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Table 2.2 Conditions Associated with Dominant Filament Types

Suggested causative conditions

Filament types

Completely mixed bioreactors

Septic wastewater/sulfide Nutrient deficiency

Low pH

H. hydrossis, M. parvicella,

S. nutans, type 1701 M. parvicella, types 0041, 0092,

0675, 1851 H. hydrossis, Nocardia spp.,

N. limicola, S. natans, Thiothrix spp., types 021N, 1701, 1851 Beggiatoa, Thiothrix spp.. types 02IN, 0914

natans, Thiothrix spp., type 021N; possibly H. hydrossis, types 0041, 0675 fungi

From Ret. 31

Although the ecosystems of activated sludge, aerated lagoons, and aerobic digestion are complex, they are not as complicated as those in suspended growth systems accomplishing biological nutrient removal. This is because biological nutrient removal systems also contain anoxic and anaerobic reactors, which provide opportunities for the growth of microorganisms that do not ordinarily grow in totally aerobic systems.

The impact of having appropriately placed anoxic zones in a suspended growth system is to allow the proliferation of denitrifying bacteria. As discussed in Section 2.2.1, they are heterotrophic organisms that use nitrate-N and nitrite-N as electron acceptors in the absence of molecular oxygen. Denitrification can be accomplished by a large number of bacterial genera commonly found in wastewater treatment systems, including Achromohacter, Aerobacter, Alcaligenes, Bacillus, Flavobacte-rium, Micrococcus, Proteus, and Pseudomonas,~ thereby making the establishment of a denitrifying culture relatively easy. However, there is uncertainty concerning the fraction of the heterotrophic bacteria in a biological nutrient removal system that can denitrify,:4 and it may well depend on the nature of the microorganisms entering the system in the wastewaters" as well as on the system configuration. Nevertheless, it is evident that the introduction of anoxic zones in suspended growth bioreactors will give a competitive advantage to denitrifying bacteria over heterotrophs that do not denitrify.

As described in Section 2.4.6, the placement of an anaerobic zone at the influent end of an otherwise aerobic suspended growth system establishes the conditions required for proliferation of phosphate accumulating organisms, thereby allowing development of a biomass that is rich in phosphorus. Although bacteria of the genus Acinetohacter were originally thought to be the major PAOs, several other bacterial types have also been found to be capable of storing polyphosphate/' " In fact, in one study, Acinetohacter was not the predominant PAO present; rather it was an unidentified gram-positive bacteria. '' Identification of phosphate-accumulating bacteria in wastewater treatment systems is not an easy task because of the complicated growth environment required for the formation of polyphosphate granules. Nevertheless, through the development and application of new techniques, we can expect to learn more in the future about the microbial ecology of these important communities.

The previous discussion has indicated the various types of organisms that can be present in suspended growth bioreactor. However, it is very important to recognize that the types that are present in any given system will depend on the reactor configuration and the biochemical environment imposed. In later chapters we will see how these conditions, which are under engineering control, can be used to select the type of microbial community required to accomplish a specific objective.

Attached Growth Bioreactors. Attached growth bioreactors are (hose in which the microorganisms grow as a biofilm on a solid support. In a fluidized bed bioreactor (FBBR), the biofilm grows on small particles of sand or activated carbon that are maintained in a fluidized state by the force of water flowing upward. Packed bed bioreactors contain similar support particles, but the water being treated flows over them without displacing them. Thus, in both bioreactor types, the biofilm is surrounded by the fluid containing the substrate being removed. In a trickling filter (TF) or rotating biological contactor (RBC), on the other hand, the biofilm grows on a large surface over which the wastewater flows in a thin film (TF) or moves through the wastewater (RBC). As a consequence, the fluid shear associated with the latter two is less than that associated with the first two. This has an impact on the type of microbial community involved.

Because FBBRs and packed beds are relatively new. few studies have been done to characterize the microbial communities involved. However, we would expect them to be very similar to those in suspended growth bioreactors, being comprised primarily of bacteria and protozoa. In contrast, TFs and RBC's contain more diverse microbial communities containing many other Eucarya, notably nematodes, rotifers, snails, sludge worms, and larvae of certain insects. " This more complex food chain allows more complete oxidation of organic matter, with the net result that less excess biomass is produced. This has the beneficial effect of decreasing the mass of solid material that must be disposed of.

The Bacteria form the base of the food chain by acting on the organic matter in the wastewater being treated. Soluble materials are taken up rapidly, while colloidal-sized particles become entrapped in the gelatinous layer built up by the bacteria to form the biofilm. There they undergo attack by extracellular enzymes, releasing small molecules that can be metabolized. The bacterial community is composed of primary and secondary saprophytes, much like suspended growth bioreactors. including members of the genera Achromobucterium. Alculigenes. Flavohtictc-rium. I'scudonwnus, Spluierolilus, and Zoogleu. Unlike suspended growth cultures, however, the species distribution is likely to change with position in the reactor. Attached growth reactors can also contain nitrifying bacteria, such as members of the genera Nitrosomonas and Nitrobacter. which tend to be found in regions of the film where the organic substrate concentration is low.

Quite extensive communities of Eucarya arc known to exist in trickling fillers. Over 90 species of fungi have been reported, and of these, more than 20 species are considered to be permanent members of the community. Their role is similar to that of the bacteria, i.e., saprophytic. Many protozoa have also been found, with large communities of Sarcodina, Mastigophora, and Ciliata being reported. Their roles are largely those of predators. During warm summer months algae can flourish on the upper surfaces of the biomass. Usually green algae and diatoms predominate. Finally, trickling filters also contain a large metazoan community, consisting of annelid worms, insect larvae, and snails. These feed on the microbial film and in some cases have been responsible for extensive film destruction.

Because of the diverse nature of the microbial community in attached growth bioreactors. the microbial interactions are extremely complex. Unfortunately, even less is known about the impact of these interactions on system performance than is known about them in suspended growth systems.

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