Factors Affecting Performance

Over its long history of use, a large data base has been assembled describing the factors affecting the performance of the trickling filter process. Unfortunately, in many cases the data are contradictory and incomplete. This arises largely because of the interrelation between trickling filter design and operational parameters. It also arises because our understanding of the trickling filter has evolved throughout its history and continues to evolve today. Our current understanding allows recognition that effects once thought to be significant are really artifacts of past design and operational practices and are not fundamental process variables. This section discusses the primary factors that affect trickling filter performance.

19.2.1 Process Loading

The performance of any biochemical operation is affected by the process loading, i.e., the amount of substrate applied per unit time per unit mass of biomass. In suspended growth systems the process loading is expressed as the solids retention time (SRT) or the process loading factor, i.e., F/M ratio. Both have physical meaning in a suspended growth system because the biomass is well mixed and can be characterized by a single parameter. In addition, the operational parameters necessary to calculate them are easily measured. This is not true for attached growth systems such as trickling filters. The biomass cannot be characterized by a single parameter because it is not uniformly distributed through the bioreactor. Furthermore, it is not possible to easily determine the biomass concentration within a trickling filter, thereby making it impossible to calculate either an SRT or a process loading factor. While some values for the biomass concentration in a trickling filter have been reported,"' no consensus exists as to the appropriateness of this approach. As a consequence, other measures of process loading must be used.

As discussed in Chapter 15, substrate removal in biofilm processes is expressed by the substrate flux, Js, which is the mass of substrate per unit time transported into and consumed by the biofilm per unit biofilm planar surface area. Logically, the biofilm process loading can be expressed in the same fashion, as the mass of substrate applied per unit time per unit of total planar biofilm area. For organic substrate, the result, referred to as the surface organic loading (SOL), \s, is expressed in units of kg substrate/(m • day). The SOL is also sometimes referred to as the applied flux. Another approach to expressing the loading on a trickling filter is the TOL, defined by Eq. 19.1 and typically expressed in units of kg substrate/(m' day). The SOL and the TOL, As, are related by the specific surface area of the media, a„:

a. A, where A, is the wetted surface area of media available for biofilm growth. Since the quantity of biomass in a trickling filter is proportional to both the media area and the media volume, both the TOL and the SOL are analogous to the process loading factor for suspended growth bioreactors. This analogy is incomplete, however, due to the fact that the biomass composition is uniform in a suspended growth system while it varies with bioreactor depth in a trickling filter, as discussed above. However, it is a useful analogy if not carried too far.

Theoretically the SOL is superior to the TOL as an expression of biofilm process loading. This is because, as discussed in Chapter 15, biofilm reactions are generally limited by mass transfer and the overall reaction rate is a function of the biofilm surface area. An inherent assumption in the concept of the SOL is that the biofilm surface area is proportional to the media surface area. A second assumption is that the substrate is dissolved and diffuses into the biofilm. While these assumptions are good ones for many biofilm processes, for some trickling filter applications they are not. This occurs for several reasons, including media plugging and inadequate wetting, the presence of colloidal organic matter in wastewater, and the occurrence of multiple substrate limitations. The result is that, for some trickling filter applications, performance correlates better with the TOL than with the SOL.

The concept of surface loading is not limited to the removal of organic substrate. Rather, it can also be applied to nitrification. For example, in a trickling filter performing only nitrification, the nitrification efficiency might be correlated with the surface ammonia-N loading (SAL) with units of kg NH,-N/(m"-day) and symbol \N,i. Alternatively, one may wish to express nitrification performance in terms of the loading of TKN on the process. In that case, one might speak of a surface nitrogen loading (SNL) with units of kg N/(m:-day) and symbol XN. These surface loadings are related to the corresponding total volumetric loadings in the same way that the SOL is related to the TOL, i.e., by Eq. 19.2.

Plugging and channeling of flow in a trickling filter results in incomplete wetting and utilization of the media surface area provided, thereby making the biofilm surface area less than the media surface area. Plugging and channeling especially occur when a trickling filter is used for carbon oxidation. This is because of high biomass production rates, which cause excess biomass to accumulate in the media. The resulting incomplete media wetting and partial media plugging produce variable and incomplete utilization of the media area provided.2 Although media with higher specific surface areas are theoretically better, beyond a point they will not result in greater biofilm surface area because the smaller openings associated with the media will cause increased plugging and reduced wetting of the available media surface area. In short, more media surface area does not necessarily mean more treatment capacity. The situation is further complicated by the fact that, in many wastewaters, a significant portion of the biodegradable organic matter is present in either a sus pended or colloidal form. This organic matter is removed initially by flocculation and entrapment, just as in suspended growth systems. It is then hydrolyzed and the soluble organic matter degraded. It is not yet clear what controls the removal rate of such organic matter in a biofilm reactor. Moreover, its retention within the trickling filter can result in additional channeling of flow.

As discussed in Section 19.1.2 and illustrated in Table 19.1, the trickling filter TOL can be correlated with its treatment objectives. Performance relationships can also be presented graphically, as illustrated in Figure 19.5 where a typical relationship between the soluble BOD5 removal efficiency of a trickling filter and its TOL is presented. The nitrification performance of trickling filters accomplishing combined carbon oxidation and nitrification can also be correlated with the TOL, as illustrated in Figure 19.6.14 The performance data presented in this figure are also correlated with the SOL in the upper axis (called areal loading).

In trickling filters accomplishing either carbon oxidation or combined carbon oxidation and nitrification, it is likely that oxygen is the limiting substance. As discussed in Sections 15.3 and 19.1.1, when both biodegradable organic matter and ammonia-N are present, heterotrophs and autotrophs compete for space in the aerobic portion of the biofilm, with autotrophs being excluded until the concentration of biodegradable organic matter drops below about 20 mg/L as COD. Therefore, if the rate of oxygen transfer into the biofilm is limiting, in the upper portion of the trickling filter, the oxidation rate of biodegradable organic matter will be limited by the rate of oxygen transfer, whereas in the lower portion of the trickling filter oxygen transfer will limit the oxidation rate of ammonia-N. If both substrates are expressed

Trickling Filter Loading Rates

Figure 19.5 Typical relationship between total organic loading (TOL) and soluble BOD5 removal efficiency for a trickling filter containing high-rate media.

Total Organic Loading, kg BOD5/(m3 day)

Figure 19.5 Typical relationship between total organic loading (TOL) and soluble BOD5 removal efficiency for a trickling filter containing high-rate media.

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