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Media Bed

Media Bed

Containment Structure

Effluent {to Further Treatment or Discharge)

(3) the wastewater application (or dosing) system, (4) the underdrain system, and (5) the ventilation system/'1The media bed provides the surface upon which the microorganisms grow. Media options consist of rock, wood, and synthetic plastic of various types and configurations.

The containment structure retains the media and applied wastewater and controls the effects of wind. Some media, such as rock and random plastic, are not self-supporting and, in these instances, the containment structure must also support the media. The containment structure is often constructed of concrete, cither poured in place or precast panels. Other materials such as wood, fiberglass, and coated steel have also been used, particularly when the media is self-supporting.

The application system uniformly applies the wastewater to the media bed. Uniform application is necessary to ensure wetting of all of the media. The application system is also used to control dosing frequency, which affects process performance.

The underdrain system has two functions. The first is to collect the treated effluent for conveyance to further treatment or to discharge. The second is to provide a plenum to allow air passage through the open media bed, thereby providing the oxygen required for aerobic metabolism. Clay or concrete underdrain blocks are often used for rock media trickling filters because of the weight that must be supported. Many types of underdrain systems, such as concrete piers, wood stringers, and reinforced fiberglass grating, are used with other media.

Oxygen required to meet the metabolic needs of the microorganisms is provided by the vertical flow of air through the media. As discussed in Section 19.2.5, ventilation to provide that air can be supplied either by natural draft or by mechanical means. In natural draft systems, the difference in density between air inside and outside the trickling filter causes air within the trickling filter to either rise or sink. This results in a continuous flow of air through the media. Density differences arise because air within the trickling filter quickly becomes saturated with water vapor and reaches the temperature of the applied wastewater. Consequently, the magnitude of the density difference depends on the temperature and humidity of the ambient air. One disadvantage of natural draft ventilation is that neutral density conditions can occur, resulting in the absence of air-flow through the trickling filter and the development of anaerobic conditions. In forced draft ventilation systems, the air is applied to the trickling filter by mechanical means. In all cases, air must be uniformly distributed across the media to ensure that oxygen is provided to the entire bioreactor.

As indicated in Figure 19.1, trickling filter effluent may be recirculated and mixed with the influent wastewater prior to its application to the trickling filter. Recirculation dilutes the influent wastewater and also allows separation of the hydraulic and organic loadings to the unit. Recirculation is an essential process component in some applications. The need for recirculation and the various recirculation configurations are discussed later.

The influent to a trickling filter must generally be pretreated to remove nonbiodegradable particulate matter such as plastics, rags, and stringy material. Materials of this type can easily plug the distributor and the media, leading to unequal flow distribution and poor performance. Debris removal by coarse screens is not generally acceptable for trickling filter applications, but adequate removal can be accomplished using fine screens (generally 1 mm opening or less) or primary clarifiers. Primary clarifiers are used most often.

The media provides a surface for the growth of microorganisms and the mechanism for retaining the microorganisms in the unit. Organic matter removal and nitrification occur by the same mechanisms as in any other aerobic biochemical operation. Soluble organic matter diffuses into the biofilm located on the media surface and is used as a carbon and energy source by heterotrophic bacteria. Colloidal and particulate organic matter also diffuse into the biofilm where they are first removed by sorption and entrapment. They are subsequently hydrolyzed into soluble organic matter by the action of extracellular enzymes. The soluble organic matter is then metabolized by the heterotrophic bacteria contained within the biofilm. Am-monia-N also diffuses into the biofilm where part is used by the heterotrophs for biomass synthesis and the remainder is oxidized to nitrate-N by nitrifying bacteria. The removal of organic matter and nitrification result in the production of additional biomass and increased biofilm thickness. When the biofilm reaches a thickness that can no longer be supported on the media, the excess sloughs off and passes into the treated effluent. Chapter 15 describes the role of diffusion in controlling the metabolic processes occurring within the biofilm.

Trickling filter effluents are usually treated in clarifiers to remove the produced biomass, although this may not be necessary in some separate stage nitrification applications because of the low yield of nitrifying bacteria. For example, consider a separate stage nitrification application in which 20 mg/L of ammonia-N is being oxidized. Since the yield coefficient for nitrifying bacteria is approximately 0.15 mg TSS/mg ammonia-N oxidized, only about 3 mg/L of nitrifying bacteria will be produced. This increase in suspended solids concentration may not be significant in relation to plant effluent suspended solids limits, and thus a liquid-solids separation device may not be required downstream of the trickling filter/

As discussed in Chapter 16, the liquid flow pattern through a trickling filter may generally be thought of as plug-flow with dispersion. Because of this flow pattern and because the microorganisms are fixed on the media, variations in the composition of the biomass often exist along the depth of the trickling filter. This is in contrast to the activated sludge process where biomass recycle results in a uniform biomass composition throughout the bioreactor. The variation in biomass composition through the depth of a trickling filter can have significant impacts on process performance. For example, carbon oxidation typically occurs in the upper portion of combined carbon oxidation and nitrification systems, while nitrification occurs in the lower portion, " as illustrated in Figure 19.2. This is due to competition between heterotrophic and autotrophic bacteria for space within the biofilm. as discussed in Chapter 15. In the upper levels of a trickling filter, both organic matter and ammonia-N concentrations are relatively high and will, generally, not limit the specific growth rate of either the heterotrophic or nitrifying bacteria. Under these conditions the heterotrophic bacteria can grow faster than the nitrifying bacteria and out compete them for space within the biofilm. As the wastewater flows down through the trickling filter, organic matter is removed, ultimately causing its concentration to limit the specific growth rate of the heterotrophic bacteria. Because the ammonia-N concentration is still high, a point is reached at which the specific growth rate of the nitrifying bacteria exceeds the specific growth rate of the heterotrophic bacteria. Under these conditions the nitrifying bacteria can effectively compete with the heterotrophic bacteria for space and will become established in the biofilm. As discussed in Chapter 15, the soluble biodegradable organic matter concentration must be reduced to about 20 mg/L as COD before this can occur.41 w

The plug-flow nature of the trickling filter can also result in reduced growth and biomass accumulation in the lower portion of the tower, leading to patchy growth as indicated in Figure 19.2."'44 This occurs because of the low yield of nitrifying bacteria and the presence of predators that consume trickling filter biomass. The reduced biofilm thickness can result in a diminished wastewater treatment capacity in the lower portion of the tower, which can be particularly important when the loading increases, such as during diurnal high flow events.

Although trickling filters are generally thought of as aerobic processes, in most cases the biofilm is relatively thick and exceeds the depth of oxygen penetration."4 Consequently, the biofilm consists of an outer aerobic layer and an inner anoxic/ anaerobic layer. This affects proccss performance in many significant ways. For example, the occurrence of a zone of low dissolved oxygen (DO) concentration within the biofilm allows denitrification to occur, although the extent is limited The removal of organic matter typically occurs in the upper levels of the trickling filter, whereas nitrification occurs in the lower levels. As a consequence, the concentration of organic matter is low in the region where nitrate-N is produced. However, some denitrification can occur when treated effluent containing nitrate-N is recirculated to the process influent.41

19.1.2 Process Options

Trickling filter process options vary with the treatment objective, the media type, and the nature of the other unit operations in the process train.

Treatment Objectives. Trickling filters are used to treat a wide variety of wastewaters to achieve various treatment objectives. Consequently, as indicated in Table 19.1, those two factors can be used to characterize the trickling filter process. Because the degree of treatment is often determined by the process organic loading rate, it can be used as a quantitative indicator of the degree of treatment. The total organic loading is the mass flow rate of biodegradable organic matter in the influent wastewater (excluding recirculation) divided by the media volume, VM. It is given the acronym TOL and the symbol As, and is calculated as:

Table 19.1 Trickling Filter Process Applications

Application/objective

Influent wastewater

Roughing

Screened wastewater or primary effluent l .5-3.5

Carbon oxidation

Screened wastewater or primary effluent

Combined carbon oxidation and nitrification Separate stage nitrification

Screened wastewater or primary effluent

Secondary effluent

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