The first large-scale success with biological wastewater treatment was achieved midway through the nineteenth century using an attached-growth process now referred to as trickling filter technology (Figure 16.5). The operative biochemical agent within these
systems develops as a thin microbial layer, commonly known as a biofilm, which grows in an attached fashion on the surface of stationary support media (see the media shown in Figure 16.6) and over which the incoming wastewater is then continuously distributed and streamed (see Figure 16.7). In turn, these attached-growth biofilters will then sorb, filter, and degrade waste contaminants as they trickle across their extensive attached-growth surface.
Compared with most other waste-processing strategies, trickling filters are fairly simple in design and operation, and as a result they still find considerable use (Water Environment Federation, 1991). While their overall effectiveness may be somewhat lower than that of other processing strategies (e.g., suspended-growth systems), the attached nature of these biofilms does provide several complementary benefits. Since the attached biofilm stays inside the reactor, there is less need to provide any sort of recycling process to return solids from the clarifier (as must be done in suspended-growth systems). Trickling filter systems also encounter fewer problems retaining their attached biomass during high-flow periods, whereas a suspended-growth reactor might overload the settler hydraulically, resulting in solids loss in the effluent. In many instances, and particularly in nitrifying
Attached Growth Reactor ^_G^ife v v
towers, where the attached biofilm is extremely thin and tightly bound, these reactors may not even require a follow-up clarifier.
The typical hardware components used with a conventional system (Figure 16.8) include a reactor filled with media, an underdrain platform at the base of the reactor on which the medium rests, a rotating influent distribution arm (or, less commonly, a collection of fixed spray distributors), and the necessary valves, pipes, and pumps required to route, and possibly recirculate, this flow through the reactor. In addition, there may be a downstream settling tank to clarify the outgoing waste stream prior to discharge, in which solids pulled or released (i.e., sloughed) from the biofilm surface can be settled.
As for the design strategies used by environmental engineers to configure and size these processes, hydraulic and substrate loadings typically represent the key parameters, as depicted in Figure 16.9. Table 16.2 provides an overview of the associated design
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