Trickling filter systems are classified under the aerobic attached growth systems (Figure 30.7). Crushed rock and stone, slag, wood, or synthetic media with higher permeability are used to fill the filter bed. The size of the media is in the range of 25-100 mm diameter. The depth of the filter
FIGURE 30.6 Schematic diagram of the activated sludge process.
FIGURE 30.7 Flow diagram of the trickling filter.
bed is about 1.8 m on average.38 The wastewater is spread over the bed from the top of the filter using a rotary distributor. During the process, microorganisms are attached and grow on the surface of the solid medium on the trickling filter bed. Organic pollutants in the incoming waste-water are degraded by the attached biofilms and complex organics are converted into a group of simple compounds such as water, carbon dioxide, and nitrate.19 Once the biofilms that are grown on the solid medium increase in thickness, they undergo the process called "sloughing" and are detached from the medium. In order to collect the detached biofilms and treated wastewater, and provide air circulation, an under-drain system is constructed for the trickling filters.36 The surface area and the void ratio of the filter bed are crucial for the performance of the reactor. Clogging of the reactor bed due to excess biofilm growth is one of the common problems in trickling filters.19 Trickling filter treatment systems can be single stage or multiple stage, depending on the level of treatment required. The design parameters and the performance of the trickling filter are shown in Table 30.4.
Trickling filter technology can be used for the treatment of pretreated dairy industry wastewater.39 A trickling filter packed with ceramic tiles was used to treat sugar wastewater. The influent BOD5 and COD were 142-203 and 270-340 mg/L; the organic loading was from 5 to 120 g BOD5/m2 d.
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