'Not applicable.

The concentration of biodegradable organic matter is typically expressed as either five-day biochemical oxygen demand (BOD5) or as chemical oxygen demand (COD), making the units for TOL kg BODs/(nr ■ day) or kg COD/(m' ■ day).

Influent wastewater containing both organic matter and ammonia-N can be treated in roughing, carbon oxidation, or combined carbon oxidation and nitrification applications. The difference between them is the degree of treatment, as indicated by the TOL/'"'2 A relatively high TOL is used in a roughing filter, resulting in residual organic matter and suspended solids concentrations that generally exceed the conventional definition of secondary treatment (30 mg/L each of BOD5 and total suspended solids [TSS]). Roughing applications are generally used to lower the concentration of organic matter prior to further biological treatment. A somewhat lower TOL is used in a carbon oxidation application so that relatively complete removal of organic matter (approaching the definition of secondary treatment) is achieved. Finally, an even lower TOL is used to achieve combined carbon oxidation and nitrification. Carbon oxidation is relatively complete in the upper portion of the trickling filter, thereby allowing the growth of nitrifying bacteria in the lower portion, as described above.

Separate stage nitrification differs in that a stream that is relatively low in biodegradable organic matter and suspended solids (BOD* and TSS generally less than 30 mg/L), but with a significant ammonia-N concentration, is applied to the trickling filter."'544^7"1"2 As a consequence, the microbial population that develops in the biofilm is enriched in nitrifying bacteria. In this instance the TOL is not an issue. Instead, the ammonia loading and other operational factors determine the degree of ammonia removal. For example, the nitrification efficiency might be correlated with the total ammonia-N loading (TAL) with units of kg NH,-N/(m'-day) and symbol ANH. Alternatively, there may be instances in which it is advantageous to express nitrification performance in terms of the loading of total Kjeldahl nitrogen (TKN) on the process. In that case, one might speak of a total nitrogen loading (TNL) with units of kg N/(nr - day) and symbol AN.

Media Type. Many types of media have been used in trickling filters, but in general, they can be divided into rock media and high-rate media." "1"2 Table 19.2 summarizes the characteristics of various rock and high-rate media, while Figure 19.3 presents photographs of several typical media.

An ideal rock media consists of rounded river rock of relatively uniform size and shape.14'"1 "2 Typical rock media are approximately 5 cm in diameter, although larger and smaller rock can be used. Rounded river rock is durable, and its rounded nature and uniform size minimize bed consolidation and plugging. Irregular materials such as slag have also been used, but they exhibit a greater potential for plugging due to the entrapment of biomass by the irregular surfaces and openings. Media attrition will also occur due to freeze-thaw cycles, resulting in the production and accumulation of fines that contribute to plugging. This is particularly true for slag media due to their rough surfaces which allow water to intrude into the media.

As indicated in Table 19.2, the primary characteristics of rock trickling filter media are high unit weight, low specific surface area, and low void space. These impose significant design and operational constraints. The higher the unit weight,

Table 19.2 Characteristics of Trickling Filter Media

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