Figures 7.21.7 and 7.21.8 are examples of common process flow diagrams for single- and multistage trickling filters. Wastewater treatment facilities often recirculate the treated effluent from the clarifier to (Atkinson and Ali 1976; Metcalf and Eddy, Inc. 1991):
• Reduce the possibility of organic shock loadings by diluting the incoming wastewater
• Maintain uniform hydraulic loadings especially under low and intermittent flow conditions
• Achieve an extensive film coverage and a relatively uniform film thickness through the filter
• Reduce the nuisances of odor and flies
However, such benefits are achieved at the expense of higher hydraulic loadings.
Trickling filters are classified by their hydraulic loadings. Typical hydraulic loadings for low-rate (without recirculation) and high-rate (with recirculation) trickling filters are 1.17-3.52 and 9.39-37.55 m3/m2-day, respectively. The corresponding loadings for super high-rate trickling filters are as high as 70.41 m3/m2-day. The effluent from a low-rate trickling filter is usually low in BOD and well nitrified. Wastewater treatment facilities commonly use two-stage trickling filters for treating high-strength wastewater and achieving nitrification at hydraulic loadings comparable to those for high-rate trickling filters (Tebbutt 1992).
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