Bacteria can be classified in many ways; however, the most important from an engineering perspective is operational. Consequently, we will focus on it.

Like all organisms, members of the domain Bacteria derive energy and reducing power from oxidation reactions, which involve the removal of electrons. Thus, the nature of the electron donor is an important criterion for their classification. The two sources of electrons of most importance in biochemical operations are organic and inorganic compounds that are present in the wastewater or released during treatment. Bacteria that use organic compounds as their electron donor and their source of carbon for cell synthesis are called heterotrophic bacteria, or simply heterotrophs. Since the removal and stabilization of organic matter are the most important uses of biochemical operations, it follows that heterotrophic bacteria predominate in the systems. Bacteria that use inorganic compounds as their electron donor and carbon

"Recognition of the distinction between Bacteria and Archaea is relatively recent. Consequently, it is still common for members of both domains to be referred as bacteria, in reference to their procaryotic nature. In this book, the term bacteria (with a lower case "b") will be used to refer to procaryotes in general, without regard to their domain.

dioxide as their source of carbon are chenioautotrophic bacteria, although most wastewater treatment engineers call them autotrophic bacteria, or simply autotrophs. The most important autotrophic bacteria in biochemical operations are those that use ammonia-N and nitrite-N. They are responsible for nitrification, and arc referred to as nitrifiers. Other autotrophic bacteria are important in nature and in sewers, but play little role in engineered treatment systems.

Another important characteristic of bacteria is the type of electron acceptor they use. The most important acceptor in biochemical operations is oxygen. Bacteria that use only oxygen are called obligately aerobic bacteria, or simply obligate aerobes. Nitrifying bacteria are the most signiticant obligately aerobic bacteria commonly found in biochemical operations. At the other end of the spectrum arc obligately anaerobic bacteria, which can only function in the absence of molecular oxygen. Between the two obligate extremes are the facultatively anaerobic, or simplv facultative, bacteria. They use oxygen as their electron acceptor when it is present in sufficient quantity, but can shift to an alternative acceptor in its absence. Thus they tend to predominate in biochemical operations. Some facultative bacteria are fermentative, meaning that they use organic compounds as their alternative terminal electron acceptor in the absence of oxygen, producing reduced organic end products. Others perform anaerobic respiration, in which an inorganic compound serves as the alternative acceptor. In Chapter 1, mention is made of anoxic environments in which oxygen is absent, but nitrate is present as an electron acceptor. Because of the prevalence of such environments in biochemical operations, the most significant facultative bacteria are those that perform denitrification, i.e., reduce nitrate-N to nitrogen gas. Other facultative and obligately anaerobic bacteria reduce other inorganic compounds. but with the exception of protons (H ), most are not of general importance in biochemical operations. Proton reduction, which occurs in anaerobic operations, yields hydrogen gas (H ). which is an important electron donor for methane formation.

Grav ity sedimentation is the most common method for removing biomass from the effluent from biochemical operations prior to its discharge. Since single bacteria are so small (— 0.5-1.0 p.m), it would be impossible to remove them in that way if they grew indiv idually. Fortunately, under the proper growth conditions, bacteria in suspended growth cultures grow in clumps or gelatinous assemblages called bio-floe. which range in size from 0.05 to 1.0 mm," Figure 2.2a shows a typical Hoc particle. The bacteria which are primarily responsible for this are called floc-lorming bacteria, and a variety of species fall into this category.

Not all bacteria arc beneficial in biochemical operations; some are a nuisance. Two forms of nuisance bacteria can grow in aerobic/anoxic systems. One grows as long strands, or filaments, which become intermeshed with biofloc particles and interfere with sedimentation. They are called filamentous bacteria. Although a small number of filaments can provide strength for the biofloc, preventing its disruption by fluid shear forces, too many can act to hold the biofloc particles apart. " as shown in Figure 2.2b. When that occurs, sedimentation is very inefficient and the biomass will not compact into a sufficiently small volume to allow discharge of a clear effluent. The other type of nuisance bacteria forms copious quantities of loam in bioreactors that are being aerated for oxygen transfer. The foam can become so deep as to completely cover both aeration and sedimentation basins, thereby disrupting, treatment and posing a danger to plant personnel. The most common nuisance or-

Sludge Good Settling
Figure 2.2 Photomicrographs of activated sludge Hoc: (a) Good settling biomass with optimal filaments; (b) poor settling biomass with excessive filaments. [Courtesy of M. G. Richard (Colorado State Univ.) and David Jenkins (Univ. California, Berkeley.)]

ganisms in anaerobic systems are the sulfate-reducing bacteria. It is generally desirable to design anaerobic operations to produce methane because it is a valuable product. If a wastewater contains high concentrations of sulfate, however, sulfate-reducing bacteria will compete for the electron donor, producing sulfide as a product. This not only reduces the amount of methane produced, but results in a product that is both dangerous and undesirable in most situations. Wastewater treatment engineers need to be aware of the growth characteristics of such nuisance organisms so that systems that discourage or prevent their growth can be designed.

Bacteria can also be classified according to their function in biochemical operations. Many act as primary degraders and attack the organic compounds present in the wastewater, beginning their degradation. If an organic compound is one normally found in nature (biogenic), the primary degraders usually will completely metabolize it in an aerobic environment, converting it to carbon dioxide, water, and new biomass. Such ultimate destruction is called mineralization and is the goal of most wastewater treatment systems. On the other hand, if an organic compound is synthetic and foreign to the biosphere (xenobiotic), it is possible that no single type of bacteria will be able to mineralize it. Instead, a microbial consortium may be required, with secondary degraders living on the metabolic products excreted by the primary degraders. The more complex the organic compounds found in a wastewater, the more important secondary degraders will be. Secondary degraders are common in anaerobic environments, however, even when biogenic compounds are being degraded, because of the specialized needs of the bacteria involved. Other functions that are important in wastewater treatment systems are the production and elimination of nitrate-N through nitrification and denitrification, respectively. Consequently, it is not surprising that bacteria are classified according to those functions, as nitrifiers and denitrifiers. While the nitrifiers constitute a highly specialized group containing a limited number of species of aerobic, chemoautotrophic bacteria, the denitrifying bacteria constitute a diverse group of facultative heterotrophic bacteria containing many species. Finally, some species of bacteria have the ability to store and release phosphate in response to cyclical environmental conditions. Because they contain quantities of phosphate well in excess of other bacteria, these bacteria are often called phosphate accumulating organisms (PAOs).

As with the classification of pollutants in wastewaters, the classifications listed above are not exclusive, but overlap, with members of the domain Bacteria playing many roles. Nevertheless, these simple classification schemes are very helpful in describing the events occurring in biochemical operations and will be used throughout this book.

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