Microbial Ecology

Microbial ecology as applied to the activated sludge process and the anaerobic digester is the review of the significant groups of wastewater organisms and the operational conditions in each biological treatment unit. This review includes the effects of abiotic and biotic factors upon the organism including their activity and growth—that is, wastewater treatment efficiency. Biological treatment units are simply biological amplifiers—that is, the removal or degradation of waste results in an increase in the number of organisms (sludge). Therefore, acceptable activity and growth of the organisms or biomass is acceptable wastewater treatment.

Collectively, all organisms and operational conditions are interrelated by the transfer of carbon and energy through a food chain (Figure 2.1) or more appropriately a food web (Figure 2.2). Within the food web there are numerous habitats, niches, and relationships (symbiotic and predator-prey) that determine the success or failure of the biological treatment unit to treat wastewater.

Abiotic factors are the nonliving components or operational conditions in a biological treatment unit that affect the activity and growth of the biomass. For example, a decrease in pH of the activated sludge process favors the proliferation of filamentous fungi and disfavors the growth of bacteria, and a decrease in pH in the anaerobic digester favors the growth of fermentative bacteria and disfavors the growth of methane-forming bacteria. Biotic factors are the living components or organisms in a biological treatment unit. Each organism has an effect upon other organisms (predator-prey and symbiotic relationships) and abiotic factors in the biological treatment unit. For example, free-swimming ciliated protozoa increase in number in the presence of large numbers of dispersed bacterial cells. However, during floc formation the number of dispersed bacterial cells decreases and, consequently, the number of free-swimming ciliated protozoa decrease in number. In the

Methane (CH4)

Methane (CH4)

Acetate, carbon dioxide, hydrogen ▲

acetogenic (acetate-forming) bacteria

Alcohols and fatty acids ▲

fermentative (acid-forming) bacteria

Simple sugars, organic acids, and amino acid hydrolytic bacteria

Complex substrates (polysaccharides, lipids, and proteins) in digester sludge

FIGURE 2.1 Transfer of carbon and energy through an anaerobic digester food chain. Carbon and energy enter the anaerobic digester in the form of large, complex organic molecules such as polysaccharides, lipids, and proteins. These compounds are degraded to smaller and simpler compounds through step-by-step biochemical reactions by a diversity of bacterial groups to methane. Through each biochemical reaction, bacteria are produced.

activated sludge process, nitrifying bacteria decrease alkalinity and pH, while denitrifying bacteria increase alkalinity and pH.

In the anaerobic digester, four different groups of bacteria have a symbiotic relationship (Figure 2.3). Fermentative bacteria increase the quantities of carbon dioxide and hydrogen (H2), while hydrogenotrophic methane-forming bacteria decrease the quantities of carbon dioxide and hydrogen. Hydrogenotrophic methane-forming bacteria combine carbon dioxide and hydrogen to form methane. By using

FIGURE 2.2 Transfer of carbon and energy through an activated sludge food web. Carbon and energy enter the activated sludge process in the form of cBOD and nBOD and alkalinity. These carbon and energy substrates are used by a variety of organisms in the activated sludge process, and many of the organisms that grow from these substrates in turn are used as substrates by other organisms. The transfer of carbon and energy in the activated sludge process is between many groups of organisms in a "web-like" pattern.

FIGURE 2.2 Transfer of carbon and energy through an activated sludge food web. Carbon and energy enter the activated sludge process in the form of cBOD and nBOD and alkalinity. These carbon and energy substrates are used by a variety of organisms in the activated sludge process, and many of the organisms that grow from these substrates in turn are used as substrates by other organisms. The transfer of carbon and energy in the activated sludge process is between many groups of organisms in a "web-like" pattern.

hydrogen to produce methane, the hydrogen pressure in the anaerobic digester decreases. This decrease in hydrogen pressure enables acetogenic bacteria to produce acetate (CH3COOH). Acetoclasitc methane-forming bacteria use acetate to produce methane and carbon dioxide. The hydrogenotrophic methane-forming bacteria also combine the carbon dioxide produced by the acetogenic bacteria with hydrogen to form methane.

However, when the hydrogenotrophic methane-forming bacteria are inhibited, the hydrogen pressure increases in the anaerobic digester. The increase in hydrogen pressure inhibits acetogenic bacteria. This results in a decrease in acetate production and consequently a decrease in methane production by acetoclastic methane-forming bacteria.

Organic acids and alcohols

Organic acids and alcohols

FIGURE 2.3 Symbiotic relationships in an anaerobic digester.

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