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Figure 19.3 Typical trickling liltcr media. (From Water Environment Federation, Design of Municipal Wastewater Treatment Plants, Manual of Practice No. 8, Water Environment Federation, Alexandria. Virginia, 1992. Copyright © Water Environment Federation; reprinted with permission.)

the greater the structural requirements and the shallower the media depth must be to avoid media crushing. For rock media trickling filters, media depths are typically on the order of 2 m, which restricts treatment efficiency to some extent. Some have been constructed to greater depths using special construction techniques, but this is not conventional practice. The relatively low specific surface area and void space restrict the organic loadings that can be applied. The low specific surface area limits the capacity for biofilm growth and consequently, the treatment capability. Similarly, the low void volume limits the space available for the passage of air, water, and sloughed biomass through the media, thereby increasing the potential for media plugging at higher organic loadings. As a consequence, rock media have typically been used at low to moderate TOLs, generally in the range of 0.5 to 1.5 kg BOD7(m'-day). More significantly, the shallow media depth and low organic loadings result in low wastewater hydraulic application rates. Although recirculation can be used to provide increased total hydraulic loadings (THLs, see Eq. 16.8), typical design practice has been to limit recirculation ratios to minimize energy requirements. As a consequence, THL to rock media is typically on the order of 0.5 m/hr, which is significantly less than that used with high-rate media. The adverse impacts of these low THLs on process performance are discussed later.

As illustrated in Table 19.2, high-rate media are characterized by significantly lower unit weight, higher specific surface area, and greater void space than rock media. As a consequence, media depths are typically greater (generally 5 to 1 m), and a wider range of TOLs can be used (up to 3.5 kg BODs/Xm'-day)). Because of the greater media depth, the cross-sectional area of a high-rate media trickling filter will be significantly less than that of a comparably sized rock media trickling filter. A THL of 1.8 m/hr is typically considered necessary to fully wet and completely utilize high-rate trickling filter media.

The effects of media type on trickling filter performance are discussed more fully in Section 19.2.6, but a brief summary is provided here. Data suggest that the performance of rock and high-rate media trickling filters are similar when they are loaded at relatively low organic loading rates, less than about 1 kg BODs/(m • day). In contrast, at higher TOLs the performance of trickling filters using high-rate media is superior. Other operating characteristics of rock and high-rate media trickling filters tend to be somewhat different. For example, excess biomass production rates may be somewhat less in rock media trickling filters than in high-rate media trickling filters.This is thought to result from the greater biomass retention characteristics of rock media, resulting in increased degradation (probably anaerobic) of the retained biomass. The biomass retention characteristics may also result in variations in the settling characteristics of the produced biosolids and in the clarity of the settled effluent.

Plastic high-rate trickling filter media are manufactured with a variety of specific surface areas. Variations in the specific surface area are accomplished by modifications in the media configuration and dimensions. Consider plastic sheet (or bundle) media as an example. Increased specific surface area is accomplished by manufacturing media sheets with slightly smaller indentations, which allows media sheets to be placed closer together. Media with a specific surface area of approximately 100 nr/m' is typically used when screened wastewater or primary clarifier effluent is being treated, whereas media with a specific surface area of approximately 140 m:/m' is typically used when secondary effluent is being treated. The more open, low specific surface area media is needed to avoid plugging problems associated with the relatively high biomass production rates experienced when screened wastewater or primary clarifier effluent is being treated. Since the biomass production rate is lower when secondary effluent is being treated, media with a higher specific surface area media can be used in that application.

As indicated in Tabic 19.2, the specific surface area of wood media is relatively low. However, experience indicates that the application of return activated sludge to a trickling filter containing this media results in significant interstitial biomass growth, which increases its effective treatment capacity.1(1 Side-by-side testing indicates that its efficiency will be equivalent to that of bundle media when RAS is recycled to the wood media. No similar improvement in performance is observed for the other media. Process configurations that recycle RAS to an upstream trickling filter are discussed in the next section.

Coupled Trickling Filter/Activated Sludge Systems. Coupled trickling filter/ activated sludge (TF/AS) systems use an upstream trickling filter combined with a downstream suspended growth biochemical operation to accomplish overall wastewater treatment.1"'-" Figure 19.4 illustrates a typical system. Such systems are referred to as containing coupled processes because no liquid-solids separation device is provided between the trickling filter and the suspended growth bioreactor. As a consequence, biomass that grows on the trickling filter sloughs off and passes directly into the suspended growth bioreactor where it is enmeshed into and becomes part of the suspended biomass. A significant portion of the biomass contained in the suspended growth bioreactor (generally 60% to 90%) is originally grown in the trickling filter.

Trade-offs exist relative to the sizes of the two biochemical operations. If a relatively small trickling filter is used, then a larger suspended growth bioreactor must be used to accomplish a specified treatment goal. Conversely, if a relatively large trickling filter is used, the treatment goal can be accomplished using a smaller

Figure 19.4 Schematic diagram of the coupled trickling filter/activated sludge (TF/AS) process.

Table 19.3 Couplcd TF/AS System Options

Unit size

Process option

Trickling filler

Suspended growth bioreaetor

RAS recycle destination

Trickling filter/solids contact (TF/SC) Activated biolilter

(ABF) Roughing filter/ activated sludge (RF/AS) Biolilter/activated sludge (BF/AS)

Large Large Small

Small

Small None Large-

Large

Suspended growth bioreaetor Trickling filter

Suspended growth bioreaetor

Trickling tiller suspended growth bioreaetor. This trade-off provides one of the criteria for classifying coupled TF/AS systems. Another criterion is the location for the return activated sludge (RAS); either around the entire system, i.e., to the trickling filter, or around only the suspended growth bioreaetor. Table 19.3 uses these two criteria to classify coupled TF/AS systems.

Both the trickling filter/solids contact (TF/SC) and the activated biolilter (ABF) processes use relatively large trickling filters and small suspended growth bioreactors. In both of these processes, removal of organic matter is achieved in the trickling filter. The suspended growth bioreaetor is used primarily to flocculate and enmesh fine suspended solids contained in the trickling filter effluent and to prepare the biosolids for efficient removal in the secondary clarifier. In the TF/SC process the RAS is recycled to the suspended growth bioreaetor, which is referred to as a solids contact basin. In the ABF process, no distinct suspended growth bioreaetor is provided. Rather, underflow from the clarifier is recycled to the trickling filter and the suspended biomass is developed and contained only in the recirculating fluid mass.

The roughing filter/activated sludge (RF/AS) and biofilter/activated sludge (BF/ AS) processes use small trickling filters. As a consequence, the removal of organic-matter in the trickling filter is incomplete and the suspended growth bioreaetor removes the remaining organic matter. In the RF/AS process, the RAS is recycled only to the suspended growth bioreaetor, whereas in the BF/AS process it is recycled to the trickling filter. In some instances (particularly the treatment of high concentration, readily biodegradable wastewaters such as from the food processing industry), recycle of the RAS to the trickling filter can improve sludge settling characteristics. This is thought to be related to a "selector effect" that is achieved by contacting the RAS with high concentrations of organic matter in the highly aerated biofilter environment."' " The use of selectors to control sludge settleability in activated sludge systems is discussed in Section 10.2.1.

19.1.3 Comparison of Process Options

The trickling filter process is a stable, reliable process that is capable of providing economical wastewater treatment. Energy requirements are typically lower than in suspended growth systems, which was one reason for the renewed popularity of trickling filters in the 1980s, a time when energy costs were escalating rapidly. The process is also relatively simple to operate. Capital costs can be high compared to other biochemical operations, and process operation cannot be adjusted as easily in response to loading and/or performance variations. The biggest drawback of trickling filters is that their performance may not meet current discharge standards. However, the performance of coupled trickling filter/activated sludge systems will generally equal that of suspended growth systems. The simplicity, stability, and low energy requirements for the trickling filter process have made it a popular option for carbon oxidation and nitrification. Primary clarification is typically provided prior to a trickling filter to minimize the debris loading.

Table 19.4 summarizes the benefits and drawbacks of the various trickling filter options. Roughing applications can provide very economical removal of organic matter, particularly from high-strength wastewaters. Further treatment of the roughing filter effluent (in addition to secondary clarification) is typically required prior to final discharge, but the size of the downstream treatment system is reduced.

Carbon oxidation applications offer the advantages of favorable economics, simple design and operation, and well known process and facility design procedures. Process performance is consistent and reliable, but may not meet the stringent standards now typically required.

Combined carbon oxidation and nitrification and separate stage nitrification applications are simple, both to design and to operate. However, experience with them is somewhat limited and performance relationships are still evolving. It is expected that this drawback will be eliminated in the future as more direct experience accumulates with these options.

A comparison of rock and high-rate media indicates substantial advantages for high-rate media. The only drawback associated with high-rate media is that media collapses can occur as a result of improper application or manufacturing. This suggests that experience is necessary in its application. Due to the associated benefits, nearly all new trickling filters are constructed using high-rate media. However, a large number of rock media trickling filters exist and many are providing effective and economical service.

The coupled trickling filter/activated sludge process was developed to take advantage of the energy efficiency and stability of the trickling filter while also achieving the excellent effluent quality obtained with the activated sludge process. Experience indicates that this objective is typically achieved. Performance differences between the four primary TF/AS options are relatively minor. An economic tradeoff exists between the more energy-efficient TF/SC and ABF processes and the less capital intensive RF/AS and BF/AS processes. Multiple modes of operation are often incorporated into full-scale facilities so that selection of a single coupled process option is not necessary.

19.1.4 Typical Applications

The trickling filter process is widely accepted and used for the aerobic biological treatment of wastewaters. It is used for both the removal of organic matter and nitrification. Its long history has resulted in a large number of operating installations. Many of the older installations use rock media, and a large number of successful

Table 19.4 Trickling Filler Process Comparison

Process

Benefits

Treatment objectives Roughing

Carbon oxidation

Combined carbon oxidation and nitrification

F.conomical, particularly for high-strength wastewaters Simple to design and operate Process and facility design well known

Economical

Simple to design and operate Process and facility design well known Simple to design and operate

Separate stage nitrification

Simple to design and operate

Media type

Rock I,argc number of existing applications Quite effective at low to moderate organic loading rates

High-rate Economical

Applicable to a wide range of process loadings and applications Process and facility design well known

Drawbacks

Further treatment typically required prior to discharge Generally requires secondary clarification

Performance is consistent, but may not reliably meet stringent performance standards

Generally requires secondary clarification Relatively new process option, performance relationships not well characterized Limited operator flexibility

Relatively new process option, performance relationships not well characterized

Relatively expensive due to structural constraints Not applicable for high loading applications Odor potential

Media collapses have occurred due to improper application and/or manufacturing

Coupled trickling filter/ activated sludge (TF/AS)

Trickling filter/solids contact (TF/SC)

Activated biofilter (ABF)

Roughing Alter/activated sludge (RF/AS)

Biofilter/activated sludge (BF/AS)

Stable, reliable performance Simple to design and operate Low energy

Process and facility design well known

Simple to design and operate Low energy Low capital cost Process and facility design well known

Stable, reliable performance Simple to design and operate Low capital cost Process and facility design well known Stable, reliable performance Simple to design and operate Low capital cost Process and facility design well known Improved sludge settling characteristics in some applications

Moderate capital cost

Process performance variable, except at low loading

Moderate energy cost

Moderate energy cost installations of various size exist. Installations constructed in the last 20 years generally use high-rate media, either plastic sheet, random, or horizontal. Plastic sheet media is currently the most popular due to its availability, cost, and good performance characteristics. Horizontal media is seldom used in new installations today due to its higher costs relative to plastic media. However, many existing installations exist because of its popularity during the 1970s.

The use of high-rate media trickling filters for separate stage nitrification has been actively considered for more than two decades, and some facilities have been in operation for a number of years.47" Interest in this application has increased recently as practitioners have identified potential cost and operational advantages. Given the level of interest currently expressed in this technology, it is reasonable to expect more applications to be constructed.

Roughing filters are often used to pretreat industrial wastewaters containing high concentrations of readily biodegradable organic matter. The roughing filter effluent (after secondary clarification) typically receives further treatment, either in another biological treatment system located at the industrial site or in a municipal wastewater treatment system. Roughing filters have also been used to pretreat mixtures of municipal and industrial wastewater containing high concentrations of organic matter.

Use of the trickling filter for carbon oxidation has declined since (he 1950s due to its general inability to meet stringent discharge standards. The coupled TF/ AS systems were developed in response to this performance short-fall and have been quite successful in meeting stringent discharge standards."Such systems can be designed for carbon removal applications alone or to accomplish combined carbon oxidation and nitrification. They have generally proven effective in achieving this goal while also retaining the basic operating characteristics of the trickling filter process.

While the trickling filter process has been used widely, it still must be considered to be a developing process. This is because of the recent process and mechanical developments, which have improved performance relative to that experienced historically. Continued development of high-rate trickling filter media has resulted in good performance and very competitive media costs. Improved understanding of trickling filter hydraulics has led to revised hydraulic loading and wetting regimes that provide better control of biofilm thickness, thereby producing a biofilm that is thinner, more aerobic, and more active. Experience with these improved facilities is accumulating, and is very encouraging. Recent experience with nitrification in trickling filter applications has caused increased interest in these options. The ongoing development of coupled TF/AS process options has resulted in improved control over effluent suspended solids concentrations, thereby allowing trickling filter based processes to produce effluent quality that rivals that of the activated sludge process.

The RF/AS and BF/AS processes have proven to be quite effective for treating higher strength industrial or industrial/municipal wastewaters. The trickling filter provides process stability and control of filamentous microorganisms, while the suspended growth bioreactor allows an effluent of excellent quality to be produced. In addition, the use of relatively small trickling filters results in a system with moderate capital and operating costs. As noted above, experience indicates that recycle of the RAS to the trickling filter (thereby converting it to a biofiltcr) generally results in the most complete control of solids settling characteristics. The ABF process has received relatively little use by itself, but many systems incorporate the flexibility to operate in this mode and it is used effectively to reduce energy costs during periods of lower process loading.

While basic information on the combined carbon oxidation/nitrification and separate stage nitrification processes has been available for nearly 20 years, significant interest in these processes has developed only recently.14"4'x They offer the potential for reduced energy costs, good process stability, and favorable economics.

Trickling filters have not proven to be as adaptable to nutrient removal as suspended growth bioreactors. While phosphorus removal in coupled TF/AS processes has been demonstrated, their removal capability is hindered by the oxidation of organic matter in the trickling filter/1 Biological nitrogen removal is also problematic for the same reason. This must be considered when selecting a trickling filter based option for a new or expanded wastewater treatment facility.

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