Aerobic Anoxic Operations

Suspended Growth Bioreuctors. Activated sludge, aerated lagoons, and aerobic digesters have similar microbial ecosystems, although they differ somewhat in the relative importance of various groups. The microorganisms in those operations are all Bacteria and microscopic Eucarya, and generally may be divided into five major classes: (1) Hoc-forming organisms, (2) saprophytes, (3) nitrifying bacteria, (4) predators, and (5) nuisance organisms."' With the exception of nitrifying bacteria, these are not distinct physiological groups and, in fact, any particular organism may fit into more than one category at a time or may change categories as the selective pressures within the community change.

Floc-forming organisms play a very important role in suspended growth biochemical operations because without them the biomass cannot be separated from the treated wastewater nor can colloidal-sized organic pollutants be removed. Figure 2.2a shows typical, well-settling biomass. Originally it was thought that the bacterium Zoogleu rumigeru was primarily responsible for floe formation, but it has now been shown that a variety of bacteria are capable of flocculation,^ although they constitute only a small percentage of the species found in a floe particle/' Classification of organisms into the floc-forming group is complicated by the fact that protozoa and fungi can also cause bacteria to flocculate." Nevertheless, the predominant floc-forming organisms are generally considered to be bacteria," with Zoogleu rumigera playing an important role."" Flocculation is thought to be caused by aggregative growth and natural polyelectrolytes, although their origin is uncertain.

Saprophytes are organisms responsible for the degradation of organic matter. These are primarily heterotrophic bacteria and include most of those considered to be floe formers. Nonflocculent bacteria are also involved, but are entrapped within the Hoc particles. The saprophytes can be divided into primary and secondary de-graders, as discussed previously, and the larger the number of substrates, the more diverse the community will be. The principal saprophytic genera are gram-negative and include Achromobacter, Alcaligenes, Bacillus, Flavobacterium, Micrococcus, and Pseudomonus."

Nitrification is the conversion of ammonia-N to nitrate-N and it may be performed by either heterotrophic or autotrophic bacteria." In spite of the fact that over a hundred heterotrophic species have been cited as forming nitrite from ammonia, v significant amounts of nitrate are not thought to be generated heterotrophicallv in natural systems.,J although studies suggest that this assumption should be investigated further." Nevertheless, nitrification in wastewater treatment systems is generally considered to be due to autotrophic bacteria, primarily of the genera Nitroso-monas and Nitrobacter, which appear to grow in close physical association.1"' Nitrosomonas oxidizes ammonia-N to nitrite-N with hydroxylamine as an intermediate product, whereas Nitrobacter oxidizes nitrite-N to nitrate-N in a single step. The fact that nitrifying bacteria are autotrophic does not mean that they cannot incorporate exogenous organic compounds while obtaining their energy from inorganic oxidation, because they can.' The amount of such uptake will be small and will vary with the growth conditions, however, so that most equations depicting the stoichiometry of nitrification ignore it and use carbon dioxide as the sole carbon source. Nitrifying bacteria have several unique growth characteristics that arc important to their impact on and survival in biochemical operations. The first is that their maximal growth rate is smaller than that of heterotrophic bacteria. Consequently, if suspended growth biorcactors are operated in a way that requires the bacteria to grow rapidly, the nitrifying bacteria will be lost from the system and nitrification will stop even though organic substrate removal will continue. Second, the amount of biomass formed per unit of nitrogen oxidized is small. As a result, they may make a negligible contribution to the MLSS concentration even when they have a significant effect on process performance.

The main predators in suspended growth bioreactors are the protozoa, which feed on the bacteria. About 230 species have been reported to occur in activated sludge and they may constitute as much as 5% of the biomass in the system."' Ciliates are usually the dominant protozoa, both numerically and on a mass basis. Almost all are known to feed on bacteria and the most important are either attached to or crawl over the surface of biomass floes. On occasion, both amoeba and flagellates may be seen in small numbers, but they are not thought to play a major role in wellsettling, stable communities. As discussed earlier, it has been suggested that protozoa play a secondary role in the formation of biomass floes and contribute to the absence of dispersed bacteria and colloidal organic material in stable communities."

Nuisance organisms are those that interfere with proper operation of a biochemical reactor when present in sufficient numbers. In suspended growth bioreactors, most problems arise with respect to removal of the biomass from the treated wastewater, and arc the result of filamentous bacteria and fungi. Although a very small number of filamentous bacteria is desirable to strengthen floe particles, too large a number is undesirable. " Even a small percentage by weight in the microbial community can make the effective specific gravity of the biomass floes so low that the biomass becomes very difficult to remove by gravity settling. This leads to a situation known as bulking. A poor-settling biomass is shown in Figure 2.2b. For many years it was thought that the bacterium Sphaerotihts nutans was the organism primarily responsible for bulking, but the conditions causing its growth were a puzzle because they appeared to be so contradictory. It was not until the pioneering work of Eikel-boom" that it was realized that many types of filamentous organisms could be responsible for bulking, and that different organisms were favored by different growth conditions. Today, effective bulking control is based on identification of the causative organism and elimination of the condition favoring its growth." Table 2.1 ranks the most abundant filamentous organisms found in bulking sludges in the United States and Table 2.2 lists the suggested causes for some. In Table 2.2, the term, "low F/M," refers to a low food to microorganism ratio; in other words, the system is being operated with a very low loading of organic matter into it. It should be noted that although Nocardia is a commonly found filamentous organism, it does not normally cause bulking because its filaments do not extend beyond the floe particle."

The other major nuisance associated with suspended growth cultures is excessive foaming. This condition is caused primarily by bacteria of the genus Nocardia and the species Microthrix parvicellaThere is still controversy concerning the conditions responsible for excessive foaming in suspended growth cultures. Because Nocardia and M. parvicella have very hydrophobic cell surfaces, they migrate to air bubble surfaces, where they stay, thereby stabilizing the bubbles and causing foam." Foaming also appears to be related to the concentration of hydrophobic organic-compounds at the air-water interface, where they are metabolized by the Nocardia and Nocardia-Wkc organisms that have collected there.

Table 2.1 Filament Abundance in Bulking and Foaming Activated Sludge in the United States

Percentage of treatment plants with bulking or foaming where filament was observed

Rank

Filamentous organism

Dominant

Secondary

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