control the thickness of the biofilm, with excess biofilm being sloughed from the discs. A sedimentation tank following the biological stage is therefore required to remove biological solids. Loading rates of 13 g BOD m 2 day-1 for domestic sewage and partial treatment of loads of 400 g BOD m"2 day"1 have been used. To achieve the 20:30 standard the loading rate should not exceed 6 g BOD m"2 day"1. Rotating biological contactors are compact, easily covered for health and aesthetic reasons, available as packaged units, simpler to operate under varying loads than trickling filters (the biofilm being wetted at all times) and are easily added onto existing treatment processes. As such they can provide a cost effective method of on-site treatment.

Ware and Pescod (1989) describe the use of full scale anaerobic/aerobic rotating biological contactors for treating brewery wastewaters. Greater than 85% COD removal was obtained in the aerobic stage, but difficulties were experienced in maintaining anaerobic populations.

Biomass support disks

Influent

Further disks in each stage Rotating shaft

Effluent

Baffle

Further disks in each stage Rotating shaft

Influent

Fig. 11.3. Schematic diagram of a rotating biological contactor.

Fig. 11.3. Schematic diagram of a rotating biological contactor.

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