Trickling Filters

The trickling filter is one of the most common attached cell (biofilm) processes. Unlike the activated sludge and aerated lagoons processes, which have biomass in suspension, most of the biomass in trickling filters are attached to certain support media over which they grow (Fig. 11).

Typical microorganisms present in trickling filters are Zoogloea, Pseudomonas, Alcaligenes, Flavobacterium, Streptomyces, Nocardia, fungi, and protozoa. The crux of the process is that the organic contents of the effluents are degraded by these attached growth populations, which absorb the organic contents from the surrounding water film. Oxygen from the air diffuses through this liquid film and enters the biomass. As the organic matter grows, the biomass layer thickens and some of its inner portions become deprived of oxygen or nutrients and separate from the support media, over which a new layer will start to grow. The separation of biomass occurs in relatively large flocs that settle relatively quickly in the supporting material. Media that can be used are rocks (low-rate filter) or plastic structures (high-rate filter). Denitrification can occur in low-rate filters, while nitrification occurs under high-rate filtration conditions; therefore effluent recycle may be necessary in high-rate filters.

AerafcLt

Supporting tmr

AerafcLt

Supporting tmr

Aownbit liyer

Figure 11 Cross-section of an attached growth biomass film.

Aownbit liyer

Figure 11 Cross-section of an attached growth biomass film.

In order to achieve optimum operation, several design criteria for trickling filters must be followed:

• roughing filters may be loaded at a rate of 4.8 kg BOD5/day/m3 filter media and achieve

BOD5 reductions of 40-50%;

• high-rate filters achieve BOD5 reductions of 40-70% at organic loadings of 0.4-4.8

kg/BOD5/day/m3; and 3

• standard rate filters are loaded at 0.08-0.4 kg/BOD5/day/m3 and achieve BOD5

removals greater than 70% [2].

The trickling filter consists of a circular tank filled with the packing media in depths varying from 1 to 2.5 m, or 10 m if synthetic packing is used. The bottom of the tank must be constructed rigidly enough to support the packing and designed to collect the treated wastewater, which is either sprayed by regularly spaced nozzles or by rotating distribution arms. The liquid percolates through the packing and the organic load is absorbed and degraded by the biomass while the liquid drains to the bottom to be collected.

With regard to the packing over which the biomass grows, the void fraction and the specific surface area are important features; the first is necessary to ensure a good circulation of air and the second is to accommodate as much biomass as possible to degrade the organic load of the wastewaters. Although more costly initially, synthetic packings have a larger void space, larger specific area, and are lighter than other packing media. Usually, the air circulates naturally, but forced ventilation is used with some high-strength wastewaters. The latter may be used with or without recirculation of the liquid after the settling tank. The need for recirculation is dictated by the strength of the wastewater and the rate of oxygen transfer to the biomass. Typically, recirculation is used when the BOD5 of the seafood-processing wastewater to be treated exceeds 500 mg/L. The BOD5 removal efficiency varies with the organic load imposed but usually fluctuates between 45 and 70% for a single-stage filter. Removal efficiencies of up to 90% can be achieved in two stages [4]. A typical unit of a trickling filter is shown in Figure 12.

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